The "Ballistic" Story

Last week, in the post about fermion conduction, I left a reference hanging:

There’s nothing physically blocking the atoms from flying right through the channel– in fact, an atom that enters the channel will always exit the other side without slowing down along the way. This is termed “ballistic,” a term that will always have a special place in my heart thanks to an incident at my Ph.D. defense.

Which was what? Let’s just leave that hanging to see if anybody actually reads this far. I can explain it in a comment if people want to know.

A couple of people asked for the explanation in comments, and I promised to explain it in a top-level post. which I thought I'd be able to write over the weekend, but that was before the epic birthday party for one of SteelyKid's classmates that wiped out Sunday, and carried into the week. But I'll provide the explanation now:

OK, so "ballistic" and your thesis defense. What's the deal with that? So, there are a bunch of rules about the composition of thesis defense committees, one of which says you need a representative from outside the degree program of the candidate. This turned out to be a little tricky, because my degree is in Chemical Physics, a program drawing from both the Physics and Chemistry (and some Electrical Engineering, I think) departments at UMD, so most of the people who do stuff closely related to what I do were already part of the program, and thus not eligible. (The usual dodge for an atomic physicist in the Physics department would be to get a small-molecule chemist, or vice versa, but all those people were part of Chem. Phys.).

Sounds tricky. to make matters worse, my advisor at NIST, Bill Phillips, had won a Nobel Prize a year and a half earlier, and was in the early stages of a sort of world tour. Bill didn't have the right sort of appointment at Maryland to officially supervise graduate theses, though, so I was officially a student of John Weiner's, and he was in the process of moving to France. It turned out there was one and only one day, the Friday before the Monday deadline for handing in the absolute final version of the thesis to the university, when John and Bill would both be in the DC area at the same time, and so the defense had to be on that one particular date.

So, how did you find anybody to be on a committee with that restrictive a schedule? Happily, I didn't have to. The director of the Chem. Phys. program, Mike Coplan, did the heavy lifting, and eventually found a guy associated with the superconductivity center to agree to be on the committee. He insisted on meeting me before he would officially agree, though, so I went to his office (which was in a weird annex of the physics building that took me a while to find, but whatever).

I take it this was not a pleasant meeting, or you wouldn't've brought it up? Um, no. It wasn't completely awful, but it basically consisted of him lecturing me at length about all the things past Ph.D. students had done to annoy him, and me assuring him that I wouldn't possibly do anything like that, and thanking him profusely for agreeing to be on my committee, and all that.

I'm sure that did wonders for your ambient stress level headed into the defense. Exactly. I was absolutely petrified of what this guy was going to do, not only because he was ridiculously demanding about when he had to have a copy of the thesis, and lectured me about grammar for a while, but because he worked in a field that I knew basically nothing about. I took one Solid State class, didn't really care for it, and got a lousy grade, so if he wanted to bust out a question from his area of research during the free-for-all part of the defense, I was basically completely screwed.

So, what happened? Well, I got to the defense expecting to be ripped apart on every slide of my talk, so I only brought the slides for the 20-minute version of my research talk, not the 50-minute seminar. I started into it, hands shaking, and was completely shocked to get no questions. At all. I was almost halfway through, absolutely terrified that they were going to let me go straight through with the short talk, and then rip me apart for an hour of free-for-all questions. And then I hit the ballistic slide.

I knew that word had to show up somewhere. Yes, that's the whole point of this, after all. Anyway, I was talking about an experiment we did looking at collisions at extremely low temperatures, which we did not by cooling the whole sample that far, but by cooling them to a moderately warm temperature by laser-cooling standards, then letting them go. As the cloud expands, the effective temperature for collisions drops.

Wait, what? That's right. The effective temperature goes down, because in order to end up in the same place after the cloud of atoms has expanded for a while, two colliding atoms need to have very similar initial velocities. Which means their relative velocity is very low, thus a very low effective temperature for the collisions.

That's a clever trick. Yeah, it was pretty cool, pardon the pun. It was also the key to this one experiment, so I spent a full slide explaining it. And in the process referred to the motion of the atoms as a "ballistic expansion" of the cloud.

At which point, the scary out-of-program guy blew up. "Who came up with this term 'ballistc' anyway?!?" he demanded, angrily. I didn't quite have the presence of mind to guess "Galileo?" and just stammered something incoherent, so he started yelling about how using that term was an outrage.

You might say that he went-- Don't. Just don't.

You're no fun. Anyway, he was really agitated about this, which baffled me for a minute, and the other members of the committee started trying to talk him down, when I finally caught what the problem was. You see, as I said in that post last week, "ballistic" is a term used to describe the flight of a bunch of particles that don't interact with each other. In order for the expansion to be ballistic, there can't be significant collisions. But here I was, talking about using a "ballistic" expansion to study collisions within the gas that was expanding.

