Insight Editions · No. 234
Antimatter
Frank Close
What antimatter is, from Dirac’s equation to CERN’s anti-atoms, why it can never be a fuel or a weapon, and how a tiny asymmetry between matter and antimatter lets anything exist.
A study guide to Antimatter by Frank Close. Shared free for personal reading; not for sale and not endorsed by the authors or publishers. Please read and buy the original.
The Book on One Page
Frank Close's Antimatter (2009) is a short, sharp book in two halves: what antimatter really is, from Dirac's equation to CERN's anti-atoms, and what it can never be, a fuel or a superweapon. Its unifying thread is symmetry and its small failures: matter and antimatter mirror each other almost perfectly, and the "almost" is why anything exists.
- Antimatter is matter with every charge reversed. An anti-atom looks, weighs and behaves like an atom; only on contact with matter does the difference show, as total annihilation into radiation (E = mc² with nothing left over), 10 billion times TNT per kilogram.
- It was predicted from pure mathematics. Making quantum mechanics obey relativity forced Dirac (1928) to use matrices, which yielded spin and unwanted negative energies; reinterpreted as holes in a filled sea, they became the positron, and by extension an antiparticle for every particle.
- Discovery was a comedy of near-misses. Skobeltzyn (1923) saw positron tracks; Anderson (1932) dared to publish; Blackett and Occhialini (1933) showed pairs are created from radiation; the antiproton (1955) and antineutron followed, then antiquarks inside every hadron.
- Taming antimatter means cages of fields. From Touschek's 1 m ring (1961) to stochastic cooling, Penning traps, single antiprotons, antihydrogen (1995, 2002) and the 27 km LEP, the technology works brilliantly for science and hopelessly for bulk.
- The mirror is slightly cracked. Only CPT is exact; CP fails in kaons (1964) and B mesons, and T fails too. This gives an absolute definition of matter (the anti-alien test) and a clue to the cosmic surplus.
- Matter survived by one part in ten billion. The Great Annihilation left one proton per ten billion photons. Quark-level CP violation is too weak; Close backs heavy Majorana neutrinos (leptogenesis).
- No fuel, no bomb. Making antimatter costs more energy than it returns, output is nanograms a year, and like charges repel, so storage fails. Tunguska was a cosmic body, not antimatter; the Air Force "programme" was a small grant; the public picture comes from Angels and Demons.
- Since 2009 the science has run on. Antihydrogen has been trapped (2010), measured to parts per trillion, laser-cooled (2021) and weighed: it falls down (2023); antiprotons have been driven around CERN in a truck (2026); baryon CP violation was seen (2025). Close's verdict on fuel and bombs still holds.
| Ch | Title | One-line payload |
|---|---|---|
| 1 | Antimatter: Fact or Fiction? | Antimatter is real and self-limiting; Tunguska and an Air Force speech set up the myths |
| 2 | The Material World | Swap every charge and matter becomes antimatter; light can make an electron-positron pair |
| 3 | Tablets of Stone | Dirac's matrices give spin and negative energy; the "sea" predicts the positron |
| 4 | A Cosmic Discovery | Positron missed in 1923, found 1932, shown to be made from radiation; PET follows |
| 5 | Annihilation | Fermions vs bosons, the four forces, antiproton (1955), quarks, anatomy of annihilation |
| 6 | Storing Antimatter | Rings, cooling, Penning traps, antihydrogen, the AD and LEP; all tiny quantities |
| 7 | The Mirror Universe | Only CPT is exact; CP violation in kaons defines matter absolutely |
| 8 | Why Is There Anything at All? | No antiworlds nearby; one-in-ten-billion surplus; neutrinos as prime suspect |
| 9 | Revelations | Tunguska solved; no antimatter fuel or bombs; Angels and Demons debunked |
| A1 | The Cost of Antimatter | 1 g = 21 kt; nanograms a year; antihydrogen gram beyond the universe's age |
| A2 | The Dirac Code | Why linear energy needs 4×4 gamma matrices, and how negative energy appears |
Antimatter: Fact or Fiction?
Antimatter is real, rare and ruinously expensive, and its mythology runs far ahead of its physics. Close opens with a childhood riddle, a Siberian fireball and a US Air Force speech to set his programme: explain what antimatter is, how it was found and made, and then judge the claims that it can power starships or make bombs.
Genesis: the founding asymmetry
The Foreword compresses the whole book into a creation story. In the beginning a burst of energy coagulated into matter and antimatter in perfect counterbalance. Antimatter is matter's faithful opposite, identical in every respect except that deep inside its atoms everything is back to front. Today it does not exist naturally on earth, a vanishing act that is one of the unexplained mysteries of the universe, yet we know it is real because physicists have made small pieces of it.
Two promises hang over it: a wonderful power source for the 21st century or the ultimate weapon of mass destruction. That is what popular literature, bloggers and "even some in the US Air Force" believe. The rest of the book tests the belief.
The all-consuming substance
Close's father posed two riddles: what happens when an irresistible force meets an immovable object, and how would you store a substance that destroys everything it touches? The first is a paradox of competing infinities, settled by "my infinity is bigger than yours". The second, young Close decided, was fiction. He was wrong.
Antimatter is a topsy-turvy shadow of matter: left becomes right, positive becomes negative. Close's images are the mould left when a cast is removed and the hole a child digs in wet sand to build a sandcastle: castle is matter, hole is antimatter. When the two meet, their complementary properties cancel in a flash of light.
- Dates. The idea was proposed about 80 years before the book (Dirac, 1928) and the first example, the positron, was seen about 75 years before (1932).
- Why we survive. Antimatter is almost nonexistent, and each annihilation destroys the antimatter too. The first positron ever seen annihilated a single electron in one atom and was gone.
- The cosmic relic. Matter is the norm everywhere we look. The destruction of antimatter was one of the first acts after the Big Bang; the radiation left over from that Great Annihilation is the microwave background, still filling space some fourteen billion years later.
Antimatter is self-limiting. It is not Vonnegut's ice-nine, which froze every ocean it touched in a chain reaction. Each piece of antimatter destroys one equal piece of matter and itself, releasing gamma rays that fly off at light speed; then the threat is over. Antimatter's range is limited, but whatever it destroys releases energy more explosively than anything else we know.
A car-boot volume of antimatter could still make a blast visible around the world. That is the frightening scenario: some antimatter left over from the Big Bang, lurking in space, falling on us in cosmic rays or meeting a spacecraft.
Did antimatter hit the earth? Tunguska, 1908
Meteorites leave craters with alien rock scattered around them; antimatter would leave only the explosion. Enthusiasts claim one such strike happened within living memory.
At 8 a.m. farmer Sergei Semenov, sitting on his steps, saw a fireball that made the sun look dark; his shirt almost burned on his body, though the blast was nearly 60 km away. Herder Vassili Ilich reported forest, reindeer and animals destroyed; some deer were charred, the rest had vanished. The fireball crossed south-east to north-west in seconds; seismic waves circled the globe and pressure waves spread across Russia and Europe. The flash was visible 700 km away, and dust scattered sunlight so far that London's midnight sky glowed like early evening. Normality took two months to return.
The expedition led by Leonid Kulik in 1927 found no crater. Under the blast lay a vast mud plain, as if a thousand bulldozers had cleared ground for a city the size of London; around it a ring of charred stumps; beyond that, trees felled like matchsticks by the blast wave. Life stayed absent for over a quarter of a century. Excavations to more than 30 metres found no meteorite material at all.
In 1965 a physicist, a chemist and a geophysicist combined the evidence: standing trees that recorded the wind, the energy needed to set trees alight, disturbances of the earth's magnetic field, seismometer readings and the brightness and length of the flash. They deduced about a million billion joules (10¹⁵ J) released in a few seconds, about an hour of the whole UK's energy use, and consistent with a nuclear explosion. In 1908 no human nuclear device existed, so a natural cause was needed. A blast plus a total absence of debris fits a lump of "antirock" as small as a metre across. Close defers the forensic verdict to Chapter 9.
The modern consensus is an airburst of a stony asteroid or comet fragment some 50-60 m across, exploding 5-10 km up, which is why it left no crater and little debris. Most yield estimates are 3-15 megatons of TNT, that is 10¹⁶ J or more, ten or more times the 1965 figure Close quotes. Microscopic silicate and metallic particles have since been reported in resin and peat at the site, and a 2007 Italian team proposed Lake Cheko as a possible fragment crater (still disputed). Note also that Kulik was a Russian mineralogist (born in Tartu, now Estonia), not Czechoslovak as the text says. Close himself rejects the antimatter explanation later; the warning is that his 2009 energy figure understates the blast.
Powerful antimatter: the energy ladder
If matter is the fuel, antimatter is the spark that releases all of it, in a way nature cannot better. The Big Bang congealed vast energy into particles; chemical and nuclear reactions only rearrange those pieces and free a trifle of what was locked in.
Close stresses that the difference between body warmth and an explosion is timescale. Your body releases chemical energy gently to hold about 37 °C (hotter in a fever, when reactions run faster to fight a virus). A meal lasts hours; compress that release into a millisecond and the result is literally explosive.
| Process | Fraction of locked-in mass-energy released | Example |
|---|---|---|
| Chemical | about one billionth | TNT, rockets, metabolism |
| Nuclear fission | about 1 part in 1,000 | Hiroshima, Nagasaki |
| Nuclear fusion | about 1 per cent | the sun, H-bomb |
| Annihilation | 100 per cent | matter meets antimatter |
- 10 billion times the energy of the same mass of TNT, per kilogram.
- 1,000 times fission and 100 times fusion, per kilogram of fuel.
- 1 gram annihilated equals the fuel tanks of about two dozen space shuttles.
- Half a gram equals 20 kilotons, the Hiroshima bomb.
- One electron annihilated yields as much as more than a billion atoms of chemical explosive.
- 50 millionths of a gram of positrons would match the Oklahoma City bomb (about 4,000 lb TNT, 1995, 168 killed).
Hence antimatter's role as the ultra-efficient fuel of Star Trek, as a theme of blue-sky NASA energy studies and as the spectre of the antimatter bomb. If Hiroshima and Bikini showed what one part in a thousand can do, through E = mc², liberating everything sounds unimaginable.
Antimatter secrets: the Air Force story
In October 2004 the San Francisco Chronicle reported that the US Air Force was "quietly spending millions" on antimatter for future weapons. In India the story inflated to defence scientists in many countries building antimatter weapons small enough to hold in one hand.
The source was a speech on 24 March 2004 by Kenneth Edwards, director of the "revolutionary munitions" team at the Munitions Directorate, Eglin Air Force Base, Florida, given as keynote at the NASA Institute for Advanced Concepts (NIAC) in Arlington, Virginia. Close notes the rhetoric:
- Present tense. These weapons "are devastating" and the destruction "is unimaginable": no "could", as if the devices already existed.
- Green bombs. Positron bombs would supposedly eject no radioactive plume, leaving only a gamma-ray burst that could "kill a large number of soldiers without touching the civilian population".
- Secrecy as proof. When reporters asked questions, the Air Force allegedly barred staff from discussing the programme, which for conspiracy theorists clinched it.
Close sets this beside the claim that Saddam Hussein was building cold fusion weapons during the first Gulf War (his book debunking cold fusion appeared on the day that war began; the BBC pulled interviews, the New York Times ran the exposé on its front page).
He is candid about his own field. After radar and the bomb, Cold War funding for radio astronomy and nuclear and particle physics was not entirely pure, and blue-sky budgets also funded near-charlatan ideas: telepathy, psychokinesis and antigravity paint. Unlike those, the evidence for antimatter is as good as that for fission in 1939, and the bomb's success confirmed a "can do" mindset among US strategists. Fifty years of secret military funding is unlikely; more plausibly the public results from CERN and Fermilab after 1996 set the military in motion.
Natural antimatter
Bulk antimatter does not exist here, but nature makes the simplest antiparticle, the positron (the mirror of the electron), all the time.
- Radioactive "positron emitters". Some nuclei release energy by rearranging, and part of it materializes as a positron. The positron did not pre-exist in the atom any more than a bark exists inside a dog. It meets an electron almost at once, and both vanish in gamma rays. Detecting those rays is the basis of the PET scanner (positron emission tomography): if you have had a PET scan, you have ingested antimatter.
