The story of antimatter
  1. 30/06/1905

    Albert Einstein publishes his theory of Special Relativity

    On 30 June 1905 the German physics journal…

    Know more
  2. 07/04/1912

    Victor Hess discovers cosmic rays

    In 1911 and 1912 Austrian physicist Victor Hess…

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  3. 27/01/1926

    Erwin Schrödinger and Werner Heisenberg devise a quantum theory

    In the 1920s, physicists were trying to apply…

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  4. 02/01/1928

    Dirac's equation predicts antiparticles

    In 1928, British physicist Paul Dirac wrote down…

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  5. 09/09/1932

    Carl Anderson discovers the positron

    In 1932 Carl Anderson, a young professor at the…

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  6. 02/02/1934

    Ernest Lawrence patents the cyclotron

    In 1929 Ernest Lawrence – then associate…

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  7. 01/04/1954

    The Bevatron starts up at Berkeley, California

    The Bevatron in 1958 (Image: Lawrence Berkeley…

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  8. 01/11/1955

    The Bevatron discovers the antiproton

    A paper titled "Observation of antiprotons," by…

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  9. 03/10/1956

    The Bevatron discovers the antineutron

    The journal Physical Review receives the paper…

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  10. 27/07/1964

    Cronin and Fitch detect a difference between matter and antimatter

    In 1964, James Cronin and Val Fitch at Brookhaven…

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  11. 01/09/1965

    First observations of antinuclei

    By 1965, all three particles that make up atoms (…

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  12. 18/08/1978

    First storage of antiprotons

    CERN issues a press release announcing the first…

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  13. 04/04/1981

    First proton-antiproton collisions

    The Intersecting Storage Rings produced the world…

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  14. 15/09/1995

    First antiatoms produced: antihydrogen, at CERN

    A team led by Walter Oelert created atoms of…

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  15. 07/02/1997

    Antiproton Decelerator approved

    In 1996 CERN's antiproton machines – the…

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  16. 18/09/2002

    ATHENA and ATRAP create "cold" antimatter

    Two CERN experiments, ATHENA and ATRAP, created…

    Know more
  17. 05/06/2011

    ALPHA traps antimatter atoms for 1000 seconds

    The ALPHA experiment at CERN reported today that…

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  18. 28/07/2011

    ASACUSA weighs antimatter to one part in a billion

    In a paper published today in the journal Nature…

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Albert Einstein publishes his theory of Special Relativity

On 30 June 1905 the German physics journal Annalen der Physik published a paper by a young patent clerk called Albert Einstein. The paper, Zur Elektrodynamik bewegter Körper, (On the Electrodynamics of Moving Bodies) set out Einstein's theory of Special Relativity, which explains the relationship between space and time – and between energy and mass – in the famous equation E=mc2.  The paper used Planck's concept of energy quanta to describe how light travels through space.

Victor Hess discovers cosmic rays

In 1911 and 1912 Austrian physicist Victor Hess made a series of ascents in a balloon to take measurements of radiation in the atmosphere. He was looking for the source of an ionizing radiation that registered on an electroscope – the prevailing theory was that the radiation came from the rocks of the Earth. To test the theory, in 1909 German scientist Theodor Wulf measured the rate of ionization near the top of the Eiffel tower (at a height of about 300 metres) using a portable electroscope. Though he expected the ionization rate to decrease with height, Wulf noted that the ionization rate at the top was just under half that at ground level – a much less significant decrease than anticipated. Victor Hess's balloon flights took such measurements further. In 1911 his balloon reached an altitude of around 1100 metres, but Hess found "no essential change" in the amount of radiation compared with ground level. Then, on 7 April 1912, Hess made an ascent to 5300 metres during a near-total eclipse of the Sun. Since ionization of the atmosphere did not decrease during the eclipse, he reasoned that the source of the radiation could not be the Sun – it had to be coming from further out in space. High in the atmosphere, Hess had discovered a natural source of high-energy particles: cosmic rays. Hess shared the 1936 Nobel prize in physics for his discovery, and cosmic rays have proved useful in physics experiments – including several at CERN – since. Find out more: About cosmic rays (from the CERN courier) - A discovery of cosmic proportions - Domenico Pacini and the origin of cosmic rays - LHCf: bringing cosmic collisions down to Earth Cosmic rays at CERN - The Large Hadron Collider forward experiment - The CLOUD experiment

Erwin Schrödinger and Werner Heisenberg devise a quantum theory

In the 1920s, physicists were trying to apply Planck's concept of energy quanta to the atom and its constituents. By the end of the decade Erwin Schrödinger and Werner Heisenberg had invented the new quantum theory of physics. The Physical Institute of the University of Zürich published Schrödinger's lectures on Wave Mechanics (the first from 27 January 1926) and in 1930 Heisenberg's book The physical principles of the quantum theory appeared. The problem now was that quantum theory was not relativistic; the quantum description worked for particles moving slowly, but not for those at high or "relativistic" velocities, close to the speed of light.

