Insight Editions · No. 86
Parallel Worlds
A Journey Through Creation, Higher Dimensions, and the Future of the Cosmos
Michio Kaku
Michio Kaku surveys modern cosmology, from the Big Bang and cosmic inflation to quantum theory, string theory, the multiverse, and possible futures of the universe.
A study guide to Parallel Worlds by Michio Kaku. Shared free for personal reading; not for sale and not endorsed by the authors or publishers. Please read and buy the original.
The Book in Brief
The argument in brief
- Precision cosmology. WMAP fixed the universe's age at 13.7 billion years and its contents at 4% atoms, 23% dark matter and 73% dark energy.
- Inflation implies a multiverse. Chaotic inflation keeps budding off new universes, and that may be how ours began.
- Gateways. Spinning black holes, wormholes and negative energy allow, on paper, travel between universes and through time, though Hawking's chronology protection may forbid it.
- Quantum branching. Measurement problems lead to Wigner's conscious observers, Wheeler's participatory universe or Everett's many worlds.
- M-theory. In eleven dimensions with branes, our universe may be a membrane floating near others, and gravity's weakness may be a clue to this.
- Design or multiverse. Rees' six finely tuned numbers argue for many universes rather than a designer.
- Test it. LIGO, LISA, WMAP's successors, dark matter detectors and the LHC look for indirect echoes.
- The end and the exit. The big freeze dooms life in our universe. A type III civilization might escape through a wormhole or send a "seed" of itself.
- Meaning. A unique final equation suggests some design, but it is up to us to create our own meaning.
Argument map: from the big bang to escaping the universe.
| Ch | Title | Core point |
|---|---|---|
| Pref | The third revolution | Precision data, inflation and M-theory are reshaping cosmology |
| 1 | Baby pictures of the universe | WMAP's portrait; a preview of the multiverse and the escape |
| 2 | The paradoxical universe | Bentley and Olbers' paradoxes; Einstein; Friedmann's fates |
| 3 | The big bang | Hubble, Gamow, Hoyle; nucleosynthesis; the CMB; dark matter |
| 4 | Inflation and parallel universes | Guth's false vacuum; Linde's chaotic inflation and budding universes |
| 5 | Dimensional portals and time travel | Black holes, Kerr rings, wormholes, negative energy, paradoxes |
| 6 | Parallel quantum universes | The cat; Wigner, Wheeler, Everett; decoherence |
| 7 | M-theory | Strings, supersymmetry, eleven dimensions, branes, holography |
| 8 | A designer universe? | Anthropic principle; Rees' six numbers; Smolin's evolving universes |
| 9 | Echoes from the eleventh dimension | GPS, LIGO, LISA, lensing, dark matter searches, LHC |
| 10 | The end of everything | Thermodynamics; crunch vs freeze; five eras; can thought survive? |
| 11 | Escaping the universe | Kardashev and Sagan scales; eleven steps; the cosmic seed |
| 12 | Beyond the multiverse | Copernican vs anthropic; quantum meaning; make your own meaning |
Argument map: from baby picture to escape plan
The Third Revolution in Cosmology
- The big freeze. The new data show the universe "expanding in a runaway mode, accelerating without limit", heading for cold and darkness in which "all intelligent life dies out".
- Three parts. Part 1 covers the universe up to inflation, "the most advanced formulation to date of the big bang theory". Part 2 covers the multiverse, wormholes, time warps, higher dimensions and M-theory. Part 3 covers the end of the universe and how an advanced civilization might escape into another universe or the past.
- Audience. Unlike his Hyperspace and Beyond Einstein, which focus on space-time, this book focuses on cosmology and needs no prior physics.
The book's arc in four moves
Baby Pictures of the Universe
Did God have a mother?
As a child raised by Buddhist parents but sent to Sunday school, Kaku asked, "Did God have a mother?" The teacher had no answer. He had stumbled on the split between creatio ex nihilo myths (Chinese P'an Ku hatching from a cosmic egg, Greek Chaos, Egyptian Ra, the Polynesian coconut, the cyclic Mayan universe) and timeless ones. The Hindu Mahapurana asks: "If God created the world, where was He before Creation?... the world is uncreated, as time itself is, without beginning and end." The two seem mutually exclusive.
The WMAP satellite
Launched on 30 June 2001 to the Lagrange point L2, a million miles from Earth, the Wilkinson Microwave Anisotropy Probe mapped the microwave afterglow released 380,000 years after the big bang. It weighed 840 kg and ran on 419 watts. Its February 2003 results marked, in John Bahcall's words, "a rite of passage for cosmology from speculation to precision science." COBE (1992) had produced only a blurry "baby picture"; if the universe were an 80-year-old, the background shows him under a day old.
How WMAP turned the microwave afterglow into precision cosmology.
- The sky is a time machine. Light from the Moon is a second old, from the Sun eight minutes, from quasars 12-13 billion years.
- The Empire State Building. If the base is the big bang, distant galaxies sit on the tenth floor, quasars on the seventh, and the microwave background half an inch above the street.
- Seeds of galaxies. WMAP's "random dots" are tiny temperature fluctuations (in a sky at about 2.725 K, predicted by Gamow's group in 1948) that grew into today's galaxy clusters.
Cosmology had a poor reputation. Lev Landau quipped that "cosmologists are often in error but never in doubt", and an old adage ran: "There's speculation, then there's more speculation, and then there's cosmology." As a Harvard student, Kaku found the field "embarrassingly primitive", full of feuds; Allan Sandage would roar, "It's war, it's war!"
Age and contents
| Quantity | WMAP (book) | Planck 2018 (now) |
|---|---|---|
| Age | 13.7 billion years (1%) | 13.8 billion years |
| Ordinary matter | 4% | 4.9% |
| Dark matter | 23% | 26.8% |
| Dark energy | 73% | 68.3% |
The universe's contents as WMAP measured them, and its age.
The heavy elements that make up our world are only about 0.03% of the universe. Earlier age estimates of 1-2 billion years had been younger than Earth (4.5 billion) and the oldest stars, an embarrassment now resolved. Dark matter outweighs the Milky Way's stars by about ten to one and is detected by how it bends light. Dark energy, the energy of empty space, is Einstein's 1917 cosmological constant revived. Theory predicts a value off by 10120, "the largest gap ever found in the history of science". Craig Hogan: "everybody's clueless about it."
Inflation, multiverse and strings
- Inflation (Alan Guth): in the first trillionth of a trillionth of a second, an antigravity force expanded space by a factor of about 1050. Space itself can expand faster than light. Our visible universe is a tiny circle on a vast balloon, which is why it looks flat. Joel Primack: "No theory as beautiful as this has ever been wrong before."
- The multiverse (Andrei Linde): since no one knows what triggered inflation, it may happen again and again, budding "baby universes" like splitting soap bubbles: eternal or chaotic inflation. Guth: "Inflation pretty much forces the idea of multiple universes upon us." Martin Rees: our universe "belongs to the unusual subset that permits complexity and consciousness."
- M-theory. Once the province of "mystics, charlatans and cranks", parallel universes became mainstream with string theory: particles are notes on tiny vibrating strings in 10- or 11-dimensional hyperspace. The "M" may stand for membrane, mystery, magic or mother.
The chain from inflation to a multiverse that string theory may describe.
| Music | String counterpart |
|---|---|
| Musical notation | Mathematics |
| Violin strings | Superstrings |
| Notes | Subatomic particles |
| Laws of harmony / melodies | Physics / chemistry |
| Symphony; music in hyperspace | Universe; "Mind of God" |
| Composer | ? (taken up in chapter 12) |
The end, and the escape
The same antigravity now accelerates the expansion. Adam Riess: the universe is like a driver who slows for a red light, then floors it on green. Within 150 billion years almost all galaxies will slip beyond view, leaving only the Local Group; future astronomers might conclude the universe is static. Then the big freeze: stars die, black holes evaporate, and "a lifeless, cold mist" of particles remains. Darwin called the prospect of total annihilation "an intolerable thought".
WMAP's baby picture in detail
WMAP, launched 30 June 2001, mapped the microwave sky with enough precision to turn cosmology into a quantitative science. Its power spectrum of hot and cold spots encodes the universe's age, geometry and contents. Kaku stresses three deliverables:
- Age. About 13.7 billion years (Planck later revised to 13.8), ending decades of debate in which some stellar ages seemed older than the universe.
- Geometry. Spots of about 1° imply spatial flatness: Omega (matter plus dark energy) equals 1 to high accuracy.
- Contents. Roughly 4% ordinary atoms, 23% dark matter and 73% dark energy in the book's figures (Planck shifted these slightly toward more dark matter and less dark energy).
The ripples are the inflated relics of quantum fluctuations: without them, galaxies would not have grown. Kaku likens the map to a doctor's first ultrasound of a baby, except the "baby" is the cosmos at 380,000 years.
What WMAP measured and why it mattered
The Local Group and the dark sky ahead
Our Local Group (Kaku says about 36 galaxies; modern counts exceed 80) is gravitationally bound and will not join the runaway expansion of distant clusters. Within it, Andromeda approaches for a merger billions of years hence ("Milkomeda"). Beyond, accelerating expansion pushes galaxies over the horizon. The book's later chapters ask whether intelligence can outlast that freeze.
Genesis, Nirvana and the bubble picture
Kaku opens with two ancient images: Genesis (a moment of creation) and Nirvana (a timeless ocean). Precision cosmology, he argues, synthesises them. Our universe is a bubble that burst into being in a fiery bang, floating in a much larger arena of eleven-dimensional hyperspace where new bubbles form continually. Big bangs may be happening "even as you read this sentence." Some sister universes may be barren; some may diverge from ours by a single quantum event; a few physicists speculate that one day, when our cosmos grows cold, intelligence might have to leave. That speculation drives the book's later chapters.
Cosmology before WMAP was "a collection of loose, highly speculative theories," with fierce feuds when data were scarce. Allan Sandage of Mount Wilson, about to speak on the age of the universe, heard the previous speaker announce "What you will hear next is all wrong," and roared of rivals' publicity, "That's a bunch of hooey. It's war - it's war!" Priests once dated creation from Adam and Eve; geologists used residual radiation in rocks; WMAP measured the echo of creation itself. String theory and M-theory now dominate the search for what happened before the bang: tens of thousands of papers, hundreds of conferences, groups at every major university, still awaiting tests by gravity-wave detectors and atom smashers. Einstein spent his last thirty years on a failed theory of everything; Kaku presents strings as the leading candidate to finish that job.
Precision as a culture change
WMAP did not only deliver numbers; it changed the sociology of cosmology. Arguments that once hinged on rhetoric now hinge on error bars. Kaku's excitement is that a subject formerly closer to myth has acquired the texture of particle physics: parameters, likelihoods, ruled-out regions. The open problems that remain (Hubble tension, dark matter's identity, dark energy's nature, the multiverse's testability) are sharper because the settled background is sharper. That is the sense in which the "baby picture" is also a coming-of-age picture for the field.
What the power spectrum hides
Acoustic peaks in the microwave spectrum record the plasma's sound waves at recombination. Peak positions fix curvature; peak heights help fix dark-matter and baryon densities; the overall amplitude matches inflation's nearly scale-invariant seed spectrum. Kaku translates that technical story into the claim that we have weighed the universe and found mostly darkness. The remaining tension between early- and late-universe expansion rates (Hubble tension) is the largest crack in that consensus picture as of 2026.
Contents of the cosmos (book figures, later refined by Planck).
The Paradoxical Universe
Newton sets the stage
Medieval comets were omens; the comet of 1066 was blamed for King Harold's defeat by William the Conqueror. When it returned in 1682, Edmund Halley asked Newton what force moved it. Newton replied that it followed an ellipse under an inverse-square law. "How do you know?" "Why, I have calculated it." Halley paid for the Principia (1687), and Pope wrote: "God said, Let Newton Be! and all was light." Halley matched the comets of 1531, 1607 and 1682 and predicted a return in 1758; it arrived on Christmas Day.
Newton had found gravity at Woolsthorpe during the plague: if an apple falls, does the Moon fall too? He invented calculus to solve the problem. His laws underpin every bridge and rocket, and they joined heaven and Earth: "the same laws that applied to the actors also applied to the set."
Bentley's paradox
Lucretius argued the universe could have no edge, since a limit needs something outside it. In 1692 the Rev. Richard Bentley wrote to Newton: gravity only attracts, so a finite universe should collapse into one fiery mass, while in an infinite one the pulls would be infinite and tear stars apart. Newton's loophole: an infinite, perfectly uniform universe in which pulls cancel. But he knew it was a house of cards; nudge one star and clusters collapse. "A continual miracle is needed to prevent the Sun and the fixt stars from rushing together through gravity."
Bentley's paradox: a finite or an infinite Newtonian universe each fails.
Olbers' paradox: why is the night sky dark?
If the universe is infinite and uniform, every line of sight ends on a star, so the night sky should blaze. Kepler escaped by enclosing the universe in a shell. A 1987 study found that 70% of astronomy textbooks gave the wrong answer.
Olbers' paradox and its resolution: a finite age and limited starlight.
| Proposed answer | Why it fails or works |
|---|---|
| Dust absorbs the light (Olbers, 1823) | Fails: over infinite time the dust heats up and glows |
| Distant stars are fainter | Fails: brightness falls as 1/r², but the number of stars rises as r² |
| Finite shell (Kepler) | Ad hoc |
| The universe has a finite age (Poe, 1848; Kelvin, 1901) | Works: light from far enough away has not reached us yet; stars also have finite lives |
The Hubble Ultra Deep Field (released 2004), built from hundreds of hours of exposure over four months, showed about ten thousand infant galaxies more than 13 billion light-years away. "We might have seen the end of the beginning" (Anton Koekemoer). Between them lies blackness, which is really the microwave background. "The night sky is not really black at all... beyond the farthest star lies creation itself."
Einstein: time slows, space bends
After graduating from Zurich in 1900, Einstein was unemployable, sabotaged by professors, fired from a tutoring job, and in despair ("It would surely be better if I did not live at all"). Mileva Maric bore a daughter out of wedlock, whose fate is unknown, and his father died thinking him a failure. Marcel Grossmann got him a clerk's job at the Bern Patent Office.
Einstein's equations forced a moving universe, so he added a cosmological constant.
- Racing light. At sixteen, inspired by Aaron Bernstein's People's Book on Natural Science, he asked what a light beam would look like if you caught up with it. Frozen light doesn't exist. Maxwell's equations say light moves at the same speed c in every inertial frame, so Newton and Maxwell clash.
- The 1905 "storm". Time beats at different rates depending on how fast you move. Lengths contract (the Lorentz-FitzGerald contraction) and mass increases, and E = mc² explains the energy of the stars. On Einstein's stage, every actor's wristwatch reads a different time.
- The vanishing Sun. Newtonian gravity acts instantly, but relativity limits signals to light speed. Planck warned him off: "you will not succeed, and even if you succeed, no one will believe you."
The birth of cosmology
Applying the theory to a uniform universe, Einstein found it would not stay still: Bentley's paradox again. In 1917 he added the cosmological constant, an antigravity "fudge factor" tuned to cancel gravity. The same year Willem de Sitter found a universe with no matter at all whose vacuum energy alone drives exponential expansion. Einstein's universe had "matter but no motion"; de Sitter's had "motion but no matter".
Kaku's moral: relativity offends common sense not because it is wrong but because "our common sense evolved in a highly unusual, obscure part of the universe", with mild temperatures and speeds.
Friedmann's three numbers
Aleksandr Friedmann (1922; he died in 1925) assumed the universe is isotropic and homogeneous (the cosmological principle). Einstein's and de Sitter's solutions became special cases of his, which depend on three parameters:
| Parameter | Meaning | Role |
|---|---|---|
| H (Hubble's constant) | Rate of expansion | The rock's upward speed |
| Omega | Density / critical density (about one H atom per three basketballs) | Gravity's brake |
| Lambda | Energy of empty space (dark energy) | Antigravity accelerator |
Friedmann's fates for Lambda = 0: matter density decides the universe's geometry and end.
An open universe is infinite in space and time, with triangles summing to less than 180 degrees. A closed one is finite and may bounce into a new big bang: the oscillating universe. WMAP showed Omega plus Lambda equals 1, a flat universe. But Friedmann's picture left a hole: an expanding universe implies a beginning, about which Einstein's equations said nothing.
Olbers, Poe and Einstein's river of time
Olbers' paradox asks why the night sky is dark if infinite stars fill an eternal universe: every line of sight should end on a star. Edgar Allan Poe guessed that light from distant stars has not had time to reach us. Expanding-universe cosmology makes that precise: the observable universe has a finite age and a finite horizon. Einstein's 1915 general relativity replaced Newton's absolute stage with a flexible space-time; gravity is the bending of that fabric. Light grazing the Sun during the 1919 eclipse bent by about 1.75" (Einstein's prediction), making him world-famous overnight. Gravity waves are ripples in the fabric; Kaku's "bouncing bed" image awaited LIGO. The Ultra Deep Field (chapter's Hubble image) shows galaxies almost to the edge of the observable past, filling Poe's dark sky with unfinished light.
