Everything we experience in our everyday lives is the result of just a few fundamental particles. A ray of sunshine is a stream of particles of light, or photons. Your own body is a complex machine made of quarks and electrons. Physicists have uncovered much about the structure of matter, but there still remain some huge unsolved mysteries. Join the Collider team as they take a light-hearted romp across the frontiers of physics, from Higgs bosons to enigmatic dark matter.
The science of particles | Dark matter | Antimatter | The Higgs boson
What is Antimatter?
It’s the favoured fodder of sci-fi writers, from fuel to power interstellar spaceships to the volatile ingredient in Vatican-annihilating bombs. But antimatter is no fiction; it’s real and what’s more, our very existence depends upon its properties.
Artist’s impression of a spacecraft proposed by NASA that is powered by antimatter. © NASA
The first particle of antimatter
We’ve known about antimatter for a long time. Paul Dirac, one of the greatest theorists of the 20th century, was the first to anticipate its existence when he predicted that the electron should have a partner with opposite electric charge – the anti-electron or positron – his only guide, a beautiful equation he first wrote down in 1927.
Few physicists took Dirac’s prediction seriously (including Dirac himself). But in 1932 Carl Anderson took a photograph that would turn the tide of scepticism for good. The photograph showed a faint white line crossing a cloud chamber, which he was using to study cosmic rays. Crucially, it curved in exactly the opposite way expected of an electron, providing conclusive evidence that anti-electrons were real.
Carl Anderson’s cloud chamber photograph confirming the existence of the positron. © Science Museum
Over the following years, physicists discovered that every particle had a corresponding anti-particle and just as particles clump together to make matter, antiparticles can clump to make antimatter.
But this begged a question: could there be parts of the universe made of antimatter? Perhaps with anti-stars, anti-planets, anti-little-green-men and anti-museums? The answer, as far as we can tell, appears to be a resounding no.
A vital imbalance
When matter and antimatter meet, they annihilate violently, releasing huge quantities of energy. If there were bits of the universe made of antimatter, then we would see constant flashes of radiation from the annihilation of matter with antimatter at the borders between the different regions. As nothing of the sort has ever been seen, it seems the whole visible universe is made only of matter.
Why? Well that’s one of the biggest unsolved problems in physics. Matter and antimatter are related to each other by a deep symmetry of nature. This symmetry is actually just a mirror reflection combined with a flipping of electric charge between positive and negative. So if you could build a perfect cosmic mirror, reflect the entire universe in it and swap all the electric charges around, you’d get a universe made of antimatter. The thing is, if this symmetry were exact we wouldn’t be here.
Why are we here?
Because of this symmetry, when everything came into existence at the Big Bang, matter and antimatter should have been created in equal amounts. As the firestorm of creation cooled, the matter and antimatter would have totally annihilated each other, leaving a cold, dark and lifeless universe. No matter, no stars, no planets or museums. Just a few lonely photons whizzing through the endless blackness.
So the fact that we exist is one of the biggest unsolved problems in physics.
To unpick this conundrum a number of experiments are underway to test the matter-antimatter symmetry. One of the largest is the LHCb detector at the Large Hadron Collider, CERN.
The LHCb cavern. © CERN
A question for quarks
The 'b' in LHCb stands for the beauty or bottom quark (depending on taste) which is a member of a family of particles called quarks.
LHCb physicists are interested in them because some composite particles containing b quarks constantly flip backwards and forwards between their matter and antimatter versions, in a sort of quantum split-personality disorder. This gives us a unique opportunity to study matter-antimatter symmetry by seeing if they spend more time as particles than anti-particles.
The interesting thing about all this is that the size of this asymmetry must be absolutely tiny. We know from astronomy that for every particle in the Universe, there were roughly a billion produced at the Big Bang. So 999,999,999 in a billion were annihilated by 999,999,999 anti-particles, leaving just one survivor.
In other words, everything that exists is a tiny one in a billion leftover from a cataclysmic battle between matter and antimatter at the beginning of time. Matter only won by a whisker. If things had been even a tiny bit different we wouldn’t be here to wonder about it at all.