Dirac's equation predicts antiparticles

In 1928, British physicist Paul Dirac wrote down an equation that combined quantum theory and special relativity to describe the behaviour of an electron moving at a relativistic speed. The equation would allow whole atoms to be treated in a manner consistent with Einstein's relativity theory. Dirac's equation appeared in his paper The quantum theory of the electron, received by the journal Proceedings of the Royal Society A on 2 January 1928. It won Dirac the Nobel prize in physics in 1933. But the equation posed a problem: just as the equation x2=4 can have two possible solutions (x=2 or x=-2), so Dirac's equation could have two solutions, one for an electron with positive energy, and one for an electron with negative energy. But classical physics (and common sense) dictated that the energy of a particle must always be a positive number. Dirac interpreted the equation to mean that for every particle there exists a corresponding antiparticle, exactly matching the particle but with opposite charge. For the electron there should be an "antielectron" identical in every way but with a positive electric charge. In his 1933 Nobel lecture, Dirac explained how he arrived at this conclusion and speculated on the existence of a completely new universe made out of antimatter: If we accept the view of complete symmetry between positive and negative electric charge so far as concerns the fundamental laws of Nature, we must regard it rather as an accident that the Earth (and presumably the whole solar system), contains a preponderance of negative electrons and positive protons. It is quite possible that for some of the stars it is the other way about, these stars being built up mainly of positrons and negative protons. In fact, there may be half the stars of each kind. The two kinds of stars would both show exactly the same spectra, and there would be no way of distinguishing them by present astronomical methods.

Carl Anderson discovers the positron

In 1932 Carl Anderson, a young professor at the California Institute of Technology in the US, was studying showers of cosmic particles in a cloud chamber and saw a track left by "something positively charged, and with the same mass as an electron". After nearly a year of effort and observation, he decided the tracks were actually antielectrons, each produced alongside an electron from the impact of cosmic rays in the cloud chamber. He called the antielectron a "positron", for its positive charge and published his results in the journal Science, in a paper entitled The apparent existence of easily deflectable positives (1932). The discovery was confirmed soon after by Occhialini and Blacket, who in 1934 published Some photographs of the tracks of penetrating radiation in the journal Proceedings of the Royal Society A. Anderson's observations proved the existence of the antiparticles predicted by Dirac. For discovering the positron, Anderson shared the 1936 Nobel prize in physics with Victor Hess. For years to come, cosmic rays remained the only source of high-energy particles. The next antiparticle physicists were looking for was the antiproton. Much heavier than the positron, the antiproton is the antipartner of the proton. It would not be confirmed experimentally for another 22 years.

Ernest Lawrence patents the cyclotron

In 1929 Ernest Lawrence – then associate professor of physics at the University of California, Berkeley, in the US – invented the cyclotron, a device for accelerating nuclear particles to high velocities without the use of high voltages. Lawrence was granted US patent 1948384 for the cyclotron on 2 February 1934. The machine was used in the following years to bombard atoms of various elements with swiftly moving particles. Such high-energy particles could disintegrate atoms, in some cases forming completely new elements. Hundreds of artificial radioactive elements were formed in this manner. Eventually, the cyclotron was able to accelerate particles such as protons to the energy of a few tens of megaelectronvolts (symbol: MeV. One MeV equals one million electronvolts). Initially driven by the effort to discover the antiproton, the accelerator era had begun, and with it the science of high-energy physics was born. In 1939 Lawrence won the Nobel prize in physics, "for the invention and development of the cyclotron and for results obtained with it, especially with regard to artificial radioactive elements".

The Bevatron starts up at Berkeley, California

The Bevatron in 1958 (Image: Lawrence Berkeley National Laboratory) In 1954, Ernest Lawrence oversaw the building of a proton accelerator called the Bevatron at the radiation laboratory in Berkeley, California. The machine's name comes from BeV, the symbol used at the time for "billion electronvolt", or 109 electronvolts. We now call this unit the gigaelectronvolt, symbol GeV – BeV is no longer used. The Bevatron was designed to collide protons at 6.2 GeV, the expected optimum energy for creating antiprotons. The following is from Experiences with the Bevatron by then Berkeley physicist Edward Lofgren, who was present for the start-up of the machine: Finally, on April 1, 1954, a feeble pulse was obtained at a magnetic field corresponding to 6 BeV. The intensity was measured by counting the tracks in nuclear emulsion that had been inserted into the beam. The intensity was in the range of 104 to 106 protons per pulse. A team of physicists headed by Italian-American physicist Emilio Segrè designed and built a detector specialized to look for antiprotons. The Bevatron was up and running.

