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 17 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.

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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

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.

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.

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.

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.

ALPHA 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.

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.

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.

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.

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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.