Question: What is antimatter?

Susan Cartwright answered on 14 Jun 2015:

Antimatter is matter with the opposite electric charge. In ordinary matter, protons have positive electric charge, and electrons have negative charge. In antimatter, antiprotons have negative charge, and anti-electrons – known as positrons – have positive charge. Other, more subtle, quantum numbers are also reversed, but global properties like mass and sensitivity to different types of force are unchanged.

Because of the reversed quantum numbers, a particle and its antiparticle together have the quantum numbers of “pure energy”. This means that a particle and its antiparticle can annihilate, converting their whole mass into energy by E = mc^2. If antimatter were plentiful, this would be *by far* the most efficient form of energy generation: the creators of Star Trek had the right idea when they decided to power the USS Enterprise by matter-antimatter annihilation. Conversely, when we create new particles in particle accelerators, we usually make them in particle-antiparticle pairs (which is a bit of a nuisance, since it means you need enough energy to create two particles rather than one!).

The interesting thing about antimatter is that it *isn’t* plentiful: our Universe seems to contain only matter, with small amounts of antimatter made “on the spot” by the same processes that make antimatter in particle accelerators on Earth. This means that there must in fact be subtle differences between matter and antimatter, so that as the Universe cooled down from its early high-energy state, somehow matter got left behind and antimatter didn’t. We have discovered some of these subtle differences, but not enough to explain the observed matter content of the universe, so this remains one of the great unsolved mysteries of particle physics and cosmology. It’s possible that neutrinos have a role to play here, which is why my colleagues and I are looking hard for differences in the behaviour of neutrinos and antineutrinos.