Antimatter is matter composed of antiparticles — particles that have the same mass and spin as their ordinary matter counterparts but opposite electric charge (and opposite values of other quantum numbers like baryon number and lepton number). The antiparticle of the electron (charge -1) is the positron (charge +1). The antiparticle of the proton (charge +1) is the antiproton (charge -1). The antiparticle of the neutron (charge 0) is the antineutron (also charge 0, but with opposite baryon number and opposite magnetic moment). Some particles are their own antiparticles — the photon is the most familiar example. When a particle meets its antiparticle, they can annihilate, converting their entire combined mass into energy (photons or other particle-antiparticle pairs) according to E = mc². A single electron-positron annihilation produces two gamma-ray photons, each with an energy of 511 keV (the rest mass energy of an electron). Antimatter is not science fiction — positrons are produced in certain radioactive decays and are used routinely in PET (positron emission tomography) medical imaging. Antiprotons and antihydrogen atoms are produced and studied at CERN.

How was antimatter predicted and discovered?

Antimatter was predicted theoretically before it was observed experimentally — one of the great triumphs of mathematical physics. In 1928, Paul Dirac formulated the Dirac equation — a relativistic quantum mechanical equation for the electron that combined quantum mechanics with Einstein's special relativity. The equation had an unexpected feature: it had solutions with negative energy, which Dirac initially interpreted as a 'sea' of filled negative-energy states (the Dirac sea). He predicted that a 'hole' in this sea would behave as a particle with the same mass as the electron but positive charge — an 'anti-electron.' In 1932, Carl Anderson at Caltech discovered exactly this particle in cosmic ray cloud chamber photographs: a particle that curved in a magnetic field like an electron but in the opposite direction, indicating positive charge. Anderson named it the positron and received the 1936 Nobel Prize in Physics. Dirac had already received the Nobel Prize in 1933 (shared with Schrödinger) for his equation. The antiproton was discovered in 1955 by Owen Chamberlain and Emilio Segrè at the Berkeley Bevatron, and the antineutron was found the following year.

Why is there more matter than antimatter in the universe?

This is one of the deepest unsolved problems in physics. The laws of physics, as we currently understand them, treat matter and antimatter almost identically. The Big Bang should have produced equal amounts of matter and antimatter, which should have annihilated each other completely, leaving a universe of pure radiation — no stars, no planets, no people. Instead, for every billion antimatter particles produced in the Big Bang, there were about a billion and one matter particles. That tiny excess — about one part in a billion — survived the great annihilation and became everything we see today. The Russian physicist Andrei Sakharov identified three conditions necessary for this imbalance (baryogenesis) in 1967: (1) baryon number violation (processes that can change the total number of baryons), (2) C and CP violation (the laws of physics must distinguish between matter and antimatter), and (3) departure from thermal equilibrium. The Standard Model of particle physics contains some CP violation (observed in the decay of kaons and B mesons), but not nearly enough to explain the observed matter-antimatter asymmetry. New physics beyond the Standard Model is almost certainly required, but its nature remains unknown.

Can we make and store antimatter?

Yes, but only in tiny quantities and at enormous cost. CERN's Antiproton Decelerator produces about 10 million antiprotons per batch by slamming high-energy protons into a metal target. The ALPHA experiment at CERN has produced and trapped individual antihydrogen atoms (an antiproton orbited by a positron) in magnetic traps, holding them for up to 16 minutes. In 2020, the ALPHA experiment measured the spectrum of antihydrogen and confirmed that it matches hydrogen's spectrum to high precision — as expected if CPT symmetry (the combined symmetry of charge conjugation, parity, and time reversal) is exact. The total amount of antimatter ever produced by all particle accelerators in history is estimated at about 10-20 nanograms — roughly enough to power a light bulb for a few seconds if annihilated. The cost of producing 1 gram of antiprotons has been estimated at about $60-100 trillion, making antimatter by far the most expensive substance ever produced. Storage is equally challenging: antimatter must be kept in perfect vacuum and away from all matter using magnetic or electric traps (Penning traps), because contact with any matter triggers immediate annihilation.

Is antimatter used in medicine?