- The sun. In the core, above 10 million degrees, protons fuse step by step into helium, which weighs less than its ingredients; the missing mass becomes energy. On the way a proton turns into a neutron and emits a positron, which annihilates instantly. The gamma rays are absorbed and re-emitted by charged particles for about 100,000 years, degrading from X-rays to ultraviolet to visible light before escaping the surface.
Daylight is partly an antimatter product. The positrons annihilating in the sun as you read this will reach earth as light a thousand centuries from now.
Positrons are commoner and more useful than people think: in medicine, technology and science. They have been accelerated to within 50 metres per hour of the speed of light and steered into head-on collisions with electrons, briefly recreating conditions of the first moments of the Big Bang. If antimatter could be made and stored until needed, it would interest both the space industry and the military, and Close's task is to measure that "if".
The Material World
A lump of antimatter would look exactly like ordinary stuff; the difference hides deep inside atoms, where every electric charge and magnetic pole is reversed. Close builds the ground floor: atoms, charge, the quantum electron, spin and Einstein's energy accounting, ending at the idea that light can congeal into an electron and a positron, and that the pair can turn back into light.
Down into the atom
Antimatter is the perfect "enemy within": outwardly indistinguishable from matter, yet able to annihilate whatever it touches. To see what is "opposite" about it, Close travels inward through ordinary matter.
- Atoms. DNA is made of molecules, molecules of atoms, the smallest piece of an element that keeps its character.
- Stardust. Hydrogen, nearly fourteen billion years old and made just after the Big Bang, is rare on earth (it floats off the top of the atmosphere) but the commonest element in the universe. Stars fuse it into the heavier elements; the oxygen you breathe and the carbon in your skin were made in stars about five billion years ago. We are stardust, or "if you are less romantic, nuclear waste".
- Scale. The full stop at the end of a sentence holds about 100 billion carbon atoms, more than all humans who ever lived. Enlarge it to 100 metres to see atoms; to 10,000 km, the size of the earth, to see a nucleus.
- Antidiamonds. Anticarbon would make antidiamond as hard and beautiful as diamond and antisoot as black as soot. Even atom by atom, matter and antimatter look the same; the contrast lies deeper, in the particles inside.
Close's central premise appears early: when relativity (the entangling of space and time) is married to the uncertainty of the quantum world, nature is forced to admit a mirror opposite for every subatomic particle, obeying the same laws.
Matter and antimatter: flipping the charges
Inside atoms are swirling currents, magnetic fields and electric forces. In anti-atoms the same forces exist with polarities reversed: north becomes south, positive becomes negative. Fire a tiny magnet past an atom and then an anti-atom: a curve to the left becomes a mirror arc to the right; attraction becomes repulsion; "where before it was safe, now it faces annihilation".
The structure behind this is simple. Like charges repel, opposite charges attract. The nucleus is positive, electrons are negative, and their attraction holds electrons in orbit; hydrogen is one electron around one proton. A further force, felt by protons but not electrons, binds protons together in nuclei despite their mutual repulsion.
Electromagnetism does not care which sign is which. Swap every positive for negative and vice versa and every force, and every structure built from them, stays the same. That swap turns matter into antimatter: antihydrogen is a negative antiproton orbited by a positive positron.
Why we don't feel these forces: electromagnetism is vastly stronger than gravity, but in bulk matter positive and negative charges almost cancel, leaving all-attractive gravity in charge of planets and falling apples. The imbalances leak out as lightning, as a magnet lifting metal against the pull of the whole earth, and as the earth's own magnetic field, driven by moving charge in its core.
That was the picture in 1928, when the first act began with three principals: Paul Dirac, Carl Anderson and Robert Millikan. Dirac, accepting his Nobel Prize in 1933, called it "an accident" that the earth is built of negative electrons and positive protons; for some stars "it is quite possible" it is the other way round. Looking at the night sky, there would be no way to tell which stars were which.
Spectra and the quantum electron
The difference between matter and antimatter shows only in the subatomic world, where Newton's certainties give way to probabilities. Billiard balls bounce predictably; beams of atoms scatter into peaks and troughs like diffracted water waves. Quantum mechanics predicts the odds for each atom, and with millions of atoms, as with millions of coin tosses, the averages become certain and Newton's laws emerge. Newton's laws treat a ball of matter and a ball of antimatter identically; it is quantum laws combined with relativity that show one form of matter is not enough.
WHY ATOMS SURVIVE
The solar-system atom was in trouble from the start. Earth has orbited the sun for over four billion years unharmed, but a hydrogen electron moving at about 1 per cent of light speed makes a million billion circuits a second: in a millionth of a second, more laps than earth has made in its whole history. Classical theory said such a charge should radiate and spiral into the nucleus in a blaze of light.
- 1896 Pieter Zeeman sees sodium's yellow lines broaden in a magnetic field.
- 1897 J. J. Thomson isolates the electron.
- 1900 Max Planck: light is emitted in packets (quanta, photons).
- 1905 Einstein: light stays in packets as it travels; also special relativity.
- 1912 Niels Bohr (summer): electron waves must fit their orbits, so atoms are stable.
- 1928 Dirac joins quantum mechanics with relativity.
Bohr's insight, pictured as a rope coiled into a lasso: a wave peaking at twelve o'clock and dipping at six fits the loop and peaks again at twelve; one dipping at five peaks next at ten and is out of step. Only orbits where the electron's wave fits exactly are allowed, so the electron cannot spiral inward.
The same rule solved a two-century mystery: atomic spectra. Heat sodium in a flame, pass the light through a prism, and you see bright lines, notably two intense yellow-orange ones (sodium street lamps). Mercury glows blue-green; the pink of nebulae is hydrogen's red line. Light is emitted only when an electron jumps from one allowed orbit to a lower one, and the photon carries the energy difference. Discrete jumps mean discrete colours, a unique "autograph" for each element, which is how we read the composition of the cosmos.
The spinning electron
The electron was suspected before Thomson isolated it, and was thought to carry both charge and a twofold magnetism, like a bar magnet. Zeeman's 1896 broadening turned out, with better instruments, to be one line splitting into two or more. The cause: the electron is a tiny magnet, so its energy shifts with its orientation in a magnetic field.
This intrinsic magnetism was named spin, with two possible orientations ("clockwise or anticlockwise"). An object of no measurable size cannot literally spin, and for years spin was a desperate bookkeeping device that fitted spectra without explanation. Its origin, like antimatter's, would appear only when Dirac combined relativity with quantum mechanics.
E is for Einstein: E = mc²
Newton's law of inertia: bodies stay at rest or steady motion unless forced, and mass measures reluctance to accelerate (a leaf versus a lump of lead). An immovable object would need infinite mass, impossible for Newton but, in a sense, real for Einstein.
- Diminishing returns. Push a resting body and it gains, say, 10 m/s. Newton says the next push adds another 10 m/s. Einstein says fractionally less, and near light speed almost nothing.
- Light speed is a wall. In Close's (older) language, mass grows with speed and becomes infinite at light speed; only massless things such as light travel at 300,000 km/s.
- Practical proof. At CERN, matter and antimatter beams must arrive on cue, and the timing only works with relativity.
- Rest energy. A stationary body holds E = mc², latent in its atoms.
- Energy of motion. Momentum times light speed, pc.
- Total energy. Not a simple sum: E² = (mc²)² + (pc)², Pythagoras for energy. Close's example: rest energy 4 J, motion 3 J, total 5 J (since 16 + 9 = 25).
- Massless light. With m = 0, a photon still has energy E = pc, so light's energy can be converted into matter's.
Close uses the older "mass increases with speed" picture. Most physicists now prefer to keep m as the invariant rest mass and let energy and momentum grow without limit (by the factor γ = 1/√(1 − v²/c²)). The physics is identical, and Close's own E² = (mc²)² + (pc)² is written in the modern invariant-mass form; only the wording differs.
From light to a pair
Energy is conserved, so light's energy can be trapped as matter. But light has no charge, and an electron is negative. Nature's answer is to make two pieces at once: the negative electron and its positive twin, the positron. The reverse also happens: an electron and a positron annihilate, their energy carried off by photons at light speed.
Relativity explains the energy accounts, but it is the union of relativity with quantum mechanics that makes antimatter inevitable: when energy congeals into matter, it must leave a negative imprint. The emergence of substance from light, Close says, is "almost biblical"; it is how matter and antimatter condensed from the heat of the Big Bang.
Tablets of Stone
Antimatter was predicted before it was seen, out of the pure mathematics of Paul Dirac's 1928 equation for the electron. Forcing quantum mechanics to obey Einstein's relativity required matrices, which delivered the electron's spin as a bonus and negative-energy solutions as an embarrassment. Dirac's reinterpretation of those solutions, a filled "sea" whose holes behave as positive electrons, became the prediction of antimatter.
Paul Dirac: the silent master
Close first met Dirac's The Principles of Quantum Mechanics as a teenager in Peterborough library, choosing it because the date-stamp sheet showed only one previous borrower in many years. He read only the preface, stumped by "orthogonality", and his father could not explain it either: the first time a parent was utterly beaten. A decade later, in his final undergraduate year in theoretical physics, he could follow it, with difficulty. The 1930 first edition baffled almost everyone; the rewritten 1935 second edition has been the classic text for over seventy years. (He later checked the library copy: only two date stamps, one his own.)
The first glimpse of the antiworld came from the beauty of equations, not from an experiment. Close likens it to written music that only a maestro hears as melody, and ranks the equation's creativity and elegance with Shakespeare or Beethoven.
- Paul Dirac (1902-84): Bristol-born son of a Swiss language teacher; raised bilingual in French and English, and taciturn in both.
- The Toronto lecture: told "I do not understand how you derived that formula", Dirac stayed silent until the chair prompted him, then said: "It was not a question; it was a statement."
- Dinner with E. M. Forster (probably apocryphal): over the main course Dirac asked, of A Passage to India, "What happened in the cave?" Forster pondered through to dessert and answered "I don't know."
Like Forster, Dirac expressed himself through written symbols. In 1928 he fused quantum theory with Einstein's special relativity and invented a new mathematical language, bizarre then, standard training for theorists now.
Two for the price of one
Mechanics relates position, time, momentum (mass × velocity) and kinetic energy, which grows with the square of speed (doubling a tennis serve needs four times the energy). In the quantum world the uncertainty in position and momentum, negligible for visible objects, becomes overwhelming, so the rules had to be rewritten as quantum mechanics. Its equations need one input: how a particle's energy, mass and momentum are related.
- Schrödinger (1926). His equation used the slow-motion (Newtonian) energy relation. It explained atomic electrons, showing the hydrogen electron moves at about 2,000 km/s, under 1 per cent of light speed, and explained Zeeman splitting from orbital motion. It had no place for spin.
- Klein's attempt. Oscar Klein generalized it using Einstein's squared relation E² = (mc²)² + (pc)². But √25 is +5 or −5: the negative-energy root was dismissed as spurious, uneasily.
- Dirac's move. Work with E itself, not E²: write energy as a linear sum of rest energy and motion energy, each appearing once.
The impossible sum. Dirac needed two quantities a and b whose squares are each 1 (a² = b² = 1) but whose cross terms cancel, a×b + b×a = 0. No ordinary numbers can do it. Matrices can, because for matrices a×b need not equal b×a.
Order matters in ordinary life too: twist a Rubik's cube top-then-side or side-then-top and you get different patterns; turn a die 90° left then top-to-front and a different face shows than in the reverse order. That is why matrices are the bookkeeping of rotations.
THE BONUS AND THE PRICE
- The bonus. Because matrices track rotation, the mathematics said the electron spins; and because the simplest (two-column) matrices sufficed, spin has two states, exactly what Zeeman's split lines required. The missing ingredient of Schrödinger's theory fell out of relativity.
- The price. The theory worked only if negative-energy solutions were as real as positive ones. Two sets of two-by-two matrices made four-by-four "γ" (gamma) matrices, the γ on the Abbey stone. Relativity insisted that an electron must both have spin and come with positive or negative energy; Dirac set out to avoid negative energy and was forced to accept it.
The infinite sea
Everyday energy is always positive and merely changes form: fuel becomes a car's motion, braking turns motion into heat and sound, and even a parked car with an empty tank holds vast mc² in its atoms (you can spend some of yours by pushing it). So what could negative energy mean? If electrons could drop into negative-energy states, all matter would collapse. Dirac used the stability of matter itself to interpret them.
