Uncertain Principles

I'd title this "Why Oh Why Can't We Have a Better Press Corps," but I don't want to infringe on Brad DeLong's trademarks.

There's been a minor kerfuffle over John Kerry's unsubtle mention of Mary Cheney in his answer to the homosexuality question in the "debate" the other night (OBDailyShow: "Gaaaaayyyyyy Daaaaughterrrrrr!!!!"). Her parents, Mr. Burns and Cruella DeVille, have been acting miffed for the last couple of days, despite the fact that Kerry didn't actually say anything disrespectful about her.

Of course, nowhere have I seen any mention of the only question that matters here: what does Mary Cheney think of this? It's not like she's in a closet somewhere (OBDailyShow2: "She's here. She's queer. Next question."), figuratively or literally-- she's working for the Bush-Cheney campaign, for God's sake. Isn't there some ace reporter out there who can call her up and ask her what she thinks? If she says she's offended, then that's a story. If she's not offended, that's the end of a story. If she refuses to comment, well, that's noteworthy, too.

But, nothing. Even in the stories that mention her position with the campaign, we don't even get a "Attempts to reach Ms. Cheney were unsuccessful." It's like nobody even thought it might be worth contacting the person at the center of the whole issue.

Similarly, there was a brief flurry of discussion after the second "debate" about Bush's lame answer to the "What mistakes have you made?" question, with people going back and forth about whether it was a good answer or not, and yet, somehow, nobody thought to ask the woman who asked the question whether she was happy with the answer. What's up with that? It's not like she was some semi-anonymous e-mailer from East Podunk-- she was in the auditorium with half the reporters in the Western world. And nobody managed to tap her on the shoulder on the way out?

In fact, that's the one bit of information I'd really like about any and all of the "town hall" questions: what did the person who asked the question think of the answers? I might consider giving a damn about what Jeff Greenfield thinks of the answers, right about the time that an asteroid strike wipes out everybody in America but the candidates, me, and Jeff Greenfield. But short of that, not so much with the Jeff Greenfield, OK?

These were questions that mattered enough to these "undecided" voters that they showed up to a "debate," and asked them of the two men for the office of "most powerful man in the world." The answers must matter to those voters, and since they matter in this election, I'd like to know what they think. Unless there was some sort of ban on talking to the actual questioners (I definitely saw comments from someone who was in the audience, but not picked to ask a question, so there can't have been a total ban on talking to the audience), they should be asked their opinion. Even if it's a confused muddle (which seems likely), that's information I'd like to see.

But again, nothing. What the hell are all these media people getting paid for?

Finally, how is it that undecided voters in the St. Louis area managed to come up with better questions than any of the professional journalists picked to "moderate" these dog and ony shows? If this "Do you love your wife?" puffery is the best that professional journalism has to offer, we might as well start choosing "debate" "moderators" by lot in 2008.

Honestly, where did we find these clowns? And can we send them back?

Posted at 10:01 AM | link | follow-ups | 7 comments

Long-time readers of this site may have noticed a lack of sport-related posts in recent weeks, despite the fact that my teams are doing pretty well at the moment. This is not really a coincidence-- I'm as surprised as anyone to see the Giants winning games (though you will note that they lost to the only really good team they've played...), but they've been winning while I haven't been talking about them, so why screw up a good thing? The Woof Gods are mighty, and utterly without pity-- just ask Curt Schilling (that "nothing better than making 55,000 New Yorkers shut up" comment didn't work out so well, did it?).

Of course, this may seem a strange attitude for a scientist to have. We are, after all, supposed to be rational and completely above superstition, at least if you believe Hollywood. I have two responses to that: first, that sports fandom transcends science-- that's a given. The second response is that people tend to underrate the role of superstition in experimental science.

I've previously mentioned the Ti:Sapph laser I used in grad school, that every post-doc in the lab had a different can't-miss trick for adjusting, none of which worked for me. We had a second laser of the same model, that served as the main trapping laser for the system, which had to be locked to a particular frequency each morning.

As a young graduate student, I had the lock procedure explained to me by the senior student who had built it. You would switch the laser controller to scan mode, turn the scan rate up to its highest setting, adjust the frequency until the laser was sweeping over the desired transition, then shut off the scan mode, use the fine control on the lock circuit to tune the frequency to the transition you wanted to lock to, and engage the lock. Then you had to turn the scan rate all the way down.

