What is Antimatter?

Antimatter is usually thought of as being rather mysterious. But in fact, it is much more abundant than you might think and it may well be the key to explaining some of the...
23 January 2011

Interview with 

Professor Andy Parker, High Energy Physics Group at Cambridge University


Diana -   Antimatter is usually thought of as being rather mysterious.  But in fact, it is much more abundant than you might think and it may well be the key to explaining some of the mysteries that surround the Big Bang.  We're joined by Professor Andy Parker from the High Energy Physics Group at Cambridge University.  Hello.  

Andy -   Nice to be here.  

Diana -   All right, very basic question to start off with then.  What is antimatter?

Andy -   Well antimatter is an interesting thing, and it takes a little bit of history to understand clearly where it comes from. So back in the '30s a theorist, a British theorist called Paul Dirac was trying to put together the latest physics of his day which was quantum mechanics and relativity.  At that time, quantum mechanics only really described things that were moving relatively slowly, relative to the speed of light.  Whereas relativity told us that physics was different close to the speed of light and there were lots of new effects.  So obviously, people wanted to put these two together.  And Dirac managed to combine the two concepts into one equation, now known as the Dirac equation, and it's a beautiful example of the power of mathematics because he wasn't expecting to find anything other than small changes to the behaviour of electrons when they got very fast.  But what his equations told him was that for every electron, which is a negatively charged particle, there was an exactly the same sort of state, but with a positive charge.  It had the same mass, it would behave exactly the same way, but it was kind of the opposite.  And furthermore, the equation said that you could take pure energy and turn it into an electron and a positron, so the antimatter partner of the electron is the positron, and he was quite baffled by this actually when he first saw it.  It's interesting that he kind of didn't believe his own maths and he tried for a while to pretend that these positively charged particles had to be the proton which was the only positively charged one he knew of, but it had the wrong mass.  Eventually, he accepted that this was actually a serious prediction of something called antimatter and that if you ran the equations for a proton, you get an antiproton, and if you did it for a neutron, you get an antineutron.  So every type of particle should have its opposite - the opposite electrical charge, and if you bring the two together, they would disappear in a puff of energy.  Cloud chamber photograph of the first positron ever observed

Diana -   So antimatter basically came about as a result of mathematics.  It was the only way to solve the equations.  

Andy -   That's right.  It was the only consistent solution to quantum mechanics and relativity.  

Diana -   Okay and how does it relate to the Big Bang then?  

Andy -   Well, okay so let me just say that this is not a theoretical concept any more. People have found antimatter.  They found the first antielectrons in the '30s and the antiproton in the '50s, and we routinely make it and it's used in hospitals but, okay, how does it relate to Big Bang?  Well the Big Bang was a state that started off with very high energy and that energy allowed the formation of lots of matter-antimatter pairs of particles, so you would expect naively that the universe were made of equal amounts of matter and antimatter because it was formed from the state of pure energy.  But this is not what we observe.  If we look at this studio for example, I've got a microphone, I don't have an antimicrophone, and there's nothing in the room which is blowing up around us so clearly, there isn't very much antimatter here.  So you might think, well, okay, it's separated out.  There's a region of the universe somewhere with all the antimatter in, and a region here with all the matter in.  But if that was the case, there would be a boundary between the two and you would see along that boundary, you would see the matter and the antimatter annihilating.  So you can look for that because it would produce a very strong source of radiation of the very particular wavelength given by the energy of the annihilation.  And we don't see that anywhere in the universe.  We've had satellites up, looking for antimatter particles coming from long away and whizzing by and the results so far have not really observed any unexpected amount of antimatter, other than what's being created from energy near us at the moment.  So it looks as if the universe is only made of matter.  So then you ask the question, why?  

Diana -   I was actually going to ask the question - so we know it should exist, but how can we actually go about detecting it?

Andy -   Okay, that's quite straightforward.  We have lots of different particle detectors that can measure particles and tell what type they are.  So, if you had one that was looking for electrons, you would test the charge that is negative by applying a magnetic field and a positron will go the other way.

Diana -   All right, so what is the why then, why isn't it there?

Andy -   Okay, so what appears to have happened is that although matter and antimatter are very symmetric, that is they look like a mirror image of each other, there is some small difference in the physics between the matter and the antimatter.  That is their interactions with the rest of the world are very slightly different and that means that when the universe started annihilating all the matter with all the antimatter, it didn't quite balance, even though they'd been created in equal amounts.  It didn't quite balance.  The rate is slightly stronger for antimatter.  We know that most of the annihilation took place because we actually see a universe full of light which are the photons that came from this annihilation.  But there's a tiny, tiny dose of matter leftover and that's what we're all made of, all the stars, all the galaxies, all the radio studios, are made of this tiny little piece that didn't manage to get annihilated.

Diana -   That's really quite profound.  Okay, so you mentioned that light and radiation is emitted after annihilation.  So, can we harness that?  Can we use that?

Andy -   Well, we could if we could make a lot of antimatter, but we're not very good at making it.  It is, in principle, straightforward.  You take something with a lot of energy, say, a proton beam and you slam it into something like a lead target and some of that energy will make antimatter.  So you'll get pairs of electrons and positrons, of protons and antiprotons made, and at CERN, that the Collider Centre in Geneva, we do this routinely and I used to work on an experiment actually where we collected antiprotons in order to annihilate them with protons and see what happened.  But you don't make very much.  It would take thousands of millions of years to make a bottle of antimatter, so although we can make enough to do experiments, we can't make enough to make large quantities that you could use as an energy source.

Diana -   Seems like a shame really, doesn't it?

Andy -   It will cost you a lot more energy to make than you would ever get back.


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