Antimatter

In 1928, British physicist Paul Dirac wrote 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.

	The Antiproton Decelerator The Antiproton Decelerator (AD) is a unique machine that produces low-energy antiprotons for studies of antimatter, and “creates” antiatoms.
	AEGIS AEGIS uses a beam of antiprotons from the Antiproton Decelerator to measure the value of Earth's gravitational acceleration.
	ALPHA ALPHA makes, captures and studies atoms of antihydrogen and compares them with hydrogen atoms.
	ASACUSA ASACUSA compares matter and antimatter using atoms of antiprotonic helium.
	ATRAP ATRAP compares hydrogen atoms with their antimatter equivalents – antihydrogen atoms.
	GBAR The Gravitational Behaviour of Antihydrogen at Rest experiment studies antihydrogen atoms falling from a height of 20 centimetres