• Question: What is the difference between matter and antimatter?

    Asked by thefreakyfox to Ben, Jony, Katharine, Mark, Peter on 16 Nov 2011. This question was also asked by kunjanddonal.
    • Photo: Ben Still

      Ben Still answered on 15 Nov 2011:

      There are mirror opposites – everything about them. Where an electron has a negative electric charge, the anti-electron (AKA positron) has a positive charge. The opposites don’t just stop there, space is also mirrored – what is left for one is right for the other and so on. These properties and many more like them are called quantum numbers, all of these quantum numbers change to a mirrored version to turn a description of matter into anti-matter and visa versa.

    • Photo: Jony Hudson

      Jony Hudson answered on 15 Nov 2011:

      Yeah, exactly as Ben said. It’s just like normal matter, but it has the opposite charge etc.

      The story of how they discovered it is quite interesting. They were taking photographs of the tracks of subatomic particles and they kept finding that every so often, very rarely, one of the particles would appear to take a wrong turn, going left instead of going right. Everything else seemed ok about these particle tracks except for turning in the wrong direction.

      A few years earlier theoretical physicist Paul Dirac hade written an equation that described how subatomic particles would behave when moving near the speed of light. The weird thing about his equation was that it predicted that there should be anti-particles with the opposite charge. But at the time, nobody had seen these anti-particles, so his theory seemed a bit crazy.

      The scientists put two and two together and realised that the wrong-turning tracks were actually the anti-particles that Dirac had predicted. Must have been really exciting to figure that out!!

    • Photo: Peter Williams

      Peter Williams answered on 15 Nov 2011:

      in short, the fact that there’s more matter than antimatter!

      But why? Well, nobody really knows. This is one of the hottest topics in physics at the moment, there are a whole load of really big experiments investigating why matter dominates over antimatter. In fact, the Italians have recently announced that they are going to build a new accelerator called Super-B to try and get to the bottom of it,
      One of the assumptions of standard big bang cosmology is that the early universe had an equal amount of matter and antimatter. Why make that assumption? Well, it’s the simplest thing to do. So then the problem is explaining what happened to all the antimatter. Our “Standard Model” of elementary particles does have room in it for a mechanism that does this. The technical term for it is “CP violation”. One of the forces by which elementary particles interact is able to “tell the difference” between matter and antimatter. In the time since the big bang, This force will have established a preference for matter over antimatter. Measurements done on accelerators over the past 20 years have found how big this effect is. And the answer so far is, not big enough. So it’s still an open question.

    • Photo: Mark Basham

      Mark Basham answered on 15 Nov 2011:

      The only thing to possibly add to the excellent answers here is that there is currently an experimental station at CERN which is trying to produce anti-hydrogen to explore the field more. Have a look at http://cool-antihydrogen.web.cern.ch/cool-antihydrogen/ if your interested

    • Photo: Katharine Schofield

      Katharine Schofield answered on 16 Nov 2011:

      As the others have mentioned, trying to find out why there’s very little antimatter around is a really important question in physics. We know that matter and antimatter behave slightly differently. We’ve seen it in experiments and we have a theory called the Standard Model that predicts this odd behaviour. The experiment I did my PhD on was looking at this very subject. What we found was that the Standard Model predictions for the types of particles I was studying were pretty much spot on. Great, big tick for the Standard Model there. BUT! (and isn’t there always a but?) the level of asymmetry observed is nowhere near enough to explain why we don’t see much antimatter around – like, a hundred million times not enough. So there must be some other source of CP violation that we don’t yet know about…so we’d like to find out what it is.

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