Now, there's no earthly reason why the scan rate should make any difference at that point. The controller had been taken out of the scan mode, so the scan rate knob wasn't really connected to anything. Matt swore that it was an important step, though, so I smiled, and nodded, and proceeded to ignore it for the first month or two that I was running the experiment.

And I'll be damned if he wasn't right. If you didn't turn down the scan rate after locking the laser, it just didn't stay locked as well as it should. There was absolutely no reason for it, but it was perfectly repeatable over a period of months. So I added it back to the routine, and it was an essential part of the start-up procedure from then on. A couple of years later, when I had to explain the system to a post-doc, he didn't believe me, either, until he saw it for himself.

Since then, I've become accustomed to superstition in science. Things get turned on in a certain order, whether or not there's a clear reason for it, because "it just works better that way." The first couple of data points you try to take are always screwy in some way, so you might as well just skip a couple of shots, whether you have reason to think there's something screwy about them or not. You always touch something metal before adjusting a laser.

Many of the little routines we go through have a logical basis-- diode lasers are very sensitive to static electricity, so it's a good idea to ground yourself before touching the laser, to prevent a static spark that could kill it. They tend to morph into something more reflexive, though-- the last time I played with an argon laser, I caught myself carefully touching metal objects before touching the controls, even though static isn't a concern there. It's just become part of the process of laser adjustment for me. In a similar way, there's obviously some logical explanation for the scan rate affecting the laser lock, back at NIST, but we never figured it out. It was easier to just take it as a fact of nature, and move on.

Talking about this process at lunch one day (specifically, the scan rate thing), a colleague described a study he'd seen in which a bunch of pigeons were put in a cage with a food dispenser and a light. After an initial period in which the light would flash before food was dispensed, they put a random interval between the light and the food. A pigeon who expected food when the light came on would be surprised to find that it didn't come out for a second or two... right after it twitched its wings in a certain way. So the next time the light flashed, the pigeon would do a little wing twitch. And then a head bob. And after a while, whenever the light would flash, the pigeon would go into this complicated and entirely random dance, in the belief that it was essential for causing the food to appear.

There are days in the lab when I think we're nothing but a bunch of dancing pigeons.

Posted at 8:12 AM | link | follow-ups | 10 comments

This week's journal club is a singleton. It was a slow week for physics papers (at least papers that I cared enough to read) in the major journals. I almost skipped Neutral Atom Quantum Register in PRL, too, as that number of Russian-sounding names in the author list is usually an indicator of Inscrutable Theory. I recognized the PI as an experimentalist, though, and it was indeed worth a look.

This is an experimental paper about a potential scheme for using neutral atoms in a quantum computer. As previously noted, a good quantum computing scheme needs to provide five things: long decoherence times (so the atoms stay in superposition states long enough for the computation to unfold); addressability (so you can target specific bits of your computer); a readout scheme (so you can extract an answer at the end of the computation); scalability (so you can add enough bits to do something worthwhile); and entangling operations (so you can create the cool quantum states that make the whole thing work). The scheme described in this paper gets four out of five, but the fifth is a doozy.

The method they propose is to trap single atoms in the anti-nodes of a standing wave laser beam, and use the two hyperfine levels of the ground state as the "0" and "1" states for the computation. This is great for decoherence, as these states have huge lifetimes, and are relatively easy to shield from outside influences. Readout is done by detecting the fluorescence when the atoms are hit by a laser, and is a well-established technology. In this paper, they demonstrate addressability by applying a linearly increasing magnetic field across the sample, so that each of the trapped atoms has a slightly different resonant frequency. They show pretty convincingly that they can pick out an arbitrary atom in the middle of the sample, and flip it from "0" to "1" in a fairly short time. Scaling is just a matter of adding more atoms to the sample, which is easy enough to do.

The one thing missing is an entangling operation. They've got a great scheme for storing neutral atoms in such a way that they can be manipulated and measured at will, but they don't have a good way to entangle two of the atoms-- that is, they can't do an operation where the state of one of the atoms ends up depending on the state of a different atom without measuring it. And without that, it's not really a quantum computer. They do mention a bit of an idea (moving atoms around to bring them close together where some reasonable interaction might entangle them), but it's not really there yet.

