Antimatter and Positrons: The Mirror Image of Ordinary Matter

Every atom you have ever touched is built from protons, neutrons and electrons. But each of those particles has a twin with the same mass and the opposite electric charge. The electron’s twin is the positron; the proton’s is the antiproton. Put these antiparticles together and you get antimatter, which behaves in almost every way like ordinary matter except for one dramatic habit: when it meets its matching particle, both vanish in a burst of energy.

A prediction from an equation

In 1928, the British physicist Paul Dirac wrote an equation combining quantum mechanics with Einstein’s special relativity to describe the electron. It worked beautifully: it naturally explained the electron’s spin and matched the hydrogen spectrum. But it had an awkward feature. It allowed solutions that seemed to describe electrons with negative energy.

After some false starts, Dirac concluded in 1931 that these solutions pointed to a new particle: something with the electron’s mass but a positive charge. He had predicted antimatter from mathematics alone.

The positron is found

The confirmation came quickly. In 1932, Carl Anderson at Caltech was photographing cosmic-ray tracks in a cloud chamber, a box of supersaturated vapour in which charged particles leave trails of droplets. A magnetic field bent the tracks: negative particles one way, positive the other. One track curved like a positive particle but was far too light to be a proton. It had to be a positively charged electron.

Anderson called it the positron, and he shared the 1936 Nobel Prize in Physics for the discovery. The heavier antiproton was produced in 1955 at the Bevatron accelerator in Berkeley by Emilio Segrè and Owen Chamberlain, and the antineutron followed a year later.

What makes something “anti”

An antiparticle has:

• the same mass as its partner;
• the opposite electric charge (for charged particles);
• opposite values of certain other quantum properties, such as the numbers physicists use to count leptons and baryons.

Particle	Charge	Antiparticle	Charge
Electron (e⁻)	−1	Positron (e⁺)	+1
Proton (p)	+1	Antiproton (p̄)	−1
Neutron (n)	0	Antineutron (n̄)	0

The antineutron is neutral like the neutron, but it is made of antiquarks rather than quarks, so it is still a distinct particle. (For what quarks are, see quarks and atoms.)

Annihilation: when matter meets antimatter

When a particle meets its antiparticle, they can annihilate: both disappear, and their combined mass becomes energy according to E = mc².

For an electron and a positron that meet at low speed, the usual result is two gamma-ray photons flying off in opposite directions, each carrying 511 keV, which is exactly the rest-mass energy of one electron. The photons go in opposite directions so that momentum is conserved.

The energy scale is enormous. Annihilating 1 gram of antimatter with 1 gram of matter converts 2 grams of mass completely into energy:

E = mc² = (0.002 kg) × (3.0 × 10⁸ m s⁻¹)² = 1.8 × 10¹⁴ J

That is roughly the energy released by 43,000 tonnes of TNT. Compare that with chemical reactions, which convert only a tiny fraction of a billionth of their mass into energy. Even nuclear fission converts less than 0.1%.

Positronium: a short-lived “atom”

A positron and an electron can briefly orbit each other before annihilating, forming an exotic atom-like system called positronium. It resembles hydrogen, except that the two particles have equal mass and circle a shared centre. It lives only a fraction of a microsecond: about 0.12 nanoseconds when the spins are paired, and about 140 nanoseconds when they are parallel. Physicists study positronium to test quantum electrodynamics with extreme precision.

Making antihydrogen

Could positrons and antiprotons make whole anti-atoms? Yes. The first antihydrogen atoms were made at CERN in 1995, just nine of them, moving at high speed. In 2002, the ATHENA and ATRAP experiments produced thousands of slow-moving antihydrogen atoms. In 2010, the ALPHA experiment managed to trap 38 anti-atoms in a magnetic bottle, and later held some for over 15 minutes.

Trapped antihydrogen lets scientists ask sharp questions:

• Does it have the same spectrum as hydrogen? Laser measurements of its energy levels agree with ordinary hydrogen to many decimal places. This is a direct test of a basic symmetry of physics.
• Does it fall down? In 2023, the ALPHA-g experiment showed that antihydrogen falls under gravity, just as ordinary matter does, ruling out the science-fiction idea of “anti-gravity” antimatter.

Despite the headlines, the total amount of antimatter ever made in laboratories is minuscule, far less than a millionth of a gram. It is also extraordinarily hard to store, because it must never touch the walls of its container.

Antimatter in everyday life: PET scans

Positrons are not just an exotic lab curiosity. They come from a common type of radioactive decay called beta-plus decay, in which a proton in an unstable nucleus turns into a neutron and emits a positron (plus a neutrino). You can read about the more familiar kinds of decay in alpha, beta and gamma radiation.

Hospitals exploit this in positron emission tomography (PET):

1. The patient receives a small dose of a tracer, most often a glucose-like molecule tagged with fluorine-18, a positron emitter with a half-life of about 110 minutes.
2. The tracer collects in tissues that use a lot of glucose, such as many tumours, the brain and the heart.
3. Each positron travels about a millimetre, meets an electron and annihilates, producing two 511 keV photons flying in opposite directions.
4. A ring of detectors catches the pairs. Each pair defines a line through the body, and a computer reconstructs a 3D image of where the tracer went.

So antimatter annihilation happens safely inside hospital patients every day. Tiny numbers of positrons are even produced naturally in your body. Potassium-40, present in all living things, very occasionally decays by emitting a positron.

The great puzzle: where did all the antimatter go?

Physics treats matter and antimatter almost identically. The Big Bang should therefore have made them in equal amounts, and they should have annihilated each other, leaving a universe full of light and almost nothing else. Instead, we live in a universe made overwhelmingly of matter. There are no known antimatter galaxies; if there were, we would see intense gamma rays where their gas met ordinary gas.

Physicists call this the matter–antimatter asymmetry. Some small difference in how matter and antimatter behave must have left a slight excess of matter, about one extra particle for every billion or so pairs, and everything in the universe today, including us, is made from that leftover. Experiments have found some tiny asymmetries (called CP violation) in the decays of certain particles, but not nearly enough to explain the imbalance. It remains one of the biggest open questions in science.

Common misconceptions

• “Antimatter has negative mass.” It has ordinary, positive mass, and experiments show it falls down under gravity.
• “Antimatter is science fiction.” Positrons are produced in hospitals every day and in natural radioactive decay.
• “We could use it as fuel soon.” Making antimatter takes vastly more energy than it releases, and storing more than a few atoms is extremely difficult.
• “Annihilation is an explosion of matter.” It is a conversion of mass into photons (and sometimes other particles), exactly balanced by E = mc².

Key takeaways

• Every particle has an antiparticle with the same mass and opposite charge; the positron is the antielectron.
• Dirac predicted the positron in 1931, and Anderson discovered it in cosmic rays in 1932.
• When matter and antimatter meet, they annihilate. An electron–positron pair usually becomes two 511 keV gamma photons.
• Antihydrogen has been made and trapped at CERN; its spectrum matches hydrogen’s, and it falls under gravity.
• PET scans use positrons from fluorine-18 decay to image the body.
• Why the universe contains so much more matter than antimatter is still an open question. For more on the particles inside ordinary atoms, start with what is an atom?