As someone who works in particle physics (I hesitate to call myself a particle physicist) one of the most embarrassing things I have to admit is that I don’t really understand the theory. That’s not so bad if you are an experimentalist (as I am) – it would be a bit of a killer blow if you wanted to set yourself up as a theorist. Nevertheless one is expected to have some sort of a grip on theory, even if you just play with plastic scintillator or write code.
And we’re not talking about quantum field theory here; I even have trouble with the basic idea that, in particle physics, forces are carried by particles. These force-carrying particles are bosons, and of course we all know about bosons now, because of the Higgs (although, just to complicate things, I’m not sure the Higgs is actually a force-carrier …)
The general gist of it is that when particles interact, they do so by exchanging bosons – though strictly speaking, it should be a boson (singular). (So for instance, when an electron scatters off another electron it does so by means of a photon.) The boson is generally the squiggly thing in the middle of the Feynman diagram – it can be a photon (but, errr, a virtual photon – not that I could really tell you the difference between a virtual photon and a real one) or a W or Z boson, or a gluon (for the strong nuclear force) or (if it ever gets discovered) a graviton, in the case of gravity. The W and Z are the carriers of the other nuclear force, the weak force – except that I’ve never been able to see quite why the weak force is described as a force, since all it ever seems to be associated with is particle decay (as in beta decay, for instance).
We are all supposed to swallow all this before rushing on to the really tricky stuff, QFT. It’s bread-and-butter stuff for particle physicists, and I for one have never really had the nerve to admit that I found it all a bit improbable and weird.
I’ll admit, though, that the boson exchange idea is a neat way of dealing with the problem of action-at-a-distance (AAAD) – just as well, since quantum mechanics tells us that the converse of AAAD, namely action by contact, is impossible since “contact” is a meaningless term at the quantum level. Now, I’ve never really understood what the problem is with AAAD, and would contend that it’s probably the simplest and easiest way of visualising forces – but I gather philosophers of science have anguished over it since Newton’s time. When bosons are exchanged they don’t make contact with other particles – they are simply emitted at one end of the interaction and absorbed at the other. But honestly – photon exchange as a way of explaining electrostatic attraction? (Repulsion is easier to follow, though you do need a medicine ball and two pairs of roller skates to really get on top of the concept even then).
I made a bargain with the textbook-writers. I decided I’d take it all on trust for now, and come back to it later. But today, quite unexpectedly, I made a bit of a breakthrough. It was while reading a history-of-science book – Peter Galison’s Image and Logic – as background material for my history of HEP research at UCL, that I came across Galison’s description of the discovery of weak neutral currents, in the caption of a bubble chamber photograph. Now this was one of the examples of force carriers I just couldn’t get. I couldn’t really even see it as an interaction. You start with a neutrino and an electron, and you end up with … a neutrino and an electron! What’s that all about? It’s like writing a mathematical equations such as 1 + 1 = 1 + 1, or a chemical equation in which an oxygen molecule turns into … an oxygen molecule. Somewhere in the middle of the neutral current interaction is a neutral Z boson – but what exactly is it doing? Well, some part of me knew, of course, that it was carrying momentum from one particle to the other – in this case, from the neutrino to the electron – but I still couldn’t see why you needed another particle to do that.
It was Galison’s terminology that started me thinking:
The electron’s trajectory goes from left to right, beginning at the arrow’s end, where it ostensibly was hit by a right-moving neutrino.
There you are, I thought at first. Who needs a Z boson? You just need one particle to hit another ….. then the penny dropped.
“Hitting one another” is just not something that fundamental particles do. In picturing them as little balls – which I find difficult not to do, even though I know they are “really” wave functions, or even worse, something with wave and particle properties that it’s impossible to visualise – we carry the analogy too far. We assume – or at least, I assumed – that two particles could bounce off each other in exactly the same way as two macroscopic objects do. Yet somewhere in another part of my brain, I also knew that the only reason objects exert forces of reaction on one another is because of electrostatic forces that come into play at very close range – crudely speaking, when the atoms at the surfaces of the two objects are separated by a distance not much bigger than an atom, so that the charge distributions of the atoms create a net repulsive charge. In point of fact, then, even at macroscopic scales, objects do not “hit” one another – they repel electrostatically. Neutrinos are, of course, electrically neutral and do not “feel” electromagnetic forces. But we know, from the bubble chamber photo, that there is an interaction, so we need to find a suitable model to explain what is going on – and just saying that one “hits” the other is not good enough. Hence the Z.
Feynman diagram for neutrino-electron interaction – from Halzen & Martin, “Quarks & Leptons”
All of which goes to show that you can find understanding in the strangest of places. Not that I should be surprised – after all, it was through reading Sir Edmund Whitaker’s wonderful History of the Theories of Aether and Electricity that I first came to understand magnetic vector potential, something that even one of my favourite textbooks of all, Grant and Phillips’ Electromagnetism, could not manage. I’m not sure whether Galison’s use of the word “hit” was a deliberate provocation, or whether it was just what he felt to be the most appropriate term for a possibly non-scientific reader to understand; Galison himself has a PhD in physics as well as one in history, I believe, so he presumably knows at least as much as me about the subject – although I have taken issue with him on a number of occasions, not least for getting a cloud chamber picture the wrong way round in the same book, or his rather sloppy take on special relativity (and railway signalling) in “Einstein’s Clocks: the Place of Time“. But on this occasion I’ll thank him for providing me – unwittingly or not – with the means to greater understanding.