# Antimatter

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?

### Check out this timeline for an overview of antimatter research

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

Not all accelerators increase a particle's speed. The AD slows down antiprotons so they can be used to study antimatter

## Antimatter experiments at CERN

In the antimatter hall at CERN, numerous experiments are using antiprotons from the Antiproton Decelerator to investigate the properties of antimatter.

# ACE

ACE brings together an international team of physicists, biologists and medics to study the biological effects of antiprotons

# AEGIS

AEGIS uses a beam of antiprotons from the Antiproton Decelerator to measure the value of Earth's gravitational acceleration

# ATRAP

ATRAP compares hydrogen atoms with their antimatter equivalents – antihydrogen atoms

# ALPHA

ALPHA makes, captures and studies atoms of antihydrogen and compares them with hydrogen atoms