Of course, neither are any of the other experimental quantum computing schemes, so it's not like they're being blown away by the competition. And if they can come up with a workable way to entangle the atoms, this system will make somebody a really neat toy.

Posted at 7:37 AM | link | follow-ups | 4 comments

As mentioned in the journal club intro, I've been meaning to try to do a good explanation of BEC for a while now. I keep getting hung up on it, though, because quantum statistics is a really difficult thing to explain.

This isn't exactly news, of course. The late, great Richard Feynman famously said that any well-understood area of physics should be explainable at the freshman level. Challenged by a colleague to explain the statistical distribution function for fermions, he set out to write a freshman-level lecture on the topic, and couldn't. He took this as evidence that we don't really understand the origin of the Fermi-Dirac distribution. (You can find a million versions of this story via Google.) Many years later, I still haven't seen a really good explanation of this problem (John Baez's has been suggested, but I don't really buy it).

This is a long way of warning my readers that what follows won't necessarily be particularly useful. If Feynman couldn't come up with a good freshman-level explanation, the odds of my finding a way to explain it to the laity are not good.

Anyway, here's the situation: all particles in the Universe can be divided into two classes, with the division being determined by a property called "spin". It gets that name because the mathematical description of the behavior of "spin" is very much like the mathematical description of a solid spinning ball, but it's important to note that the particles themselves are not, in fact, spinning (if you try to calculate the necessary rotation rate for a proton, you find that a point on the surface should be moving faster than the speed of light). "Spin" is an intrinsic property of the particles themselves, and has nothing to do with actual motion.

Spin comes in two varieties: integer (0, 1, 2, etc.) and half-integer (1/2, 3/2, 5/2, etc.). Particles with integer spin are "bosons," after the Indian mathematician Satyendra Nath Bose, while particles with half-integer spin are "fermions," after Enrico Fermi.

String theorists and high energy types will tell you that all material particles are fermions, and that bosons are only force carriers. That's true, as long as you're only talking about single particles: protons, neutrons, and electrons are all fermions. For those of us who work in the real world, with whole atoms, though, it's possible to have atoms of either type: if you stick two fermions (spin of 1/2) together, you get a composite boson (spin of 0 or 1, depending on which way the spins are arranged). You can keep doing this with more and more particles-- as many as you want. If your atom has an even number of protons, neutrons, and electrons (all together), it's a boson. If it has an odd number of protons, neutrons, and electrons, it's a fermion.

"OK," you're saying, "But what's the point?" The point of all this is that the two types of particles are subject to very different rules. If you have a large number of bosons, they will arrange themselves in a dramatically different way than a large number of fermions.

The more familiar of the two is the distribution for fermions, which is determined by the Pauli Exclusion Principle, which says that no two fermions can occupy a single state. If you took chemistry in high school, you spent a bunch of time drawing little up and down arrows to indicate the filling of electron shells in atoms: that's Pauli Exclusion. Electrons are fermions, so no two of them can be in the same state. You can put two electrons in the same energy level, but they have to have their spins pointing in opposite directions (hence the arrows). Once you've got one "up" electron and one "down" electron, that state is full, and you have to move on to the next level. The whole discipline of chemistry is a consequence of Pauli Exclusion.

Bosons are different. There is no prohibition against two bosons occupying the same state. In fact, they're perfectly happy to pile into a given state in huge numbers. If electrons were bosons, all the little arrows would point in the same direction, and all the electrons would be in the lowest energy level, and chemistry exams would be really, really easy.

So, why is this? That is, why is it that half-integer spin is the characteristic that determines whether a particle is subject to Pauli Exclusion? That's the question that Feynman couldn't come up with a freshman-level answer to. The consequences of this split are many, varied, and very cool indeed, but the reasons for it are pretty opaque. I can't say I really understand the origin of it myself-- I understand the mathematics, but the "why" is pretty hazy.

So, much as I hate to do this, I'm going to have to punt. You'll just have to take my word for it that this is how the world works, and I'll explain the consequences (including BEC) a little later.

Posted at 1:51 PM | link | follow-ups | 9 comments