# 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

# ASACUSA

ASACUSA compares matter and antimatter using atoms of antiprotonic helium

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## Featured updates on this topic

### ALPHA experiment shows antihydrogen charge is neutral

21 Jan 2016 – ALPHA shows the most accurate measurement yet of the electric charge of antihydrogen atoms in a new Nature paper

### BASE compares protons to antiprotons with high precision

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

### Antihydrogen at CERN: 20 years and going strong

24 Sep 2015 – In September 1995 physicists at CERN synthesised the first antihydrogen atoms, paving the way for today’s growing antimatter research

### TEDed animation asks if antimatter falls up

10 Oct 2014 – TEDed and CERN physicist Chloé Malbrunot team up to test the principle of universality of free fall for antimatter

### New results from the AMS experiment in space

18 Sep 2014 – Latest measurements from the AMS experiment unveil new territories in the flux of cosmic rays

### CERN’s ALPHA experiment measures charge of antihydrogen

3 Jun 2014 – ALPHA reports a measurement of the electric charge of antihydrogen atoms, finding it to be compatible with zero to eight decimal places

### Antimatter experiment produces first beam of antihydrogen

21 Jan 2014 – The ASACUSA experiment at CERN has succeeded for the first time in producing a beam of antihydrogen atoms

### Join the dots to measure antimatter

8 Aug 2013 – Help the AEGIS experiment at CERN to work out how antimatter is affected by gravity. Just join the dots!

### Lighting the torch for ELENA

17 Jun 2013 – A ground-breaking ceremony today marked the start of construction of an extension to CERN's antimatter facility

### ALPHA: Novel investigation of gravity and antimatter

30 Apr 2013 – The ALPHA collaboration has published a paper describing the first direct analysis of how antimatter is affected by gravity

### LHCb experiment observes new matter-antimatter difference

24 Apr 2013 – The LHCb collaboration has made the first observation of matter-antimatter asymmetry in the decays the B0s

### ATRAP: Never a dull moment for the antiproton

25 Mar 2013 – The ATRAP experiment presents most precise measurement yet of the antiproton magnetic moment