His key was the rule behind Mendeleev's Periodic Table. Families of elements recur periodically: the inert gases helium, neon, argon; the water-loving metals sodium, magnesium; the acid-forming fluorine, chlorine, iodine. Quantum mechanics explains the recurrence by the Pauli exclusion principle: no two electrons can occupy the same quantum state. Electrons are like cuckoos: two in one nest is one too many.
- The ladder. The vacuum is a bottomless pit whose rungs are all the negative-energy states.
- Already full. Every rung is occupied, so no electron can fall in and matter is stable.
- Sea-level. This calm, unnoticeable filled sea defines zero energy.
- The hole. A missing negative-energy, negative-charge electron looks like a particle with positive energy and positive charge: the positron.
- Making one. A gamma ray with enough energy kicks an electron out of the sea, leaving an ordinary electron plus a hole: light has become an electron-positron pair.
- Identical copies. Every particle drawn from the same sea is identical, which is why all electrons and all positrons are exactly alike.
What is this positive electron?
In 1928 the particle world was cosy: electrons and protons, believed immutable, plus the expected but unfound neutron. Dirac's theory implied particles could be created and destroyed, and its new positive particle had no place. At every talk someone asked "where is the anti-electron?", to nervous laughter. To silence them, Dirac suggested the positive particle might be the proton, despite a mass mismatch of nearly 2,000. Historians argue how seriously he meant it; Peter Kapitsa claimed it was a joke, made to park the mass question as a detail.
Robert Oppenheimer, later head of the Manhattan Project, saw the full vision. Run pair creation backwards and an electron and its positive partner annihilate into gamma rays. If the partner were the proton, every hydrogen atom would self-destruct, and all matter would vanish in a flash. Dirac accepted the point at once.
In September 1931 Dirac published the conclusion: the hole is "a new kind of particle, unknown to experimental physics", with the electron's mass and opposite charge, an "anti-electron". His theory had "complete and perfect symmetry" between positive and negative charge, so "it must be possible to reverse the charge of any kind of particle": he predicted the antiproton too. Today every particle has an antiparticle is an established law. Dirac also had protons fill a sea; we now say their constituents, quarks (Chapter 5), obey the exclusion principle and can likewise be materialized from the vacuum as particles and antiparticles.
Dirac's sea was a brilliant scaffold, but modern physics no longer uses it. It only works for particles obeying the exclusion principle (fermions); in 1934 Pauli and Weisskopf showed spin-zero particles (bosons, which cannot fill a sea) also have antiparticles. Quantum field theory, and the Stückelberg-Feynman reading of negative-energy solutions as antiparticles moving forward in time (Close returns to this idea in Chapter 7), replaced the infinite sea. The equation and its prediction stand; the picture of a filled vacuum is a historical device. Close also understates Hermann Weyl, who argued in 1930-31 that the theory's symmetry required the positive particle to have the electron's mass, ruling out the proton.
A Cosmic Discovery
The positron was photographed in 1923, unrecognized, predicted in 1928 and identified in cosmic rays in 1932. Close tells it as a comedy of near-misses: Skobeltzyn, Millikan, Blackett and Dirac, and the Joliot-Curies all had it in hand. Anderson won the credit by daring to publish; Blackett and Occhialini proved positrons are made from radiation on earth, not visitors from space, and the particle went on to become a working tool in PET scanners and materials testing.
The prize missed twice: Skobeltzyn
High above us, cosmic rays (high-energy particles and gamma rays from space) smash into the upper atmosphere and spawn cascades of secondary particles; most are absorbed, leaving a harmless drizzle at ground level. Among them is exotic matter unknown on earth.
The key instrument was the cloud chamber: a glass box of moist low-pressure air with a piston; sudden expansion makes vapour condense on charged particles, leaving trails like a jet's contrails. It was to early atomic physics what the telescope was to astronomy.
Dmitry Skobeltzyn wanted to detect invisible gamma rays by the electrons they knocked out of atoms, as H. G. Wells's invisible man betrays himself by jostling a crowd. The rays also kicked electrons out of the chamber walls, so he placed the chamber between the poles of a large magnet to sweep them away. The cleaner view showed some "electrons" curving the wrong way: positrons, though nobody expected them. Other tracks ran nearly straight, far too energetic for any known radioactivity: he was the first to see tracks of cosmic rays themselves, almost certainly including more positrons. He showed the pictures at a conference in Cambridge in 1928, the same year and town as Dirac's prediction. No one could explain them, and he missed the prize a second time by not following up.
A magnetic field bends a charged particle's path: light or slow particles curve more than heavy or fast ones, and the direction of curvature gives the sign of the charge. That simple rule decided everything that followed.
Anderson, Millikan and the lead plate
Robert Millikan at Caltech (Nobel 1923 for measuring the electron's charge) coined the name "cosmic rays" and believed they were gamma rays, the "birth pangs of creation". In 1930 he asked his research student Carl Anderson to build a magnet strong enough to bend them. With engineers from the nearby aeronautical laboratory Anderson built one ten times stronger than Skobeltzyn's and found negative and positive particles in roughly equal numbers.
- Millikan's view. Gamma rays knock charged particles from atoms, so the negatives are electrons and the positives protons.
- Anderson's objection. Protons leave dense trails; all these were thin and wispy like electrons. Perhaps the "wrong way" tracks were electrons moving upwards.
- The test. Anderson put a lead plate across the chamber. A particle loses energy crossing it, so it curves more tightly afterwards, revealing its direction and hence its charge.
- The verdict. Both were wrong: the tracks were downward-moving positive electrons.
Ironically, Anderson's first positron really was moving upward, a stray made in the air below the plate, which confused him; then came a clean example moving down through the lead, clearly far lighter than a proton. The editor of Science News Letter printed a track and coined the name "positron". Working on the US west coast, Anderson and Millikan had only a passing awareness of Dirac's theory.
Blackett, Occhialini and creation
Patrick Blackett, in Rutherford's group at the Cavendish Laboratory, loved gadgets. His automatic cloud chamber fired about every ten seconds onto cinema film; in 1921-24 he took more than 20,000 photographs of alpha particles hitting nitrogen and caught nuclear transmutation on film. But only about 1 picture in 20 showed anything.
In 1931 Giuseppe Occhialini arrived, expert in Geiger counters, which fire when a charged particle passes but say nothing about it. They combined the two strengths.
- Success rate rose from 1 in 20 to 4 in 5.
- First triggered photographs: June 1932.
- Nearly 1,000 pictures by late autumn 1932.
- Showers of up to 20 tracks from one point, about half positive, half negative.
- Paper sent to the Proceedings of the Royal Society, February 1933.
Blackett saw wrong-way tracks and talked to Dirac about them, yet neither made the connection: perhaps Dirac's cautious, non-selling manner, perhaps Blackett did not take the theory seriously. Only on hearing of Anderson did they realize what they had.
They had more than Anderson, though. The showers sprang from the copper casing of the chamber. A single cosmic-ray electron, braked in the intense fields inside copper atoms, radiates gamma rays; each energetic gamma ray makes an electron-positron pair; and the cascade multiplies. Because positrons do not exist naturally on earth, equal numbers of positive and negative tracks meant they were being created on the spot. Blackett and Occhialini had demonstrated for the first time the creation of matter and antimatter from radiation, E = mc² run in reverse, proving the positron was no extraterrestrial interloper.
Their proof also forced Millikan to concede Anderson was right. Anderson kept the priority because he had gone public in 1932 despite his supervisor's scepticism. Fortune favours the brave.
Close places the Science News Letter photograph in December 1931. Standard histories date Anderson's decisive lead-plate photograph to 2 August 1932, reported in Science in September 1932 and in Physical Review in 1933, where the word "positron" appears. Late-1931 publicity concerned Anderson's earlier "positive" tracks, then read as protons. The order of events in Close's story stands; the dating of the naming is loose.
Positrons on earth
Once the news spread, physicists found positrons in their old photographs. Among the losers were Irène and Frédéric Joliot-Curie, who had already missed the neutron (produced in January 1932 and mistaken for gamma rays) and now the positron. Luck turned: they won the 1935 Nobel Prize for chemistry for making short-lived artificial radioactive nuclei, among them positron emitters. Anderson's Nobel came in 1936.
Becquerel's 1896 radioactivity is, we now know, a neutron turning into a proton and emitting an electron. The mirror process, a proton turning into a neutron and emitting a positron, is just as natural. The difference is what happens next. An electron can join an atom and take part in chemistry for ages; a positron, a stranger in our land, is partnered by an electron in a brief "dance of doom" and annihilates within a microsecond, usually as two gamma rays almost back to back. That back-to-back signature is the key to its uses.
- PET scanning. Tag sugar with positron emitters such as carbon-11, nitrogen-13, oxygen-15; active tissue (brain, heart, muscle) takes up the sugar; a halo of detectors catches the paired gamma rays and reconstructs the source in slices. Isotopes are short-lived (oxygen-15's half-life is about two minutes), so a small proton accelerator makes them near the patient.
- Materials testing. Annihilation in metals reveals metal fatigue earlier than other methods; used on aircraft turbine blades, it lets safety margins be narrowed and profits raised.
- Antimatter chemistry. An electron and a positron form positronium, an "atom" living under a millionth of a second. Positronium molecules have been made, with speculation about a gamma-ray laser.
- The sun. Solar fusion turns four protons into helium plus two positrons and neutrinos; the positrons annihilate, and Close estimates about 10 per cent of visible sunlight began as positron annihilation.
Positronium research has accelerated. In 2024 two groups, AEgIS at CERN and a team at the University of Tokyo, laser-cooled positronium for the first time, a step towards a Bose-Einstein condensate of positronium that could in principle seed the gamma-ray laser Close mentions. Precision positronium spectroscopy has also thrown up small, still-unexplained tensions with theory. On PET, today's total-body scanners (such as EXPLORER, 2018-19) image the whole body at once, so positron medicine has grown well beyond the brain studies Close describes.
Positrons are unfamiliar only because they are hopelessly outnumbered and quickly killed; they are made naturally, used daily, and lit the daylight you see. That is the real antimatter story: common, useful and fleeting.
Annihilation
Nature has two kinds of particle: fermions, the bricks of matter and antimatter, and bosons, the force carriers that are neither. Close sorts the four forces and their carriers, tells how the antiproton (1955) and antineutron completed the antiworld's basic kit (and started a Nobel lawsuit), shows protons and neutrons are made of quarks with matching antiquarks, and dissects annihilation itself: not instantaneous, but a short courtship ending in pions and gamma rays.
Neither matter nor antimatter
Besides matter (electrons) and antimatter (positrons) there is non-substance. All electromagnetic radiation, from gamma rays to radio, is made of photons, and annihilation turns substance into them; with the right conditions, photons turn back into substance.
Energy is the other non-substance: it changes form (electrical, chemical, motion) and can transubstantiate into matter and antimatter at Einstein's exchange rate. The minimum to make an electron-positron pair is 2mc², one mc² for each. Made at rest, the pair would annihilate at once; extra energy becomes motion and lets them fly apart and survive.
| Fermions | Bosons | |
|---|---|---|
| Named after | Enrico Fermi | Satyendranath Bose |
| Role | Bricks of substance | Carriers of force |
| Antimatter? | Particle and antiparticle | Neither matter nor antimatter |
| Rules | Dirac's equation, exclusion principle | Different rules; can pile up |
| Fate | Decay to electrons, protons, neutrons | Come and go freely |
| Examples | electron, quarks, proton | photon, gluon, W, Z, pion |
Dirac, Close says, was lucky. In 1928 the only massive particles known, the electron and proton, happened to be fermions; the one boson, the photon, was massless. The first massive boson, the pion, turned up in cosmic rays twenty years later. Had it been known in 1928, Dirac might never have pursued his equation so hard. (The alpha particle is also a boson, but was known to be a helium nucleus, not fundamental.)
Four forces and their carriers
Gravity and electromagnetism reach across effectively infinite distances: gravity holds planets in orbit, currents in the earth's molten core swing a compass, and even GPS works by radio waves, the same force. Naming the go-between "the electromagnetic field" is a label, not an explanation. One result of Dirac's work was that the field is itself quantized: photons flit between charges and carry the force. An electron oscillating in a London antenna makes electrons move in your radio; "the force is with you".
- Electromagnetism. Carried by the photon; shapes atoms and everything built from them.
- Gravity. Believed carried by the graviton, never detected.
