Category Archives: Physics

Dun-dun-dun-dun-dun-dun-dun-dun

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Just when you thought it was safe to go back in the water.

what if: Spent Fuel Pool

What if I took a swim in a typical spent nuclear fuel pool? Would I need to dive to actually experience a fatal amount of radiation? How long could I stay safely at the surface?

Spoiler alert, but necessary for my comment

In fact, as long as you were underwater, you would be shielded from most of that normal background dose. You may actually receive a lower dose of radiation treading water in a spent fuel pool than walking around on the street.

This jibes with my experience in the navy — the crew had their exposure monitored, and the word was that it wasn’t unusual for someone on a sub not directly being exposed to radioactive/contaminated materials or hanging out near the reactor compartment to have their dose be lower than what one would get on the surface. The benefitted from spending most of their time several tens of meters blow the surface, and all of the attenuation that afforded.

More PITA Than PEW

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I align lasers for a living. Or so it seems.

That’s not actually my job description, but if you observed what I do in the lab over a long enough period, it wouldn’t be an unreasonable description of the plurality of my effort. That job doesn’t occupy as much of my time as it once did; because of some engineering advancements we’ve made, once lasers are aligned they stay aligned for much longer, but there’s still a bit of time spent doing this.

And this is the bane of my existence

That’s a multi-axis fiber port. And they are a complete pain in the butt to align.

The reason for using them is the “engineering advancement” I mentioned — if you couple your laser into an optical fiber, it’s very easy to send that light somewhere else. If you sent it through open space, you leave yourself open to all sorts of problems — dust and dirt on your optics, things blocking the beam, and misalignment issues creeping in — if you send the light just 1 meter, each milliradian of error moves your beam a millimeter, and thermal changes can “walk” a mirror mount to steer in a subtly different direction. If you need precision, long-term alignment, fiber is the way to go.

Free-space alignment is relatively easy. This is a mirror pair we call a dog-leg (golfers might observe it’s two dog legs, but this isn’t golf)

I’ve drawn in a laser beam (you wouldn’t see an actual beam unless there was dust or other some particulates to scatter the light, and we try to minimize that) bouncing off of the two mirrors. Each mirror mount has two knobs: one pushes on a corner for horizontal tilting, while the other pushes on the opposite corner for vertical tilting; there is a small ball bearing at the third corner, and a couple of springs to hold it all together. Adjusting a knob will tilt the mirror and deflect the beam, and change the angle of reflection off of the mirror (and also off of the next mirror). You can un-do the angle change with the second mirror, so that the exiting beam is headed in the same direction as you started, but having been translated to one side. The adjustments are orthogonal, so you can “walk” a beam anywhere you want by adjusting pairs of knobs, as long as the beam continues to hit the mirrors.

That’s pretty easy, in principle; in practice it’s a little tougher, because you often do this “blind” — you are looking at the target or a display, telling you how well you are hitting the target, and the lights might be off. But it’s a skill that’s picked up pretty quickly.

The fiber port, though, is tougher. The chuck in the middle is for an optical fiber, and in my case it’s a single-mode optical fiber. The core of such a fiber is a few microns across and is very sensitive to the spatial mode (the shape) of the light it accepts. I’ve previously shown what a poor mode looks like, but assuming you have a nice zeroeth order mode, you need to send the light in with the fiber positioned just right — not only hitting the core of the fiber, but at the proper angle and with the fiber tip just the right distance from the fiber, to match the mode characteristics.

There’s a little divot on the left side of the fiber port that houses the screw for moving the fiber chuck in the x direction, and one at the top for y, and they are both sensitive and subject to a bit of hysteresis (turning through some angle and then getting back to the original position doesn’t reposition the holder exactly), and also have the annoying habit of walking off when you remove the allen key/driver. The small black dots on the face of the holder are the tip/tilt controls, which also suffer a bit from hysteresis. In addition, to translate in the z-direction (moving the fiber closer or further from the lens) you have to change all three tilt controls. In practice this means your coupling efficiency goes down as you change the first two settings, and you have to hope you can recover your signal as you work on the third. If you can’t, it may be because you are moving the wrong way along z, or because you changed one of the tilt settings too much, so you have to try many iterations to find out what’s going on.

Just so you know it’s not all pew, pew, pew when you work with lasers.

More Attitude

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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

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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

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You need to a flashplayer enabled browser to view this YouTube video

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.

This Just In: Relativity Works

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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.