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 – which won Dirac the Nobel prize in 1933 – 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", for example, identical in every way but with a positive electric charge. The insight opened the possibility of entire galaxies and universes made of antimatter.
But when matter and antimatter come into contact, they annihilate – disappearing in a flash of energy. The big bang should have created equal amounts of matter and antimatter. So why is there far more matter than antimatter in the universe?
At CERN, physicists make antimatter to study in experiments. The starting point is the Antiproton Decelerator, which slows down antiprotons so that physicists can investigate their properties.
In the antimatter hall at CERN, numerous experiments are using antiprotons from the Antiproton Decelerator to investigate the properties of antimatter.
ACE brings together an international team of physicists, biologists and medics to study the biological effects of antiprotons
AEGIS uses a beam of antiprotons from the Antiproton Decelerator to measure the value of Earth's gravitational acceleration
ATRAP compares hydrogen atoms with their antimatter equivalents – antihydrogen atoms
ALPHA makes, captures and studies atoms of antihydrogen and compares them with hydrogen atoms
ASACUSA compares matter and antimatter using atoms of antiprotonic helium
21 Jan 2016 – ALPHA shows the most accurate measurement yet of the electric charge of antihydrogen atoms in a new Nature paper
12 Aug 2015 – In a paper published today in Nature, BASE reports the most precise comparison of the charge-to-mass ratio of the proton to the antiproton