Monthly Archives: February 2012

How Would George Washington Feel About This?

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Ol’ George was wary of foreign entanglements, but then he wasn’t a physicist living in the quantum age.

A Schrödinger cat with eight lives: quantum entanglement of eight photons

One of the most mind-blowing areas of quantum mechanics is entanglement: two or more particles separated in space can have physical properties that are correlated. A measurement performed on one particle will tell us the result of the same measurement taken on an entangled particle.

Yes! They avoided the common pitfalls of entanglement reporting. There’s even a link to the paper at the end. Good job.

I Grow More Powerful

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SMBC: Grace Hopper

For the record, RADM (which was Commodore, at the time) Grace Hopper gave the commencement speech when I graduated. I was in the navy, though not yet commissioned, and I took Hopper’s famous “It’s easier to ask for forgiveness than it is to get permission” as a direct order.

Getting Poolsick

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As the ship moves the pool table adjusts so that the billiard balls don’t move at all, even in rough weather.

There’s no bias toward any of the pockets, that’s for sure.

Fly Like an Eagle

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I intend to write up parts of my recent presentation into a blog post, but here is something on timekeeping from The Virtuosi: Time Keeps On Slippin’

However, there is a subtlety to one of their arguments that requires more detail

The second measure of “good-ness” is precision or, in watch parlance, stability. This is essentially a measure of the consistency of the watch. If I have a watch that is consistently off by 5 minutes from the official time, then it is not accurate but it is still stable.

This is true but does not extend far enough. If a watch is consistently off by 5 minutes (or constant amount) from another source, then both must be running at the same frequency — they are accumulating phase at the same rate. But stability goes one step further. Even if the clocks were running at different rates, and phase was accumulating between them (i.e. one is running fast), you can make the same statement. The fractional frequency stability — given by the Allan deviation — depends on how the difference of the two frequencies, as measured over different intervals, changes. But the difference doesn’t have to be zero. A clock that consistently gains e.g. a second per day is also a very stable device: the frequency difference is always ~1/86400, but it’s constant, and the Allan deviation looks at the difference between subsequent frequency comparisons.

The danger here is in the assumption that two stable clocks will give you the same time readings. That’s not what we mean by stability. Stability is a measure of how the frequency is changing. An analogue would be a common conceptual mistake in using Newton’s second law: an object at rest feels zero force, but a force of zero does not mean the object is at rest — it simply means an object has a constant velocity.

It's Pure … Something

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One of the more popular crackpottish ramblings I run across concerns “pure energy” and an explanation of what it is. Here is a good counterpoint:

Pure Energy?
Energy is a property of “stuff”; that is a physical system. A configuration of a physical system will have a property that we can indirectly measure, which we call energy. One cannot have energy as some independent “thing”.

Morium about Thorium

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Here’s an article about Thorium reactors I found via Nick at Fine Structure. It’s not particularly detailed, so I thought I might be able to fill in some of the gaps.

Th-232 is the naturally occurring isotope of Thorium, with a 14 billion year half-life, and the idea is that you let it absorb a neutron to become Th-233, which then beta decays twice (to Pa-233 with a half-life of 22 minutes and then to U-233 with a half-life of 27 days). The neutrons, as the article says, are produced by bombarding lead with protons, and would also be produced by fissions in the U-233. But since the mass is subcritical, this extra source of neutrons is required in order to run at steady-state and produce macroscopic amounts of power.

[T]here are downsides to the use of uranium-235 as fuel: first, it produces plutonium as waste. Second, the uranium-235 fuel cycle is what engineers call “critical”: once it gets going it’s self-sustaining, so there is a risk – albeit a tiny risk – of loss of control.

The unwanted plutonium “waste” is a byproduct of having U-238 around; most Uranium is U-238, and it, like Th-233, can absorb a neutron and beta decay twice. It becomes the somewhat long-lived fissile Pu-239. Starting with Thorium bypasses this particular issue.

The use of a subcritical mass shouldn’t be specific to this system, though. You should be able to do this with U-235 as well, but running a reactor as a critical mass is a much easier system to build.

The other advantage appears to be the fission products. The article states

[T]he small amount of toxic waste generated by the thorium/uranium-233 fuel cycle ceases to be radioactive after a few hundred years, rather than the thousands of years during which uranium waste remains toxic.

This makes sense to me; the fission products have excess neutrons and tend to beta-minus decay and with U-233 you start with fewer neutrons, so you will have a slightly different fission yield and your fission products are going to generally be closer to stability. This would also imply less decay heat, which is the issue that has been plaguing the Fukushima reactors — the radioactive fission products must be cooled long after shutdown. What the article doesn’t explain is how much smaller this is, so it’s hard to tell if this article is overplaying the operational safety improvements.

We Now Return You to Your Regularly Scheduled Program

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I’m back from my jaunt down to Augusta, GA. I had a good time talking with the faculty and students and my talk was well-received — I got some really good questions from the students and even the biology folks said they understood most of it (I may have lost them temporarily when I talked about the guts of atomic clocks, such as the mention of Zeeman splitting of atomic sublevels). The room was reasonably full, too — maybe 60-70 people. The only real glitch was the previously mentioned swapping of NRL for USNO. If I had had time to add a slide to the talk I would have put up a map showing the geographical difference in our locations, and then say how wonderful it is to be in South Carolina (which, like NRL, is right across the river), but I instead opted to mildly embarrass my navy buddy by calling him Ensign and telling about calling up the USNO Master Clock voice announcer. But I kid because I love, and I think he’s done a great job in helping to build up the physics program there and I had a wonderful time visiting with some old friends. Back to regular blogging soon!

How Black Light Works

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How Black Light Works.

Bananas are extremely cool in black light, especially around their spots, which have deposits of phosphors. Banana spots glow in leopard-like patterns under black light. Chlorophyll glows red, and vaseline glows blue. Tonic water glows in black light. Vitamin A and the B vitamins thiamine, niacin, and riboflavin will all glow under black light, especially if they’re soaked in vinegar. Simply grind up vitamin tablets, soak them in a little vinegar, and either leave them around or serve them as salad dressing.

I know that the B-vitamins fluoresced, but not bananas. Now I need to check this out.