From Olbers' dark sky to an expanding, finite-age cosmos.
Critical density and the bent starlight
The critical density that separates eternal expansion from recollapse is only about 10 hydrogen atoms per cubic metre in the book's figure (modern value closer to 5-6 per cubic metre). Below it the universe is open; above it, closed; at it, flat. Inflation drives the cosmos to that knife-edge. Eddington's 1919 teams at Principe and Sobral measured starlight deflection near the Sun; modern radio measurements confirm Einstein to better than 0.01%. Gravity waves remained unheard until LIGO; the chapter's "bouncing bed" is no longer metaphor. JWST's early galaxies push Poe's unfinished light still closer to the big bang, without erasing the dark-sky resolution that a finite age provides.
Geometry of expansion
Friedmann's solutions of Einstein's equations classify universes by curvature and contents. Hubble's redshift-distance relation (now the Hubble-Lemaître law) showed recession; Lemaître had the expansion interpretation earlier. Dark energy's discovery remade the classification: a flat universe can still accelerate. The chapter's path from Olbers to Einstein to deep fields is Kaku's way of showing that each paradox, once resolved, opens a sharper one, ending at inflation and the multiverse in the next chapters.
Absolute space retired
Newton needed a fixed stage; the bucket argument was his proof. Einstein removed the stage: only relative intervals matter, and matter tells space-time how to curve. The chapter's sequence from Olbers through 1919 to gravity waves is the public history of that retirement. What remains absolute is the speed of light in vacuum and the demand that laws look the same to all free-falling observers. Those leftovers are still enough to forbid easy time machines and to permit hard ones on paper.
Retiring Newton's stage
The Big Bang
In March 1995 Time declared "Cosmology is in chaos": Hubble data on one galaxy, M100, implied a universe of 8-12 billion years, younger than its oldest stars. "You can't be older than your ma" (Christopher Impey). But the loopholes were "big enough to drive the Starship Enterprise through". Any rival theory must explain all the data points the big bang already fits.
Edwin Hubble, patrician astronomer
Born in 1889 in Marshfield, Missouri, Hubble read Jules Verne, boxed well enough to be offered a fight with Jack Johnson, and went to Oxford as a Rhodes scholar, returning with tweeds, a pipe, a British accent and duelling scars (rumoured self-inflicted). He switched from law to the 100-inch telescope at Mount Wilson, "a man in a hurry".
Hubble's two results: other galaxies exist, and they recede in proportion to distance.
- The Great Debate (1920). Harlow Shapley said the Milky Way was the whole universe; Heber Curtis said the spiral nebulae were Kant's "island universes".
- Standard candles. A bright distant star looks like a dim near one. Henrietta Leavitt had found that a Cepheid variable's pulsation period reveals its true brightness. Hubble found one in Andromeda with a 31.4-day period, placing it about a million light-years away (really about 2.5 million), far outside our 100,000-light-year galaxy. In one stroke the universe went from one galaxy to billions.
- Redshift. Light from a receding source is stretched toward red (the Doppler effect, as in police speed guns). Vesto Slipher (from 1912) found that most galaxies are redshifted. Strictly, the redshift comes from space itself stretching between galaxy and Earth.
Lemaître's superatom
The Belgian priest Georges Lemaître reasoned that an expanding universe began fantastically hot and dense, a "superatom" that exploded. "Standing on a well-chilled cinder, we see the slow fading of the suns, and we try to recall the vanished brilliance of the origin of worlds." (Poe had again anticipated him.) Eddington found an abrupt beginning "repugnant", but Lemaître's persistence wore down resistance.
George Gamow, cosmic jester
Gamow (born Odessa, 1904) was a prankster, cartoonist and author of twenty popular books that raised generations of physicists, Kaku included. Edward Teller: "Ninety percent of Gamow's theories were wrong... But he didn't mind." The other ten percent changed science. As a boy he examined communion bread under a microscope and found nothing changed: "the experiment which made me a scientist." He studied under Friedmann, tried to flee the USSR by raft across the Black Sea, and finally defected via a Brussels conference.
Gamow's nuclear kitchen: a hot big bang cooks light elements and leaves an afterglow.
The nuclear kitchen and the afterglow
Gamow's "prehistoric kitchen of the universe" proposed that fusion in the hot early universe built the elements by adding particles to hydrogen. His student Ralph Alpher did the work, and Gamow added Hans Bethe's name for the pun: the alpha-beta-gamma paper (1948). It explained why the universe is about 75% hydrogen and 25% helium by mass. David Spergel: "Every time you buy a balloon, you are getting atoms... made in the first few minutes of the big bang." But nuclei with 5 and 8 particles are unstable, so the chain broke there.
- Fossil heat. A hot origin should leave black-body radiation, the characteristic glow whose colour gives temperature (Wedgwood noticed it in his kilns in 1792; Planck's 1900 formula launched quantum theory). The Sun's yellow means about 6,000 K; Betelgeuse's red, about 3,000 K.
- Alpher and Herman (1948) predicted a microwave afterglow at about 5 K. The universe became transparent 380,000 years after the bang, when it cooled to about 3,000 degrees and atoms could form.
- Ignored. "Nobody bit; nobody said it could be measured... over the period 1948 to 1955, we sort of gave up" (Alpher). Gamow later pushed the figure, wrongly, to 50 K.
Fred Hoyle, contrarian
Born in 1915, the son of a Yorkshire textile merchant, Hoyle scorned Roman numerals ("How could anybody be so daft as to write VIII for 8?"), and was struck on the ear, later deaf, for bringing a six-petalled flower to prove his teacher wrong. With Thomas Gold and Hermann Bondi he devised the steady state theory, allegedly inspired by the circular ghost film Dead of Night (1945). Galaxies recede, but new matter is created continuously, so the universe has no beginning or end.
Steady state against big bang, and the evidence that decided it.
Nucleosynthesis in stars. Hoyle showed how stellar cores build elements up to iron. He bridged Gamow's gap by predicting an unstable, excited state of carbon formed from three helium nuclei, later found in the lab. Beyond iron, fusion yields no energy, so heavier elements (copper, zinc, gold, uranium) are forged in supernovae. The landmark B²FH paper (1957: Burbidge, Burbidge, Fowler, Hoyle) set out the steps. Gamow conceded in mock-Genesis style: God forgot mass five, "And God said, 'Let there be Hoyle.'"
| Element source | Champion | Elements |
|---|---|---|
| Big bang (first minutes) | Gamow | Hydrogen, deuterium, helium-3, helium-4, lithium-7 |
| Stellar cores | Hoyle | Carbon up to iron |
| Supernovae | B²FH | Beyond iron (copper, zinc, gold, uranium) |
Evidence against the steady state
- Evolution. Steady state predicts the young universe looked like today's. But quasars (huge young galaxies powered by black holes) blaze only at great distances: they existed then and are gone now.
- Too much helium. Helium (first seen in the Sun's spectrum in 1868, found on Earth in 1895, hence a misnamed "metal") is about 25% of all atoms and uniformly spread, as Gamow predicted. Stars alone cannot make that much.
Ignition at about 10 million degrees: a main sequence star burns for about 10 billion years (the Sun is halfway).
Red giant: helium burning, swelling to the orbit of Mars.
Sun-like stars end as an Earth-sized white dwarf.
Stars of 10-40 solar masses: an iron core, collapse, a supernova, leaving a neutron star (a city-sized nucleus; a spinning one is a pulsar).
The largest remnants, over about 3 solar masses, become black holes.
Our true "mother" is not the Sun but unnamed stars that died in supernovae and seeded our nebula: "our bodies are made of stardust." A supernova within about ten light-years could end life on Earth. Betelgeuse and Spica are far enough away to be safe; a minor marine extinction about 2 million years ago may trace to one about 120 light-years away.
Bird droppings and the big bang
In 1965 Arno Penzias and Robert Wilson, using the 20-foot Holmdel Horn antenna in New Jersey, found static from every direction. Scrubbing off "a white coating of dielectric material" (pigeon droppings) did not remove it. Meanwhile Robert Dicke's Princeton group had rediscovered Gamow's idea and was building a detector. Bernard Burke connected them, and the "Keystone Kops" story ended with a Nobel for Penzias and Wilson in 1978.
Hoyle conceded in Nature in 1965: "what really killed the steady-state theory was psychology... an important phenomenon which it had not predicted... knocked the stuffing out of me." Gamow privately resented that his group's prediction was rarely credited.
Dark matter
- Zwicky (1930s). Galaxies in the Coma cluster moved so fast that the cluster needed hundreds of times more mass than was visible. Astronomers ignored him, partly because Newton had been rescued before (Neptune, 1846), partly because of his abrasive personality. He co-coined "supernova", predicted neutron stars, and denounced the "spherical bastards" of the astronomy elite ("bastards any way you look at them").
- Vera Rubin. From 1962, she found that stars orbit galaxies at the same speed regardless of distance: flat rotation curves. Galaxies must be about ten times heavier than they look. She faced open sexism ("Have you ever considered a career in which you paint pictures of astronomical objects?"). By 1978 her study of eleven spirals, with Albert Bosma's radio data, convinced the field.
- Candidates. MACHOs (dim ordinary matter: brown dwarfs, black holes), hot dark matter (neutrinos, too fast to clump) and cold dark matter or WIMPs, the leading candidate. Gravitational lensing maps dark matter across the sky.
How dark matter was found: galaxies spin and clusters bind too strongly for visible mass.
Gamow's kitchen and the steady-state fight
Big bang versus steady state: how the afterglow decided
Dark matter enters the story
Fritz Zwicky (1930s) found Coma-cluster galaxies moving too fast for visible mass to bind them. Vera Rubin and Kent Ford (1970s) measured flat spiral rotation curves: stars in the outskirts orbit as fast as those inside, implying massive dark halos. By the book's date, dark matter outweighed atoms several times over; its particle identity remained unknown.
Alpha, beta, gamma and the Cadillac miss
Hoyle's BBC file once read "DO NOT USE THIS MAN"; the network ignored it and his series The Nature of the Universe inspired a generation, including Wallace Sargent. A 1993 Sky and Telescope contest drew 13,000 alternative names for the big bang and found none better. Steady-state continuous creation could not explain why distant radio sources and quasars were more common in the past, nor the black-body spectrum COBE nailed. The nuclear kitchen's light-element yields remain one of the theory's strongest pillars, even while the lithium-7 abundance stays stubbornly low compared with predictions.
Stars as factories
After the nuclear kitchen comes stellar alchemy. Protostars contract and ignite near 10 million degrees; Sun-like stars burn about 10 billion years; red giants swell toward the orbit of Mars; white dwarfs, neutron stars and black holes are the endpoints by mass. Supernovae seed the interstellar medium with carbon, oxygen, iron and the rest of chemistry. Without that sequence, Gamow's light elements would never become planets or people. Dark matter, meanwhile, provides the gravitational scaffolding on which visible galaxies assemble, even while its particle nature stays hidden from detectors.
From rivalry to consensus
The mid-century feud between big bang and steady state was not polite. Hoyle's rhetorical gifts and Gamow's showmanship filled the vacuum left by missing data. Penzias-Wilson, COBE, light-element abundances and source counts ended the feud in the bang's favour without ending controversy inside the bang paradigm: dark matter, dark energy, inflation's details and the lithium problem are unsettled children of a settled parent theory. Kaku's narrative uses that history to prepare the reader for inflation and the multiverse: today's speculative edge may be tomorrow's textbook, or may join steady state in the museum of beautiful wrong ideas.
Evidence that ended the steady state.
Inflation and Parallel Universes
Poul Anderson's novel Tau Zero (1967) imagines a starship that cannot stop accelerating, sees the universe age, survives the big crunch and witnesses a new big bang. It reflects the open debate of its time: freeze, crunch, oscillation or steady state. Since then "a new theory called inflation has emerged."
Four forces and the dream of unification
"SPECTACULAR REALIZATION," Guth wrote in his diary in 1979. Born in 1947 in New Jersey to parents who never attended college, he was drawn at MIT to the search for unification: the idea that today's universe is the shattered remnant of a greater simplicity.
| Force | Carrier | Role | If switched off |
|---|---|---|---|
| Gravity | (graviton, unseen) | Holds planets and stars; 1036 times weaker than electromagnetism | Stars explode, Earth disintegrates |
| Electromagnetism | Photon | Light, electronics, chemistry | Civilization goes dark (as in the 2003 blackout) |
| Weak nuclear | W and Z bosons | Radioactive decay; heats Earth's core | No decay; nuclear medicine fails |
| Strong nuclear | Gluon | Binds nuclei; powers the stars | Atoms fall apart, universe darkens |
A comb with static electricity lifts paper against the gravity of the whole Earth. In the 1950s particle physics was chaos: so many particles that Oppenheimer joked the Nobel should go to whoever did not find a new one. Then came a series of advances:
- Quarks (Gell-Mann and Zweig, early 1960s): three quarks make a proton or neutron.
- Electroweak unification (Weinberg and Salam, 1967; Nobel 1979 with Glashow): W, Z and photon treated on the same footing.
- Quantum chromodynamics (1970s, from SLAC data): gluons bind quarks.
Phase transitions and the false vacuum
At the big bang all forces may have been one "superforce". As the universe cooled it "cracked" like freezing water: the universe is broken because it is old and cold. A false vacuum is like a dammed river: apparently stable, but under pressure, ready to burst toward the true vacuum of lowest energy. Guth's insight was that in the false vacuum space expands exponentially, as de Sitter found in 1917, driven by the vacuum's energy.
Guth's inflation mechanism: a false vacuum drives exponential expansion until it decays.
| Problem | The puzzle | Inflation's answer |
|---|---|---|
| Monopole | GUTs predict many single magnetic poles; none seen ("Like the unicorn...") | Diluted across a vastly larger universe |
| Flatness | Omega near 1 today requires Omega = 1 to one part in 1014 at one second, like a pencil balanced on its tip for years | Inflation stretched space flat, as Earth looks flat to us |
| Horizon | Opposite sides of the sky (90 million light-years apart at 380,000 years) match to a thousandth of a degree without time to exchange heat (Dicke's name) | Our visible universe was a tiny uniform patch blown up by 1050 |
Reaction
Guth presented his theory in 1980, nervous of revealing himself as "a greenhorn cosmologist". Gell-Mann: "You've solved the most important problem in cosmology!" Glashow said Weinberg was "furious": "he just didn't think of it himself." Job offers poured in; a fortune cookie ("An exciting opportunity lies just ahead of you if you are not too timid") nudged him to call MIT, which hired him.
- The astronomers' objection. Inflation predicts Omega (plus Lambda) equals exactly 1, but the observed matter density was only about 0.3. Robert Kirshner: "This 'inflation' idea sounds crazy." Roger Penrose: "a fashion the high-energy physicists have visited on the cosmologists... Even aardvarks think their offspring are beautiful."
- The graceful exit problem. Guth's bubbles of true vacuum failed to merge, leaving a lumpy universe, like a pot of steam bubbles that never fully join. In 1981 Linde, and separately Steinhardt and Albrecht, proposed new inflation: a single bubble inflating long enough to fill everything (like a thick dam delaying tunnelling). But what prolongs inflation by 1050? Over fifty mechanisms have been proposed; Kaku has tried several himself.
Chaotic inflation: the multiverse
Linde ("it was impossible for God not to use such a good possibility") proposed that symmetry breaking happens at random points. Most bubbles inflate a little; by chance some inflate enormously. Inflation becomes eternal: universes bud from universes, and "the multiverse is eternal, even if individual universes are not." High-Omega bubbles crunch at once; low-Omega ones expand forever. Kaku argues the multiverse is forced on us: quantum theory gives a probability to unlikely events, and an electron exists as a "cloud" of many positions at once. Once the universe was smaller than an electron, it too may exist in many states.
Linde's chaotic inflation: inflation sprouts new regions endlessly, each a universe.
This would explain why the universe has zero net spin (a spinning universe would allow time travel; see chapter 5) and zero net charge, balanced to one part in 1021. A 0.00001% imbalance in your body would tear you apart. The exception is matter over antimatter: Sakharov proposed a tiny breaking of CP symmetry, leaving one extra particle per billion. Its origin is still unknown.
