Monday, June 04, 2007

Link plus a couple of papers

The Incoherent Ponderer has a fascinating analysis up of the statistics of the PhD-to-faculty pipeline in physics. The one thing missing (for lack of a good source of statistics) is how many physics PhDs go on to become faculty in a different discipline. This is increasingly common in this age of interdisciplinary work. For example, while by the IP's rankings Rice only places 1.9 percent of its PhDs as faculty members in top-50 physics departments, I can think of a few who are now faculty in, e.g., EE, Mat Sci, BioE, Chemistry, etc. It would be very interesting to look at the trends over the last twenty or thirty years. One reason for the pedigree effect is that good science is correlated with having cutting-edge resources - as fancier facilities (at least in condensed matter) have trickled down to the masses, so to speak, have things become more egalitarian?

Two more points.... First, I have some nagging doubts about the validity of some of those numbers. I can already count 7 Stanford PhD alumni that I know who have assistant/assoc. faculty positions in top-50 universities. According to the AIP numbers, that's 25% of all of the ones out there. That seems hard for me to believe. Second, Chad Orzel has a very valid observation that goes to the heart of a pathology in our field. 93% of all colleges and universities are not in the top 50. As a discipline I think we do real sociological damage to our students when we brain-wash them into thinking that the only successful outcome of a graduate degree is a tenured job at Harvard. That kind of snobbery is harmful, and probably has something to do with attrition rates. People should not decide that they're failures because R1 academia isn't what they want to do. I thought hard about taking a job offer from a college, and I still resent the fact that some people clearly thought I was loopy for even considering that path.

arxiv:0706.0381 - Fiebig et al., Conservation of energy in coherent backscattering of light
This paper is at once a very nice piece of experimental work, and an example of the kind of argument that I really don't like. In mesoscopic physics, there is a phenomenon known as weak localization for electrons. Consider an electron moving through a disordered medium, and look at one particular trajectory that contains a closed loop (made up of straight propagation pieces and elastic scattering events). Feynman says that the amplitude corresponding to this trajectory is a complex number whose phase is found by adding up the phase from propagation along the straight segments plus the phase shifts from the scattering events. Now consider a second trajectory, identical to the first, but traversing the loop in the opposite direction. It turns out that the amplitudes of these two trajectories interfere constructively for backscattering by the loop. That is, the quantum probability for getting through the loop is below the classical value, and the quantum probability for getting reflected by the loop excedes the classical value. It turns out something very analogous to this can happen for light propagating through a diffusive medium, and this can be the basis for some really cool things, like random lasers (where the back-scattering itself acts like an effective cavity!). The authors of this paper show the physics of this beautifully, but they present it in the form of a straw man argument, saying that the coherent scattering result (with greater than classical backscattering) looks at first glance like it violates conservation of energy. No, it doesn't. It looks like coherent scattering. It doesn't look like a violation of conservation of energy any more than typical diffraction does.

arxiv:0705.4260 - Huang et al., Experimental realization of a silicon spin field-effect transistor
For nearly 17 years people have been trying to make a spin transistor of the type discussed here. The idea is that spins are injected from a magnetically polarized source, traverse a channel region, and then try to leave through a magnetically polarized grain. Depending on the gate electric field, the moving spins precess and either get out of the system or not depending on their eventual alignment relative to the drain magnetization. This has historically been extremely difficult for many reasons, not the least of which are the difficulty in injecting highly polarized carriers into a semiconductor and the annoying fact that spin polarization, unlike charge, can relax away to nothing. Well, this is a pretty convincing demo of a device quite close in concept to the original idea, though it's not a field-effect geometry as first conceived. Very pretty data.

5 comments:

Incoherent Ponderer said...

Doug, as usual any kind ranking riles up a lot of people. I did top 50 simply because this is what I had time for. I got pretty tired around top 40, but decided to extend it a little. In the end - these are just numbers, and objective numbers, unlike various "perceived quality" rankings.

I would love to see numbers for EE, MSE etc. as well - as I discuss in my post. A lot of my classmates who got degrees in physics went on to get faculty jobs in EE or MatSci - one of them did so without any postdoc experience, which is almost unheard of in physics. Perhaps this is one of the reasons places like MIT or Urbana don't stack up in normalized numbers as well - perhaps their graduates go into many other disciplines.

In several of my previous posts I mentioned the mentality of postdocs going on the job market who would only apply to top research places, and only within physics departments and would scoff at anything lesser. One postdoc I knew ridiculed me for even bringing up a school that is typically ranked in top 100 but probably not top 50, and similarly had very low opinion of materials science programs. Well, he is now unsuccessful on the job market for the second time in a row and is scrambling to find something - anything really, since his postdoc is up.

Well, the table, however limited in size, provides some perspective as to how many people get hired at these places and how hugely competitive this whole job search enterprise is.

The numbers should be correct, though - I could send you my Stanford column if you want to check against your friends. I am not at all that surprised you know a large fraction of them.

Incoherent Ponderer said...

Here's the Stanford column.

Harvard University 1
Princeton University 2
Massachusetts Inst of Technology 2
University of California-Berkeley 1
California Institute Technology
Cornell University 1
University of Chicago 1
U of Illinois at Urbana-Champaign 2
Stanford University
Univ of California-Santa Barbara
University of Texas at Austin
Columbia University
Yale University 2
University of Washington 1
Univ of California-Los Angeles
Univ of California-San Diego 1
University of Pennsylvania
University of Maryland College Park
University of Michigan
Rutgers State Univ-New Brunswick
University of Wisconsin-Madison 2
State U of New York-Stony Brook
University of Minnesota
Ohio State University
University of Rochester a
Brown University
University of Rochester
Carnegie Mellon University
Johns Hopkins University 1
Rockefeller University
Purdue University 2
Michigan State University
University of California-Irvine 2
Indiana University
CUNY - Grad Sch & Univ Center
University of Florida
Northwestern University 1
University of Colorado 2
Boston University
University of Pittsburgh
Duke University
Florida State University
Rice University 1
Brandeis University
University of Arizona
University of Virginia 1
Texas A&M University 1
Univ of California-Santa Cruz
Iowa State University
University of Southern California

Let me know if you find any discrepancies - perhaps I relied on AIP Graduate Program listings (most of which use 2005-2006 data) too much.

Doug Natelson said...

Hmm. Yeah, I can match up the names I had in mind with that listing. I'm surprised that I'm that well-networked. I know we basically agree about the culture of physics, btw....

The new NRC rankings should be very interesting. While the top 8-10 are unlikely to shift much, I bet there's been some shuffling since 1995. Though, as we know, trying to encompass something as complex as this in a single ranking number is silly.

Kun Gao said...

From an applicant's view, the rank more or less has its meaning as a guide for the choice of phd study.Of coures,the reasons e.g. individual preference,weather condition,city or country are all the key points except the pure academic level.

Bernard said...

If you think you got the "he's gone loopy" look for considering taking a job offer from a college... try telling people you're considering teaching high school after doing graduate work... :)

Of course, it's the same problem with everything (and the irony of the information age): The big names get a disproportionate share of attention, which allows them to command a disproportionate share of attention. It's a networking thing, I think -- isn't something like 90% of all traffic routed through something like 5 top sites? Not all nodes are created equal! :)