Daily Archives: November 16, 2010

Oh, Say, Can You See This?

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How Do You See Interference Between Independent Lasers?

[T]his is traditionally done with a single laser and two slits. In principle, you ought to be able to do it with two completely independent lasers of the same wavelength, and see an interference pattern that way. In practice, though, this is very hard to do, as the phase difference between two independent lasers will jump around randomly at a very high rate. At any given instant, the two lasers will interfere with each other to make a nice double-slit pattern, but the positions of the bright and dark spots will shift around randomly, so fast that your eye can’t follow them.

Clang II: The Wrath of Oersted

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When last we left our intrepid physicists, they (meaning we) had, after many trials and tribulations, found a magnetic washer that was messin’ with our clock. We removed it, killed it and had it stuffed and mounted on the wall. A couple of you posted congratulatory notes on the success of the mission. I had delayed posting the story until we had confirmed that the vacuum was intact, because Murphy has a way of penalizing premature celebration. And the vacuum was fine the next day. Nope, no problems with the vacuum.

But I still should have waited. The problem is that there was still an icky nasty foreign magnetic field. At first we thought that maybe we (meaning I) had screwed up the shields by getting a magnet too close. And by too close, I mean I accidentally waved my hand too close to one part and heard a resounding THUNK! as the magnetic attraction (however the funk that works) overwhelmed my grip. We routinely degauss the shields, but there’s always the spectre of “burning” in a strong enough localized field that the degauss-o-tron can’t handle. That made for a bit of stomach acid on my part until we confirmed that the shields were fine. The problem was localized to the location of another bolt.

Yikes.

But it wasn’t just a washer this time. There was another one, even though we had asked the magic eight-ball if the problem was fixed and we got a “Signs point to YES” answer. At this point we were incredibly paranoid and with good reason — the pathology was far more sinister. We had noticed that some of the titanium bolts we used would have titanium chips inside the socket head, from the broaching process used to create the hexagonal shape. But they’re titanium chips, right? It’s only a problem if they keep you from tightening the bolt. Or so we had thought, and while we did clean the bolts up, we weren’t as anal meticulous as we needed to be: the problem (most likely one, at least) is that the tool itself is steel, and tools will occasionally chip. And that chip can get caught in with the broaching chips, lodged inside the cap-screw, and that, my friends, is pure evil.

So we scraped and tested — I did my best Nick Stokes impersonation and gathered the specks on some tape so I could wave it in front of the detector, and pretty soon we found one that buried the needle (metaphorically, at least; it was a digital meter)

That’s the one who slimed me. The ugly little spud above the 5″ line. (Yes, we have English-system rulers in the lab. They are a nanosecond long.)

Everything seems to be fine now. This time I waited until I could check that things were running well, and confirm we’re moving on to tackling the next gremlin in the lab.

Thinking in a Different Corner of the Box

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I often despise the phrase “think outside the box” because when I see it on the scienceforums.net boards, it’s usually proffered by a crackpot who is using it to mean “Pay no attention to the violation of the first or second law of thermodynamics behind the curtain.” In that sense, the box is physics, and everything is inside the box. You might find the box is a slightly different shape, or there is something interesting in the corner, but everything is inside the box.

Here’s a story from an older article. The thinking was inspired.

Open minds reap rewards

The year before, in 1952, Ed Salpeter, a researcher in New York, had pointed out that the beryllium barrier might be leapfrogged if, in the heart of “red giant stars”, three helium nuclei collided almost simultaneously, gluing together to make carbon-12. It was the nuclear physics equivalent of three shopping trolleys colliding simultaneously in a car park. Unfortunately, this process was fantastically unlikely.

Enter Hoyle. His argument, as as far as Fowler could make out, was that the process would be speeded up if, by a bizarre coincidence, carbon-12 had an energy state exactly equal to the energy of three colliding helium nuclei at the 100 million-degree temperature inside a red giant. That energy was 7.65 MeV. The state had to exist, reasoned Hoyle, because life existed and life was based on carbon.

Scientific thought like this gives me a hadron.