What other universes might look like
Symmetry is beauty: an equation is beautiful if it stays the same when its parts are interchanged, as a snowflake looks the same rotated by 60°. "The secret of nature is symmetry, but much of the texture of the world is due to mechanisms of symmetry breaking" (David Gross). A spherical embryo is symmetric but useless; it breaks to a bowling-pin shape (sphere to cylinder), while a starfish breaks to fivefold symmetry. How the symmetry breaks determines the creature.
| Theory | Symmetry | Reshuffles |
|---|---|---|
| Strong force (QCD) | SU(3) | Three quark "colours" |
| Weak + electromagnetic | SU(2) × U(1) | Electron and neutrino; light |
| Standard Model | SU(3) × SU(2) × U(1) | Quarks and leptons separately |
| Minimal GUT | SU(5) | All five together (protons can decay) |
The GUT symmetry can break in many ways. Ours yields the Standard Model with its 19 parameters. Others could have different force strengths (too weak a nuclear force means no stars; too strong means stars burn out before life forms), unstable protons (a lifeless mist), extra stable particles (richer chemistry), or several kinds of light, with eyes to match. There may be hundreds, or infinitely many, ways to break the symmetry.
Testable predictions
The multiverse cannot be tested directly, but inflation itself can. The uncertainty principle forbids a perfectly smooth fireball; inflated quantum ripples should show in the microwave background and grow into galaxy clusters. A smooth sky would have undermined quantum theory itself. COBE found ripples of one part in 100,000, as required. Guth: "I'm completely snowed."
Inflation's checkable predictions against the microwave sky.
10-43 s, GUT era: gravity splits off; inflation by perhaps 1050; 1032 degrees.
10-34 s, end of inflation: the strong force splits; quark soup; matter-antimatter annihilation leaves one part in a billion.
3 minutes: nuclei form (75% hydrogen, 25% helium).
380,000 years: atoms form; the universe becomes transparent; the background radiation is released.
1 billion years: stars and galaxies condense from quantum ripples.
6.5 billion years: de Sitter acceleration begins.
13.7 billion years: today, 2.7 K, runaway expansion.
The "smoking gun" would be primordial gravity waves from inflation, a target for detectors (chapter 9). Inflation draws on Einstein (many universes) and quantum theory (tunnelling between them); M-theory may settle the rest.
Graceful exit and the bubble picture
Guth's first model pictured the false vacuum like a pot of water just before boiling: it wants to enter the true vacuum but needs an impurity to start a bubble. Once bubbles form they grow, yet in his original equations they never fully coalesce, leaving a lumpy universe of bubble walls. That is the graceful exit problem. In 1981 Andrei Linde, and independently Paul Steinhardt and Andreas Albrecht, proposed new inflation: a single bubble inflates long enough to fill the entire observable cosmos, like a thick dam that delays tunnelling. The delay must last long enough for expansion by about 1050. Over fifty microphysical mechanisms have been proposed for that delay; Kaku notes he has tried several himself. Guth accepted the fix and predicted that data would eventually show the universe is flat (Omega near 1).
Flatness, monopoles and the horizon in numbers
- Monopoles. GUTs predict heavy magnetic monopoles as leftovers of the GUT phase transition. None have been seen ("Like the unicorn..."). Inflation dilutes them across a vastly larger volume, so fewer than one should remain in our Hubble volume.
- Flatness. Omega (plus Lambda) near 1 today requires Omega = 1 to about one part in 1014 at one second after the bang, like balancing a pencil on its tip for years. Inflation stretches any initial curvature flat the way Earth's surface looks flat to someone standing on it.
- Horizon. Opposite sides of the microwave sky were about 90 million light-years apart at recombination (380,000 years) yet match to a thousandth of a degree, with no time to exchange heat (Dicke's name for the puzzle). Inflation says our visible universe was once a tiny, causally connected patch blown up by that same factor of 1050.
Three puzzles inflation solves at once
Symmetry, GUTs and other possible worlds
An equation is beautiful if it stays the same when its parts are reshuffled, as a snowflake looks the same rotated by 60°. "The secret of nature is symmetry, but much of the texture of the world is due to mechanisms of symmetry breaking" (David Gross). A spherical embryo is symmetric but useless; it breaks to a bowling-pin shape, while a starfish breaks to fivefold symmetry. How the GUT symmetry breaks chooses the low-energy laws:
| Outcome of breaking | Consequence for life |
|---|---|
| Nuclear force too weak | No stable stars |
| Nuclear force too strong | Stars burn out before chemistry evolves |
| Unstable protons | A lifeless mist of decay products |
| Extra stable particles | Richer chemistry, perhaps different biology |
| Several kinds of light | Eyes (or sensors) tuned to each |
Ours yields the Standard Model with its 19 free parameters. Minimal SU(5) GUTs also predict proton decay; none has been seen. Other breakings, Kaku argues, could be the physics of other bubbles in Linde's multiverse. High-Omega bubbles crunch at once; low-Omega ones expand forever. Quantum theory assigns a probability even to rare outcomes, and once the universe was smaller than an electron it too may have existed in many states at once.
Why Omega looked wrong until 1998
Astronomers measured matter density near 0.3, not 1. Robert Kirshner called inflation "crazy"; Roger Penrose dismissed it as "a fashion the high-energy physicists have visited on the cosmologists... Even aardvarks think their offspring are beautiful." The missing piece was Lambda. Type Ia supernovae (white dwarfs pushed past Chandrasekhar's 1.4 solar-mass limit) are standard candles. Saul Perlmutter's Supernova Cosmology Project and Brian Schmidt's High-Z team expected slowing; instead they found acceleration. Omega 0.3 plus Lambda 0.7 equals 1, exactly as inflation wants. George Jacoby: anyone claiming Lambda was nonzero "was treated as kind of nuts." Schmidt feared "we were going to get massacred." Edward Witten: "the strangest experimental finding since I've been in physics."
From Omega puzzle to acceleration: Lambda completes inflation's flatness prediction.
Guth's diary and the unification race
"SPECTACULAR REALIZATION," Guth wrote in his diary in 1979. Born in 1947 in New Jersey to parents who never attended college, he was drawn at MIT to unification: today's universe as the shattered remnant of a greater simplicity. After two thousand years of work, physics counts four forces (searches for a fifth have been negative or inconclusive). A comb with static electricity lifts paper against the gravity of the whole Earth, showing how weak gravity is (1036 times feebler than electromagnetism). The particle zoo of the 1950s was so crowded that Oppenheimer joked the Nobel should go to whoever did not find a new particle. Then quarks (Gell-Mann, George Zweig), electroweak unification (Weinberg, Abdus Salam, Sheldon Glashow, Nobel 1979) and QCD from SLAC data produced the Standard Model: every non-gravitational experiment fits, some to one part in a hundred million (Gordon Kane), yet 19 free parameters and three duplicate generations leave it looking "cobbled together." GUTs mix quarks with leptons so protons can decay; minimal SU(5) is now ruled out by the non-observation of that decay.
When Guth presented inflation in 1980 he feared looking like "a greenhorn cosmologist." Gell-Mann: "You've solved the most important problem in cosmology!" Glashow said Weinberg was "furious" he had not thought of it. A fortune cookie ("An exciting opportunity lies just ahead of you if you are not too timid") nudged Guth to call MIT, which hired him. Edward Tryon had already proposed in Nature (1973) that the universe is a vacuum fluctuation, "one of those things which happens from time to time." Positive matter energy can cancel negative gravitational energy (a donkey in a hole needs energy added just to reach zero), so the net cost may be about an ounce: "the ultimate free lunch."
Eternal inflation's logical fork
If inflation is eternal, bubble nucleation never ends and the global universe has no single beginning. Each bubble's local big bang is a regional event. Critics (Steinhardt and others) argue that a theory that realises every outcome predicts nothing; defenders reply that measure problems are technical, not fatal, and that local predictions (flatness, ripple spectrum) still bite. Kaku sides with the defenders in spirit while admitting the multiverse cannot be photographed.
Guth to Lambda: who closed each gap
Dimensional Portals and Time Travel
If the world is a stage, relativity allows trapdoors leading not to a basement but to parallel stages with "new laws, new rules, and a new script." These anomalies may even offer a way out of the big freeze.
Black holes: the magic sphere
- Dark stars (1783). John Michell asked what happens if a star's escape velocity (25,000 mph for Earth) reaches the speed of light: it would be invisible.
- Schwarzschild (1916). Serving on the Russian front, Karl Schwarzschild found the simplest exact solution of Einstein's equations. Around a compact mass lies a "magic sphere", a point of no return: 3 km for the Sun, about a centimetre for Earth.
- Droste and Weyl. Light orbits in circles at 1.5 times the radius; to a distant observer, a falling clock slows and freezes at the sphere. Hermann Weyl found what seemed another universe on the far side.
- The "Hadamard disaster" (1922). Einstein: if the singularity were real, "It would be a true disaster for the theory."
- Resolved (1932). Lemaître showed the singularity at the sphere was an artefact of the coordinates; H. P. Robertson showed time stops only for outside observers. The faller crosses in a fraction of a second. Today it is the event horizon.
Falling in, you would see light captured over the hole's whole history, then be "spaghettified" by tidal forces, and inevitably crushed at the centre. Leaving would need faster-than-light travel.
The Einstein-Rosen bridge
Mathematicians call them multiply connected spaces: a lasso around a doughnut's hole cannot shrink to a point. Lewis Carroll's looking glass was a wormhole. In 1935 Einstein and Nathan Rosen joined a black hole solution to a flipped copy, hoping to model the electron without a singularity. It failed as particle theory, but the Einstein-Rosen bridge became a possible gateway to another universe (exiting via a white hole). For a static black hole the bridge closes too fast to cross, and tidal forces kill: wormholes may exist, but no one could pass through one "and live to tell about it."
The Einstein-Rosen bridge: a black hole solution links two universes but collapses too fast to cross.
Kerr's spinning ring
In 1963 Roy Kerr solved for a spinning black hole, the realistic case, since collapsing stars spin up like skaters pulling in their arms. The star collapses into a ring; fall through it and you are not crushed. "Pass through this magic ring and, presto, you're in a completely different universe where radius and mass are negative!"
Kerr's spinning black hole: a ring singularity one might pass through.
Seeing black holes
As late as 1998 Douglas Richstone recalled that someone claiming a galactic black hole was thought "a little nuts". Now hundreds are known via Hubble, Chandra and the Very Large Array. Most galaxies with central bulges seem to host one. Astronomers infer them from accretion disks (the lowest-energy shape for spinning matter) and stellar orbits, while magnetic fields channel gas into twin polar jets.
| Object | Mass (book) | Notes |
|---|---|---|
| Milky Way centre (Sagittarius) | 2.5 million suns | Hidden by dust; otherwise brighter than the Moon |
| Andromeda | 30 million suns | Double nucleus: remains of a swallowed galaxy |
| NGC 4261 | 1.2 billion suns | 400-light-year disk feeding radio jets |
| M87 | 3 billion suns | Its "tail" is an outgoing plasma jet |
| RX J1242-11 | 100 million suns | Seen swallowing a star (2004): "stretched beyond its breaking point" |
Chandra counted 600 black holes through one gap in the dust, implying at least 300 million across the sky.
Gamma-ray bursters
The Vela satellites were launched to detect secret nuclear tests by their characteristic double flash, and instead found huge explosions in deep space. Gamma-ray bursts release in seconds as much energy as the Sun over its 10-billion-year life, from a region tens of miles across. They are now seen about three times a day, and rapid-response telescopes swing to them within seconds. Candidates: colliding neutron stars, or (evidence from 2003) a "hypernova" forming a black hole and beaming energy along its poles, which makes it seem brighter if pointed at us.
Time machines
Newton's time was an arrow; Einstein's a river that speeds and slows. Could it loop back, or fork?
| Solution | Year | How it works | Catch |
|---|---|---|---|
| Van Stockum cylinder | 1937 | An infinite cylinder spinning near light speed drags space-time (frame-dragging); circle it like a maypole to go back in time | Infinite length; it would fly apart |
| Gödel universe | 1949 | The whole universe rotates (once per 70 billion years); a wide round trip reaches any past event | The universe expands, it doesn't spin |
| Thorne wormhole | 1988 | A traversable wormhole held open by negative energy | Needs huge negative energy |
| Gott cosmic strings | 1991 | Circle two cosmic strings passing near light speed | Half a galaxy's mass-energy for a one-year trip |
Gödel saw the danger: a traveller could meet his younger self and "do something to this person which, by his memory, he knows has not happened to him." Einstein, his Princeton neighbour, admitted the problem had "disturbed me already at the time of the building up of the general theory", and excluded it "on physical grounds".
Thorne's time machine and negative energy
In 1985 Carl Sagan, writing Contact, needed a two-way trip to Vega and asked Kip Thorne. In 1988 Thorne, Michael Morris and Ulvi Yurtsever showed that a wormhole held open by exotic matter or negative energy is traversable, with forces milder than an airline flight.
Thorne's wormhole time machine: hold the throat open and shift one mouth in time.
- Negative matter would fall up, repelled by everything; any on early Earth would have floated away. Never seen.
- Negative energy is real. Casimir (1948 prediction) showed that two uncharged plates attract, because fewer virtual particles fit between them. Steven Lamoreaux measured the force in the 1990s: about 1/30,000 of an ant's weight.
- Other sources. Moving mirrors (Davies and Fulling), squeezed laser states (pulses of 10-15 s), and the horizon of an evaporating black hole (Bekenstein, Hawking).
- Hawking's theorem. Light converging into a wormhole mouth must defocus inside, which requires negative energy, so every stable wormhole needs negative energy.
Chronology protection
T. H. White's ants: "Everything not forbidden is compulsory." In 1992 Hawking proposed a chronology protection conjecture to "make history safe for historians". He used Misner space: your bedroom is the whole universe, with opposite walls joined, so you see endless copies of your own back. If the walls close in, each circuit boosts your speed until you travel back in time. But light circling the room is blueshifted without limit; the energy-momentum tensor diverges and the wormhole collapses.
- The tide turned. Sergei Krasnikov: "not a grain of evidence to suggest that the time machine must be unstable." Li-Xin Li proposed an anti-chronology protection conjecture. In 1998 Hawking conceded that back-reaction "does not enforce chronology protection."
- Verdict. Matthew Visser: the question "requires a fully developed theory of quantum gravity." Hawking now calls time travel highly unlikely, not impossible. And no tourists from the future? Perhaps no machine has been built yet.
Time paradoxes
| Paradox | Example |
|---|---|
| Grandfather | Step on the furry mammal ancestral to humanity |
| Information | Give your younger self the secret of time travel; it has no origin (The Terminator's chip) |
| Bilker's | See you'll marry Jane; marry Helen instead |
| Sexual | Father yourself (Heinlein's "All You Zombies") |
Time paradoxes and the proposed ways out.
Relativity forbids time simply ending: you can't vanish. Two solutions dominate:
- Self-consistency (Igor Novikov): an "invisible hand" prevents paradoxes. "We cannot send a time traveler back to the Garden of Eden to ask Eve not to pick the apple." He compares it to gravity limiting your will to walk on the ceiling. But inanimate objects (machine guns sent to Alexander before his battle with Darius) and the butterfly effect make this hard.
- Branching (many worlds): time forks; you kill your parents' duplicates in another universe. Radiation passes through the wormhole once, removing Hawking's divergence. This leads to quantum theory's deepest question: how can a cat be dead and alive?
From Michell to Oppenheimer: why collapse wins
John Michell (1783) asked what happens if a star's escape velocity reaches the speed of light: it would be invisible, a "dark star." Karl Schwarzschild (1916), serving on the Russian front, found Einstein's simplest exact solution: a "magic sphere" of no return (3 km for the Sun, about a centimetre for Earth). Light can orbit at 1.5 times that radius (Droste, Weyl). Einstein feared a real singularity would be a "true disaster for the theory" (the "Hadamard disaster," 1922). Lemaître (1932) and H. P. Robertson showed the sphere singularity was a coordinate artefact: the faller crosses in a fraction of a second while a distant clock appears to freeze. Today that surface is the event horizon. Inside, light collected over the hole's whole history greets you; tides "spaghettify" you; the centre crushes you. Leaving needs faster-than-light travel.
Einstein (1939) modelled swirling dust and concluded "'Schwarzschild singularities' do not exist in physical reality." Eddington wanted "a law of Nature to prevent a star from behaving in this absurd way." The same year Oppenheimer and Hartland Snyder modelled a burnt-out giant (say 40 solar masses, radius about 80 miles) imploding and showed a black hole must form. Observation has sided with Oppenheimer: billions of stars may end that way.
