How antimatter went from Dirac's equation to PET scans and CERN traps

Every known charged particle has a counterpart with the same mass and the opposite electric charge. The electron has the positron. The proton has the antiproton. When a particle meets its counterpart, the pair annihilates, and their mass turns into energy. That partner material is called antimatter. It is a measured, tested part of physics, not only a science-fiction device.

So what is antimatter? It is ordinary matter's mirror image in electric charge and certain other quantum properties. An antiproton carries a negative charge instead of a positive one. A positron is positive where an electron is negative. Combine one antiproton with one positron and you get antihydrogen, the simplest anti-atom. Physicists have built it in laboratories and held it in magnetic traps.

Prediction came before detection. Paul Dirac's 1928 equation for the electron pointed to a particle no one had yet observed. Within a few years, a physicist studying cosmic rays found it. Since then, antimatter has moved from a theoretical result to a working tool and a central question in physics.

It shows up in hospitals. Positron emission tomography, better known as PET, relies on positrons released by a radioactive tracer inside the body. It shows up in nature, too. Thunderstorms and Earth's radiation belts produce antimatter without any human help. Detectors in orbit search for it in the cosmic rays that stream through space.

It also sits at the center of an unsolved problem in cosmology. Current theory suggests the early universe should have made matter and antimatter in near-equal amounts. Yet the observable universe is dominated by matter. Explaining that imbalance, known as matter-antimatter asymmetry, remains an open question. Experiments at CERN, the European physics laboratory near Geneva, test whether antimatter behaves exactly like matter. That includes how it responds to gravity.

The antimatter facts below are a selection of 10 topics, not a complete or ranked account of the field. They cover the prediction and discovery of the positron, the physics of annihilation and the medical scanners that use it. They also cover the antihydrogen experiments at CERN, natural sources of antimatter on and around Earth and particles that may be their own antiparticles.

Illustration: Quartz

Antimatter began as a mathematical result, not an observation. In 1928, British physicist Paul Dirac published an equation for the electron whose negative-energy solutions later led him to predict the electron's positively charged twin.

Dirac was working at the University of Cambridge on a mismatch between two theories. Quantum mechanics, as developed in the mid-1920s, did not fit with Albert Einstein's special theory of relativity. An electron moving close to the speed of light needed a description that respected both frameworks. Dirac's answer appeared in a paper titled "The Quantum Theory of the Electron" in the Proceedings of the Royal Society.

The equation explained more than its author first set out to explain. It accounted for the electron's spin, a property that earlier theories had to add by hand. It also gave the correct value for the electron's magnetic moment. Those results gave physicists strong reasons to take the equation seriously.

It also produced a complication. For every solution describing an ordinary electron, the mathematics allowed a matching solution with negative energy. Physicists could not discard these solutions without breaking the theory.

Dirac spent several years working out what they meant. In 1930, he proposed that empty space is a "sea" of filled negative-energy states. A gap, or hole, in that sea would behave like a positively charged particle. He first suggested the hole might be the proton, the only known positive particle at the time.

That idea ran into trouble. Hermann Weyl showed the hole would need the same mass as the electron. A proton is roughly 1,836 times heavier. J. Robert Oppenheimer also argued against the proton interpretation.

In 1931, Dirac accepted the consequence. He proposed a new particle, which he called the anti-electron. It would share the electron's mass and carry an equal but positive charge. The same paper suggested the proton should have its own counterpart, an antiproton. The anti-electron later became known as the positron.

Dirac shared the 1933 Nobel Prize in Physics with Erwin Schrödinger for their work on atomic theory. The positron had been described in detail on paper before any experiment offered evidence of it.

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Carl Anderson detected the first known antiparticle, the positron, at Caltech in 1932. He found it in cloud chamber photographs of cosmic rays. In those images, a particle with an electron's mass left a track that curved the opposite way from an electron's.

Anderson was studying cosmic rays in Robert Millikan's group at Caltech. His instrument was a cloud chamber placed inside a strong magnetic field. Charged particles passing through the chamber left trails of condensed droplets. The magnetic field bent those trails. The direction of each bend revealed the sign of the particle's charge.

Some tracks matched electrons in every respect but one. They curved in the opposite direction, as a positively charged particle would. One alternative explanation was that these were ordinary electrons traveling backward through the chamber.

Anderson ruled that out with a lead plate set across the chamber. A particle crossing the plate loses energy. Its path then curves more tightly on the far side. That change showed which way the particle had moved. Combined with the direction of the bend, it confirmed a positive charge.