Now that you mention it, that does sound pretty damning. So, did they fail you, or did you fall down and beg piteously until they let you graduate anyway? Neither. In fact, this was the best thing that happened during the talk.

You see, when we sent the paper off to a journal a few months earlier, one of the referees raised the same issue, though not in those words. And as a result, I had looked into this, and found that the density we were working at was low enough that the number of collisions was completely negligible. If we held the atoms in a tight trap for the full duration, so the cloud remained at the highest density of the whole experiment, the number of collisions per atom would've been something like 0.4. But in fact, the density of the cloud dropped by a couple of orders of magnitude over the course of the expansion, leading to a corresponding drop in the collision rate.

So, while the expansion wasn't perfectly ballistic in the strictest technical sense, it was so close that there was no practical difference. Any given atom would fly out happily in a straight line, colliding with at most one other atom, if those two atoms happened to be in the tiny fraction of the atoms in the cloud that happened to collide in a given run of the experiment.

That must've felt good. Yeah, it was pretty awesome. The other faculty on the committee were trying to get him calmed down, but I was able to say "No, no-- I got this" and give a good explanation of why there wasn't anything wrong with the term "ballistic" as I was using it.

And, even better, that outburst seemed to break some sort of block, and from that point on, I was interrupted regularly, and the talk took a decent amount of time. I still got hit with some tough questions afterwards, but the scary out-of-program guy was mollified by my explanation, and didn't really push me after the formal talk. I got through it, and got my degree.

And, apparently, learned to love the word "ballistic." Exactly. Every time I hear it, I remember that story, and how good it felt when I realized I understood exactly why the scary guy was angry, and how to solve the problem.

(And, as a bonus, I have a smart question to throw at anybody using "ballistic" in a condensed matter context, should I ever need one...)


More like this

So, what's the difference between a chemical physicist and a physical chemist? (I have a friend who's a physical chemist at the U. of Cincinnati.)

A chemistry professor at Williams, a physical chemist, once started a research talk by saying that a chemist is somebody who buys an apparatus and makes a sample to test in it, while a physicist is someone who builds an apparatus then buys a sample to test. So, a physical chemist is someone who builds and apparatus and makes a sample to test in it. By that logic, I suppose, a chemical physicist would be someone who buys an apparatus and buys a sample...

In more practical terms, I suspect that both chemical physicists and physical chemists work on the same problems, but chemical physicists have academic jobs in physics departments while physical chemists have academic jobs in chemistry departments.

My friend is Tom Beck, who has numerous publications under the name Thomas L. Beck, and is also a fantastic acoustic bass player. (I know Tom because we play music together.) Tom's always asking me about playing music more melodically, and I'm always asking Tom about stochastic modeling of chemical interactions. I'm not a scientist, but a huge science fan.

Last time I talked to Tom about chemistry/physics, he was working on a computational model for the free energy of a single proton in water. He was totally jazzed because his numerical results were orders of magnitude away from both the theoretical model and experimental results.

It taught me an important lesson: Huge errors are really exciting to scientists. You can generally tell where someone is coming from by how they react to errors and uncertainty.

So, while the expansion wasn’t perfectly ballistic in the strictest technical sense, it was so close that there was no practical difference.

There are lots of times when this works. My own research involves collisionless plasmas, where the probability of any given ion or electron interacting with another particle is vanishingly small, but you still get collective effects, and for sufficiently slow oscillations fluid (MHD) theory is perfectly valid.

Be careful, however. Systems with small but nonzero viscosity behave much differently from systems with zero viscosity. That happens because the viscosity term in the Navier-Stokes equation is attached to a boundary condition, so you can't neglect it in most real applications.

By Eric Lund (not verified) on 13 Sep 2012 #permalink

My out-of-program guy (appointed by someone outside the department) was a geologist (one of "the rock people", as Sheldon Cooper would put it). At one point, I said that two (of the three) temporal logics that I used were "incomparable", which in set theory means that neither is a subset of the other. I then described the logics, where their expressiveness overlapped, and examples of propositions that could be expressed in one but not the other. "But..." the geologist interrupted, "you *just* compared them!" Silence as I and my in-program committee mentally reviewed what I'd said that could make him think so. And then it hit me that this was a case of "ceci n'est pas une pipe" -- "No, sir. I compared the *descriptions* of the logics. In set theory, incomparable sets..."

By Christopher Bohn (not verified) on 13 Sep 2012 #permalink

As a physical chemist who has spent his career as an instrument builder in a field dominated by physicists, I'd say the difference between chemical physics and physical chemistry is primarily in the program in which they earned their degree.