Observing holes and bursts
As late as 1998 claiming a galactic black hole could make you sound "a little nuts" (Douglas Richstone). Hubble, Chandra and the Very Large Array then found hundreds via accretion disks (the lowest-energy shape for spinning gas) and stellar orbits; magnetic fields channel twin polar jets. Chandra counted 600 holes through one dust gap, implying at least 300 million across the sky. The Vela satellites, built to catch secret nuclear tests by their double flash, instead found gamma-ray bursts: in seconds they release as much energy as the Sun over 10 billion years, from a region tens of miles across, seen about three times a day. Candidates were colliding neutron stars or a "hypernova" forming a black hole and beaming along its poles (evidence from 2003 favoured the latter for long bursts).
Black hole evidence ladder from math to image
Negative energy, Casimir and chronology protection
Kip Thorne's 1988 traversable wormhole needs negative energy to hold the throat open. The Casimir effect (predicted 1948, precision-measured by Lamoreaux in 1997) squeezes parallel plates with a tiny negative energy density from vacuum fluctuations. Scale that up, shift one mouth near light speed so its clock lags, and stepping into the Earth mouth exits at the ship's earlier time: a time machine that cannot go back before it was built. Hawking's chronology protection conjecture says quantum effects destroy any such machine before it opens (a burst of radiation). J. Richard Gott's 1991 cosmic-string machine uses only positive-density matter: two strings passing at 99.999999996% of light, about 10 million billion tons per centimetre, half a galaxy's mass-energy for a one-year trip. Gödel's 1949 rotating universe (once per 70 billion years) also allows closed timelike curves, but our universe expands rather than spins. Van Stockum's 1937 infinite spinning cylinder is likewise unphysical.
Paradoxes: consistency or many worlds
Gödel saw the threat: meet your younger self and "do something to this person which, by his memory, he knows has not happened to him." Resolutions Kaku surveys include the consistency constraint (you can only do what already happened), Novikov's self-consistency principle, and many-worlds branching so the changed past is another universe. Einstein admitted the problem had "disturbed me already at the time of the building up of the general theory" and excluded it "on physical grounds." Whether nature enforces that exclusion is still open.
Asimov's warning and Wells's invisible man
Isaac Asimov's The Gods Themselves dramatises what happens if a parallel universe's stronger nuclear force leaks into ours: free energy first, then a Sun that brightens toward explosion. The fiction tracks the physics: change a coupling and stellar lifetimes collapse. H. G. Wells's invisible man and Edwin Abbott's Flatland (1884) supply the dimensional intuition: hover inches above a two-dimensional tabletop and you are invisible to its inhabitants, see their insides, and look like a god. Tap a fourth spatial dimension and the same tricks apply in our world. Kaku uses these stories not as evidence but as intuition pumps for wormholes and branes.
Thorne's group asked whether a sufficiently advanced civilization could hold a wormhole open. The answer in classical relativity is yes if negative energy is allowed; quantum inequalities then try to snatch that answer back. Gott's cosmic strings and Alcubierre's warp metric (1994) widen the zoo of time machines without removing the energy condition problems. The chapter's sober close is that mathematics permits portals; engineering and quantum gravity may still forbid using them.
White holes, foam and a universe in the bedroom
A white hole is a time-reversed black hole: nothing can enter, everything can exit. Some early wormhole pictures placed a white hole at the far mouth. Stability arguments and the lack of observed white holes make them unlikely astrophysical objects, yet they remain useful as mathematical bookends. At the Planck length, Wheeler's space-time foam froths with tiny wormholes and baby universes; Zeno's paradox about the smallest distance reappears as a physical question. Kaku's "universe in your bedroom" riff is that a sufficiently advanced technology might pluck a foam wormhole and inflate it, linking distant times or places without crossing the space between. No experiment has seen foam directly; it is an extrapolation of quantum gravity ideas still under construction.
Paradox table in prose
Consistency: you cannot kill your grandfather because you did not. Many worlds: you can, but the murder is in a branch that was never yours. Chronology protection: the machine explodes or seizes before the loop closes. No-go energy conditions: the machine cannot be built. Kaku presents these as live options rather than a settled theorem. The chapter's evidence ladder for black holes (math, collapse proofs, accretion, shadows, gravity waves) is far stronger than its evidence ladder for time machines (exact solutions plus speculation). That asymmetry is the point: relativity's Trojan horse contains both treasure and contraband, and we have cashed only some of the treasure.
Time-machine zoo and each catch
Parallel Quantum Universes
"I think I can safely say that nobody understands quantum mechanics" (Feynman). "Anyone who is not shocked by the quantum theory does not understand it" (Bohr). Douglas Adams' Infinite Improbability Drive crosses space by controlling quantum probabilities. Kaku asks his PhD students to calculate the chance that they will dissolve and reappear on the far side of a brick wall, or on Mars: small, calculable, and longer than the life of the universe to wait for. Yet electrons do exactly this inside every computer.
Worlds one event apart
- Philip K. Dick, The Man in the High Castle. An assassin kills Roosevelt in 1933; the US loses the war and is partitioned by Germany and Japan. A banned novel within the novel, The Grasshopper Lies Heavy, imagines the Allies winning.
- The Twilight Zone. A man wakes to find his wife, friends and parents don't know him. A single cosmic ray striking an embryo's DNA could cause a miscarriage: one quantum event can separate a world with you from an identical one without you.
John Wheeler, "monster mind"
Born in 1911 in Jacksonville, Wheeler lost part of a thumb to dynamite as a boy. He coined "black hole" (1967), worked on the atomic and hydrogen bombs, and wrestled all his life with quantum paradoxes. Feynman called him one of the "monster minds". When he was young, followers of Ernst Mach scoffed at atoms, and Boltzmann killed himself in 1906, partly from the ridicule. Then in 1925-27 the atom's secrets "came tumbling out", and Wheeler went to Copenhagen to learn from Bohr.
- Schrödinger's wave equation described the wave (psi) that accompanies the electron. Dirac boasted that the laws of "the whole of chemistry are thus completely known."
- Max Born proposed that psi gives the probability of finding the electron. A person's wave function looks like the person but seeps out across the universe, so there is a tiny chance of finding them on the Moon.
| Copenhagen postulate | Content |
|---|---|
| a. Quanta | Energy comes in packets: photon, W and Z, gluon, graviton (unseen) |
| b. Probability waves | Particles are points, but their location is given by a wave obeying a wave equation |
| c. Collapse | Before observation, an object exists in all states at once; observation "collapses" it into one |
Determinism or uncertainty?
Quantum theory is the most successful theory ever, tested to one part in ten billion, yet its postulates set off eighty years of philosophical storms. Banquo begged to "look into the seeds of time". Newton and Laplace thought they could: given every particle's position and velocity, "the future just like the past would be present before his eyes." Laplace to Napoleon: "Sire, I had no need of that hypothesis." Einstein: "I am a determinist... I know philosophically a murderer is not responsible for his crimes, but I prefer not to take tea with him." Predestination also divided theologians; Kaku links it to Luther's 1517 attack on indulgences.
Physicists mock philosophers (Einstein: philosophy is "as if written in honey... Only mush remains"), but philosophers may laugh last: the quantum tree in the forest exists in all states (fallen, burnt, sawdust) until someone looks, echoing Bishop Berkeley. Feynman: quantum mechanics "describes nature as absurd from the point of view of common sense. And it fully agrees with experiment." Weinberg admits "discomfort in working all my life in a theoretical framework that no one fully understands." Planck: "we ourselves are part of the mystery we are trying to solve."
The cat, the box and the Solvay showdown
Schrodinger's cat: one atom's superposition spreads to a cat until someone looks.
At the 1930 Solvay conference, which Wheeler called the greatest intellectual debate he knew, Einstein fired thought experiments "like a jack-in-the-box, popping up fresh every morning" (Paul Ehrenfest). His best was a box of light with a timed shutter: weigh the box before and after one photon escapes, and both its energy and its time of release are known exactly, violating the uncertainty principle. Bohr spent the evening "extremely unhappy"; Ehrenfest found him mumbling "Einstein... Einstein... Einstein." After a sleepless night Bohr found the flaw: the lighter box rises in gravity, and by Einstein's own general relativity a clock's rate depends on its height, restoring exact uncertainty. Bohr used Einstein's theory of gravity to refute Einstein.
To "God does not play dice," Bohr reportedly replied, "Stop telling God what to do." Einstein conceded that the theory "contains a piece of definitive truth" but treated it as an approximation to be absorbed, as relativity absorbed Newton. These thought experiments are now real experiments: buckyballs of 60 carbon atoms have been put into two-place superpositions, and Bohr's wall "is rapidly crumbling."
The bomb
Rutherford had called atomic power "moonshine". In 1939 Bohr brought Wheeler news of fission (Hahn and Meitner), and they calculated chain-reaction probabilities: you cannot know which neutron splits which nucleus, but you can predict billions. Szilard later got Einstein to write to Roosevelt. Heisenberg was ordered to build a bomb for Hitler. At their mysterious 1941 meeting in Copenhagen, letters released by the Bohr family in 2002 show, Heisenberg said a Nazi victory was inevitable and Bohr should cooperate. Bohr, appalled, later escaped and nearly suffocated on the flight to Britain. Wheeler joined Fermi to build the first reactor at Chicago, helped design the Hanford plutonium works (whose output destroyed Nagasaki), and witnessed the first hydrogen bomb in 1952.
Feynman's sum over paths
To cross a room, Feynman said, you must consider every possible path: via Mars, the nearest star, even back to the big bang. Assign each a number by precise rules and add them up. Crazy paths largely cancel (they are the quantum fluctuations), while the Newtonian path survives with the largest total. "Our commonsense notion of the physical universe is simply the most probable state among an infinite number of states." Your body "sniffs out" all paths. The path integral reproduces Schrödinger's equation, earned the 1965 Nobel for one of its consequences, and now underpins GUTs, inflation and string theory. Kaku writes every equation this way: "quantum theory is much more alien than the mind-bending consequences of relativity." When Wheeler explained it to Einstein, Einstein still rejected dice but said he had "earned the right to be wrong."
Feynman's sum over paths: add every possible route and the classical path emerges.
Four ways out of the cat problem
| Interpretation | Champion | How the cat is settled | Problem |
|---|---|---|---|
| Copenhagen | Bohr, Heisenberg | Observation collapses the wave | "Observation" is undefined; two physics |
| Consciousness | Wigner, Linde | A conscious mind collapses it | Who observes the observer? (Wigner's friend) |
| Decoherence | Dieter Zeh (1970) | Air molecules split dead and live waves before you look | Doesn't say which outcome nature "chooses" |
| Many worlds | Hugh Everett III (1957) | Both happen, in separate branching universes | Endless proliferating universes |
Four answers to the cat problem.
- Consciousness. Wigner: "the content of the consciousness is the ultimate reality." Linde: "In the absence of observers, our universe is dead." On this view dinosaur fossils spring into existence when examined, as if millions of years old. A camera doesn't help: who observes the camera?
- Decoherence. Live and dead waves must vibrate in step (coherence). One collision with an air molecule knocks them out of phase, and they never interact again. Collapse needs no mind; but who picks the result?
- Many worlds. Drop postulate (c): the wave never collapses, just branches. "There is a universe where Elvis is still alive" (Guth). Frank Wilczek: a single-cell mutation could give Helen of Troy a wart, so "there are many, many worlds in which Helen of Troy did have a wart at the tip of her nose." Olaf Stapledon's Star Maker imagined it first.
It from bit
Hailed by New Age fans, Wheeler disowned three parapsychologists on one programme: "Where there's smoke, there's smoke." "I am just driven crazy by that question," he said, quoting Leibniz that the world may be a dream but is "real enough if using reason well we are never deceived by it." Finding many worlds "too much excess baggage", he proposed "It from bit": information is fundamental, and the universe came into being by observing itself in a loop, a "participatory universe".
Quantum computing and teleportation
Moore's law (doubling every 18 months) will hit atomic limits as chip layers shrink from twenty atoms to five and electrons tunnel out. Atoms can compute with qubits, spins in many states at once, and, as David Deutsch stresses, in effect sum over parallel universes. Factorizing a 100-digit number, the basis of most codes, takes centuries classically but might be easy quantum mechanically. In 2004 the record was seven atoms (factoring 15); a useful machine needs thousands to millions kept coherent.
Teleportation and computing both run on entanglement and fail through decoherence.
Teleportation uses entanglement to get around the uncertainty principle. Entangled twins B and C are separated; B "scans" object A, and C becomes an exact replica while A's state is destroyed.
1997-98: Caltech, Aarhus and Wales teleport a photon across a tabletop; Samuel Braunstein compares entangled pairs to lovers who "could answer for their lover even if separated by long distances."
2003: Geneva teleports photons 1.2 miles through fibre (Nicolas Gisin: perhaps a molecule in my lifetime).
2004: NIST teleports the state of a beryllium atom.
Limits: the original is destroyed (one copy only), nothing travels faster than light, and coherence is fragile. A human would involve so much information that the transfer "would take the age of the universe" (Braunstein), though a virus might be teleported this century.
The wave function of the universe
Hawking said that when he hears about the cat he reaches for his gun. Schrödinger's psi lives in ordinary space; Hawking's lives in Wheeler's superspace, the space of all possible universes, and obeys the Wheeler-DeWitt equation. In the early 1990s Hawking summed over universes connected by wormholes (a sea of soap bubbles joined by filaments) and found the cosmological constant most likely zero. Critics replied that such sums are unreliable without a theory of everything, and many physicists think string theory, or M-theory, is that theory.
Decoherence, umpires and why the cat splits
Wheeler's three umpires capture the dispute: calling them as you see them, as they are, or as nothing until called. Bohr drew a wall between quantum and classical; Schrödinger's cat was meant to show that wall fails when a Geiger counter couples an atom to a macroscopic animal. Hugh Everett (1957) removed collapse: the wave function of the universe always evolves, and measurement entangles observer with system so that both branches persist. Decoherence (Zeh, Zurek and others) explains why we do not see interference between live and dead cats: the environment records which branch we are in, and the branches' phases scramble. Weinberg's radio analogy: many stations fill the room, but one receiver tunes to one frequency. Parallel worlds differ by trillions of atomic energies, so they no longer interact.
Four common escapes from the cat paradox
From EPR to teleportation
Einstein, Podolsky and Rosen (1935) thought entanglement proved quantum theory incomplete. Bell (1964) made the dispute experimental; Aspect (early 1980s) and later loophole-free tests sided with quantum theory. Teleportation protocols (1993 proposal; tabletop photons by 1997-98; atoms at NIST in 2004) move quantum states, not matter, and still need a classical channel, so they do not beat light speed. Quantum computers exploit superposition and entanglement until decoherence destroys the calculation; Kaku saw them as a laboratory for many-worlds language even if the metaphysics stays optional.
Wheeler, the bomb and sum over histories
John Wheeler, Feynman's mentor, coined "black hole" and "wormhole" and asked whether observers participate in bringing reality about ("it from bit"). His work on the delayed-choice experiment suggests that how we choose to measure a photon can seem to rewrite its past path, without sending usable signals backward. Feynman's sum over histories says a particle takes every path at once; amplitudes interfere, and the classical path is the one whose neighbouring paths reinforce. The atomic bomb project forced many of these theorists into applied work; after the war the conceptual puzzles returned sharper. Kaku treats many-worlds not as science fiction but as the reading that takes the Schrödinger equation literally for the whole universe, including its observers. Whether that reading is metaphysics or physics remains unsettled; what is settled is that entanglement is real, teleportation works in the lab and decoherence explains why the everyday world looks classical.
Wave function of the universe
If quantum theory applies to everything, the universe has a wave function (Hartle-Hawking and others). Boundaries may be replaced by a no-boundary proposal in imaginary time; classical space-time emerges only approximately. Linde's more speculative suggestion that observers somehow participate in making their cosmic histories real sits at the edge of the chapter's science. Kaku includes it to show how far many-worlds thinking can be pushed once collapse is denied. The safer core remains: decoherence plus entanglement plus laboratory teleportation already force a richer ontology than nineteenth-century materialism expected.
Molecules as proof of parallel states
Kaku's molecular opening is deliberate: chemistry works because electrons occupy spread-out clouds, not miniature planetary orbits that would collapse under radiation. Superposition is not an optional oddity; it is why solids are solid. Extending that lesson to the cosmos is the controversial step. Between the molecule and the multiverse sit decoherence, measurement theory and the preferred-basis problem. The chapter walks that bridge; it does not pretend the last span is as sturdy as the first.
Entanglement becomes a tool
M-Theory: The Mother of All Strings
"I feel that we are so close with string theory that... any day, the final form of the theory might drop out of the sky and land in someone's lap." (Edward Witten)
Hyperspace in fiction and art
In H. G. Wells' The Invisible Man (1897), Griffin finds "a geometrical expression involving four dimensions." Kaku uses Edwin Abbott's Flatland (1884) to show why. A Flatlander who hovers an inch above the tabletop becomes invisible, can step out of any prison (just a drawn circle), lift gold from a sealed vault (an open rectangle) and operate without cutting skin. We may be the Flatlanders, with whole universes "right above our noses." In Wells' The Wonderful Visit (1895), an angel from a parallel universe with cylindrical space, non-inverse-square gravity and "four-and-twenty primary colours" is shot by a vicar: "There may be any number of three-dimensional Universes packed side by side."