- Strong force. Gluons glue quarks into protons and neutrons; pions help grip protons and neutrons into nuclei.
- Weak force, neutral form. Carried by the Z⁰, a "heavy light" more massive than an iron atom.
- Weak force, charged form. Carried by W⁺ and W⁻, which move electric charge between particles: a proton becomes a neutron and the W⁺'s energy materializes as a positron; in neutron decay a W⁻ hands its charge to an electron.
The weak force makes the sun shine: it converts four protons into a helium nucleus (two protons, two neutrons), carrying away charge as positrons. It is so feeble that in five billion years only about half the sun's protons have been used: slow enough for intelligent life to have time to emerge, fast enough to supply the conditions for life at all.
Matter won the war fourteen billion years ago. Bosons come and go; fermions decay to the stable electrons, protons and neutrons, which then last indefinitely, "so long as they do not meet antimatter". Our atoms are billions of years old and will outlive the arrangement that currently "thinks it is us".
More antiparticles: the antiproton and the antineutron
Dirac's equation is "the script that all fermions read", so the proton and neutron must have antiparticles. The antiproton has the proton's mass and negative charge. The antineutron has zero charge like the neutron, so what distinguishes them? The neutron's inside is full of swirling positive and negative charges that sum to zero but generate magnetism; in the antineutron every inner charge is reversed, so its magnetic poles are reversed and it follows the mirror-image path in a magnetic field. The inner charges are quarks; the antineutron is made of antiquarks.
An antiproton is about 2,000 times heavier than a positron and needs that much more energy to make. By 1950 cosmic rays had yielded the muon (a heavy electron), the pion (a boson about one-seventh of a proton's mass) and long-lived "strange" kaons, but no antiproton. Physicists were so sure of Dirac that Berkeley built a machine for the purpose: the Bevatron, named for a billion electron volts (BeV), enough to make an antiproton together with a proton (pairs are always created).
Several teams proposed ways to pick the antiproton "needle" from a haystack of electrons, positrons and pions. A committee chose Owen Chamberlain, Emilio Segrè, Clyde Wiegand and Tom Ypsilantis; they announced the antiproton in 1955. A team led by Oreste Piccioni, who went second, found the antineutron. Chamberlain and Segrè shared the 1959 Nobel Prize. Piccioni believed his ideas, submitted in his proposal, were used by the winners; in 1972 he sued for damages, but the case was thrown out on the statute of limitations, thirteen years having passed. Opinion split: some said Piccioni deserved a share, some that nobody did, others that credit belonged to the machine's designers.
So within thirty years of Dirac's prediction the antiworld's building blocks, positron, antiproton and antineutron, were all in place, and for a time antiparticles seemed to hold no special interest.
The antineutron discovery by Cork, Lambertson, Piccioni and Wenzel is usually dated 1956 (Close says 1957), and the text spells the name both "Piccione" and "Piccioni"; Piccioni is correct. Since Close wrote, heavier antimatter has been assembled from collisions: antihypertriton (2010) and antihelium-4 (2011) at Brookhaven's RHIC (STAR), and antihyperhydrogen-4 (2024), the heaviest antinucleus yet. All match their matter twins within errors. Also since publication: the Higgs boson (2012), a boson outside Close's force-carrier scheme that gives particles mass; the graviton remains undetected.
Quarks and antiquarks
The neutron was found in 1932, the same year as the positron, and the menu was still short. Within thirty years, cosmic rays and accelerators had produced so many particles that a Dirac-style prediction would have caused little stir. Most were unstable: some lived only as long as light takes to cross a nucleus, effectively dying at birth; "long-lived" ones survived under a billionth of a second, the time for light to cross your hand. Near light speed, though, they travel measurable distances before decaying.
- The bubble chamber. In 1952 at the University of Michigan, Donald Glaser, musing on bubbles in beer, invented it: particles leave gas bubbles in a liquid, the reverse of the cloud chamber. Its spiralling, branching tracks revealed whole families of new particles, and the cloud chamber became a museum piece.
- The alphabet soup. Λ, Ω, Σ, Ξ, Δ, then φ, η, ω, ρ and on into Latin letters. Rutherford's jibe that all science except physics is stamp collecting became extreme irony.
- The echo of Mendeleev. Patterns among particles hinted at families, as the periodic table had hinted at atomic structure (later proved by Thomson's electrons and Rutherford's nucleus).
- Inside the proton. Rutherford found the nucleus on a Manchester tabletop; probing protons needed the 3 km electron accelerator at Stanford. Electrons sometimes bounced sharply, as from hard points: the proton's charge sits on three quarks, confined within 10⁻¹⁵ m, like an ant hill that looks like a brown lump until you see the seething ants.
| Particle | Quarks | Charge sum | Antiparticle |
|---|---|---|---|
| Proton | u u d | ⅔ + ⅔ − ⅓ = +1 | antiproton: anti-u anti-u anti-d, −1 |
| Neutron | u d d | ⅔ − ⅓ − ⅓ = 0 | antineutron: anti-u anti-d anti-d, 0 |
| Delta | uuu, ddd (etc.) | +2 ... −1 | anti-Delta |
| Lambda, Sigma | one s | 0, ±1 | anti-Lambda, anti-Sigma |
| Xi | two s | 0, −1 | anti-Xi |
| Omega | s s s | −1 | anti-Omega |
| Pion, kaon | quark + antiquark | 0, ±1 | neither matter nor antimatter |
The up quark has charge +⅔, the down −⅓; the strange quark has the down's charge and is about 20 per cent heavier, and the more strange quarks in a trio, the stranger the particle. Each antiquark has the same mass and size and opposite charge (anti-up −⅔, anti-down +⅓). Every anti-Greek-letter particle has been made in accelerator collisions, and each one, to the best accuracy achieved, is a perfect mirror of its partner.
When quark meets antiquark
The strong force can also bind one quark to one antiquark: the pion and the kaon. Such a particle is neither matter nor antimatter, and in a space of 10⁻¹² m the pair soon meet and annihilate, so pions and kaons live only briefly.
Proton-antiproton experiments show annihilation is a courtship, not a collision.
- Capture. The proton's electric field reaches about 10⁻¹⁰ m, some 10,000 times the proton's size. A slow antiproton is caught and orbits like an electron in an atom.
- Cascade. It drops from outer to inner orbits, emitting gamma rays whose energies are "like the skid marks at the scene of a crash".
- Doom. Within range of the strong force, after a dance lasting up to a hundredth of a second, the end comes in less than a billionth of a billionth of a second: pions and gamma rays, then the pions decay to more gammas, electrons, positrons and neutrinos.
The faster the collision, the more pions and gamma rays come out. We now know enough about annihilation that if anyone ever builds an antimatter power source or bomb, or a lump of antirock hits the atmosphere, physicists can predict what would happen. The catch is the obvious one: in a world of matter, antimatter must be contained away from all matter until it is needed.
Storing Antimatter
The answer to Close's father's riddle is a cage without walls: an ultra-high vacuum in which magnetic and electric fields hold antiparticles away from all matter. Close traces the technique from Touschek's suitcase-sized ring (1961) through stochastic cooling, the Penning trap, single trapped antiprotons, the first antihydrogen (1995) and the Antiproton Decelerator, to LEP, where positrons circled for days. Each step showed matter and antimatter mirror each other precisely, and each showed how slow, small and costly taming antimatter is.
Touschek and the storage ring
The man who answered the riddle was Bruno Touschek, though nobody saw it as an antimatter store at the time. CERN's 27 km LEP ring later kept positron bunches circling for weeks, within 55 metres per hour of light speed (300,000 km/s), for as long as the magnets were powered and stray gas atoms were avoided. No one carries a 27 km ring around, but the principle came from much humbler hardware.
- Rolf Widerøe: Norwegian; invented acceleration by many small voltage pushes (straight-line prototype), still the principle of every accelerator; in 1943 applied to patent colliding counter-rotating stored beams, refused as "obvious".
- Ernest Lawrence: American; bent the path into a circle so particles cross the same gap repeatedly, the cyclotron (Nobel Prize).
- Bruno Touschek: Austrian, worked on wartime radar in Hamburg with Widerøe; in 1960 saw that one ring of magnets can steer electrons one way and positrons the other.
- Gersh Budker: Russian; tamed protons and antiprotons with electron cooling (by 1974).
- Carlo Rubbia: drove CERN's proton-antiproton collider to find the W and Z.
- Simon van der Meer: Dutch engineer; stochastic cooling; shared the 1984 Nobel Prize.
Colliding single particles is hopeless: they almost always miss. Store intense beams and fire them at each other and some particles will coincide. Americans built the first pair of storage rings in 1959, with electrons going clockwise in one ring and anticlockwise in the other. Touschek's insight: a positron has the electron's mass and opposite charge, so the same magnetic field sends it the opposite way along exactly the same path, provided the energies match.
At Frascati near Rome, Touschek's group built ADA (Anello di Accumulazione), just one metre across, and stored electrons and positrons: the first time antiparticles were tamed. The suitcase-sized machine moved to Orsay near Paris for its more intense electron beam, and in 1963 stored positron beams were passed head-on through electron beams, occasionally annihilating. A ring that stores antiparticles is also a machine that annihilates them on demand: store them if you wish, collide them if you prefer.
Thirty years of ever larger rings followed, several Nobel Prizes, and finally LEP. Smaller rings at Stanford, in Japan and back at Frascati were later tuned to hunt for matter-antimatter differences (Chapter 8). The rings work only because electrons and positrons respond identically to the magnets; timing protons and antiprotons around a ring shows them alike to better than one part in a billion.
Storing antiprotons: cooling the wild ones
Making antiprotons is easy given energy: fire protons into metal and about once in 250,000 collisions a proton-antiproton pair is born. Controlling them is hard. They emerge near light speed in all directions, and fields that hold positrons cannot stop them flying sideways into the walls. They must be "cooled": brought into step.
- Electron cooling (Budker). Pass antiprotons through clouds of cold electrons. Electrons and antiprotons pose no danger to each other (an electron fears only a positron; an antiproton only protons and neutrons). The antiprotons' jitter drains into the electrons. By 1974 it worked, but not for intense beams.
- The motive. The 1979 Nobel Prize went to Glashow, Salam and Weinberg for the electroweak theory, though its key prediction, the W and Z, was unproved. Rubbia planned to make them in proton-antiproton annihilations, counter-rotating in CERN's Super Proton Synchrotron (SPS), fed from the older PS.
- Stochastic cooling (van der Meer). A particle going round a semicircle takes about 50 per cent longer than a signal crossing the diameter at light speed. Sense the beam's error on one side, compute the correction, and send it across the ring to arrive before the antiprotons do. Light covers one-third of a metre per nanosecond (about one foot, the length of an adult foot).
- A new burst of antiprotons every 2 seconds from protons out of the PS hitting a target.
- Two rings in one: newcomers precooled in an outer ring, shuttered into an inner stack before the next burst.
- About 10¹¹ (a hundred billion) antiprotons in just over a day.
- Result: the W and Z discoveries and van der Meer's 1984 Nobel.
- Price tag: millions of dollars and great patience.
The Penning trap: cold storage
Can antiparticles be held cold, at room temperature or below, rather than racing at high energy? The trick descends from 1930s vacuum-tube electronics. Electricity passed through gas in a glass tube produced the glows that fascinated Victorians, and led J. J. Thomson to the electron by steering beams with fields. A strong magnetic field bends electrons into tight circles so no current crosses the tube, unless residual gas knocks them free; Frans Penning used that as a vacuum gauge.
Hans Dehmelt set the voltage so current never flowed: electrons wander for ever in a canister made of fields, no bigger than a soft-drink can (a hollow cylindrical anode with angled end caps as cathode). He modestly named it the Penning trap.
In 1973 Dehmelt trapped a single electron, whose circling made it a tiny radio transmitter. Tuning to its frequencies he measured its magnetism to one part in ten billion, and found it larger than Dirac's equation predicts by about one part in a thousand. Far from refuting Dirac, this confirmed the fuller theory: Feynman and others had shown the field around an electron seethes with virtual electrons and positrons, and Dehmelt was measuring the electron plus its disturbed vacuum. In 1984 he reversed the fields, held a single positron for three months in a cylinder half the size of a thumb, and found exactly the same excess. Nobel Prize 1989.
