## Archive for January, 2013

### Monocoil…Monocoil…Monocoil

Don’t try this at home, unless you’ve got more skill than is portrayed here

### The Method

The main theme of this month’s Physics Today is Norman Ramsey, and while most of it is paywalled, the first article is not: Norman Ramsey and his method

His separated oscillatory fields method paved the way for atomic clocks and was the reason for his winning the Nobel prize in 1989.

### More Attitude

The link in yesterday’s post, pointing to the Foundational Attitude Towards QM survey/poll/snapshot, has generated interest elsewhere. Sean Carroll has deemed the histogram of the interpretations of QM The Most Embarrassing Graph in Modern Physics

For quantum mechanics, by contrast, all we really have to do (most people believe) is think about it in the right way. No elaborate experiments necessarily required (although they could help nudge us in the right direction, no doubt about that). But if anything, that makes the embarrassment more acute. All we have to do is wrap our brains around the issue, and yet we’ve failed to do so.

Chad Orzel has responded with Experiments Are Not Afterthoughts

This plays into a pet peeve of mine, which I’ve ranted about before, namely the idea that experiments are somehow an afterthought, just cleaning up the loose ends once theorists have done the hard work of thinking about things.

This is emphatically wrong. Experiment is at least an equal partner in this, and every other scientific question. If we ever do determine that there is One True and Correct Interpretation of quantum mechanics, it will be because that intepretation produces makes concrete predictions that are testable by experiment. Full stop.

(Oooh. Schrödinger’s catfight!. OK, no, not really.)

I’ve already said that this kind of discussion isn’t something I spend a lot of effort on, and that’s probably because I’m an experimentalist, and because I am, I tend to agree with Chad here — the only way you know you’re right is if you compare your model with nature. We’re looking at a black box, labeled what’s really going on here and we aren’t going to simply think our way inside. We have to look at what goes in and what comes out of the box, or come up with a clever way to take a peek inside, but that’s all code for experiment.

A comment I read recently regarding a peripherally related subject (paraphrasing) is that theoretical physicists generally tend to be more prone to blurring the line between models and observation, which I think might be true (I’ve seen it happen, at least, but that’s merely anecdotal) and also might come into play here, in the form that experimentalists may be more demanding of, well, experiment. And since I think of the interpretations as just a tool to help with intuition about what’s going on, and not the theory itself, I see no need for embarrassment. I don’t see this as being much different than picking your favorite analogy to explain a concept, and finding that within a group of physicists, there is more than one analogy that individuals favor. The analogy doesn’t change the underlying physics.

I had mentioned that the commentary on the survey responses was fun to read, and this question’s commentary was

Finally, looking back, we regret not to have included the “shut up and calculate” interpretation

The shut up and calculate “interpretation” (or, if you like, “we don’t need no stinking interpretation”) is another approach I favor on some occasions. I like this simply because it removes the controversy that Sean has pointed to. It ignores any of the worry about what’s really going on inside the black box because, until we can come up with a test — which requires a model that distinguishes the options — we won’t know and can’t know. So why worry about it? The thing to focus on is the answer we get — the result of the QM calculation. If we ever do come up with that model that lets us test an interpretation, it becomes a new black box, inside of which we can’t see. That just pushes the problem back a step.

Sean closes his post by saying that he’s confident this will be resolved, but one has to recognize that this is not a slam-dunk. There is no fine print that says that all of nature will be understandable or double our money back. We have a track record that shows us that science is a great set of tools to help us understand nature and has allowed us to dig deeper and deeper, but success is not guaranteed.

### You've Got an Attitude

A Snapshot of Foundational Attitudes Toward Quantum Mechanics

I dont get all that caught up in the issues of foundational questions of quantum mechanics; I think, for example, that the interpretations are bridges to understanding, so while I’m happy to cite the Copenhagen interpretation, I’ll also mention many-worlds if that helps — I don’t feel locked into one or the other. (There are some, though, that don’t “feel” right to me and/or seem to have been discredited in some way, and I don’t call upon them. I’m also not nearly as familiar with them). I don’t think you’re going to solve many of these foundational problems unless they aren’t truly foundational, in which case you then have to wrestle with the issues one level down.

Regardless, I think this “snapshot” is interesting, in part because others do spend more time on these issues. Also for the author and respondent comments on many of the questions.

### Uncertain if This is Just Diffraction

via zapperz

One thing that would have been useful, I think, is a calculation in support of this, even if it’s just an order-of-magnitude one, to tell us if this is reasonable. The size of the spot (maybe a cm?) gives you a bound on the uncertainty of the transverse momentum. It’s green light (probably 532 nm) and the screen is of order a meter behind the slit. That tells us what the transverse component of the momentum should be — the momentum of the photon multiplied by the sine of the angle (or just the angle, in the small-angle approximation, which is around 0.01 radians)

So the uncertainty of the momentum is around 0.01 of the momentum, given by $$h/lambda$$, so Planck’s constant drops out when we pop this into the uncertainty relation. Rearranging the equation tells us that $latexDelta{x} > frac{lambda}{4pitheta}$ or that we shouldn’t see this effect until the slits are separated by less than about 10x the wavelength of the light — which is around what we expect just from experience in diffracting light, that it only becomes an important consideration as you approach the wavelength of the light.

### Getting Below Zero

How Do You Make Negative Temperatures, Anyway?

There’s a also an explanation of negative temperatures in spin states

### This Just In: Relativity Works

A thought experiment for the relativity skeptics

An interesting thought experiment that shows how general relativity must hold, though I doubt it will work on the crackpot demographic, as it’s proof-by-contradiction, and ends up with a perpetual motion machine as the contradiction. If you think those are real, the proof doesn’t work. And also that nobody expects the Spanish Inquisition nothing works on the crackpot demographic.