Weinberg on BCS

For some reason I don’t understand, I occasionally get a copy of some random (as far as I can tell) issue the International Journal of Modern Physics A in the post. It just arrives. The people at World Scientific are presumably hoping I’ll get hooked by it and take out a personal subscription or something. Not sure. Well, it has not happened yet (just as it did not with the unsolicited regular arrivals of Angeleno a while back). But you never know.

Anyway, before putting the one that arrived recently on to my shelf (I can’t easily throw things like that away… it’s a problem, I know) I glanced through the contents to see if there were clues as to why this one was sent to me. I found no obvious ones, but the very first article caught my eye. It was a rather good essay by Steven Weinberg (1979 Nobel Prize in Physics) on BCS (J Bardeen, L N Cooper and J R Schrieffer – No, nothing to do with College Football’s BCS.) theory of superconductivity and the importance of the idea of spontaneously broken symmetry in condensed matter physics and then in particle physics (see last year’s Physics Nobel, by the way). I thought I’d point it out to you, since it is rather nicely written and very instructive (whether or not you’re already familiar with the phenomenon/idea).

He begins with some overstatement, however. He expounds upon the difference between particle physicists and condensed matter physicists and although it is amusingly written in parts (I assume intentionally) he really over-eggs the pudding, in my opinion. Look at this:

[…] There are deep differences in our aims, in the kinds of satisfaction that we hope to get from our work.

Condensed-matter physicists are often motivated to deal with phenomena because the phenomena themselves are intrinsically so interesting. Who would not be fascinated by weird things, such as superconductivity, superfluidity, or the quantum Hall effect? On the other hand, I don’t think that elementary-particle physicists are generally very excited by the phenomena they study. The particles themselves are practically featureless, every electron looking tediously just like every other electron.

He’s got to be joking here, I think. It’s funny, but misses the point. (For a start, the fact that every electron is just like every other electron everywhere in the universe is an amazing phenomenon in its own right! He must know this.) He goes on to talk about the reductionist view of the particle physicist vs the approach of the condensed matter physicist to study emergent phenomena, and so on and so forth.

Now granted, he has a Nobel Prize, and I, er… don’t, but with respect, I sort of think of this way of separating things as quite an old-fashioned view (and I’m not really sure if it was ever true), or a view that in current times you might have when you’re a 21 year old physicist, and have not yet learned your history, or much physics. This of course does not apply to the great man, who knows more physics than I’ll probably ever know, and from whose bibliography just one paper selected at random likely would trump anything I’ve ever written… but I’ll soldier on with my view on this.

It seems to me to be much more deliciously tangled up than he suggests in his essay, and that actually the distinction between “fundamental” and “emergent” is so blurred as to be non-existent. This is my own view on things, anyway, given established examples and lessons in the history of physics, and the direction of current research ideas. So much of what interests me in my own study of “fundamental” physics may well have a great deal to gain from ideas concerning emergent phenomena. I’m quite sure that the very fabric of spacetime itself, something so prized in particle physics as being fundamental – at the core of the subject itself – is likely to be an emergent phenomenon: The nice smooth spacetime in which physics “takes place” – the place you keep your stuff and where stuff happens – is in my view likely to be an illusion of sorts. It will turn out to be an approximation to some microscopic physics described in terms of new variables in a quantum gravity description, rather like the fluidity of water, with other characteristics such as wetness, viscosity, and so forth, emerges from the collective behaviour of its constituent molecules. We’ve good examples of this already in toy models derived in the string theory context (see also another post here). It may turn out to be all wrong (we’ve not put much of this to direct tests yet), but it has a good feeling to it that suggests at least some of the story is on the right track. (I take the recent encouraging progress described here as encouragement too.) We shall see.

Overall, I think that Nature recycles ideas, or better, recycles mechanisms and phenomena. So the study of things we think are “fundamental” can be informed a great deal by the things we think of as “emergent”, and often erroneously taught to ignore or think of as less important when we are young.

Lesson: We ignore other fields of physics at our peril.

I’m sure Weinberg knows all this, since the core subject of his essay (which is from 2007) – which I urge you to read – is about the cross-pollination of ideas between condensed matter and particle physics. I don’t think that it stops with the examples we’ve seen so far. I bet it’ll continue.

I found a version of the essay at CERN Courier’s site. You can read it here.


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