I've managed to leave string theory alone for a while, but a post came across Mixed States today that I can't avoid commenting on. Lubos Motl points to a news article about a recent measurement at MIT and NIST, in which Dave Pritchard's group used their cyclotron mass spectrometry technique to mesure the change in mass of a nucleus after emitting a photon. They pitch this as a test of E=mc2, and Pritchard is quoted thusly:
"In spite of widespread acceptance of this equation as gospel, we should remember that it is a theory," said David Pritchard, a professor of physics at MIT, who along with the team reported his findings in the Dec. 22 issue of Nature. "It can be trusted only to the extent that it is tested with experiments."
Motl dismisses this, writing:
Realistically speaking, the formula - and many other formulae - can be trusted well beyond these experiments. Everything depends on the amount of reasoning that we are allowed to perform with our brains in between the experiments. It is not true in science that every new experiment is really new. The whole goal of science is that we know the result of a huge class of experiments without actualling performing them. We can make predictions. Very general predictions and less general predictions. And science is able to do such things, indeed. If we are allowed to think a lot, the experiment is not terribly thrilling and its result is known in advance. There is just no way how we could design a theory in which the results will be different.
I think this is a nice illustration of what I see as a fundemantal attitude split between string theory and the sort of experimental physics I do. To Motl, it's inconceivable that the theory could work out any other way, so it's hardly worth bothering to do the experiment. To my mind, the fact that any other theory would be inconceivable is that whole reason to do the experiment in the first place. Yeah, you probably won't find anything (and if you did, you'd have a horrible time convincing anyone you were right) but if you did, it'd turn physics inside out.
(I've said this before, without the dig at string theorists, back in the very early days of this blog. It was the combination of trolling through my old archives (for reasons that will be explained later) followed by reading Motl's post on Mixed States that brought this post about.)
A few disclaimers: In general, I have very little use for Lubos Motl: he combines the worst sort of string-theorist attitude with political views that I find obnoxious, and a website design that I find appalling (in fact, if you'd like to read the whole post (which goes off into a big thing about global warming), I recommend using RSS in some form (it's still on the front page of Mixed States as I type this)). It's entirely possible that I'm putting an uncharitable interpretation on what he writes because of this, and because I know Dave Pritchard, and know that he's a smart guy.
Regarding the actual experiment, I suspect it's being over-sold, as Dave is somewhat prone to that, and everybody's all Einstein-happy this year. I can't read the original article at home, because the publishers of Nature are a bunch of bastards, but I don't think it's all that incredibly exciting-- it's the most precise direct test of the equivalence of mass and energy, but there are indirect constraints that are tighter than the current measurement (as Motl points out elsewhere in his piece). I don't think it's a trivial result, though-- if nothing else, it's technically very impressive, and I think there is something to be said for direct measurements over indirect ones.
Of course, that doesn't mean I'd like to be doing those measurements-- this is in a large class of fundamental experiments that I think are important in a philosophical sort of way, and that I'm glad to see somebody doing, but I'm just as glad that the person doing them isn't me.
(Originally posted at my steelypips blog, where there's some lively discussion in comments.)
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