The fourth dimension also shaped art (per Linda Dalrymple Henderson): Picasso's cubist faces seen from all sides at once, Dalí's Christus Hypercubus (crucified on an unfolded tesseract) and melting clocks, Duchamp's time-lapse Nude Descending a Staircase.
The chasm between relativity and quantum theory
| General relativity | Quantum theory | |
|---|---|---|
| Domain | The very large: black holes, big bang, expansion | The very small: atoms, quarks |
| Mathematics | Smooth curved surfaces | Discrete quanta |
| Predictions | Definite | Probabilities only |
Higher dimensions give "enough room" to unify all the forces. Previous attempts were humiliations:
- Einstein (1928): a premature unified theory set off a press stampede, and Selfridges pasted its six pages in a London shop window.
- Schrödinger (1946): announced his theory at a press conference attended by Ireland's de Valera ("I shall look like an awful fool if I am wrong"). He had rediscovered a theory Einstein had already discarded.
- Heisenberg and Pauli (1958): Bohr told Pauli, "We in the back are convinced that your theory is crazy. But what divides us is whether your theory is crazy enough."
Wheeler compares the search to Darwin's single theory explaining all life. Weinberg compares it to early maps that all pointed to a north pole no one had visited.
A theory discovered backward
String theory "was discovered quite by accident, applied to the wrong problem, relegated to obscurity, and suddenly resurrected as a theory of everything." Normally physics goes from principle to classical equations to quantum corrections; string theory began with its quantum answer, and its guiding principle is still unknown. Because any small adjustment destroys it, it will be either a "theory of everything" or a "theory of nothing."
String theory found backward: a formula first, the theory behind it later.
Late 1960s: Sakita, Virasoro and Kikkawa treat it as the first term of an infinite series; Kaku and L. P. Yu calculate the full set of corrections.
About 1970: Nambu, Goto (and Susskind, Nielsen) identify the underlying object: a vibrating string.
1971: Schwarz, Neveu and Ramond add spin (integer and half-integer, as nature requires).
1974: Kaku and Kikkawa write string field theory, the whole theory in an equation "an inch and a half long".
1974: Lovelace finds the original model needs 26 dimensions; the superstring needs 10. Scherk and Schwarz reinterpret the unwanted massless spin-2 particle as the graviton.
1984: Schwarz and Michael Green show the theory is free of anomalies as well as infinities. The bandwagon starts.
Witten: "By rights, string theory shouldn't have been invented." As a Berkeley graduate student, Kaku watched physicists shake their heads at a theory that skipped observation and simply guessed the answer. Like delicate crystal, the Veneziano model shattered at any tweak; only papers exploring its symmetries survived. After Kaku's 1974 Aspen seminar on string field theory, Feynman told him: "I may not agree totally with string theory, but the talk you gave is one of the most beautiful I have ever heard." (Feynman once shouted "Wrong!" when Weinberg explained the W on his blackboard, the angle that later won Weinberg a Nobel.)
Ten dimensions and the graviton
No theory had ever chosen its own dimension. Newton's inverse-square law works in any dimension (inverse-cube in four). String theory works only in 10 (or 26). String theorists became a joke; in an elevator Feynman teasingly asked Schwarz how many dimensions he was living in that day.
Relativity versus quantum: why unification is hard.
Infinities and anomalies
Quantizing gravity normally produces two fatal problems. Infinities: quantum fluctuations, normally tiny corrections, become infinite. Anomalies: quantum effects break the theory's symmetry. Kaku's analogy is a hypersonic rocket: wing vibrations that are harmless at low speed grow until they tear the wing off (divergences), and hairline cracks spread through the hull's cylindrical symmetry (anomalies). For decades physicists applied "Band-Aids"; string theory is a new design with new materials. Einstein wondered whether God had any choice in creating the universe; perhaps only one theory is free of these inconsistencies.
Cosmic music
Einstein said a good theory should offer a picture a child can grasp. Here it is: inside an electron is a vibrating string. Pluck it differently and it becomes a neutrino or a quark, just as A, B flat and G are all notes of one violin string, none more fundamental. "The 'harmonies' of the string are the laws of physics"; strings splitting and rejoining give the interactions of atoms and nuclei; melodies are chemistry. The Pythagoreans first linked music and number (halve the string, go up an octave), and string theory revives their dream that "All things are numbers." Yehudi Menuhin: "Music creates order out of chaos." Leibniz: music is "the hidden arithmetic exercise of a soul unconscious that it is calculating."
Why strings, and why ten then eleven
Relativity treats space-time as smooth; quantum theory fills it with violent foam at the Planck length (10-33 cm). Point particles make gravity's Feynman diagrams diverge. Strings smear interactions over a tiny length, taming the infinities. The theory was found backward: Gabriele Veneziano (1968) wrote a dual resonance formula that fitted the strong force; only later did physicists see it described vibrating strings. Early versions needed 26 dimensions (bosonic) or 10 (superstrings). Anomaly cancellation by Green and Schwarz (1984) launched the first superstring revolution: five consistent theories in ten dimensions. Edward Witten's 1995 insight (second revolution) showed the five are limits of one 11-dimensional framework, M-theory, which also recovers supergravity. Extra dimensions can be curled up (Kaluza-Klein) so small we do not notice them, or, in brane-world pictures (Lisa Randall and others), we live on a 3-brane floating in a higher-dimensional bulk, with gravity leaking into the extra directions and thereby appearing weak.
From dual models to M-theory
Cosmic music and the Standard Model
Each vibrational mode of a string is a particle: graviton, photon, electron, quark. Mass and charge are notes on the string. Supersymmetry pairs every boson with a fermion, softens hierarchy problems and supplies dark-matter candidates (if the lightest superpartner is stable). Deriving the exact Standard Model spectrum from a compactification remains unfinished; millions of Calabi-Yau shapes give different low-energy laws, the "landscape" that later chapters treat as a multiverse of vacua.
Flatland powers and the failed unified field
Hover above Flatland and you see inside every creature, vanish by stepping off the page and reach any point without crossing the intervening lines. Those "powers of a god" are Kaku's metaphor for what access to higher dimensions would mean. Einstein's late unified-field attempts failed; a 1928 media stampede even put his unfinished equations in the New York Times, with over a hundred reporters outside his house. Eddington wrote that a Cambridge student was asked whether Einstein had solved the universe and replied he had not heard about it, being on the river. Schrödinger, Heisenberg, Pauli and Eddington all tried unification and failed. The chasm remains: relativity is smooth geometry of the very large; quantum theory is discrete probability of the very small. Only in ten or eleven dimensions, Kaku claims, is there "enough room" to fit all the forces into one framework. Critics reply that "enough room" is not the same as evidence.
Supersymmetry's promise and bill
Supersymmetry doubles the particle spectrum: every fermion gets a bosonic partner and vice versa. Benefits advertised in the book include a darker-matter candidate (lightest superpartner), help with the hierarchy problem and better meeting of force strengths at a GUT scale. The bill came due at the LHC: no superpartners at the masses that would have solved the hierarchy problem most neatly. The deeper string structures (dualities, branes, holography) do not automatically fall with low-energy SUSY, but the pre-LHC optimism of chapter 7 must be read with that later data in mind.
Kaluza-Klein: light as a ripple in the fifth dimension
In 1921 the obscure mathematician Theodor Kaluza wrote to Einstein: write general relativity in five dimensions, separate out the extra pieces, and Maxwell's equations of light fall out "almost by magic". Light is a ripple in the fifth dimension, as fish under lily pads, unaware of the third dimension, can still see the shadows of raindrops rippling the surface. Add more dimensions and their vibrations give the W, Z and gluons. But the theory was riddled with infinities, and why don't we see the fifth dimension? Smoke never vanishes into it, so (with Oskar Klein) it must be curled into a circle smaller than an atom.
Kaluza-Klein: write gravity in five dimensions and Maxwell's light falls out.
String theory faces the same problem. The universe began in ten dimensions; the ten-dimensional state was unstable, and six dimensions curled into a tiny ball (compactification) while four expanded in the big bang. A garden hose looks one-dimensional from afar; up close it is a two-dimensional tube.
Why strings work: topology and supersymmetry
Newton's force on a point particle grows as 1/r², infinite at r = 0. Feynman's tricks tame this for other forces but not for quantum gravity. Strings cure it in two ways:
- Topology. A string has a finite length (about the Planck length), so forces grow only to about 1/L², and divergences are "smeared out". The mathematics involves the elliptic modular function of Srinivasa Ramanujan, the self-taught genius from near Madras who died of tuberculosis in 1920 at 32 (the model for Good Will Hunting's "next Ramanujan"). His function has special properties in 24 dimensions; adding two for physics gives 26, and its eight-dimensional version gives 10. "Ramanujan was doing string theory before World War I!"
- Supersymmetry. The largest possible symmetry, reshuffling all particles. Matter particles are fermions (spin ½, 3/2: electrons, neutrinos, quarks); force carriers are bosons (spin 1 for photons, W, gluons; spin 2 for the graviton). Supersymmetry pairs each with a sparticle: selectron, squark, slepton, gravitino. Boson and fermion contributions to the infinities have opposite signs and cancel exactly, like a rocket whose wings are designed so that their vibrations cancel.
Deriving the Standard Model
In 1984 Candelas, Horowitz, Strominger and Witten showed that curling six dimensions into a Calabi-Yau manifold while keeping supersymmetry yields something remarkably close to the Standard Model. The puzzling three generations correspond to the number of holes in the Calabi-Yau space (a doughnut and a coffee cup have one; eyeglass frames have two).
Five theories and a jungle of solutions
By the mid-1990s the bandwagon had stalled. String equations had billions of solutions, each a self-consistent universe, many resembling ours but none exactly ours. Multiverse fans welcomed this; others were embarrassed (David Gross: millions of solutions in three space dimensions and "no good way of choosing among them"). No one knew how to break supersymmetry. And there were five string theories. Why would God need five?
| Theory | Strings | Notes |
|---|---|---|
| Type I | Open and closed | The original Veneziano theory; five interactions (Kaku and Kikkawa) |
| Type IIA, IIB | Closed only | Strings split like dividing cells |
| Heterotic E(8)×E(8) | Closed | Princeton "string quartet" (Gross, Harvey, Martinec, Rohm); big enough to swallow GUTs; the most realistic |
| Heterotic SO(32) | Closed | One set of vibrations lives in 26 dimensions |
Two more 11-dimensional theories were puzzles. Supergravity (van Nieuwenhuizen, Ferrara, Freedman, 1976) added a spin-3/2 gravitino to Einstein's theory. Cremmer, Julia and Scherk (1978) showed its most general form lives in eleven dimensions; it inspired Hawking's Lucasian inaugural lecture on "the end of theoretical physics" being in sight, but it was not finite. Supermembranes (two-branes and five-branes) were also consistent in 11 dimensions, but intractable and unstable.
1994-95: the eleventh dimension
Five string theories as limits of one eleven-dimensional M-theory.
Kaku was at Cambridge to give a talk on the tenth dimension when Townsend excitedly warned that his title might soon be obsolete. ("Uh oh.") Kaku fired objections: supermembranes are intractable and unstable; supergravity diverges; no one knows how membranes interact; and M-theory has no equations. Townsend conceded most points but was confident. Kaku's image: a pebble in the desert that turns out to be the tip of a buried pyramid, with chambers still to be excavated before we find the ground floor.
Branes and duality
- The brane menagerie. A point is a zero-brane, a string a one-brane, a membrane a two-brane; our universe may be a three-brane. (In p dimensions, a "p-brane", pronounced "pea-brain"; a chart of them is a "brane-scan".) Hořava and Witten showed that slicing a ribbon off a membrane's equator yields the heterotic string. There are five ways to reduce 11 dimensions to 10, hence five string theories.
- Duality. Maxwell's equations look the same if electric and magnetic fields swap, and exactly the same if monopoles exist and charge e swaps with 1/g: weak electricity equals strong magnetism. Similarly the five string theories are dual. Type I (open and closed strings, five interactions) and heterotic SO(32) (closed strings only, partly 26-dimensional) look nothing alike, yet crank up the coupling strength of one and it turns into the other. This convinced Kaku.
Branes, collisions and Randall's warped space
In M-theory the fundamental objects include not only strings but p-branes: membranes of various dimensions. Our universe may be a 3-brane. Lisa Randall (with Raman Sundrum) showed a warped extra dimension can explain why gravity is so weak: gravitons spread into the bulk while other forces stick to our brane. Ekpyrotic and cyclic models (Steinhardt, Turok and others) replace the hot big bang's singularity with a collision of branes: each smash heats a new big bang, and the cycle can repeat. Kaku presents these as competitors and complements to inflation, still speculative.
Brane-world options: live on a surface, collide, or warp the bulk.
Black holes, information and holography
Hawking showed black holes radiate and can evaporate, raising the information paradox: does information that falls in vanish, violating quantum unitarity? String theory and the AdS/CFT correspondence (Juan Maldacena, 1997) suggest a holographic answer: the information is preserved on the horizon's surface, and a gravity theory in a volume is equivalent to a quantum field theory on its boundary. Kaku treats this as evidence that space-time itself may be emergent from more fundamental degrees of freedom. The paradox remains actively debated; recent calculations using "islands" and replica wormholes (2019-) favour information escape with the radiation.
Colliding universes and the information bet
Kaku sketches brane collisions as an alternative genesis: two empty branes approach in the bulk, smash, and dump energy that looks from within like a hot big bang. Cyclic versions hope to avoid a beginning of time. Separately, the information paradox asks whether Hawking radiation is truly thermal. If it is, quantum mechanics breaks; if information escapes, black holes are dramatic but law-abiding. Kaku bet on escape; the post-2019 Page-curve calculations using quantum extremal surfaces lean his way, though the physical mechanism (islands, replica wormholes) is still being digested. Holography goes further: the degrees of freedom of a region may live on its boundary, so three-dimensional space is a kind of image. That idea, once wild, is now mainstream theoretical lore even among physicists who doubt strings.
Randall's warped throat in plain language
If a fifth dimension is severely warped, a small proper distance in the bulk can correspond to a huge difference in energy scales. Fields stuck to our brane feel ordinary TeV physics; gravity, spreading through the bulk, looks weak here. That picture offered a geometric reason for the hierarchy without low-energy supersymmetry. Experiments that look for Kaluza-Klein graviton modes or short-distance gravity changes are the tests; so far they have not appeared. Ekpyrotic collisions use related higher-dimensional geometry for a different job: replacing the bang's singularity with a bounce. Both ideas show how M-theory talk reaches cosmology without yet locking in a unique model.
Lisa Randall and the weakness of gravity
Since branes allow some extra dimensions to be large, even infinite, M-theory offers more than tiny curled-up spaces. Lisa Randall (raised in Queens, an undergraduate at Harvard in the early 1980s, drawn to physics by simple pictorial models) attacked the hierarchy problem: why is gravity so weak? "The two mass scales are separated by sixteen orders of magnitude!" Gravity's weakness is why stars are so big: it takes a star's whole mass to squeeze protons together. Kaku adds a plea for mathematics as "the language of nature": Galileo said the universe "is written in mathematical language."
ADD (Arkani-Hamed, Dimopoulos, Dvali): the extra dimension is only about a millimetre wide, so deviations from Newton's law appear only below a millimetre, a testable prediction.
Randall-Sundrum: the extra dimension may be infinite, but the brane's own gravity traps gravitons "like flies trapped on flypaper", preserving the inverse-square law. A second parallel brane can explain the weakness numerically.
If so, gravity is as strong as the other forces but diluted, and quantum gravity might appear at trillions of electron volts rather than the Planck energy of 1019 billion: within reach of the LHC. A hidden galaxy on a brane "just above" ours would be invisible yet pull on our galaxy, like Wells' invisible man: dark matter might be a parallel universe.
Colliding universes: three brane cosmologies
| Scenario | Proposers | Idea | Critique |
|---|---|---|---|
| String-gas cosmology | Brandenberger, Vafa | Strings and antistrings wound around all dimensions keep them small; only in 3 or fewer space dimensions do they meet and annihilate, letting those dimensions spring open | Explains why we see 3 + 1 dimensions only "roughly" |
| Ekpyrotic ("big splat") | Steinhardt, Ovrut, Turok | Two flat, cold three-branes drift together and collide; the impact energy becomes matter and radiation; the cycle repeats every trillions of years | Linde, Kallosh, Kofman: a singularity; why start perfectly flat? |
| Pre-big bang | Veneziano | An infinitely old, cold universe forms black holes; each collapses to the Planck length and bounces into a big bang | Speculative; few tests |
Brane cosmology: our universe as a membrane, and big bangs as brane collisions.