The vacuum is full of virtual antimatter. We live in a world of matter, but empty space teems with fleeting particle-antiparticle pairs that never materialize yet measurably shift the electron's magnetism. Antimatter is more than rare stuff far away; it is part of what every electron is.
Antiprotons in the trap
High-energy beams are useless for trapping, so CERN built the Low Energy Antiproton Ring (LEAR) to slow antiprotons down. CERN's summary of the challenge: make them at temperatures far hotter than the sun's core, then store them colder than outer space in a vacuum better than the moon's. Liquid helium cannot be the coolant (its protons and neutrons would annihilate the cargo), so Gerald Gabrielse, a Dehmelt colleague, used cold electrons, as Budker had.
- The trap. About 15 cm long; once antiprotons enter, a voltage rise shuts the trapdoor.
- Isolating one. Start with about 10,000 antiprotons plus electrons; pulse the voltage to let light electrons escape; lower the barrier until a dozen antiprotons remain; use a tuned laser to remove the fastest one by one; then close the trap.
- The result. Proton and antiproton have opposite charge and the same charge-to-mass ratio to better than 90 parts per trillion.
DOES ANTIMATTER FALL UP?
Symmetry says antimatter falls towards an anti-earth; whether it falls towards earth had never been measured. Antiprotons, nearly 2,000 times heavier than positrons, were the best bet. Close sat on the CERN committee that reviewed a 1980s Los Alamos proposal; nobody expected it to succeed, but everyone expected the attempt to advance antimatter technology. Experiments do show all kinds of matter fall at the same rate, which, if general relativity is right, implies antimatter falls normally too. In 2009 Close could only say: "we still don't know."
In 2023 CERN's ALPHA-g experiment released trapped antihydrogen atoms in a vertical trap and watched where they annihilated: antimatter falls down, with acceleration about 0.75 ± 0.13 (stat. + syst.) ± 0.16 (simulation) g, consistent with ordinary gravity and ruling out "antigravity". Precision versions (ALPHA-g upgrades, AEgIS, GBAR) now aim at per-cent accuracy. Meanwhile the BASE experiment has pushed Gabrielse's comparison much further: proton and antiproton charge-to-mass ratios agree to 16 parts per trillion (2022) and their magnetic moments to about 1.5 parts per billion (2017); in 2025 BASE held a single antiproton's spin in a coherent quantum superposition for about a minute, an "antimatter qubit".
Antihydrogen and the antimatter factory
Charged antiparticles cannot be piled up in bulk: like charges repel, the field bottle leaks, and the cargo dies. Combine positrons and antiprotons into neutral antihydrogen and fields lose their grip, so the atoms hit the walls. Before 1995, perhaps not one anti-atom had ever existed in the universe: in cosmic rays antiprotons and positrons pass too fast to combine.
- 1995 At LEAR, antiprotons passing heavy nuclei create electron-positron pairs; rarely, an antiproton captures the positron: nine antihydrogen atoms, announced early 1996, each living a fraction of a second.
- 1996 LEAR closes; the Antiproton Decelerator (AD), a rebuilt Antiproton Accumulator with better vacuum and LEAR's cooling, slows antiprotons to about 10% of light speed.
- 2002 ATHENA (August, the centenary of Dirac's birth) and ATRAP make tens of thousands of cold antihydrogen atoms; a month later ATRAP reports the first look inside an anti-atom.
ATHENA's recipe: catch about 10,000 antiprotons per AD bunch and slow them to a few millionths of light speed; collect about 75 million cold positrons from a radioactive isotope in a second trap; merge both in a mixing trap, where antihydrogen forms. Proof is unambiguous: a neutral anti-atom drifts out of the fields and hits the wall, where its antiproton and positron annihilate at the same spot and instant.
The goal is to compare hydrogen and antihydrogen in electromagnetic and gravitational fields; any difference would be profound. For energy or propulsion you would need many billions of times more, safely stored. The AD is "the best antimatter factory on the planet".
| Milestone | Close's picture (2009) | Since publication |
|---|---|---|
| Holding antihydrogen | Forms, escapes, annihilates in microseconds | ALPHA traps 38 atoms (2010), then ~300 for 1,000 s (2011) |
| Spectroscopy | A hope | 1S-2S matches hydrogen to ~2 parts per trillion (2018); hyperfine, Lamb shift measured |
| Cooling anti-atoms | Not possible | Laser-cooled antihydrogen (ALPHA, 2021) |
| Production rate | Thousands per day | 15,000 trapped in under 7 h via beryllium-ion positron cooling (2025), 20 times faster |
| Slower antiprotons | AD at ~5 MeV | ELENA ring (fully running 2021) cuts to 100 keV, ~100-fold more trapped |
| Gravity | Unknown | Falls down (ALPHA-g, 2023) |
| Moving antimatter | Fixed at CERN | BASE-STEP: protons trucked round CERN (2024); 92 antiprotons driven 8 km in about 30 min (announced March 2026) |
LEP: the largest antimatter store
Through the 1990s, antimatter was made, contained and annihilated routinely in the Large Electron Positron collider, 50 m below Switzerland and France in a tunnel as long as London's Circle Line.
- Ring 27 km: 8 curved sections of nearly 3 km, with 500 m straight sections.
- 3,500 bending magnets plus 1,000 focusing magnets.
- Positrons made by firing electrons into tungsten, stacked in an accumulator, then stepped up through accelerators "like a car moving up through the gears".
- The longest ultra-high vacuum ever built, below lunar pressure.
- Positrons crossed the Franco-Swiss border 11,000 times a second, passing under Voltaire's statue at Ferney.
- Four crossing points where bunches met; most particles missed, a few annihilated.
Each annihilation recreated, in a tiny region, the universe a billionth of a second after the Big Bang; detectors recorded what particles and antiparticles emerged from this "mini bang".
The moon moves LEP. Beams began arriving fractionally early, then late, by under a nanosecond, on a 28-day cycle. The lunar tide that raises oceans by metres also flexes the earth's crust: the 27 km ring stretched and shrank by a few millimetres each month. And because hundreds of scientists worldwide needed to share LEP data, CERN invented the World Wide Web: antimatter destroys matter but indirectly created the Web.
WHAT LEP REVEALED: THREE GENERATIONS
Nature did not stop at the electron and the up and down quarks. LEP clarified the Standard Model: each charged lepton and each quark comes in three generations, and every one has an antiparticle.
| Generation 1 | Generation 2 | Generation 3 | Charge | |
|---|---|---|---|---|
| Charged leptons | electron | muon (~200 × e) | tau (~3,500 × e) | −1 |
| Neutrinos | electron-neutrino | muon-neutrino | tau-neutrino | 0 |
| Up-type quarks | up | charm | top | +⅔ |
| Down-type quarks | down | strange | bottom | −⅓ |
| Force carriers | photon (electromagnetic) | W⁺ W⁻ Z⁰ (weak) | gluons (strong) | photon + W, Z = electroweak |
LEP showed energy congealing into matter and antimatter in perfect balance. Yet among hundreds of particle types, a handful are made symmetrically but behave asymmetrically in their lives and deaths. Explaining those exceptions might explain why the Great Annihilation left something rather than nothing.
LEP closed in 2000 and its tunnel now houses the Large Hadron Collider (protons, running since 2009-10; the Higgs boson, 2012). Close's footnote calls an antiproton LHC "science fiction", and it remains so. A successor electron-positron collider, the FCC-ee (about 91 km), completed its feasibility study in 2025 and awaits a decision. Note: the tau is about 3,500 times the electron's mass; Close's "4,000" overstates it slightly.
The Mirror Universe
A positron can be described as an electron running backwards in time, but nothing actually travels into the past: the true symmetry between matter and antimatter appears only when charge (C), mirror image (P) and time (T) are all reversed together. Reverse fewer and nature shows a small, real asymmetry, first seen in neutral kaons (1964) and later, larger, in B mesons. That asymmetry gives an absolute definition of matter, good enough to tell whether a friendly alien is safe to touch.
Backwards in time?
Richard Feynman, a young member of the Manhattan Project, became famous for Feynman diagrams: pictures of particles moving through space and time, emitting and absorbing photons, that are really a code for deep mathematics and now the staple of physics calculation. In wartime neutral Switzerland, Ernst Stueckelberg had a similar, less powerful idea, published in a Swiss journal in 1941, including the notion that an antiparticle is a particle travelling backwards in time. Feynman reached his version eight years later and, decisively, made it a working accounting system. Stueckelberg felt under-credited and said wartime made it impossible to get an artist to draw the diagrams, an excuse Close finds hard to believe for "a few straight lines connected by wavy ones".
The phrase invites fantasies of positrons arriving from the future and anti-aliens growing younger by the antiday. Close dismantles them by asking how physics treats time.
TIME'S ARROW IS STATISTICAL
- Laws without direction. Newton's laws do not distinguish past from future: a film of planets run backwards looks like a mirror image of the real thing. Combine a mirror (P, parity) with time reversal (T) and you get exactly the original.
- Disorder gives the arrow. Flowers wither and eggs don't reassemble because there is one way to make an egg and countless ways to smash it. (Even planets show an arrow eventually: tidal friction makes the moon recede, measured with sensors left by Apollo astronauts.)
- Snooker. After the break, ten scattered reds never regroup into a triangle, so a reversed film is obvious. With only two balls, a "stun" shot run backwards looks normal, apart from a black ball striking a white one.
- Paint them both white, or for charged particles swap the charges too, and the reversed film is indistinguishable from reality.
A positron is not a time traveller. Reverse the movie, look in a mirror and swap every charge, and electrons behave exactly like positrons. In that bookkeeping sense a positron is an electron going backwards in time; physically, positrons have positive energy and move forwards in time like everything else. Feynman's "backwards" trick merely avoids pitfalls in handling negative-energy states.
Film electrons and positrons circling LEP from Monday to Friday, then run the film backwards: the positrons' current looks like the reversed electrons' current, no more and no less than the electrons look like reversed positrons. Antimatter is ordinary except for its capacity to annihilate.
| Operation | What it flips | Exact symmetry alone? |
|---|---|---|
| C (charge conjugation) | particle ↔ antiparticle | No |
| P (parity) | mirror image, left ↔ right | No |
| T (time reversal) | film runs backwards | No (seen 1998) |
| CP | antiparticle in a mirror | No (seen 1964) |
| CPT | all three together | Yes: required by the CPT theorem (gravity aside) |
In Stueckelberg's and Feynman's day it was assumed any one reversal would expose the symmetry. We now know one or two are not enough: only CPT is exact, a theorem with fine print that ignores gravity. Reverse only one or two of charge, mirror and time, and matter and antimatter can be told apart absolutely.
The strange behaviour of strange particles
The yin and yang symbol looks perfectly balanced, but compare it with the mirror image of its negative and the symmetry fails, as it does for matter and antimatter.
The culprit is the neutral kaon, K⁰, a boson made of a down quark and a strange antiquark, charges summing to zero. Swap matter for antimatter (strange quark plus down antiquark) and you get the anti-K⁰.
An experiment at Brookhaven National Laboratory tested CP, swapping charges and looking in a mirror, which everyone assumed nature obeyed. It does not: kaons showed a small CP violation. When Jim Cronin and Val Fitch received the Nobel Prize (1980), a Swedish newspaper's headline read: "The laws of nature are wrong!" They are not wrong, only subtler than expected.
JEKYLL AND HYDE
A strange quark is heavier than a down quark but otherwise similar, so a strange can shed energy and become a down, and a down given energy can become strange (likewise for antiquarks). Inside a kaon this lets the particle change identity.
- Oscillation. K⁰ and anti-K⁰ swap back and forth. If matter and antimatter were symmetric, each direction would be equally likely and a film of the oscillation would look the same run either way.
- How to look. Start with a 50:50 beam and examine what remains when they die: the decay products reveal which form died. Anything other than 50:50 means either uneven oscillation or one form dies more readily; either way, matter and antimatter differ.
- CERN, 1998. Experiments found anti-K turns into K slightly faster than the reverse. You can therefore tell which way a film of kaon oscillation runs: a direct arrow of time at the level of single particles. Start with equal K and anti-K and a small excess of K develops. Trifling in the kaon's short life and far too small to explain the cosmos, but proof in principle that asymmetry can arise.