The track also ran too far to belong to a proton with that curvature. That eliminated the other familiar positive particle. Anderson published the finding in 1933 under the title "The Positive Electron," and the particle became known as the positron.

Anderson shared the 1936 Nobel Prize in Physics with Victor Hess. Hess was honored for discovering cosmic rays, the radiation in which Anderson had found the positron. Anderson was cited for the positron itself.

The positron showed that antiparticles existed in nature. Heavier antiparticles took more than two decades to confirm. Creating them required particle accelerators with far more energy than earlier machines could deliver.

The antiproton was found in 1955 at the Bevatron in Berkeley, California. The team was Emilio Segrè, Owen Chamberlain, Clyde Wiegand and Thomas Ypsilantis. Segrè and Chamberlain won the 1959 Nobel Prize in Physics for the discovery.

The antineutron followed in 1956, also at the Bevatron. Unlike the positron and the antiproton, it carries no electric charge. Its discovery showed that antimatter counterparts exist for neutral particles as well as charged ones.

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Matter-antimatter annihilation converts the full mass of a particle and its antiparticle into energy. The amount follows Albert Einstein's equation E=mc², which links mass to energy through the speed of light squared.

The simplest case involves an electron and a positron. If both are at rest, they usually vanish and leave behind two gamma-ray photons. Each photon carries 511 keV, which matches the rest-mass energy of an electron. The two photons fly off in opposite directions, as conservation of momentum requires.

An electron and a positron can also briefly orbit each other before they annihilate. This short-lived bound state is called positronium. The form with opposite spins typically decays into two photons. The form with aligned spins decays into three photons and lasts longer.

Heavier particles behave differently. A proton and an antiproton usually annihilate into a spray of pions rather than a clean pair of photons. Those pions then decay into other particles, including neutrinos. The neutrinos escape with part of the energy. That makes the full output hard to capture.

The clearest contrast is with nuclear reactions. Annihilation converts 100 percent of the mass involved. Nuclear fission and fusion convert only a small fraction of their fuel's mass, well under one percent. Splitting uranium or fusing hydrogen releases energy from changes in how tightly nuclei are bound. The particles themselves are not destroyed.

A simple calculation shows the scale. One gram of antimatter meeting one gram of matter would release about 1.8 × 10^14 joules. Both grams count toward the total, because annihilation consumes the matter as well as the antimatter. That equals roughly 43 kilotons of TNT. It is also about 50 gigawatt-hours of energy.

That scale is why antimatter appears in proposals for spacecraft propulsion. Those proposals remain theoretical. Antimatter cannot sit in an ordinary container. It would annihilate on contact with the walls. Laboratories hold small amounts in magnetic and electric traps inside a vacuum, which keeps the particles away from matter.

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Positron emission tomography, or PET, puts antimatter to work in hospitals. The scan maps where a positron-emitting tracer collects inside the body by catching the gamma rays released when those positrons meet electrons.

The most widely used tracer is fluorodeoxyglucose, usually shortened to FDG. It is a glucose analogue tagged with fluorine-18, a radioactive isotope of fluorine. Fluorine-18 decays by emitting positrons. Its half-life is about 110 minutes. That short lifespan means the isotope is produced in a cyclotron and must reach the patient within hours.

A scan begins with an injection of the tracer into a vein. The patient then rests while it circulates. Cells take up FDG much as they take up ordinary glucose. Tissues with high energy demands absorb more of it. The tracer then emits positrons from wherever it has gathered.

Each positron travels only a few millimeters at most through tissue before it meets an electron. The two annihilate, typically producing a pair of 511 keV gamma-ray photons. The photons fly away from each other in nearly opposite directions.

That geometry is what makes the image possible. The patient lies inside a ring of detectors. When two detectors on opposite sides register photons at almost the same instant, the scanner records a line between them. The annihilation happened somewhere along that line.

A single scan collects millions of these paired detections. Reconstruction software combines the lines to calculate where the tracer is concentrated. The result is a three-dimensional map of metabolic activity rather than a picture of anatomy alone.

Oncology is a major use. Many tumors consume glucose faster than the tissue around them. On a scan, they show up as areas of concentrated tracer. Doctors use PET to help detect cancer, judge how far it has spread and check how a tumor responds to treatment.

The technique also appears in neurology and cardiology. In the brain, it can show patterns of metabolism. In the heart, it can assess blood flow to the muscle. Many modern scanners pair PET with CT, laying the metabolic map over a detailed anatomical image.

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Antihydrogen, made of one antiproton bound to one positron, was first produced at CERN in 1995 by a team led by Walter Oelert. Later experiments at the same laboratory learned to cool these anti-atoms, trap them and measure their light.