The ekpyrotic model gets inflation's results in reverse: flatness because the branes started flat ("Flat plus flat equals flat"), and uniformity because they had ages to equilibrate in slow motion. Steinhardt notes that today's acceleration might herald another collision: "It is not a pleasant thought." Linde: the collision is like "throwing a chair into a black hole... and saying it somehow preserves the shape of the chair." Steinhardt: "What looks like a singularity in four dimensions may not be one in five." Guth was open-minded but challenged string theorists to derive inflation. Joel Primack: "essentially untestable." But the LISA gravity-wave mission could distinguish violent inflationary waves from the ekpyrotic model's milder ones.
Mini black holes
If an extra dimension is near, unification energies fall within reach. Einstein and Rosen had already imagined electrons as tiny black holes (1935), and Hawking showed that black holes evaporate and shrink. The LHC, colliding protons at 14 TeV, might produce mini black holes of about a thousand electron masses that live 10-23 s and leave clear tracks. They pose no danger: cosmic rays striking Earth carry more energy. The Pierre Auger Observatory in Argentina might see ten cosmic-ray showers a year triggered by them. A detection would not prove string theory but would strongly support parallel universes.
The information paradox
Hawking radiation has a temperature linked to the horizon's area, but counting a smooth black hole's quantum states seemed impossible. Strominger and Vafa asked for the black hole's dual: a collection of one-branes and five-branes, whose states are countable. The count reproduced Hawking's formula exactly, perhaps string theory's most elegant result.
The black hole information paradox: Hawking radiation seems to erase information.
- The paradox. Black holes "have no hair": only mass, spin and charge survive. Quantum mechanics says information can never be lost. Hawking: "spacetime ties itself in knots and... information gets lost in the folds."
- 2004 reversal. Hawking announced he was wrong: a book's information leaks out, mangled, in the radiation, settling a bet whose prize was an encyclopedia, "from which information can be easily retrieved." Kaku expects string theory to show that information "subtly reappears somewhere else", perhaps even in another universe.
The holographic universe
A hologram stores a 3-D image on a 2-D plate. Bekenstein and Hawking found that a black hole's information is proportional to its surface area, not its volume: you can judge a black hole by its cover. In 1997 Juan Maldacena showed that a five-dimensional anti-de Sitter universe (negative cosmological constant) is exactly dual to a four-dimensional theory on its boundary.
Is the universe a computer program?
The Matrix asks whether reality could fit on a CD. A Newtonian world resists simulation: weather forecasts fail after days because of the butterfly effect, and "the smallest model that can accurately describe the weather is the weather itself." But tile a black hole's horizon with Planck-sized squares, one bit each, and you get its information content. Bekenstein: "Field theory, with its infinity, cannot be the final story"; the final theory may be about "information exchange among physical processes." A centimetre-sized black hole holds about 1066 bits; the visible universe at least 10100 (a googol), which in principle fits in a sphere a tenth of a light-year across. A CD of that capacity would give "a God-like ability to rewrite the script", unless, as Bekenstein concedes, the smallest model of the universe is the universe itself.
The end of physics?
No: knowing the rules of chess does not make you a grand master. M-theory is unfinished, still evolving backward, and its field equations are unknown. Physicists are "drowning in p-branes" (doughnuts, multi-holed doughnuts, intersecting branes), like blind men and an elephant, each sure the beast is a one-, two- or three-brane. History suggests that a proliferating spectrum hides something simpler: spectral lines led to the atom, particle zoos to quarks, quarks to strings. Kaku's own bet is that branes are "condensations" of space itself, reviving Einstein's dream of matter as geometry. Brian Greene: string theorists are "an Einstein bereft of the equivalence principle." Without a unique solution among millions, a growing view holds that there is none: all are realized in a multiverse, which leads to the anthropic principle.
Landscape, anthropic selection and testability
When string compactifications yield 10500 or more vacua, each with different constants, prediction looks impossible unless something selects our vacuum. Anthropic reasoning says we observe this vacuum because only here can observers exist. Lenny Susskind and others embraced the landscape; David Gross and many others call anthropic arguments a retreat from physics. Kaku is sympathetic to the multiverse reading but insists experiment must eventually decide: gravity waves from the big bang, deviations from Newton's law at submillimetre scales, cosmic-ray or collider relics of extra dimensions, or mathematical consistency alone. As of the book's date none of those signals had appeared; chapter 9's Update tracks what has and has not since.
Blind men and the elephant
Physicists drowning in p-branes each grasp one part of M-theory: one-branes, two-branes, five-branes, intersections, dualities. Kaku compares them to blind men describing an elephant, each sure the beast is a rope or a wall. History's pattern, he hopes, is that a proliferating spectrum hides a simpler object: spectral lines to atoms, particle zoos to quarks, quarks to strings, branes perhaps to condensations of space itself. Brian Greene's line that string theorists are "an Einstein bereft of the equivalence principle" captures the missing organising insight. Until it arrives, the theory advances by dualities, consistency checks and the hope that experiment or mathematics will cut the vacuum count down from 10500.
M-theory's report card, today
A Designer Universe?
Kaku's second-grade teacher told him that "God so loved the earth, that He put the earth just right from the sun." Too far and the oceans freeze, too close and they boil. Scientists call this the Goldilocks zone, and the teacher's argument is an instance of the anthropic principle: the laws of nature are arranged so that life and consciousness are possible. Whether by design, accident or a multiverse is the question of the chapter.
A planet inside many Goldilocks zones
| Zone | Why Earth is "just right" | What goes wrong otherwise |
|---|---|---|
| Distance from Sun | Liquid water, the "universal solvent" | Mars: frozen desert with permafrost. Venus: CO2 greenhouse at 900 °F, acid rain, 100 times Earth's pressure |
| Moon's size | A large Moon steadies Earth's tilt | Axis could swing up to 90°, wrecking the climate stability DNA needed |
| Jupiter | Its gravity cleared out asteroids in the "age of meteors" | Continuous impacts on a young Earth |
| Planet mass | Keeps oxygen, loses primordial poisonous gases | Too small: no air. Too large: toxic atmosphere |
| Orbits | Nearly circular planetary orbits, rare close encounters | A gas giant could fling Earth away |
| Place in galaxy | About two-thirds of the way out | Near the centre: radiation from the black hole region. Far out: too few heavy elements |
The Moon recedes about 4 cm a year; in roughly 2 billion years it may be too distant to steady Earth's spin. Mars, with tiny moons, has wobbled by tens of degrees. Peter Ward and Donald Brownlee (Rare Earth): "Without the Moon there would be no moonbeams, no month, no lunacy, no Apollo program, less poetry", and probably no advanced life. They argue intelligent life may be unique in the galaxy.
Hunting other worlds
By 2002 astronomers had found about 100 extrasolar planets, mostly by the wobble method: a star and a giant planet are like dance partners clasping hands, the star tracing a tiny circle (Chris McCarthy). Doppler shifts reveal velocities as small as 3 m/s, a brisk walk. Other methods: dips in brightness as a planet transits its star, and planned interferometer satellites that cancel starlight. Most early finds were Jupiters on eccentric or tight orbits that would expel any Earth from the habitable zone, so a "normal" system around HD 70642 made headlines in 2003. Paul Butler (part of the team behind early planet discoveries) aimed to survey all 2,000 sun-like stars within 150 light-years.
Destiny forks for the cosmos
Anthropic filters on constants.
Cosmic accidents
Stability for hundreds of millions of years is hard to arrange:
- Proton vs neutron. The proton is slightly lighter. If it were about 1% heavier it would decay into a neutron and atoms would fall apart.
- Proton stability. Its lifetime far exceeds the universe's age, as DNA requires.
- Strong force. A bit weaker and deuterium flies apart, so no stellar nucleosynthesis; a bit stronger and stars burn out too fast.
- Weak force. Neutrinos carry energy out of supernovae and help forge elements beyond iron. Too weak and they escape without effect; too strong and they are trapped in the core.
Change any of these and stars never form, DNA never exists or Earth freezes. Kaku attributes to Hugh Ross the image of a tornado in a junkyard assembling a Boeing 747 (it is originally Fred Hoyle's).
Four anthropic principles
| Version (Don Page's list) | Claim |
|---|---|
| Weak | "What we observe about the universe is restricted by the requirement of our existence as observers." |
| Strong-weak | "In at least one world... of the many-worlds universe, life must develop." |
| Strong | "The universe must have the properties for life to develop at some time within it." |
| Final | "Intelligence must develop within the universe and then never die out." |
Four readings of the anthropic principle, from bland to bold.
Against: Steven Weinberg admits the appeal of believing "we were somehow built in from the beginning" but calls the strong principle "little more than mystical mumbo jumbo." Heinz Pagels dropped it for lack of predictive power, an endless tautology: we are here because we are here. Guth: "The anthropic principle is something that people do if they can't think of anything better to do."
Martin Rees: six numbers and a multiverse
The Astronomer Royal, an unlikely radical, argues that so many coincidences imply many universes, most of them dead. "The apparent fine-tuning on which our existence depends could be a coincidence. I once thought so. But that view now seems too narrow."
Rees' six numbers: each must sit in a narrow band for life
Fred Hoyle had found that a 4% shift in the nuclear force would block carbon formation through its beryllium "bridge". On dimensions: in one dimension particles pass through each other; a two-dimensional Flatlander with a digestive tract would split in half, and neural networks of 100 billion neurons, each linked to 10,000 others, would short-circuit; in four or more dimensions gravity follows an inverse-cube law and, as Paul Ehrenfest showed in 1917, orbits of planets and electrons are unstable. Three dimensions are special.
Goldilocks zones for Earth imply other planets; Goldilocks zones for the universe imply other universes. Weinberg likes the multiverse because it removes a sudden beginning of time. Rees adds a quirky point: the universe has a little "ugliness". Earth's orbit is slightly elliptical and lambda is small but not zero: our universe is "no more special than our presence requires", just what random generation predicts. As an astronomer he insists such ideas be testable, perhaps within twenty years.
Universes that evolve
Lee Smolin proposes cosmological natural selection: if universes sprout from black holes with slightly mutated constants, the universes making the most black holes leave the most "children", and the multiverse fills with them. Our universe should then be close to optimal for making black holes, a testable claim: if tweaking a constant (say, a stronger nuclear force creating many short-lived stars and supernovae) produced more black holes, the idea would be falsified. One still has to show that black-hole-rich universes favour life. Kaku closes on a hopeful note: satellites, space telescopes, gravity-wave detectors and lasers are bringing these questions into the lab.
Searching for Echoes from the Eleventh Dimension
Ken Croswell warns: "Other universes can get intoxicating: you can say anything you want about them and never be proven wrong." But much of astronomy is indirect: no one has visited a star, yet spectra reveal its hydrogen and helium; no one sees a black hole, yet accretion disks betray it. Kaku's case is that computers, lasers and satellites put echoes of the eleventh dimension within reach.
GPS: relativity at work
The Global Positioning System (24 satellites) synchronizes pulses to within 50 billionths of a second to fix a position within 15 yards. Without relativistic corrections the clocks would drift by about 40,000 billionths of a second a day and the system would fail. Clifford Will knew relativity had "come of age" when he had to brief an Air Force general on it.
Why GPS needs relativity: two clock effects that do not cancel.
Gravity waves
"Every time we have looked at the sky in a new way, we have seen a new universe" (Gary Sanders). Einstein predicted gravity waves in 1916: remove a bowling ball from a trampoline and ripples spread at the speed of light. They are fantastically weak.
How LIGO senses a passing gravity wave
LIGO (Hanford, Washington, and Livingston, Louisiana) switched on in 2003: L-shaped pipes 2.5 miles long, the world's largest artificial vacuum, mirrors so smooth that on an Earth-sized version "the average mountain wouldn't rise more than an inch" (GariLynn Billingsley). Its enemies are mundane: daytime logging 1,500 feet away, freight trains at midnight and 6 a.m., ocean waves arriving every six seconds ("a huge headache during the Louisiana hurricane season"), and lunar tides. Mirrors sit on stacked spring-damped steel platforms on 30-inch concrete slabs. Partners include VIRGO (Pisa), TAMA (Tokyo) and GEO600 (Hanover). Cost: $292 million plus $80 million, the NSF's most expensive project. Detection rates seemed bleak: one event every ten years, maybe a black-hole collision once in a thousand. Peter Saulson compared the team to medieval cathedral builders. LIGO II promised a thousandfold improvement: "an event every three days."
Einstein lenses and rings
Soldner (1801) computed light bending with Newton, missing a factor of 2. Einstein considered gravitational lenses in 1912, and in 1936 answered Czech engineer Rudi Mandl that a star could act as a lens, producing double images or a ring, but with "not much hope of observing this phenomenon directly"; privately, the paper "is of little value, but it makes the poor guy happy."
1986: first giant arcs in galaxy clusters; Paczynski proposes microlensing to find MACHOs
1988: first Einstein ring, MG1131+0456
1990s: EROS, MACHO and OGLE log 500+ microlensing events
1997: Hubble and MERLIN catch the complete ring 1938+666, a second of arc, "a perfect Einstein ring!" (Ian Brown)
1998: lens statistics put lambda at no more than 62% (WMAP later: 73%)
Uses: about one quasar in 500 is lensed; arcs (Abell 2218) map dark matter; microlensing finds MACHOs and planets; delays between twin quasar images give the Hubble constant, independent of Cepheids and supernovae; and lens counts over time estimate the cosmological constant.
Dark matter in your living room
The Sun moves around the galaxy at 220 km/s through a "dark matter wind": perhaps a billion particles cross every square metre per second, yet a kilogram of detector might register 0.01 to 10 events a year. Blocks of sodium iodide, germanium, silicon or freon wait for a nucleus to recoil. UKDMC, ROSEBUD, SIMPLE and Edelweiss saw nothing; DAMA (100 kg of sodium iodide, near Rome) claimed a signal in 1999 that others could not reproduce. David Cline: a verified signal "would go down as one of the great accomplishments of the twenty-first century."
Digital astronomy
The internet ended the "priesthood" hoarding telescope time. SETI@home sent Arecibo data to about 5 million PCs. The Sloan Digital Sky Survey (2.5 m telescope in New Mexico, 30 CCDs cooled to -80 °C, under $20 million, a hundredth of Hubble) mapped a quarter of the sky, 100 million objects, a million galaxy distances and 15 terabytes, freely shared. In 2003 Sloan studies of 250,000 galaxies weighed dark-matter halos via satellite orbits and cast doubt on the 1983 rival theory of modified Newtonian dynamics; a study of 120,000 galaxies found 20,000 with growing black holes, suggesting galaxies and black holes form together.
Adaptive optics (pioneered by Claire Max at Livermore, tested 1996): a laser measures the atmosphere's twinkle and a deformable mirror (changing shape 670 times a second on Keck II since 1999) cancels it, giving Keck's twin 10 m, 36-segment mirrors a tenfold sharper view: stars orbiting the Milky Way's black hole, Neptune, Titan, and the transit of HD 209458's planet. Radio dishes are "lashed together": the VLBA (ten 82-foot dishes from Hawaii to the Virgin Islands, online 1993 for $85 million) acts as a 5,000-mile telescope, like reading a Los Angeles newspaper from New York.
Measuring the eleventh dimension
Testing for extra dimensions: look for gravity leaking at short distances.
The Large Hadron Collider
| LHC fact (as Kaku describes it) | Value |
|---|---|
| Ring | 27 km (circumference, not diameter as Kaku says), straddling France and Switzerland |
| Collision energy | 14 TeV protons; lead nuclei for quark-gluon plasma |
| Magnets | 1,232 dipoles, 8.3 tesla, 12,000 A, cooled to -271 °C |
| Reach | 10-19 m, 10,000 times smaller than a proton |
| Goals | Higgs (115-200 GeV), quark-gluon plasma of the first 10 μs, mini black holes, supersymmetry |
What the LHC can test for M-theory and cosmology.
CERN serves more than half the world's experimental particle physicists. Superpartners would be "lower octaves" of the string and a dark matter candidate. The LHC is a second choice: the Superconducting Supercollider (50-mile ring near Dallas, 40 TeV, $11 billion) was cancelled by Congress in 1993, a blow that "set the field back for an entire generation." Kaku blames a split scientific community, "big science versus small science" arguments, the end of Cold War rivalry with the Soviet UNK machine, and physicists' failure to speak to the public.