Yes — positron emission tomography (PET) is one of the most important medical imaging techniques and relies directly on matter-antimatter annihilation. In a PET scan, the patient is injected with a tracer molecule labelled with a positron-emitting radioactive isotope — most commonly fluorine-18 attached to a glucose analogue (fluorodeoxyglucose, FDG). The fluorine-18 undergoes beta-plus decay, emitting a positron. The positron travels a short distance through tissue before encountering an electron and annihilating — for fluorine-18 the mean range is around 0.6 millimetres, with a maximum near 2.4 millimetres — producing two 511 keV gamma-ray photons travelling in almost exactly opposite directions. A ring of gamma-ray detectors surrounding the patient detects these coincident photon pairs, and tomographic reconstruction produces a three-dimensional image showing where the tracer accumulated in the body. Because cancer cells consume glucose at a higher rate than most normal tissues, FDG-PET is widely used for cancer detection, staging, and monitoring treatment response. A typical 370 MBq dose of fluorine-18 contains about 3.5 × 10¹² atoms, so a PET scan involves on the order of a few trillion individual positron-electron annihilation events — antimatter physics applied routinely to save lives.

Universe 14 min read

The Physics of Antimatter: Why the Universe Chose Matter

Every particle has an antiparticle, yet the cosmos is made of matter. Annihilation, PET scans, trapped antihydrogen and Sakharov's conditions.

World of Physics Editorial July 1, 2026 Updated October 6, 2026

antimatterpositronDirac equationpair annihilationbaryogenesisCP violationPET scanCERNantihydrogencosmology

Table of Contents

The Missing Mirror

In 1928, a 25-year-old British physicist named Paul Dirac wrote down an equation. He was trying to describe the electron — the simplest particle of matter — in a way that was consistent with both quantum mechanics and Einstein’s special relativity. The equation worked beautifully. It predicted the electron’s spin, its magnetic moment, and its behaviour in electromagnetic fields, all from first principles.

But it also predicted something else. Something nobody had asked for.

The Dirac equation had two sets of solutions. One set described electrons — particles with negative charge, positive energy, behaving exactly as observed. The other set described a particle with the same mass as the electron but with positive charge — a mirror image of the electron that had never been seen.

Dirac was initially reluctant to accept this prediction. He tried to reinterpret the negative-energy solutions as protons (which are positive, but 1,836 times heavier — it didn’t work). Eventually, he accepted what his equation was telling him: for every particle of matter, there must exist a corresponding antiparticle — identical in mass but opposite in charge.

Four years later, Carl Anderson found it.

The Positron: Matter’s Mirror

Anderson was studying cosmic rays at Caltech, using a cloud chamber — a device filled with supersaturated vapour in which charged particles leave visible trails of condensation. A magnetic field curved the trails: positive particles curved one way, negative the other. The curvature radius revealed the particle’s momentum.

In 1932, Anderson photographed a track that curved like an electron’s but in the wrong direction — indicating a positive charge. It wasn’t a proton (too light — the curvature and ionisation density were wrong for a particle 1,836 times heavier than an electron). It was a new particle: the same mass as the electron, but positively charged.

Anderson named it the positron. He received the 1936 Nobel Prize for the discovery. Dirac had already received his Nobel in 1933 for the equation that predicted it.

The positron was the first antiparticle. It would not be the last.

The Antiparticle Zoo

Every particle in the Standard Model has a corresponding antiparticle:

The electron (e⁻) has the positron (e⁺) — same mass (0.511 MeV/c²), opposite charge.

The proton (p) has the antiproton (p̄) — same mass (938.3 MeV/c²), opposite charge. Discovered in 1955 by Chamberlain and Segrè at the Berkeley Bevatron (Nobel Prize 1959).

The neutron (n) has the antineutron (n̄) — same mass, zero charge for both, but opposite baryon number and opposite magnetic moment. Discovered in 1956.

Each quark has an antiquark — up (u) has anti-up (ū), down (d) has anti-down (d̄), and so on. An antiproton is made of two anti-up quarks and one anti-down quark (ūūd̄).

Neutrinos have antineutrinos — whether they are truly distinct or whether neutrinos are their own antiparticles (Majorana fermions) is an open question, currently being investigated by neutrinoless double beta decay experiments.

Some particles are their own antiparticles: the photon, the Z boson, the gluons, and (probably) the graviton. These are all electrically neutral and carry no conserved charge that distinguishes particle from antiparticle.

In principle, you could build an entire anti-atom — antiprotons and antineutrons forming an antinucleus, orbited by positrons. Anti-hydrogen, anti-helium, anti-carbon, anti-oxygen, anti-water, anti-people. The physics would be identical. The chemistry would be identical. An anti-person looking in an anti-mirror would see nothing unusual.

Until they tried to shake hands with a person. Then both would vanish in a flash of gamma rays.