Hydrogen is the simplest atom in nature. It has one proton and one electron. Its antimatter counterpart swaps each part for its antiparticle. That simplicity makes antihydrogen the natural candidate for a direct comparison with ordinary matter.

The 1995 atoms moved at high speed and lasted only an instant. They annihilated almost as soon as they formed. Physicists had nothing to hold or examine. The result proved that antihydrogen could be made, but it did not allow detailed study.

The next step came in 2002. The ATHENA and ATRAP collaborations, both working at CERN, produced cold antihydrogen. Slow atoms are far easier to handle than fast ones. Cold production opened the way to confining the atoms.

Confinement is difficult because antihydrogen cannot touch any container. Contact with ordinary matter destroys it. The atom also carries no net electric charge. Electric fields therefore cannot grip it the way they hold bare antiprotons. ALPHA used a magnetic trap that acts on the atom's small magnetic moment instead.

ALPHA reported trapping antihydrogen atoms in 2010. In 2011, the collaboration held them for about 1,000 seconds, or more than 16 minutes.

That holding time made precise measurement possible. In 2016, ALPHA measured the 1S-2S transition of antihydrogen. This is the energy jump between the atom's ground state and an excited state. The result matched ordinary hydrogen within the experiment's precision.

The choice of this transition was deliberate. Hydrogen's 1S-2S frequency is among the most precisely measured quantities in physics. Current theory predicts that hydrogen and antihydrogen share identical energy levels. Any measured difference would point to a gap in that theory.

All of this work depends on a steady supply of slow antiprotons. CERN's Antiproton Decelerator provides them to the experiments. The laboratory later added the ELENA ring to slow the antiprotons further before they reach the traps.

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Antimatter falls down, not up. In September 2023, the ALPHA collaboration at CERN reported in the journal Nature that antihydrogen atoms released from its vertical ALPHA-g apparatus moved downward under gravity rather than rising.

The question had stayed open for decades because gravity is hard to measure on antimatter. Gravity is by far the weakest of the fundamental forces. On a single charged particle such as a positron or an antiproton, stray electric and magnetic fields overwhelm the gravitational pull.

Antihydrogen solves part of that problem. Each atom pairs one antiproton with one positron. The two opposite charges cancel, leaving the atom electrically neutral. That neutrality lets gravity's small effect show up instead of being swamped by electromagnetic forces.

The atoms are still difficult to handle. They cannot touch the walls of any container. Contact with ordinary matter would annihilate them. ALPHA holds them in a magnetic trap that acts on the atoms' magnetic properties rather than on electric charge.

ALPHA-g stands that kind of trap upright, with its axis running vertically. The team gradually weakened the magnetic fields holding the atoms and let them escape. Detectors recorded where each escaping atom annihilated. That showed whether it had left through the top or the bottom of the trap.

More atoms left through the bottom. That pattern matched what a downward gravitational pull would produce. At the precision of this measurement, it ruled out the idea that Earth repels antimatter — a hypothesis often called antigravity.

The finding has limits. It does not establish that antimatter and matter respond to gravity in exactly the same way. Albert Einstein's general relativity predicts identical behavior through a rule called the weak equivalence principle. Testing that prediction closely would require a more sensitive measurement.

That is why the result counts as a first step rather than a final answer. Any small difference between how matter and antimatter fall would point to physics beyond current theory. For now, the measured result is that antihydrogen falls toward the ground.

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Physics has no confirmed explanation for why the universe is made almost entirely of matter. Models of the Big Bang call for matter and antimatter to have formed in close to equal amounts, yet antimatter is scarce in the cosmos today.

This gap is known as the matter-antimatter asymmetry, or the baryon asymmetry problem. If the two had formed in exactly equal amounts, they would have destroyed each other. The result would have been a universe filled with radiation, with no atoms left to build stars, planets or people.

Some small imbalance must have tipped the outcome toward matter. The galaxies, gas and dust seen today are what remained after the rest annihilated.

Astronomers have not found large regions of the cosmos made of antimatter. Wherever matter and antimatter domains met, annihilation would produce gamma rays that telescopes could detect.

In 1967, Soviet physicist Andrei Sakharov outlined the conditions any process would need to create this imbalance. There are three. First, some interaction must violate baryon number, the count of particles such as protons and neutrons minus their antiparticles. Second, the laws of physics must violate two symmetries, known as C and CP. Third, the process must happen out of thermal equilibrium. Otherwise, reverse reactions would wipe out the imbalance as it forms.