Tabletop accelerators
Conventional machines are relay races where RF energy kicks the baton each lap, and RF is near its limit. In plasma wakefield acceleration a laser or particle beam makes a plasma wave and particles "surf" its wake. Rutherford Appleton produced 400 MeV protons with a 50-terawatt laser; École Polytechnique gave electrons 200 MeV in a millimetre; SLAC accelerated over 1.4 m in 2001. Energies rose tenfold every five years; scaled to a metre, 200 GeV, and "the LHC [would] look like the last of the dinosaurs."
Long shots and pure thought
Witten's favourite: a cosmic string stretched by the big bang, "seeing a string in a telescope." Brian Greene lists five possible confirmations: neutrino masses explained; small violations of the Standard Model; new long-range forces revealing a Calabi-Yau shape; dark matter particles matching predictions; and a calculation of dark energy. Kaku's own bet is mathematics: a complete theory should compute the proton, neutron and electron masses with no adjustable parameters. Einstein: "pure thought can grasp reality, as the ancients dreamed."
What would count as an echo of the eleventh dimension
Kaku's experimental wishlist mixes tests that have succeeded with tests that still wait:
- Gravity waves from astrophysical mergers (LIGO) confirm relativity; primordial waves from inflation would probe GUT-scale physics and possibly stringy relics.
- LISA in space would see supermassive black-hole mergers and extreme-mass-ratio inspirals that ground detectors miss.
- Tabletop gravity tests look for deviations from 1/r2 at submillimetre scales, a signature of large extra dimensions.
- Dark matter detectors (CDMS, DAMA, and today's LZ and XENONnT) hunt WIMPs that supersymmetry often predicts.
- Colliders hunt superpartners, mini black holes and Kaluza-Klein modes.
- Precision CMB polarisation hunts the inflationary B-mode spectrum.
A null result in any one channel does not kill M-theory; a positive, distinctive signal in several would strongly favour it. Kaku also notes the fallback of "verification from pure mathematics," which critics treat as abandoning empiricism.
Echo channels: which tests speak to strings, which only to Einstein.
LIGO's enemies and cathedral builders
LIGO's 2003 debut used L-shaped arms 2.5 miles long, the world's largest artificial vacuum, and mirrors so smooth that on an Earth-sized optic "the average mountain wouldn't rise more than an inch" (GariLynn Billingsley). Daytime logging 1,500 feet away, freight trains at midnight and 6 a.m., ocean waves every six seconds and lunar tides all shake the detectors. Mirrors sit on stacked spring-damped steel platforms on 30-inch concrete slabs. Partners: Virgo (Pisa), TAMA (Tokyo), GEO600 (Hanover). Cost about $292 million plus $80 million, then the NSF's most expensive project. Early estimates were bleak: one event per decade, maybe a black-hole collision once in a thousand years. Peter Saulson compared the team to medieval cathedral builders. LIGO II (Advanced LIGO) promised a thousandfold gain and "an event every three days"; that promise was kept after 2015. Kaku's chapter, written before the first detection, reads now as a bet that paid out for astrophysics even while the "eleventh dimension" signals he hoped for have not arrived.
Colliders, dark matter and the SSC ghost
The cancelled Superconducting Super Collider haunts the chapter: Kaku's partisan history of its death in Congress still matches how many physicists tell the story. The LHC took up part of the job and found the Higgs, not the hoped-for superpartners. Dark-matter searches (then CDMS and DAMA; now LZ, XENONnT, PandaX) continue to report nulls for WIMPs, while DAMA's annual modulation remains contested by ANAIS and COSINE-100. Submillimetre torsion balances (Eöt-Wash) push limits on large extra dimensions. Each null narrows the simplest string-inspired models without touching the deeper landscape of high-scale compactifications.
SETI, Arecibo and other ears
The End of Everything
Darwin found it "an intolerable thought" that humanity, after long progress, faces "complete annihilation." Norse Ragnarok begins with three winters, wolves devouring sun and moon, Loki breaking free, and Surtur's fire engulfing heaven and earth, yet a new earth rises from the sea. Myths worldwide pair catastrophe with renewal; physics asks whether there is any renewal at all.
WMAP shows a dark energy proportional to volume: more space means more push, a runaway exponential expansion. Eventually only the 36 galaxies of the Local Group will remain visible as the rest race beyond our horizon. "The universe will become terribly lonely," then freezing.
The three laws of thermodynamics
Helmholtz (1854) first applied thermodynamics to the whole universe.
Why the universe runs down: the laws of thermodynamics applied to cosmos.
| Law | Statement | Cosmic meaning |
|---|---|---|
| First | Matter plus energy is conserved (E = mc2 converts one into the other) | Nothing is created from nothing |
| Second | Total entropy always increases: paper burns, smoke never reassembles | Everything runs down: "heat death" |
| Third | No refrigerator reaches absolute zero; quantum uncertainty forbids zero motion | The cold approaches but never arrives |
Life seems to defy the second law, but local order is paid for by sunlight: count the Sun and Earth together and entropy still rises. A refrigerator lowers entropy locally only by raising it outside. An oscillating universe does not escape either: carried-over entropy would make each cycle longer, and every version ends intelligent life.
Death by fire: the big crunch
Martin Rees' 1969 paper "The Collapse of the Universe: An Eschatological Study" assumed Omega = 2. Expansion halts when galaxies are twice as far apart, redshift turns to blueshift, and about 50 billion years from now galaxies merge in the last 100 million years. Stars dissolve before they collide: blueshifted starlight and a soaring background temperature exceed their surface temperatures. Dyson: "we have no escape from frying." A bounce, as in Poul Anderson's Tau Zero, might work in Newton's universe if galaxies missed each other, but Penrose and Hawking proved that in Einstein's theory a collapse ends in a singularity: sideways motion carries energy, which itself gravitates.
Five eras of the universe
Fred Adams and Greg Laughlin divide cosmic history into five eras on a logarithmic clock (20 means 1020 years).
Adams and Laughlin's five eras (exponents of years)
Hazards of the stellar era
| Time scale | Threat | Possible response |
|---|---|---|
| Tens of thousands of years | Next ice age (orbital wobbles let polar air spread); Earth was once entirely ice-covered | Move underground |
| Thousands to millions of years | Impacts: a ~10 km object at Yucatán ended the dinosaurs; 1 in 100,000 chance of global damage in 50 years; asteroid 1950 DA might hit on 16 March 2880 with 400-ft tsunamis | Track and deflect asteroids |
| 3.5 billion years | Sun 40% brighter; oceans boil | Push Earth outward with asteroid slingshots |
| 5 billion years | Sun becomes a red giant about Earth's orbit, swallowing Mercury and Venus, then a white dwarf of 0.55 solar masses, ending as a black dwarf | Space arks, or capture a passing star (Croswell) |
Before 10,000 years ago humans lived "like wolves in packs"; the warm interglacial let civilization start. Ken Croswell: "As long as people get smarter faster than the Sun gets brighter, the Earth should thrive." The biosphere's flowering lasts only about a billion years, and we are halfway through. "Mother Nature wasn't designed to make us happy" (Donald Brownlee).
The black hole and dark eras
A proton-sized black hole radiates 10 billion watts for the solar system's lifetime; a solar-mass one evaporates in 1066 years, a cluster-mass one in 10117, ending each life in an explosion. Life might huddle around them "like homeless people... next to the dying embers." In the dark era protons may have decayed, leaving positronium: an electron and positron orbiting each other in "atoms" 1012 megaparsecs across, millions of times today's observable universe, with chemistry on unimaginable timescales. Tony Rothman: "For this too is written in the Book of Destiny."
Can intelligence survive?
Two problems: machines need a temperature difference to do work, and the smallest transmittable bit scales with temperature. Freeman Dyson's answer: engineer cooler bodies, move to robots, then to pure consciousness, think slower and hibernate. Subjective time stays the same while physical time stretches; an ingenious life form could think forever on finite energy, and might even see baby universes and quantum leaps flicker past like everyday events.
Death is "the final cessation of all information processing." Fortunately there is ample time, and the next chapter asks whether physics permits escape to a parallel universe.
Heat death, proton decay and the long future
If dark energy is a true cosmological constant, expansion accelerates forever. Stars burn out; galaxies go dark; protons may decay (lifetime above 1034 years so far); black holes evaporate by Hawking radiation over absurd timescales; only a thin heat bath near absolute zero remains. Adams and Laughlin periodise this future into named eras (stelliferous, degenerate, black-hole, dark). Kaku asks whether intelligence can migrate, slow its clock or leave. Planetary defence is the near-term version of the same instinct: deflect asteroids (1950 DA's 2880 risk, Apophis in 2029) before cosmic carelessness ends the experiment early.
Long-future eras if the freeze wins
Near-term extinction versus cosmic extinction
Before the heat death comes the ordinary apocalypse: asteroid impacts, supervolcanoes, nuclear war, engineered pandemics. Kaku treats planetary defence as practice for thinking on cosmic timescales. The dinosaur-killing impact about 66 million years ago left the Chicxulub crater; objects such as 1950 DA keep a nonzero impact probability centuries out. A type I civilization that cannot deflect rocks will never reach type II. The chapter's darker joke is that we may fail the easy exam and never sit the hard one that chapter 11 sets.
Milkomeda and the disappearing sky
In the accelerating cosmos the Local Group remains bound. Andromeda and the Milky Way should merge into "Milkomeda" on a timescale of billions of years, while exterior clusters slip beyond the horizon. Future astronomers in that merged galaxy may see a nearly empty sky and invent myths to explain it. Kaku uses that image to stress urgency: the evidence for other galaxies and for the big bang is perishable on cosmic timescales. Recording and understanding the universe is not only curiosity; it is a race against accelerating erasure.
Proton decay and the dark era
GUTs that unify quarks and leptons usually predict proton decay. Decades of underground searches have not seen it, pushing lifetimes above 1034 years and killing the simplest SU(5). If protons are stable or nearly so, the degenerate era lasts longer; if they decay, all ordinary matter eventually becomes a thin lepton-photon haze before black holes finish evaporating. Either way the qualitative freeze remains unless dark energy itself evolves, the DESI question that now hangs over the chapter.
Near-term versus cosmic threats.
Escaping the Universe
In Bear's Eon, a hollow asteroid settles into Earth orbit after mass panic. Physicist Patricia Vasquez finds seven chambers with lakes, forests and cities, and a library holding Tom Sawyer republished in 2110: the ship is from Earth 1,300 years ahead. Its records describe a nuclear war two weeks into her future. The asteroid holds "the Way", a tunnel of gateways to every alternate history her theories made possible. She meets descendants who reshape bodies, resurrect personalities from computer banks and tap infinite information; homesick, she scans for a world where her family lived, leaps, and by a tiny mathematical slip lands where the Egyptian empire never fell. Kaku asks whether a real civilization might find refuge in another universe. For a physicist the question is not resources but proof of principle: do the laws of physics allow it? Martin Rees: wormholes and extra dimensions "could transform our entire universe eventually into a 'living cosmos.'"
Kardashev's ladder
Nikolai Kardashev (1960s) ranked civilizations by energy, each type 10 billion times the last. Even a civilization trying to hide would leak waste heat, by the second law of thermodynamics.
Kardashev scale, with Kaku's growth estimates at 2-3% a year
Carl Sagan added decimals: type I.1 is 1017 W, so we are about 0.7, a thousand times short of type I. At 2-3% growth a year (tied to GDP), Kaku times type I at 100-200 years, type II at 1,000-5,000, type III at 100,000 to a million.
Type I in practice
Type I will stay home for centuries sorting nationalism and sectarianism. Orbit costs $10,000-40,000 a pound ("John Glenn made out of solid gold"); a shuttle flight about $800 million; a Mars mission $100-500 billion with almost no economic return; launch failure about 1 in 70. Space elevators of carbon nanotubes to a geosynchronous satellite more than 20,000 miles up (Jack and the Beanstalk) could cut the cost of reaching Earth orbit, not the planets; NASA was already funding studies. Slow solar and ion engines heating cesium could build an interplanetary highway. Terraforming Mars with redirected comets, greenhouse methane or even underground nuclear charges remains wildly speculative. Eventually fusion ramjets scooping interstellar hydrogen might reach other stars, a century or more away because proton-proton fusion is hard even on Earth. Type I weather control would mean steering hurricanes and droughts, not just forecasting them.
Type II: the star
Type II resembles Star Trek's Federation without warp drive. Freeman Dyson's spheres, built from a dismantled Jupiter and enclosing a star, would glow in the infrared; Jun Jugaku's search to 80 light-years found none (the galaxy is 100,000 light-years across). Matter-antimatter drives approach light speed at near 100% efficiency but need atom smashers for antiprotons. Such a civilization could deflect comets, steer climate and flee supernovae: practically immortal except for its own folly. Dyson also imagined "astroidal" habitats that migrate when a star dies.
Type III: the galaxy
Dyson's Carroll eras: how a type III galaxy may fragment without FTL links.
Type IV: the continuum
A ten-year-old at the London Planetarium insisted on a type beyond III: power from the "continuum", the dark energy that makes up 73% of the cosmos. Like Nikola Tesla's dream of vacuum energy (against Thomas Edison's coal and copper), or gold dissolved in the oceans beyond Fort Knox: vast but too dilute to harvest cheaply with present technology. A type IV civilization would treat the cosmological constant as a resource.
Ranking by information
Exponential growth risks drowning in waste heat, like bacteria in a petri dish; the escape is miniaturization. The brain (100 billion neurons, as many as visible galaxies) is a parallel learning network, not a Turing machine with Windows or a Pentium, and thinks "without working up a sweat"; messages are chemical and slow, but parallel and plastic. Princeton DNA computers (bases A, T, C, G instead of 0 and 1) solved the travelling salesman problem for several cities; molecular transistors and early quantum computers point the same way. Sagan's letter scale counts bits:
| Sagan type | Bits | Example |
|---|---|---|
| A | 106 | Oral culture (20 yes/no questions split the world into 220 ≈ 106 pieces) |
| C | 109 | Surviving writings of ancient Greece (Philip Morrison, MIT) |
| H | 1015 | Today's libraries (~1013 from books) plus photographs: we are 0.7 H |
| J-K | above 1016 | First alien contact likely with type 1.5 J or 1.8 K |
| Q | huge | A galactic type III; III Q is the minimum for a wormhole leap |
| Z | vaster still | Information of a billion galaxies |
Escape needs this computing power: curvature anywhere depends on every mass in the universe plus quantum corrections Einstein's equations make hard even for one star. Rees: life "may not forever be an unimportant trace contaminant of the universe"; snuffing ourselves out would destroy "genuine cosmic potentialities."
Eleven steps out of a dying universe
| Step | Task | Key obstacle |
|---|---|---|
| 1 | Find and test a theory of everything (atom smashers, gravity-wave detectors on stable moons free of erosion) | Are wormholes stable under quantum corrections? Does radiation pile up infinitely in time-travel loops (unless many worlds splits it)? Is there enough negative energy? |
| 2 | Look for natural wormholes, cosmic strings, negative matter and white holes (time-reversed black holes that might sit at a black hole's other end) | None ever seen; too risky to bet all life on them |
| 3 | Send shielded probes into a Kerr black hole | Blueshifted radiation; signals freeze and redshift near the horizon, so data must be sent from a distance; one-way trip |
| 4 | Build a black hole in slow motion: add neutron stars (Manhattan-sized, heavier than the Sun) one by one to a swirling cluster until it collapses into a controllable Kerr ring, or assemble them past ~3 solar masses | One-way trip; radiation; stability needs quantum gravity; avoid a supernova-like blast |
| 5 | Create a baby universe from a false vacuum (Alan Guth) | Densities of 1080 g/cm3 or ~1029 K; Penrose's theorem makes it collapse to a black hole unless negative energy is used; umbilical Hawking burst ~500 kilotons (~25 Hiroshimas) |
| 6 | Build atom smashers of stellar size (beams bent by magnets on moons and asteroids, or a 10-light-year line) | At 200 GeV per metre, Planck energy needs ~10 light-years; background photons may cap beam energy (GZK); explosive magnetic pulses may destroy coils each pass |
| 7 | Use laser implosion (Livermore-style banks on asteroids) to heat a false vacuum or squeeze Casimir plates to the Planck length (10-33 cm) | Lasing materials crack; magnetic squeezing is "like squeezing a balloon"; plates must not collapse |
| 8 | Build an Alcubierre warp drive (1994): shrink space ahead, stretch it behind (lasso the table across a bunching carpet) | Needs negative energy at Planck separation; Sergei Krasnikov showed you cannot steer from inside, so it suits a timetabled "railway to the stars"; cannot leave the universe, but can help build Gott's colliding cosmic strings |
| 9 | Harvest negative energy from squeezed light | Lawrence Ford's three laws (1978, Tufts): bigger pulses are briefer; a negative pulse is paid back by a larger positive one; a shutter adds its own positive pulse; a 1 m wormhole needs negative energy in a band of 10-22 m |
| 10 | Think slowly and wait for quantum transitions | Gateways open unpredictably, to unknown destinations; subjective time may make them seem common |
| 11 | Send a seed of the civilization | Encoding about 1024 bits in a nanoscale package |
Kaku's escape plan compressed: find a theory, find a portal, send the smallest seed.
