After Our Time

7 October 2007

Antimatter

Posted by Adrian under Science

Despite having a background in science, I generally don’t look forward to the occasions when In Our Time covers scientific topics. There are two reasons for this - firstly, if it’s a subject that I’m interested in, I probably won’t learn anything new since I’ll have read about it elsewhere. Secondly, and more importantly, the contributors for scientific subjects are just not quite as good at explaining themselves as those for other subjects. I don’t know why this is the case, but it’s something I’ve definitely observed.

Antimatter (audio stream/wiki) is potentially a very tricky, very dry topic. It’s not something like evolution, where you can get a good argument going between Dawkins and someone else about punctuated equilibrium - it’s pure science, where the concepts are foreign and abstract. I wouldn’t have blamed anyone if In Our Time stumbled here.

To my delight, this was one of the clearest and most comprehensible science editions I’ve ever listened to. Val Gibson provided an exceptionally good introduction into the nature of matter and antimatter, and Frank Close and Ruth Gregory were equally skilled speakers. I suppose they may have covered this ground many times in lectures and elsewhere, which would explain the smoothness of their explanations, but it’s impressive nonetheless.

A good example comes about five minutes in, when Frank Close talks about Dirac’s equation that predicted the existence of antimatter. Equations are tricky - on the one hand, scientists don’t want to get bogged down in the details, but on the other, if you simplify it too much, you aren’t explaining anything at all. Here’s how he handles it:

Dirac came up with the idea of trying to combine quantum mechanics with the other great pillar of twentieth century physics, Einstein’s theory of relativity, and to apply it to the simplest thing then known, the electron. The surprise was that he found that he couldn’t do it, at least, not just by writing a single equation. He set out to write a single equation, to describe the energy of the electron, and the equation insisted on splitting into four parts - in mathematical jargon, he had to use matrices, but for our purposes, there were four equations where he only wanted one.

Now, they all had to mean something, and the question was, what? He quickly realised what two of them meant. As Val has said, the electron has a sort of corkscrew - think of it as a spinning top, going clockwise or anticlockwise - and that spin had been recognised must exist because people knew from the way atoms behaved that when electrons were in magnetic fields, they would spin one way or the other. For the first time, Dirac’s equation was saying, Aha! That is why there is a doubling-up of these two spin possibilities.

But what about the other two? That was the great puzzle, because as he looked at the equations, they seemed to be saying, the electron can exist with negative energy. Now, at this point, I imagine Dirac’s thoughts were probably like the listeners’ - ‘er, what’s going on here? Negative, with respect to what?’ Clearly, it’s a nonsense, phrased that way.

And then he had what to me was the great insight, which was there was another way of interpreting this doubling-up. It was that the negatively-charged electron with negative energy could also be read as saying a positively-charged electron with positive energy. So, now once he’d at least gotten something sensible - positive energy had appeared, which made sense - but positively-charged electron? There was no such thing - nobody knew of any such thing!

What Frank does well here is not merely explaining the equation, but importantly explaining Dirac’s great insight in a way that makes sense to listeneres. Frank goes on to say that the positron (the positively-charged electron) was later discovered by Carl Anderson in 1932, who used a cloud chamber to see its trail. The fact that it happened a mere four years after Dirac’s work was something that was somewhat disturbing for Frank.

I agree that it’s strange - almost as if the creation of a theory for the existence of positrons somehow removed a barrier that prevented their discovery. And perhaps it did; no doubt people were using cloud chambers for all sorts of things at the time, but even if you found evidence for a positively-charged electron, you were likely to think your equipment was malfunctioning rather than you’d made a major breakthrough. This theory may have not only spurred people to look for the positron, but given its discoverer a license to publish his findings. Einstein’s general theory of relativity, published in 1915/6 and subsequently ‘proved’ in 1919 by Arthur Eddington’s observations of stars close to the eclipsed sun is another notable example (although of course Eddington was looking for the effect).

From there, we move on to the fun stuff - what happens when matter and antimatter meet? Now, we already know the answer - an absolutely enormous explosion. How enormous? Well, the first nuclear bomb used in the Trinity test had about 6kg of plutonium. Only 1 gram of that plutonium was converted into energy during the explosion, which is still equivalent of a huge 20 kilotons of TNT. But if you replaced that plutonium with matter and antimatter, then the resulting explosion would be 6000 times bigger, because all of the 6kg would be converted into energy.

However, the central point of this edition was not about the nature of antimatter per se or explosions, but the question of why there is essentially no antimatter to be found in nature, when matter and antimatter are supposed to be identical? At the Big Bang, why weren’t equal amounts of matter and antimatter formed, and why didn’t they just annihilate each other? 18 minutes in, Val Gibson employs an analogy that will work particular well with fans of Sharpe:

At the beginning of the universe, we’ve got all this matter and antimatter around, and you can consider them as two armies that meet. If you had an army, say in the Napoleonic times, where you’ve got the ranks and you’ve got muskets* and they are firing at each other, and as they fire, the kneel down and reload and so you get waves that go through the matter and antimatter armies. You find the the matter is just slightly quicker than the antimatter army. The effect of that is that it soon annihilates all the antimatter, and you’re just left with the matter.

As later pointed out, this flaw in the symmetry between matter and antimatter is so small that there is only one extra particle of matter for every ten billion particles of antimatter; so in a way, the entire universe is just one ten-billionth of what was present at the Big Bang (yes, I know that this is probably a simplification, but it’s a wonderful image).

Speaking of images, Ruth Gregory comments around 33 minutes in that it’s hard for us to comprehend what conditions were like in the ultra-hot birth of the universe. Someone without an understanding of physics would think that as we heat up water and it turns from solid to liquid to gas, we are seeing three completely different substances; yet we know that at zero and one hundred degrees celsius, H2O goes through phase changes due to the breaking of particular types of intermolecular bonds, and that explains things. Imagine, she asks us, the same sort of thing, but with matter as a whole - when you heat it up enough, it acts completely different, just as steam acts completely differently to ice.

I could go on, but you should just listen to the programme.

I’ve sometimes thought that one reason why shows on science aren’t as interesting as those on history, politics or art is because science aims to describe the world as it objectively is, and as such, there can only be one correct description. Now, if you talk about science that is ‘known’ (or at least, that we have a very high level of confidence in) then you’re just describing stuff; and even worse, you’re describing concepts that are often non-intuitive and complicated (unlike history, which deals in incest, wars, assassinations and the like).

Alternatively, you could go down the different and recently very popular route of talking about ‘controversial’ science, where people don’t agree about how the world is. In theory, this is fine notwithstanding the obvious difficulties in explaining the concepts involved; practically all branches of science, physics in particular, have become so specialised that the disagreements are incomprehensible to even informed laymen (I read a recent interview with an eminent physicist who, when asked his opinion of string theory, said that he would need several years of study to provide a useful response). Unfortunately, in practice, ‘controversial’ science actually means pseudoscience (e.g. homeopathy) or politics (e.g. climate change).

I’m making one false assumption here though, and that’s that ‘describing stuff’ must be boring. On the contrary, scientists get to explain how the universe really, genuinely works! Who else gets to do that? Not historians or artists, that’s for sure. Explaining how the universe works ought to be fascinating and fun, and it’s practically a crime that it can be made boring. This programme on Antimatter shows us how it’s supposed to be done.

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