THREE GENERATIONS MAKE ASYMMETRY POSSIBLE
The clue is a gift of nature: LEP showed three generations of quarks and leptons. Extend Dirac's ideas to three generations and matter and antimatter need no longer be exact counterparts. Replace the strange quark in a kaon with the third-generation bottom quark and the resulting B and anti-B mesons show an even larger asymmetry, as theory predicted.
Close's account ends at kaons and early B-factory results (BaBar at Stanford and Belle in Japan, 2001 onwards, behind the 2008 Nobel to Kobayashi and Maskawa, whose three-generation theory he describes). Since then: BaBar (2012) observed time-reversal violation directly in B mesons; LHCb at CERN has measured CP violation in B⁰ₛ mesons (2013), in charm mesons (2019), the first in an up-type quark, and in baryons (2025), via Λ_b decays at about 2.5 per cent asymmetry, the first in the class of particles that make up ordinary matter. Every result fits the three-generation Standard Model, and its total CP violation remains billions of times too small to explain the cosmic excess of matter (Chapter 8).
Don't shake hands with an anti-alien
You are hovering over a distant planet whose friendly, advanced inhabitants talk to you by radio. They naturally say they are made of matter: nobody calls their own stuff "anti". How can you check their dictionary against yours? If matter and antimatter were perfectly symmetric, the only test would be risky: approach, or fire a probe into their atmosphere (and an anti-alien might not take kindly to matter exploding in its anti-atmosphere).
The long-lived neutral kaon settles it. It decays to a charged pion plus an electron or a positron, and the two are not equally likely.
- Out of every 2,000 long-lived neutral kaon decays of this type, about 1,003 give a positron and 997 an electron.
- Kaon mass: slightly more than half a proton's.
- Neutral kaons come in short-lived and long-lived forms; the long-lived shows the larger asymmetry here.
- Name the particle by mass. Names won't translate, so specify a particle slightly more than half the mass of the heavy particle at the centre of your simplest atom (proton or antiproton). Nothing else fits.
- Specify the neutral one. The property holding atoms together is "charge"; we mean the kaon with no charge, not K⁺ or K⁻.
- Choose the long-lived form.
- Ask the question. "Is the light particle produced most often in its decays the same as the one in your atoms, or the opposite?"
- Read the answer. "The same": their atoms hold positrons; they are antimatter, so look but don't touch. "The opposite": they are matter; it is safe to land.
The protocol works only because CP is broken: the asymmetry gives a convention-free definition of matter that any physicist anywhere in the universe can check.
Why Is There Anything at All?
If the Big Bang made matter and antimatter equally, they should have annihilated each other completely; the real puzzle is why matter survived. Close surveys the evidence that no antimatter worlds exist within hundreds of millions of light years, reconstructs the Great Annihilation (ten billion and one protons for every ten billion antiprotons), shows that quark asymmetries are too small to explain the surplus, and backs the heavy-neutrino route, now called leptogenesis, as the best theory, still unproven.
The mystery of the missing antimatter
The fireball of the Big Bang, fourteen billion years ago, turned radiant energy into matter and antimatter in perfect balance. In the dense cauldron they would have collided constantly; matter born in the first moment should have been annihilated the next. The mystery is less why antimatter disappeared than why any matter survived, and the answer must be that matter and antimatter are not perfect mirrors.
Strange and bottom particles show subtle differences, but electrons, protons and neutrons match their antiparticles to the limits of measurement. And nothing forbids antimatter in bulk: an anti-periodic table of positrons around antinuclei would obey the same quantum rules, giving anti-amino acids, anti-DNA and anti-life. Antistars shine with identical spectra, so looking cannot tell stars from antistars. Earth is atypical anyway: hydrogen is rare here but commonest in the cosmos, and a random volume of space millions of light years across is almost free of carbon, iron or gold.
HOW WE KNOW THERE ARE NO ANTIWORLDS NEARBY
- The solar system. Astronauts landed on the moon and probes on Mars without annihilating. The solar wind washes every planet; were it antimatter, the planets would glow in gamma rays. They do not.
- Comets. Cults claiming comets are antimatter are talking nonsense: an anticomet in the solar wind would blaze, each gram annihilated releasing twice a Hiroshima bomb. The Giotto probe flew inside Comet Halley and sent pictures. Anticomets and antimeteorites are under one part in a billion of solar-system matter.
- Cosmic rays. The positrons and antiprotons in cosmic rays are debris made by ordinary cosmic rays hitting gas, not remnants of antistars. An exploded antistar would send antinuclei; searches by the AMS detector and a balloon above the South Pole had found not even antihelium.
- Interstellar space and colliding galaxies. Space is nearly empty, not empty; antimatter travelling light years would hit something and flash. Galaxies that collide and distort would light up with gamma rays at an antimatter boundary. None is seen.
- The galactic centre. For thirty years, gamma rays have shown clouds of positrons there. In 2008 the Integral satellite linked them to X-ray binaries: ordinary stars being eaten by neutron stars or black holes, whose infalling gas gets hot enough to make electron-positron pairs.
- A 2002 solar flare. Its fast particles struck the solar atmosphere and made up to half a kilogram of positrons, enough, if their annihilation energy were captured, to power the UK for two days.
So everything within several hundred million light years is matter, apart from transient antiparticles. That is a huge volume but only part of the visible universe. Separate domains are conceivable: as a cooling universe freezes like water into snowflakes, random local excesses of matter or antimatter might have grown into matter regions and antimatter regions. Most models disfavour this.
AMS-02 (installed on the International Space Station in 2011) has recorded billions of cosmic rays. It confirmed an unexpected excess of high-energy positrons (first seen by PAMELA, 2008-09), probably from pulsars, with dark-matter annihilation not excluded. Since 2016 its leader Samuel Ting has reported a handful of antihelium candidates, including antihelium-4, in talks; they remain unpublished and unconfirmed, and if real would be very hard to explain without antimatter reservoirs. The galactic 511 keV positron source is still unexplained: the 2008 X-ray-binary link has not held up as decisive, and candidates include supernova-made radioactive nuclei, the central black hole and dark matter. The balloon programme Close mentions (BESS-Polar) saw no antihelium; the dedicated GAPS balloon is designed to look for antinuclei at low energy.
The Great Annihilation
- Today, on average, every five cubic metres of space holds 1 proton, 0 antiprotons and 10 billion quanta of radiation.
- Just after the Big Bang, the same budget: 10,000,000,001 protons, 10,000,000,000 antiprotons and 10 billion photons.
- After the Great Annihilation, the 1-in-10-billion survivors make every galaxy, star and person; the annihilated rest is the microwave background.
So something, even earlier, tipped the balance by one part in billions. Before asking what, Close reviews how matter itself emerged.
Replaying the Big Bang
Matter has evolved. Above about 10,000 degrees atoms lose their electrons and form a plasma; the sun's core (above a million degrees) is a plasma of electrons and protons, and beam experiments at those energies confirm the sun is a fusion machine doing the first stage of "chemical cookery". Electrons seem unchanged at all temperatures, but protons and neutrons are not: above about a million billion degrees they melt into a quark-gluon plasma.
Nowhere is that hot now except in accelerator collisions, where antimatter is the perfect tool. A proton hitting a fixed target wastes most of its energy; head-on particle-antiparticle annihilation frees all of mc².
The irony: we learned how matter formed using antiprotons and positrons as time machines. Had they been plentiful, they would have made antistars and anti-elements just as easily. The imbalance must have arisen earlier than the billionth of a second LEP could probe.
Close does not name them, but cosmologists use Andrei Sakharov's three conditions (1967) to judge any theory of the surplus:
- Baryon-number violation. Some process must change the net count of matter minus antimatter.
- C and CP violation. Matter and antimatter must behave differently (Chapter 7).
- Departure from equilibrium. A rapidly cooling universe must freeze the imbalance in before it is undone. Close's neutrino story is designed to meet all three.
Neutrinos
CP violation for strange and bottom quarks was sensational, but quantitatively far too small. Attention has shifted to the leptons, and especially to neutrinos, now "the prime suspect".
Neutrinos are among the commonest particles and the most elusive: no charge, tiny mass, passing through the earth "like a bullet through a bank of fog". They are fermions, so the matter-antimatter distinction applies, yet a neutrino has no inner structure to tell it apart from an antineutrino. It is a spinning "piece of nothingness".
- The bookkeeping. When a proton becomes a neutron it emits a positron and a neutrino: charge is conserved by the positron, and the net count of fermions (matter minus antimatter) by the neutrino. Neutron decay gives an electron and an antineutrino. In reverse, a neutrino turns a neutron into a proton plus electron; an antineutrino turns a proton into a neutron plus positron.
- Handedness. Spin along the direction of flight is left-handed or right-handed, like a corkscrew. For fifty years neutrinos were only left-handed and antineutrinos only right-handed; a mirror swaps one for the other.
- What "anti" means. Had the positive electron been named something else, and the negative proton a "negaton", they would seem just two new particles. For neutrinos it may be better to say there is one neutrino: left-handed it prefers electrons, right-handed positrons.
- Mass changes everything. A massless fermion at light speed keeps its handedness; a massive one can be flipped. Neutrinos turn out to have mass (Close: discovered "in the last five years"), so either handedness is possible for neutrinos and antineutrinos alike. Close puts the mass at under 1/10,000 of an electron's.
Dirac neutrino Neutrino and antineutrino are distinct particles: "left-hand matter, right-hand antimatter", just as for the electron and positron.
Majorana neutrino Neutrino and antineutrino are the same object, both matter and antimatter. Proposed by Ettore Majorana soon after Dirac's equation; it could explain why there is something rather than nothing.
Apocalypse, not quite
Explaining why neutrino masses are so tiny has led theorists to propose very heavy Majorana neutrinos (Close calls them "majorons"). Made in the Big Bang's heat, they could decay by radiating a Higgs boson into any neutrino or antineutrino, with no reason to favour either equally.
Then came the Great Annihilation, destroying all antimatter with its matching matter and leaving a handful of excess quarks for every ten billion that vanished, which cooled into a universe of stable protons. Close calls this the best current theory, to be tested at the LHC and in cosmic rays. One consequence is decisive for the final chapter: the asymmetry formed at energies far beyond any experiment, so we cannot convert matter into antimatter in the laboratory by recreating those conditions. Antimatter as an energy source needs some other solution.
Terminology. In the literature a "majoron" is a hypothetical massless boson from broken lepton number; the heavy fermions Close describes are usually called heavy right-handed (sterile) Majorana neutrinos, and the mechanism leptogenesis (Fukugita and Yanagida, 1986). In the standard version they weigh around 10⁹-10¹⁵ GeV, far beyond the LHC, which has seen none. Dates and numbers. Neutrino mass was established by oscillations at Super-Kamiokande in 1998 (Nobel 2015), over a decade before the book. KATRIN (2025) caps the mass at 0.45 eV, so it takes at least a million neutrinos, not 10,000, to weigh one electron; cosmology suggests the three masses sum to under about 0.1 eV. Open questions. Majorana nature is tested by neutrinoless double-beta decay (KamLAND-Zen, LEGEND, nEXO): unseen so far. Leptonic CP violation is hinted at by T2K (2020) but the first joint T2K-NOvA analysis (2025) is not decisive; DUNE and Hyper-Kamiokande aim to settle it in the 2030s.
Revelations
Close's verdict: antimatter will never be a fuel or a weapon, because nature allows it only as a storage battery that returns less energy than it costs, made a few atoms at a time and impossible to store densely. Tunguska was not an antimatter strike; the Air Force "weapons programme" amounted to a $3 million research grant and a student paintball drone; and much public belief traces back to Dan Brown's Angels and Demons. Antimatter's real value is as a tool that recreates the early universe and saves lives in medicine.
The law that kills the dream
Energy congealed into matter and antimatter billions of years ago; we free a sliver with chemistry and fission. Antimatter would be the perfect spark, since touching it releases all the energy of anything. But there is none left nearby, so we must make it, and there nature sets the terms.
Antimatter is a battery, not a fuel. Making antimatter from energy via E = mc² always makes equal amounts of matter and antimatter, and annihilating them can at best return what was put in; in practice most is wasted. This is not a problem for more research or better technology to solve: "that is the nature of nature". Antimatter could be a primary energy source only if we found natural deposits, like oil.
Tunguska solved
The only hint of natural antimatter is the claim about Tunguska. Close sets the scene of the solar system's traffic:
- Comets. Probably the oldest members of the solar system: gravel and ice with frozen ammonia and methane, mostly lurking beyond Pluto. Near the sun the ice vaporizes into a coma up to 100,000 miles across around a nucleus about a mile wide; the solar wind blows out the tail (as in the Bayeux record of the comet of 1066).