Step 1 must settle three questions: wormhole stability under quantum corrections; whether radiation piles up infinitely in a time-travel loop (the many-worlds view splits the loop and may evade the pile-up); and whether negative energy can be produced in usable amounts. Gravity-wave detectors on airless moons free of wind and erosion would test the theory. Step 4 echoes Einstein's 1939 paper showing swirling dust never reaches the Schwarzschild radius: inject mass slowly and a civilization could tip it over and open a Kerr ring as gently as it liked. Allen Everett showed two Alcubierre ships enable time travel, and J. Richard Gott quipped that Roddenberry "was indeed right to include all those time-travel episodes!" Gott also argued a warp-drive network might be easier than a wormhole network because it alters existing space rather than punching new holes.
Step 9's quantum inequalities are severe. Ford's three laws (1978) say a large negative-energy pulse must be brief; it is paid back by a larger positive pulse; and any shutter that tries to catch only the negative part adds its own positive pulse. A one-metre wormhole would need negative energy concentrated in a band of about 10-22 m, far thinner than an atom. Step 10 is patience: if gateways open by quantum tunnelling, a civilization that thinks slowly (or hibernates between openings) may treat rare events as routine on its subjective clock.
The last hope: a seed through the wormhole
If only subatomic wormholes are stable and bodies cannot survive tides and radiation, a civilization must send information. Nature shows the way: hibernation, antifreeze in frogs and fish, fungal spores, and above all the acorn: small, resilient, carrying the full DNA, built to travel, stocked with food, and able to live off new soil. Stephen Hawking: "quantum theory allows time travel on a microscopic basis."
Kaku concedes that every step "must read like science fiction", but says nothing in physics or biology forbids it. The unified field theory, far from a "useless but elegant curiosity", may be the blueprint for survival, and another universe's refugees might one day burrow into ours. The physicist who treats M-theory as pure math may be missing its ultimate use: as an escape plan.
Why proof of principle is enough
Kaku repeats that he is not writing an engineering manual. A type III civilization with III Q information resources might solve problems that look absurd today, just as GPS would look absurd to Newton. The value of the eleven steps is to show which laws must stretch (negative energy, quantum gravity, information survival) and which already permit the outline (general relativity's wormholes, quantum theory's tunnelling, biology's seeds). If Ford's inequalities permanently forbid macroscopic negative energy, the plan fails at step 9 and the seed must wait for natural microscopic gateways (step 10) or a baby-universe birth (step 5). That honesty about failure modes is part of the "proof of principle" standard: map the forks, do not pretend the road is paved.
Type I politics and the waste-heat ceiling
Kaku's type I checklist is sociological as much as technical: a planetary language beside local ones, trading blocs that outgrow nations, information that embarrasses dictatorships, environmental problems that ignore borders. He wrote amid NAFTA and early EU deepening; later decades mixed confirmation (internet scale, climate diplomacy) with reversal (surveillance states, nationalist turn). The physics constraint is clearer than the politics: exponential energy growth without miniaturisation cooks the planet in waste heat. Hence the turn to information ranking and to seeds rather than arks. A civilization that cannot compress itself will not fit through any portal physics is likely to allow.
Energy leakage and the immortal type II
Even a civilization trying to hide would radiate waste heat, by the second law, so Kardashev's ladder is in principle observable. Type II's Dyson swarm would glow in the infrared; searches to tens of light-years have found none, but the galaxy is vast. Matter-antimatter drives are nearly 100% efficient yet need atom smashers to make antiprotons, a type I budget-breaker and a type II routine. Kaku's "immortal except for folly" claim means comets, ice ages and even nearby supernovae become manageable by moving or shielding, not that physics grants eternal life. Type III's communication problem without FTL returns: Dyson's Carroll eras predict fragmentation until wormholes or other shortcuts stitch the galaxy back into one culture. That is why escape technology and galactic civilisation are the same research programme in Kaku's framing.
Computing the other side
Einstein's equations couple every mass to the curvature at every point; quantum corrections make the system harder. A civilisation about to leave must simulate the destination's laws, constants and hazards. Sagan's III Q threshold is Kaku's way of saying: without that much organised information, the seed is a message in a bottle with no address. Step 1 (a working theory of everything) and step 11 (the seed) therefore bracket the whole plan: know the laws, then send the smallest package that can rebuild knowers of those laws on the far side.
Step-by-step failure modes
Kaku's plan is only as strong as its weakest step. If M-theory (or any quantum gravity) forbids stable macroscopic wormholes, steps 2-4 and 8-9 collapse together. If Ford's inequalities cap negative energy at microscopic amounts, Thorne mouths and Alcubierre bubbles stay chalkboard toys. If baby-universe creation always hides behind a black-hole horizon from the parent's view, step 5 may birth a cosmos no one can enter. If GZK-like limits or engineering crack every stellar accelerator, step 6 never reaches the Planck energy. The honest reading is therefore conditional: escape is allowed by some extrapolations of known laws and forbidden by others; experiment and better theory must cut the list. That is still a sharper claim than "anything goes" or "nothing is possible."
Seed logistics
A nanobot seed needs: error-corrected archives of science and culture; constructors that can bootstrap from ambient materials; a policy for what to build first (power, shelter, cloning, teaching); and an ethic for meeting any life already present. Kaku barely sketches that politics. The physics point stands without it: information is more portable than flesh, and nature already ships genomes in spores and seeds. Moravec's gradual neuron replacement and whole-brain emulation are offered as carbon-to-silicon bridges, each philosophically contested.
Sagan information ladder beside Kardashev energy
Beyond the Multiverse
Three epigraphs frame the chapter: Cardinal Baronius via Galileo ("The Bible teaches us how to go to heaven, not how the heavens go"), William James on why there is something rather than nothing, and Einstein: whoever "can no longer wonder... is as good as dead." Thomas Huxley, "Darwin's bulldog", called "man's place in Nature" (1863) "the question of all questions."
Three giants, three theologies
| Thinker | View of God and humanity | Words |
|---|---|---|
| Newton | Laws imply a lawgiver | "The most beautiful system of the sun, planets, and comets could only proceed from the counsel and dominion of an intelligent and powerful Being" |
| Einstein | The "Old One", who does not intervene; wanted to "read the Mind of God" | "Science without religion is lame. But religion without science is blind" |
| Darwin | Torn: could not see the universe as "blind chance or necessity" | "My theology is simply a muddle" |
Moving humanity has been dangerous. Copernicus's De Revolutionibus appeared in 1543, the year he died; Galileo, shielded by the Medici, eventually faced the Vatican over the telescope; Giordano Bruno was burned in Rome in 1600, having proclaimed God glorified "in an infinity of worlds." Their real offence, Kaku says, was dethroning humanity. The Vatican acknowledged its error over Galileo in 1992; Bruno received no apology.
Copernican versus anthropic
Kaku feels two emotions at once: Pascal's terror at "the eternal silence of those infinite spaces", and wonder at life's complexity.
Two principles in tension: we are not special, yet the universe suits us.
Anthropic principle: a "ridiculously narrow band" of parameters allows us. Hawking: had the expansion rate at one second been smaller by one part in a hundred thousand million, the universe would have recollapsed: "I think there are clearly religious implications." Liquid water is precious (Earth and perhaps Europa), and the brain may be the most complex object for light-years around.
Kaku: the first confronts us with enormity, the second with rarity; neither settles our role without quantum theory.
Quantum meaning
On Wigner's reading, an infinite chain of observers ends in a cosmic observer, perhaps God, as in Ronald Knox's limerick and its reply: "I am always about in the Quad / And that's why the tree / Will continue to be, / Since observed by Yours faithfully, God." James Jeans: "the universe seems to be nearer to a great thought than to a great machine." Wheeler's "it from bit" and "participatory universe": "The universe is also adapted to us", a "Genesis by observership." George Wald: "A physicist is the atom's way of knowing about atoms." Gary Kowalski: our purpose is to "preserve and perpetuate our world as well as to study it." On this view the universe needs observers in order to exist, a comforting thought that the many-worlds reading undercuts.
Meaning in the multiverse
Human cloning, "a matter of time", poses a milder version: are we responsible for our clones, or our quantum copies? Kaku's resolution: within each universe causality holds on average; commit a crime and you will most likely go to jail. A Russian physicist in Las Vegas bet everything on one wager: "in one quantum universe, I shall be rich!" In this one he left broke, and he must live with that.
What physicists think
Steven Weinberg (The First Three Minutes): "The more the universe seems comprehensible, the more it also seems pointless", though understanding it "lifts human life a little above the level of farce, and gives it some of the grace of tragedy." He also wrote that "for good people to do evil, that takes religion," and that "the tragedy is that there is no script." Richard Dawkins agrees: "nothing but blind, pitiless indifference." Yet in Alan Lightman and Roberta Brawer's interviews of cosmologists only a handful sided with Weinberg.
| Cosmologist | Position |
|---|---|
| Sandra Faber | "Completely pointless from a human perspective"; we arose naturally |
| Margaret Geller | Life is short: have "as rich an experience as you possibly can," create, educate |
| Don Page | God created man for fellowship; physical laws are "the grammar and the language that God chose to use" |
| Charles Misner | Religion holds "serious truths"; we "owe honor and awe to its Creator" |
Did God have a choice?
Paul Tillich: physicists are the only scientists who can say "God" without blushing. In string theory particles are notes, chemistry is melody, physics is harmony, the universe a symphony, and "the mind of God" is "cosmic music vibrating through hyperspace." Is there a composer? Einstein suspected God had no choice, and M-theory's symmetries, the only cure for divergences and anomalies, may make a unique theory. Einstein's God was Spinoza's, revealed "in the orderly harmony of what exists, not in a God who concerns himself with fates and actions of human beings"; he rejected reward, punishment and personal survival. Frank Wilczek suspects most scientists will end up in Dante's First Circle with Plato and Aristotle, alongside Mark Twain ("believing what any darn fool knows ain't so").
Creating our own meaning
A single orderly equation "implies a design of some sort", but no formula will fulfil billions of people emotionally. "The real meaning in life is that we create our own meaning." Einstein could not comfort those who wrote asking for life's meaning; Guth: life's purpose is "the purpose that we've given it." Kaku takes Freud's work and love and adds two more: fulfil your talents (rather than be haunted by what you might have become) and leave the world better than you found it, through science, the environment, peace and justice, or mentoring.
Design, chance and the multiverse as explanation
The final chapter weighs the anthropic principle: life sees life-friendly constants because lifeless universes have no observers. Critics call that a tautology; defenders say the multiverse makes it predictive in a weak sense (we should find ourselves in a typical life-allowing vacuum). Kaku surveys theological and philosophical responses without settling them, and returns to the practical hope that a complete theory will either derive the constants uniquely or embed them in a testable ensemble. The book's arc closes where it began: a baby picture of one universe that may be one of many, and a species just learning to read it.
Who designs the designer?
If constants look tuned, options include necessity (a unique theory fixes them), chance (multiverse plus anthropic selection), and design. Design invites the regress: who designed the designer? Necessity has so far failed to derive the Standard Model's 19 parameters. Chance shifts the burden to showing that life-allowing vacua are not absurdly rare in the ensemble. Kaku leaves the reader with humility: we have a baby picture of one universe and equations that hint at more, not a final metaphysics. The practical stance he favours is to keep testing, keep building the theory and keep the escape routes on the blackboard until experiment erases them.
Closing the arc
Baby picture, paradoxes, bang, inflation, portals, quantum copies, strings, destiny, experiments, freeze, escape, meaning: the twelve chapters form one argument. Precision data made cosmology scientific; unification ideas made a multiverse thinkable; eschatology made it urgent. Whether the reader leaves believing in other universes or only in better error bars, Kaku's demand is the same: take the data seriously, take the equations seriously and do not confuse hope with evidence.
Three answers to apparent fine-tuning
Main Points
- Cosmology became precise. WMAP measured the universe's age and contents to about 1%, and Planck refined them.
- The big bang is secure. Expansion, the light elements and the CMB all support it, and the steady-state theory lost.
- Inflation explains flatness and uniformity, and its quantum ripples seeded the galaxies.
- Chaotic inflation implies a multiverse, with new universes budding off endlessly, each with its own laws.
- Most of the cosmos is dark. Atoms are a few per cent; the rest is dark matter and dark energy, both still unidentified.
- Black holes may be gateways. Kerr rings and wormholes allow inter-universe travel on paper, but stability and radiation remain problems.
- Time machines need negative energy, and Hawking's chronology protection or quantum effects may forbid them.
- Time paradoxes have three proposed fixes: self-consistency, many worlds, or physics blocking time travel.
- Quantum measurement forks. It leads to Copenhagen, Wigner's consciousness, Wheeler's participation, Everett's many worlds or decoherence.
- Strings need ten or eleven dimensions. M-theory unites five string theories and supergravity.
- Branes change the picture. Our universe may be a membrane, with gravity leaking into nearby dimensions, which would explain its weakness.
- Holography. A region's information scales with its surface area, and a 5D universe can be dual to a 4D boundary theory.
- Fine-tuning is real. The explanations on offer are design, accident, or a multiverse, which Rees favours.
- Rees' six numbers (epsilon, N, Omega, lambda, Q, D) must each sit in a narrow band for life.
- Experiments test only indirectly. Gravity waves, lensing, the LHC and dark matter searches mostly test Einstein and the Standard Model.
- The big freeze looms. In an accelerating universe stars die, black holes evaporate and thinking eventually stops.
- Dyson's slow thinkers fail if dark energy fixes a lowest temperature (Krauss and Starkman).
- Kardashev types. Civilizations climb by factors of 10 billion in energy, and we are about type 0.7.
- Eleven steps to escape run from a theory of everything to a nanoscale seed sent through a wormhole.
- Meaning is made, not found. Work, love, developing your talents and leaving the world better than you found it.
| Name | What it is | Use |
|---|---|---|
| Friedmann's Omega and lambda | Density and dark energy fix the universe's fate | Read crunch, freeze or rip |
| Inflation / false vacuum | Exponential early expansion | Explain flatness, uniformity and ripples |
| Many worlds | Everett: every quantum outcome happens | Remove wave function collapse |
| Brane world (ADD, Randall-Sundrum) | Matter on a membrane, gravity in the bulk | Explain gravity's weakness |
| Holographic principle | Information scales with area | Black hole entropy; AdS/CFT |
| Anthropic principle | Observers select compatible laws | Explain fine-tuning without a designer |
| Rees' six numbers | Constants that set the universe's character | Quantify fine-tuning |
| Kardashev / Sagan scales | Energy and information ranks | Gauge civilizations, including ours |
| Adams-Laughlin eras | Five logarithmic ages of the universe | Place the present in deep time |
Master chain: what must be true for Kaku's story to hold.
| Person | Role in the book |
|---|---|
| Albert Einstein | General relativity, cosmological constant, Einstein-Rosen bridge |
| Edwin Hubble | Galaxies and expansion |
| George Gamow | Hot big bang and its afterglow |
| Fred Hoyle | Steady state; carbon resonance; coined "big bang" |
| Alan Guth / Andrei Linde | Inflation / chaotic inflation and the multiverse |
| Stephen Hawking | Black hole radiation, chronology protection, information bet |
| Kip Thorne | Traversable wormholes and time machines |
| Hugh Everett / John Wheeler | Many worlds / participatory universe, "it from bit" |
| Edward Witten | M-theory |
| Lisa Randall | Warped brane worlds |
| Martin Rees | Six numbers, the multiverse |
| Freeman Dyson | Thinking forever; Dyson spheres |
| Steven Weinberg | "The more comprehensible... the more pointless" |
Read as a whole, the book is less a proof of the multiverse than a brief for taking it seriously: data made cosmology precise, unification made other vacua conceivable, and cosmic eschatology made the stakes practical. Hold the critical lenses beside the hope; both are part of studying the argument.
About this book
Parallel Worlds: A Journey Through Creation, Higher Dimensions, and the Future of the Cosmos by Michio Kaku, 2004. This is a chapter-by-chapter study of the book, with summaries, maps, tables and diagrams; short quotations keep their page references. It is shared free for personal study, is not for sale, and is not endorsed by the author or publisher. Please read and buy the original.
Insight Editions · rajeshrmahapatra.com/insight-editions