The Bevatron discovers the antiproton

A paper titled "Observation of antiprotons," by Owen Chamberlain, Emilio Segrè, Clyde Wiegand, and Thomas Ypsilantis, members of what was then the Radiation Laboratory of the University of California at Berkeley in the US, appeared in the 1 November 1955 issue of Physical Review Letters. It announced the discovery of a new subatomic particle, identical in every way to the proton – except its electrical charge was negative instead of positive. A month before the paper appeared, The New York Times had put the news on the front page: New Atom Particle Found; Termed a Negative Proton With the discovery of the antiproton, Segrè and colleagues had further proof of the essential symmetry of nature, between matter and antimatter. Segrè and Chamberlain were awarded the Nobel prize in physics in 1959 "for their discovery of the antiproton".

The Bevatron discovers the antineutron

The journal Physical Review receives the paper Antineutrons Produced from Antiprotons in Charge-Exchange Collisions by a second team working at the Bevatron – Bruce Cork, Glen Lambertson, Oreste Piccione and William Wenzel. The paper – which announces the discovery of the antineutron – is published in the issue dated November 1956.

Cronin and Fitch detect a difference between matter and antimatter

In 1964, James Cronin and Val Fitch at Brookhaven National Laboratory in the US did an experiment with particles called neutral K-mesons, or "kaons". The types of kaons they chose to study can be regarded to consist of one half ordinary matter and the other half antimatter. They started with two types of kaon that had seemingly identical masses but different lifetimes. Kaons of the long-lived type exist for 5.2 × 10-8 seconds before each decays into 3 pions. Kaons of the short-lived type exist for only 0.89 × 10-10 seconds before each decays into 2 pions. Cronin and Fitch shot the two types of kaon down a 17-metre beamline and detected the resulting pion-decays at the other end. Given the different lifetimes of the kaon types and the length of the beamline, you would expect only to see decays from the long-lived kaon type at the detector. Cronin and Fitch expected the short-lived kaon type to decay long before it reached the end of the beamline, and so its decay products would not be detected. In other words you would expect to detect only 3-pion decays and no 2-pion decays at all. But in their experiment, Cronin and Fitch did detect 2-pion decays: 45 of them, out of a total of 22,700 decay events – a ratio of about 1 in 500. The result violated a fundamental principle of physics – the symmetry between matter and antimatter. The pair announced their result in the paper "Evidence for the 2-pion Decay of the K Meson", published in the journal Physical Review Letters on 27 July 1964. They shared the 1980 Nobel prize in physics "for the discovery of violations of fundamental symmetry principles in the decay of neutral K-mesons."

First observations of antinuclei

By 1965, all three particles that make up atoms (electrons, protons and neutrons) were known to each have an antiparticle. So if particles, bound together in atoms, are the basic units of matter, it is natural to think that antiparticles, bound together in antiatoms, are the basic units of antimatter. But are matter and antimatter exactly equal and opposite, or symmetric, as Dirac had implied? The next important step was to test this symmetry. Physicists wanted to know how subatomic antiparticles behave when they come together. Would an antiproton and an antineutron stick together to form an antinucleus, just as protons and neutrons stick together to form the nucleus of an atom? The answer to the antinuclei question was found in 1965 with the observation of the antideuteron, a nucleus of antimatter made out of an antiproton plus an antineutron (while a deuteron – the nucleus of the deuterium atom – is made of a proton plus a neutron). The goal was simultaneously achieved by two teams of physicists, one led by Antonino Zichichi using the Proton Synchrotron at CERN, and the other led by Leon Lederman, using the Alternating Gradient Synchrotron (AGS) accelerator at the Brookhaven National Laboratory, New York. The CERN paper, Experimental Observation of Antideuteron Production was published in the Italian particle-physics journal Il nuovo cimento on 1 September 1965 (the journal ended when it was merged into the European Physical Journal in 1999.