CP symmetry combines charge conjugation, which swaps a particle for its antiparticle, with parity, which mirrors space. If CP held exactly, matter and antimatter would behave as perfect mirror images. Nature would have no way to favor one over the other.

Evidence that CP symmetry breaks arrived three years before Sakharov's paper. In 1964, James Cronin and Val Fitch observed CP violation in the decay of neutral kaons, particles that contain a strange quark or strange antiquark. Their experiment ran at Brookhaven National Laboratory in New York. The two physicists shared the 1980 Nobel Prize in Physics for the discovery.

Later experiments found CP violation in other particles, including B mesons. The Standard Model of particle physics describes these effects. Yet the CP violation it contains is far too small to account for the amount of matter in the universe.

That shortfall points to physics beyond the Standard Model. Proposed sources include undiscovered particles and extra CP violation among neutrinos. None has been confirmed. Where the missing antimatter went remains one of the largest open questions in cosmology.

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Antimatter is not made only in particle accelerators. Thunderstorms produce positrons, and Earth's inner radiation belt holds a band of trapped antiprotons. Space-based instruments reported both findings in 2011.

The thunderstorm link comes from terrestrial gamma-ray flashes. These are brief bursts of gamma rays generated inside storm clouds. They usually last less than a millisecond. NASA's Compton Gamma Ray Observatory first recorded them in 1994.

In 2011, NASA reported that its Fermi Gamma-ray Space Telescope had detected positrons tied to these flashes. The telescope's Gamma-ray Burst Monitor picked up gamma rays at 511 keV. That energy is the signature of a positron and an electron annihilating.

The process starts with the strong electric fields inside a storm. Those fields accelerate electrons to very high speeds. When the fast electrons strike air molecules, they give off gamma rays. Some of those gamma rays then convert into electron-positron pairs.

The charged particles can escape the atmosphere and travel along Earth's magnetic field lines into space. That path means a spacecraft can detect positrons from a storm that is not directly beneath it.

The second finding involves the Van Allen belts. These are zones of charged particles held in place by Earth's magnetic field. They were discovered in 1958 from data returned by Explorer 1, the first U.S. satellite.

PAMELA, short for Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics, launched in 2006 aboard the Russian Resurs-DK1 satellite. In 2011, its team reported a belt of antiprotons trapped in Earth's inner magnetosphere. The measurements came from the South Atlantic Anomaly. In that region, the inner belt dips closest to Earth's surface.

These antiprotons trace back to cosmic rays. High-energy particles from space collide with atoms in the upper atmosphere. The collisions create showers of secondary particles, and some of them are antiparticles. Earth's magnetic field catches a portion of these antiparticles and confines them in the belt.

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The Alpha Magnetic Spectrometer, known as AMS-02, is a particle physics detector mounted on the International Space Station. It sorts cosmic-ray particles by charge and energy. One of its main results is a positron fraction that climbs as energy rises.

Space shuttle Endeavour carried the instrument to orbit on mission STS-134. Astronauts installed it on the station in May 2011. That flight was Endeavour's last. The experiment is led by Samuel Ting, the MIT physicist who shared the 1976 Nobel Prize in Physics.

The detector works by bending particles. A magnet curves the path of any charged particle passing through it. Matter and antimatter particles of the same mass bend in opposite directions. The direction of the curve reveals the sign of the charge. A silicon tracker records each path in fine detail.

Several other detectors surround the magnet. Time-of-flight counters measure speed and direction of travel. A transition radiation detector and an electromagnetic calorimeter help tell light positrons apart from much heavier protons. A ring imaging Cherenkov detector adds a separate measurement of velocity. Taken together, the readings let researchers identify each particle type with high confidence.

The positron measurement is the experiment's best-known finding. Cosmic rays striking interstellar gas produce some positrons as a byproduct. In that simple picture, the share of positrons among electrons and positrons should fall at higher energies. AMS-02 found the opposite. Above roughly 10 billion electron volts, the positron fraction rises. The earlier PAMELA satellite had reported a similar trend in 2009.

The source of the extra positrons is still debated. Pulsars, the rapidly spinning remnants of collapsed stars, can produce and accelerate electron-positron pairs. Some dark matter models predict positrons from particle collisions or decays. Neither explanation has been confirmed. Separating them requires more data at the highest energies, where particles arrive rarely.

Keeping the instrument running has required work in orbit. Its cooling system began failing years after installation. Between November 2019 and January 2020, astronauts Luca Parmitano and Andrew Morgan completed four spacewalks to install new pumps. Those repairs extended the detector's working life.