Annihilation: E = mc² Made Literal

When a particle meets its antiparticle, they can annihilate — their combined mass is converted entirely into energy, following Einstein’s most famous equation:

E = mc²

For an electron-positron annihilation at rest:

E = 2 × 0.511 MeV = 1.022 MeV

This energy appears as two gamma-ray photons, each carrying 511 keV, flying off in exactly opposite directions (to conserve momentum). The particles are gone. Their mass has become light.

For a proton-antiproton annihilation, the energy is about 1,877 MeV — roughly 2,000 times more, because protons are 1,836 times more massive than electrons. The annihilation products are typically multiple pions (which then decay into muons, electrons, neutrinos, and gamma rays).

The energy density of antimatter annihilation is extraordinary:

1 gram of antimatter + 1 gram of matter → 1.8 × 10¹⁴ joules

That’s about 43 kilotonnes of TNT — roughly three Hiroshima bombs — from two grams of material. It’s the most energy-dense reaction possible in physics: 100% mass-to-energy conversion, compared to ~0.7% for nuclear fusion and ~0.1% for nuclear fission.

This is why antimatter features prominently in science fiction as a fuel source. It’s also why antimatter weapons are not a concern in practice — the total amount of antimatter ever produced by all particle accelerators in human history is about 10–20 nanograms, enough to power a light bulb for a few seconds.

Pair Production: Creating Something From Nothing

Annihilation has a reverse process: pair production. A sufficiently energetic photon (gamma ray), passing near an atomic nucleus, can spontaneously convert into a particle-antiparticle pair:

γ → e⁻ + e⁺

The photon must have at least 1.022 MeV of energy (the rest mass energy of the electron-positron pair). The nucleus is needed to conserve momentum — a photon travelling in a straight line cannot produce two massive particles without a recoiling body to absorb the momentum imbalance.

Pair production happens continuously in the upper atmosphere (from cosmic ray interactions), in particle accelerators, near black holes and neutron stars, and even in thunderstorms (lightning can produce gamma rays energetic enough for pair production).

The process reveals something profound about the vacuum. Empty space is not truly empty — it seethes with the possibility of particle-antiparticle pairs, which can momentarily appear and disappear (virtual pairs) without violating energy conservation, as long as they exist for a time shorter than Δt ~ ℏ/ΔE (the Heisenberg uncertainty principle).

This vacuum fluctuation underlies the Casimir effect (a measurable force between closely spaced metal plates), Hawking radiation (black hole evaporation by pair production near the event horizon), and the Lamb shift (a tiny change in hydrogen’s energy levels caused by the electron interacting with virtual pairs in the vacuum).

PET Scans: Antimatter Saves Lives

The most widespread practical application of antimatter is in positron emission tomography (PET) — a medical imaging technique used to detect cancer, monitor heart disease, and study brain function.

The physics is elegantly simple:

Step 1: The patient receives an injection of a tracer molecule tagged with a positron-emitting isotope — most commonly fluorine-18 (half-life 110 minutes) attached to fluorodeoxyglucose (FDG), a glucose analogue.

Step 2: The FDG accumulates in metabolically active tissue. Cancer cells, which consume glucose at abnormally high rates, take up disproportionately more FDG.

Step 3: Fluorine-18 undergoes beta-plus decay, emitting a positron. The positron travels 1–2 mm through tissue before encountering an electron.

Step 4: The positron and electron annihilate, producing two 511 keV gamma rays travelling in exactly opposite directions (180° apart).

Step 5: A ring of scintillation detectors surrounding the patient registers the two gamma rays in coincidence — detecting both within a narrow time window (~5–10 ns). The line connecting the two detector hits passes through the annihilation site.

Step 6: Millions of such coincidence events are recorded and reconstructed tomographically into a three-dimensional image of tracer concentration.

The numbers are worth doing. A typical injected dose of 370 MBq of fluorine-18 contains about 3.5 × 10¹² atoms — the activity divided by the decay constant — so a PET scan involves on the order of a few trillion individual positron-electron annihilation events, each one producing a pair of gamma rays that map the location of metabolically active tissue. Our fuller treatment of the technique is in PET scans and positron annihilation.

The technique is exquisitely sensitive. PET can detect metabolic changes before structural changes appear on CT or MRI, making it invaluable for early cancer detection and for monitoring whether a tumour is responding to treatment.

Every PET scan is a practical demonstration of E = mc², pair annihilation, and special relativity — physics of the most fundamental kind, applied routinely in hospitals worldwide.