First storage of antiprotons

CERN issues a press release announcing the first storage of antiprotons. It reads:  Antimatter, in the form of antiprotons, has been stored for the first time in history.  This scientific first occurred at CERN, the European Organisation for Nuclear Research, at the end of July during tests conducted in view of using the SPS European accelerator as a colliding device between protons and antiprotons.  Several hundred antiprotons of 2.1 GeV/c were produced by protons from the PS accelerator and were kept circulating in a machine called ICE (Intial Cooling Experiment) for a period of 85 hours i.e. about 300, 000 seconds (3 × 105). The previous best experimental measurement of antiproton lifetime, acquired during bubble chamber experiments, was about 10-4 second, i.e. a ten-thousandth of a second. 

First proton-antiproton collisions

The Intersecting Storage Rings produced the world’s first proton-antiproton collisions on 4 April 1981, paving the way for proton-antiproton collisions in the Super Proton Synchrotron (SPS), and the Nobel prize for Simon van der Meer and Carlo Rubbia. The ISR proved to be an excellent instrument for particle physics. By the time the machine closed down in 1984, it had produced many important results, including indications that protons contain smaller constituents, ultimately identified as quarks and gluons.

First antiatoms produced: antihydrogen, at CERN

A team led by Walter Oelert created atoms of antihydrogen for the first time at CERN’s Low Energy Antiproton Ring (LEAR) facility. Nine of these atoms were produced in collisions between antiprotons and xenon atoms over a period of 3 weeks. Each one remained in existence for about 40 billionths of a second, travelled at nearly the speed of light over a path of 10 metres and then annihilated with ordinary matter. The annihilation produced the signal that showed that the anti-atoms had been created. This was the first time that antimatter particles had been brought together to make complete atoms, and the first step in a programme to make detailed measurements of antihydrogen. The hydrogen atom is the simplest atom of all, made of a single proton orbited by an electron. Some three quarters of all the ordinary matter in the universe is hydrogen, and the hydrogen atom is one of the best understood systems in physics. Comparison with antihydrogen offers a route to understanding the matter–antimatter asymmetry in the universe.

Antiproton Decelerator approved

In 1996 CERN's antiproton machines – the Antiproton Accumulator (AC), the Antiproton Collector and the Low Energy Antiproton Ring (LEAR) – were closed down to free resources for the Large Hadron Collider. But a community of antimatter scientists wanted to continue their LEAR experiments with slow antiprotons. Council asked the Proton Synchrotron division to investigate a low-cost way to provide the necessary low-energy beams. The resulting design report for the Antiproton Decelerator concluded: The use of the Antiproton Collector as an antiproton decelerator holds the promise of delivering dense beams of 107 protons per minutes and low energy (100 MeV/c) with bunch lengths down to 200 nanoseconds. The Antiproton Declerator project was approved on 7 February 1997.

ATHENA and ATRAP create "cold" antimatter

Two CERN experiments, ATHENA and ATRAP, created thousands of atoms of antimatter in a “cold” state in 2002. Cold means that the atoms are slow moving, which makes it possible to study them before they meet ordinary matter and annihilate. Antihydrogen formed in the experiments when cold positrons and antiprotons were brought together and held in a specially designed “trap”. Once formed, the electrically neutral antihydrogen atoms drifted out of the trap and annihilated. Find out more

ALPHA traps antimatter atoms for 1000 seconds

The ALPHA experiment at CERN reported today that it succeeded in trapping antimatter atoms for over 16 minutes: long enough to begin to study their properties in detail. ALPHA is part of a broad programme at CERN’s antiproton decelerator investigating the mysteries of one of nature’s most elusive substances. ALPAH studied 300 trapped antiatoms. Trapping antiatoms will allow antihydrogen to be mapped precisely using laser or microwave spectroscopy so that it can be compared to the hydrogen atom, which is among the best-known systems in physics. Any difference between matter and antimatter should become apparent under careful scrutiny.

ASACUSA weighs antimatter to one part in a billion

In a paper published today in the journal Nature, the Japanese-European ASACUSA experiment at CERN reported a new measurement of the antiproton’s mass accurate to about one part in a billion. Precision measurements of the antiproton mass provide an important way to investigate nature’s apparent preference for matter over antimatter. To make these measurements antiprotons are first trapped inside helium atoms, where they can be ‘tickled’ with a laser beam. The laser frequency is then tuned until it causes the antiprotons to make a quantum jump within the atoms, and from this frequency the antiproton mass can be calculated. However, an important source of imprecision comes from the fact that the atoms jiggle around, so that those moving towards and away from the beam experience slightly different frequencies. A similar effect is what causes the siren of an approaching ambulance to apparently change pitch as it passes you in the street. In their previous measurement in 2006, the ASACUSA team used just one laser beam, and the achievable accuracy was dominated by this effect. This time they used two beams moving in opposite directions, with the result that the jiggle for the two beams partly cancelled out, resulting in a four times better accuracy.