- Debris. Comet fragments form rings and trails; when earth crosses one we get meteor showers, the Leonids (about 12 November) and Perseids (mid-August). Extinct comet heads become some of the asteroids, several of them earth-crossing and up to a mile wide.
- Monsters. About once in 100 million years: the Manhattan-sized asteroid that hit 65 million years ago at 40 km/s, made the Chicxulub crater in Yucatán and killed the dinosaurs. The Arizona crater, over a mile wide, came from an oil-tanker-sized rock.
Tunguska is odd only because, without eyewitnesses and instrument records, nothing permanent would mark it. Antimatter in the solar system is at most one part in a billion, so an antirock is a long shot; but forensic science settles it anyway.
| Evidence | Antimatter strike predicts | Comet airburst predicts | Observed |
|---|---|---|---|
| Crater, debris | None | None: energy spent in the air | None |
| Neutrons | Annihilation pions eject floods of neutrons | None | Not measured directly; see carbon-14 |
| Tree-ring carbon-14 | Spike from neutron bombardment | Normal | Normal in 1908 (Cowan, Atluri, Libby, 1965) |
| Night-sky glow | No reason for direction | Dust tail pointed north-west, lighting Russia and Europe but not the US | Bright nights in Europe for a week |
| Heat flash, burns | Gamma burst | Shock-heated air and burning comet chemicals | Burns at 60 km, felled forest |
The carbon-14 test is decisive. CERN experience shows that annihilation products hitting material knock out many neutrons, which would have stamped extra carbon-14 into the 1908 tree rings; there is nothing unusual. Close's conclusion: a comet fragment exploded high in the atmosphere, the flash and shock heating doing the damage. Such events have happened many times unwitnessed; Tunguska was "rather small" by world standards, and no antimatter has ever hit the earth.
Close's verdict on antimatter holds. Current work favours a stony asteroid (about 50-60 m) over a comet, though a cometary body is not excluded; yields of 3-15 megatons are typical. The Arizona (Barringer/Meteor) crater is now dated to about 50,000 years ago, not 30,000, and Chicxulub to 66 million years.
The power of antimatter
Your body temperature is E = mc² at work: in metabolism a tiny part of the food's mass, about one part in a billion (a microgram per kilogram), becomes energy, far too small to weigh against the sweat and DNA you shed. That one part in a billion powers chemistry, biology, life and gunpowder, all via outer electrons. The nucleus offers up to ten million times more per atom: about 1 per cent of mass-energy. Antimatter promises 100 per cent.
The difference is supply. Chemistry works because a gram contains up to 10²⁴ atoms, all contributing; uranium can be dug from the ground, its energy locked in by nature aeons ago. Every antiparticle must be made by us, and friction and waste mean far more energy goes in than ever comes out.
Antimatter at large: the numbers
Whether for Brown's Vatican bomb, Star Trek or a power plant, the simplest antimatter will do (no "anti-TNT" chemistry needed), made one particle at a time.
- A gram of antiprotons needs nearly a trillion trillion (10²⁴) of them; a gram of positrons 2,000 times more.
- All the antiprotons made since 1955 at CERN and Fermilab: under a millionth of a gram, enough to light one bulb for a few minutes; the energy spent could have lit Times Square or Piccadilly Circus.
- At present rates one gram would take a hundred thousand centuries, or, from the nanogram a year (cost: tens of millions of dollars) Close quotes later, hundreds of millions of years and over $1,000 trillion.
- Largest number of antiprotons ever stored in one trap: about a million, a billion billion short of a gram.
Storage is the second wall. Like charges repel: the more antiprotons or positrons you cram in, the harder the bottle must push, and "a leaky bottle of antiparticles is not the answer to my father's challenge".
- Charged particles in a Penning trap. Works for small numbers for weeks; mutual repulsion caps the load around a million.
- Neutral antihydrogen. Charges cancel, but fields can no longer grip the atoms, which drift to the walls. Trapping by the atom's magnetism in rapidly varying fields was proposed but not achieved even for a few atoms. Bulk magnetism (anti-iron) would interfere with the bottle's own fields.
- Positronium (positron + electron). Neutral and of interest to the US military, but it self-annihilates in under a microsecond: useless for a voyage to Mars.
Fantasies: antimatter bombs
The same reasons that rule out energy storage rule out bombs: you cannot accumulate enough at high enough density. And if you could build a force field strong enough to hold a gram of antiprotons, releasing the field would produce a blast from the charges flying apart greater than their annihilation. Close's advice to bomb makers: forget the antimatter and use the containment technology.
After Kenneth Edwards's 2004 speech, Rex Swenson of Eglin's Munitions Directorate said in July 2004 that everyone was "very excited". Within a month higher officials in the Air Force and Pentagon overruled interviews: "we're not at the point where we need to be doing any public interviews". Conspiracy theorists saw suppression; Close sees no weapons, just a dream. The real activity: an open research grant of over $3 million to Gerald Smith, ex-chair of physics at Penn State, whose LEAR antiproton work Close had reviewed at CERN. Smith founded Positronics Research in Santa Fe in 2001, advertising energy storage, destroying chemical and biological agents, nuclear medicine and propulsion. No demonstration of bulk production or storage has been made.
Rolf Landua of CERN notes the irony: scientists foresaw the atom bomb years before the surprised public saw it; the antimatter bomb is the reverse, imagined by the public while scientists have long known it is not practical.
To boldly go: antimatter propulsion
Annihilation gamma rays could heat a propellant to huge temperatures, or evaporate silicon carbide from a surface to make exhaust. The selling point is weight. In Cassini-Huygens, over half the probe's weight was fuel and oxidizer, and the launcher weighed more than 180 times the probe. Promoters say a crewed Mars trip's three tonnes of chemical propellant could be replaced by under a hundredth of a gram of antimatter, a rice grain. They say less about the containment hardware: even a millionth of that charge would push on the tank walls with tons of force.
- Penn State's plan (1997). A trap holding a billion antiprotons for ten days as prototype for one holding 10¹⁴ antiprotons for 120 days (a Mars round trip), "confident of achieving this goal", then a thousand such traps. Close calls it a management plan, not a technology: ten years later nothing like it exists, and no CERN work aims at it.
- Photon rockets. Eugen Sänger proposed one powered by electron-positron annihilation in the 1950s; never built.
- Stretched positronium. Ackermann, Shertzer and Schmelcher (1997-98) predicted that combined electric and magnetic fields could stretch positronium like a dumbbell, its partners thousands of times further apart, so it survives far longer; Smith suggested a "practically infinite" lifetime. Close: unconfirmed, and even if it worked for a few atoms it returns us to the old problem of holding trillions of separated charges.
The Air Force aircraft dream ended up as a student design project linked to Eglin: a positron-powered craft with a wingspan under three feet, able to hover, firing paint balls as simulated ammunition. "There in a nutshell you have the US Air Force's hopes … laid bare."
Angels and Demons: fiction taken as fact
Edwards's "No Nuclear Residue" line, and the press's "clean superbomb", echo Dan Brown's 2000 novel, then climbing the best-seller lists, whose antimatter bomb produced "no pollution or radiation". Most people knew CERN, if at all, as the birthplace of the Web; after Brown it became "the lab that makes antimatter". Both true; much else in the novel is not.
| Brown's "FACT" | Close's reality |
|---|---|
| No pollution or radiation | Annihilation is radiation: gamma rays. Brown's own hero is told to "shield your eyes"; gamma rays are invisible, so shield your body |
| Ignites on contact with anything | Only on contact with matter; that is the whole problem of storage |
| 1 g = a 20-kiloton bomb | Close's own figure: half a gram of antimatter (with its matter) gives 20 kt; but a gram is unattainable |
| CERN recently made the first antiparticles | Antiparticles have been made for eighty years; CERN made a few anti-atoms in the last decade |
| The AD makes antimatter in much larger quantities | The AD makes fewer antiprotons than LEAR did |
| Something from nothing; Genesis proved | Making a pair costs exactly the energy it releases; "cod theology" |
| Antimatter could save the planet | It is a lossy battery: less out than in |
Brown's demonstration device used positrons; later his bomb seems to be antihydrogen. Close's deeper objection is to the Creation analogy. The Big Bang created all energy, matter, space and time, something "beyond my comprehension"; laboratories only study what came after. Energy is not nothing: the power company bills you for it.
Close's central argument has aged well. CERN estimates that all antimatter it has ever made would light a bulb for only minutes; the AD plus ELENA produce on the order of picograms to nanograms over years, and the closure of Fermilab's antiproton source (2011) left CERN as the only low-energy antiproton supplier. Storage has improved for science, not bulk: BASE has kept antiprotons in a reservoir trap for over a year, and ALPHA now traps thousands of antihydrogen atoms (Chapter 6 table), but these are still attograms. In March 2026 BASE-STEP drove 92 antiprotons around CERN by truck. The Angels and Demons film (2009) partly shot at CERN prompted the lab to publish its own fact sheets. Antimatter propulsion studies continue only as NIAC-style concept work.
Antimatter fact-ory
Antimatter will not interest power companies, but in regions smaller than atoms its annihilation is invaluable: in medicine (PET), in technology and above all in fundamental science. Head-on beams carry little total energy, but concentrated into a volume smaller than a nucleus they recreate conditions from when the universe was less than a billionth of a second old. Groups of atoms able to think have built machines that revisit their own origins in the Big Bang, and the tool that made it possible is antimatter. "With such inspirations in fact, who needs fiction?"
The Cost of Antimatter
Close's back-of-envelope accounts behind the book's claims: one gram of antimatter holds about 21 kilotons, so half a gram with its matter partner equals Hiroshima; the world's best antiproton source made about 1.5 nanograms a year; and making useful amounts of antihydrogen would take longer than the age of the universe.
A gram versus Hiroshima
- Unit. One kiloton of TNT = 4.2 × 10¹² J (4.2 terajoules). One joule is the kinetic energy of 2 kg moving at 1 m/s (KE = ½mv²).
- One gram. E = mc² = 10⁻³ kg × (3 × 10⁸ m/s)² = 9 × 10¹³ J = 90 TJ, or 21.4 kilotons.
- Two for one. The same energy sits in the gram of matter it annihilates, so half a gram of antimatter matches the Hiroshima bomb (20 kt).
- Caveat. That assumes all the energy comes out at once. Annihilating atom by atom, a hoard might "fizzle rather than explode".
How long to make a gram?
A gram needs 6 × 10²³ antiprotons or 10²⁶ positrons.
| Source (2007-09) | Rate | Per year | Mass per year |
|---|---|---|---|
| Fermilab (record, June 2007) | 10¹⁴ per month | ~10¹⁵ | ~1.5 ng |
| CERN AD | ~40,000 per second | ~10¹³ (Close) | ~1% of Fermilab |
| AD upgraded ×10 to ×100 | world total still ~3 ng | ||
| Darmstadt (planned) | might match Fermilab | ||
| Antihydrogen at CERN | several hundred atoms/s | 1 ng in ~1,000 centuries |
Annihilating a year of Fermilab's output with matter would, Close says, release about 270 joules, five seconds of a feeble bulb. All the antiprotons ever made would light a bulb for a few minutes, and they were lost long ago; the number actually stored is trifling. CERN's antiprotons are fewer but colder, made for trapping and building antihydrogen. To fill a toy balloon with antihydrogen, let alone make a gram, would take longer than the universe has existed.
- Fermilab energy. 1.5 ng of antiprotons plus 1.5 ng of matter is 3 × 10⁻¹² kg × 9 × 10¹⁶ = 2.7 × 10⁵ J: 270 kilojoules, not 270 joules. That runs a 100 W bulb for about 45 minutes. The order-of-magnitude point (trivial energy) still stands, but the figure is out by a factor of 1,000.
- AD output. 40,000 per second for a full year is about 1.3 × 10¹², nearer 10¹² than 10¹³; given downtime, real yearly output is lower still.
- Since publication. Fermilab's antiproton source closed in 2011 with the Tevatron. The Darmstadt facility (FAIR) is still being built, with its low-energy antiproton programme deferred, so CERN's AD and ELENA are the world's only source of slow antiprotons. ELENA raised the number of trappable antiprotons about a hundredfold, not the total produced.
The Dirac Code
How Dirac turned Einstein's E² = (mc²)² + (pc)² into a linear equation for E: the coefficients cannot be numbers, only matrices; three dimensions of motion use up all the 2×2 matrices; adding mass forces a doubling to 4×4 gamma matrices, and the doubled half is the negative energy that became antimatter.
The impossible sum
Dirac wanted the energy as a plain sum: E = a·(pc) + b·(mc²), with no square root. Squaring it gives
E² = a²(pc)² + b²(mc²)² + (ab + ba)(pc)(mc²)
and to match Einstein the coefficients must obey a² = 1, b² = 1 and ab + ba = 0. Close uses the 3-4-5 triangle (rest energy 3, motion 4, total 5: 9 + 16 = 25): write the "5" as some amount of "3" plus some amount of "4". No ordinary numbers square to 1 while their cross terms cancel, whatever the triangle.
Matrices do it
A matrix is an array of numbers with its own rules: addition is element by element; multiplication combines rows with columns, so a×b need not equal b×a. The ordinary number 1 becomes the matrix with 1s down the diagonal. One pair that solves Dirac's problem:
| Matrix | Row 1 | Row 2 | Property |
|---|---|---|---|
| a | 0, 1 | 1, 0 | a² = 1 |
| b | 1, 0 | 0, −1 | b² = 1 |
| a×b | 0, −1 | 1, 0 | |
| b×a | 0, 1 | −1, 0 | a×b + b×a = 0 |
Three dimensions, then mass
Momentum has three components (x, y, z), so Dirac needs a(x), a(y), a(z), each squaring to 1 and anticommuting in pairs. For a massless electron the three 2×2 Pauli matrices do it: σₓ = (0, 1 | 1, 0), σᵧ = (0, −i | i, 0), σ_z = (1, 0 | 0, −1), where i = √−1. (Close's printed σᵧ omits the i, so it squares to −1; his footnote says it must be multiplied by −i.)
But these exhaust the independent 2×2 matrices (a(z) is the old b). The mass term needs a fourth anticommuting matrix and none is left. Dirac's way out was to double everything to 4×4.
- γ⁰ (the "b"). +1 on the top two diagonal places, −1 on the bottom two: the positive-energy and negative-energy halves.
- γˣ, γʸ, γᶻ. Each Pauli matrix σ placed in the top-right corner and −σ in the bottom-left; zeros elsewhere ("opposite corners, lower half of opposite sign").
- The rule. All four anticommute in pairs and square to ±1, so the cross terms vanish and E² = (mc²)² + (pc)² is recovered.
- On the stone. These are the γ in iγ·∂ψ = mψ.
Running the two-by-two solution twice, once with b as plus one and once as minus one, gives the electron both its two spin states and its negative-energy twin. That doubling is the mathematical birth of antimatter.
The source edition's matrix layouts (printed as images) did not survive extraction: the trial equations, the example matrices and the displayed gamma matrices were missing. The matrices above are the standard Pauli and Dirac-representation matrices, consistent with Close's verbal description; the 2×2 example pair is the simplest choice satisfying his conditions and may differ from the one he printed.
Master Takeaways
- Antimatter is a battery, never a fuel. Making it from energy always yields equal matter and antimatter, and annihilation can at best return what went in; with real losses, far less. No research programme can change that.
- Antimatter is self-limiting. Each antiparticle destroys one particle of matter and itself; there is no chain reaction, no "ice-nine". The danger is the energy released, 100 per cent of mc², not runaway spread.
- The mirror is almost perfect, and "almost" is why we exist. Masses, charges, magnetism and atomic spectra of particles and antiparticles agree to parts per trillion, yet CP violation lets the early universe leave one proton in ten billion after the Great Annihilation.
- Mathematics predicted a new form of matter. Forcing quantum theory to obey relativity made Dirac use matrices, which delivered spin for free and demanded negative energies; taking them seriously predicted the positron, found four years later.
- Storage is limited by physics, not engineering. Like charges repel, so traps of antiprotons or positrons cap out in the millions; neutral antihydrogen slips the fields' grip unless held by its weak magnetism. Bulk antimatter has nowhere to be kept.
- The numbers end the fantasy. World output was about a nanogram a year; a gram would take hundreds of millions of years and over $1,000 trillion, while half a gram is a Hiroshima.
- Energy and matter are interconvertible, but charge and "matter-number" are conserved. Light makes pairs, never a lone electron; a proton turning into a neutron emits a positron and a neutrino to balance the books.
- Antiparticles are everyday phenomena. Positron emitters drive PET scans, positron annihilation detects metal fatigue, and about 10 per cent of sunlight began as positron annihilation in the solar core.
- Discovery rewards the bold publisher. Skobeltzyn, Blackett, Dirac and the Joliot-Curies all held the positron; Anderson got the Nobel because he went public despite his supervisor.
- Annihilation is a courtship, then a flash. A slow antiproton is first captured into an atom-like orbit and cascades inward, emitting gamma "skid marks", before the strong force ends it in under 10⁻¹⁸ s.
- Only CPT is an exact symmetry. Reversing charge, mirror image and time together leaves physics unchanged; reversing one or two does not, and kaon oscillations show an arrow of time for single particles.
- Matter can be defined without convention. The long-lived neutral kaon decays to a positron 1,003 times for every 997 electrons, so any civilisation can tell whether it is matter or antimatter, a test for anti-aliens.
- No antiworlds lie within hundreds of millions of light years. No gamma glow from the solar wind, colliding galaxies or interstellar boundaries, and no confirmed antinuclei in cosmic rays.
- Antimatter beams are time machines. Head-on annihilation frees all the collision energy, recreating the universe at under a billionth of a second old; LEP mapped the three generations of quarks and leptons this way.
- Three generations make asymmetry possible. Matter-antimatter differences need at least three quark families; B mesons show bigger effects than kaons, as theory predicted, but quark CP violation is far too small for the cosmos.
- Neutrinos are the prime suspects. If neutrinos are Majorana particles, very heavy partners decaying unevenly in the early universe could have seeded the quark surplus (leptogenesis).
- Tunguska was not antimatter. An annihilation would have flooded the forest with neutrons and raised carbon-14 in the 1908 tree rings; it did not. A cosmic body exploded in the air.
- Military antimatter was a speech, a grant and a paintball drone. Secrecy was taken as proof; the reality was $3 million of open research and a three-foot student aircraft.
- Popular belief can be traced to a novel. "No radiation", "a gram equals 20 kilotons", "Genesis proved": Angels and Demons supplied the script that the 2004 press coverage followed.
- Energy is not nothing. Making particles "from energy" is not creation from nothing; it costs exactly the energy the pair holds, a point Close turns against "cod theology".
- Since 2009, antimatter has been trapped, measured, cooled, weighed and moved. ALPHA holds antihydrogen for minutes, matches hydrogen's 1S-2S line to ~2 parts per trillion, saw it fall down (2023); BASE trucked antiprotons across CERN (2026); none of this changes the energy verdict.
Frameworks & mental models
| Name | What it says | Use it when |
|---|---|---|
| Energy ladder | Chemistry frees ~10⁻⁹ of mc², fission ~10⁻³, fusion ~10⁻², annihilation 1 | Sizing any energy claim |
| E² = (mc²)² + (pc)² | Total energy is the hypotenuse of rest and motion energy | Relating mass, momentum, energy |
| Pair creation and annihilation | Light ↔ particle + antiparticle; charge always balanced | Explaining where antimatter comes from |
| Dirac sea | Filled negative-energy states; holes act as antiparticles | Historical intuition (now replaced by field theory) |
| Fermions vs bosons | Bricks of matter vs force carriers that are neither | Classifying any particle |
| Cooling (electron, stochastic) | Drain a beam's jitter so it can be stored | Understanding accelerator antimatter |
| Penning trap | Fields, not walls, hold charged antiparticles | Any cold-storage claim |
| C, P, T and CPT | Only all three reversed together is exact | Thinking about matter-antimatter differences |
| Anti-alien protocol | Kaon decay asymmetry defines matter absolutely | Explaining why CP violation matters |
| Great Annihilation ledger | 10¹⁰ + 1 protons vs 10¹⁰ antiprotons → 1 survivor | Grasping the size of the cosmic puzzle |
| Sakharov conditions | Baryon violation, CP violation, non-equilibrium | Judging theories of the matter surplus |
| Battery, not fuel | You only get back what you put in, minus losses | Evaluating antimatter energy or weapons hype |
| Forensic test | Annihilation leaves neutrons and carbon-14 | Checking "antimatter impact" stories |
Close is a reliable, lucid guide whose central verdict, no antimatter fuel or bombs, has held up completely. Three caveats. First, the science has moved fast since 2009: antihydrogen trapping (2010), spectroscopy at parts per trillion, laser cooling (2021), the gravity measurement (2023), ELENA, antiproton transport (2024-26), positronium laser cooling (2024) and CP violation in charm (2019) and baryons (2025) all postdate the book, and his "we still don't know whether antimatter falls" is now answered. Second, some details are loose or wrong: the Tunguska energy (10¹⁵ J is ten or more times too low), Kulik's nationality, the antineutron date, the tau mass, the Fermilab energy (270 kJ, not 270 J), the term "majorons" for heavy neutrinos, and neutrino mass described as recent (1998) and as 1/10,000 of an electron (it is under one millionth). Third, the pedagogy is dated in places: "relativistic mass" and the Dirac sea are historical devices, and the cosmology chapter presents leptogenesis more confidently than the evidence warrants; neutrinoless double-beta decay and leptonic CP violation remain unconfirmed in 2026.
Who's who
| Name | Role | Why they matter |
|---|---|---|
| Paul Dirac | Cambridge theorist | 1928 equation; predicted positron and antiproton |
| Robert Oppenheimer | Theorist | Showed the "hole" cannot be the proton |
| Hermann Weyl | Mathematician | Argued the antiparticle must share the electron's mass |
| Dmitry Skobeltzyn | Leningrad experimenter | Photographed positrons (1923) without recognizing them |
| Carl Anderson | Caltech student | Identified the positron (1932); Nobel 1936 |
| Robert Millikan | Caltech physicist | Named cosmic rays; resisted Anderson's reading |
| Patrick Blackett, Giuseppe Occhialini | Cavendish | Triggered cloud chamber; proved pair creation (1933) |
| Irène and Frédéric Joliot-Curie | Paris | Missed neutron and positron; Nobel for artificial radioactivity |
| Chamberlain, Segrè, Wiegand, Ypsilantis | Berkeley | Found the antiproton (1955) |
| Oreste Piccioni | Berkeley | Antineutron; sued over the 1959 Nobel |
| Bruno Touschek | Frascati | First electron-positron storage ring (ADA) |
| Gersh Budker | Novosibirsk | Electron cooling of antiprotons |
| Simon van der Meer, Carlo Rubbia | CERN | Stochastic cooling; W and Z (Nobel 1984) |
| Hans Dehmelt | Penning trap | Single electron and positron trapped; Nobel 1989 |
| Gerald Gabrielse | Harvard, CERN | Trapped a single antiproton; ATRAP |
| Ernst Stueckelberg, Richard Feynman | Theorists | Antiparticles as particles running backwards in time |
| Jim Cronin, Val Fitch | Brookhaven | CP violation in kaons (1964) |
| Ettore Majorana | Theorist | Neutrino as its own antiparticle |
| Kenneth Edwards | US Air Force, Eglin | 2004 speech behind the antimatter-weapon stories |
| Gerald Smith | Penn State, Positronics Research | Air Force-funded antimatter propulsion work |
| Dan Brown | Novelist | Angels and Demons, source of popular myths |
Connect it
- Hype audits. Close's method, check the energy budget, the production rate and the storage limit, is a template for evaluating any technology pitch where the physics or the economics of "making the fuel" is glossed over, from novel batteries to grand AI compute claims.
- Secrecy is not evidence. The Air Force episode shows how silence plus a vivid narrative becomes "proof"; in risk and governance work, demand the measurable artefact (the trap, the gram, the demo) before the scenario.
- Precision as discovery. Dehmelt's one-part-in-a-thousand excess and the kaon's 1,003 versus 997 show that tiny, well-measured deviations carry the deepest information, a useful stance for model monitoring and anomaly detection in production systems.
About this book
Antimatter by Frank Close, 2009. 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.
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