Know Your Geeks

Alex Palazzo offers a taxonomy of biologists, and takes some heat in the comments for leaving people out or mischaracterizing subdisciplines. This reminded me that I did a similar post about physics quite some time ago-- almost four years! That's, like, a century in blog-time...

I'll reproduce the geek taxonomy after the cut, and clean up a few rotted links.

Geek Taxonomy

So, what, exactly, is it that I do for a living? (Other than come in to work every morning and respond to disgruntled emails about the grades I hand out, that is...). Depending on the context, I have a bunch of different answers to this question, depending on the degree of detail required. "College professor" or "physicist" seems to satisfy most people (who generally respond with some variant of "Eww. I hated that class."). A slightly more specific answer would be "I study atomic, molecular and optical physics" (I used to just say "atomic physics" but then people would assume I built bombs for a living...), though "Quantum Optics" is also a fairly accurate characterization of some of what I do, and sounds cooler. More specific yet would be "I study laser cooling of atoms and molecules" or even "I study ultra-cold collisions between atoms in laser-cooled samples." I've also worked in Bose-Einstein condensation, though that's not what I'm doing right at the moment.

Since most people are generally lost after "physicist" (and tend to have a somewhat distorted view of what physicists do-- white lab coats are less common than you might think), I'll save the explanations of quantum optics, laser cooling, and BEC for later posts, and give a quick run-down of the various sub-species of physicists. As with all academic disciplines, of course, the categories of physics are infinitely subdivided (and probably fractal in nature), but there are at least a handful of broad and generally recognized categories based on areas of study. In no particular order (and using the topic group headers favored by Physical Review Letters to remind myself of a few of them), these are:

Astrophysics and Cosmology: In a sense, this is the Department of Big Questions: Where did the Universe come from? Where is it going? How do planets/ stars/ galaxies/ clusters of galaxies form? What makes all the neat stuff we see through telescopes actually work?

Strictly speaking, Cosmology (dealing with the questions of the origin and ultimate fate of the whole Universe) is a somewhat separate field from Astrophysics (which deals with how the stuff in the Universe now works), but I'll lump them together here. I'm not entirely clear on where the line between "Astronomy" and "Astrophysics" is located, and as I know this is a subject which sometimes provokes ugly arguments, I won't attempt to draw one.

This is the sub-field most likely to produce amusingly obscure paper titles (my favorite, encountered while scanning the Table of Contents of Phys. Rev. Letters, was "Black Holes Have No Short Hair"), and also one of the most photogenic areas of physics. This stuff turns up in the New York Times science section with great regularity, usually illustrated with nifty pictures from the Hubble Telescope.

Elementary Particle Physics: These are the people with the massive particle colliders, and the whole zoo of quarks, leptons, mesons, muons, kaons, neutrinos, and all the rest. This is probably what most people think of when they think of physics. Experimental work in this field consists of getting a bunch of very small particles (generally protons), accelerating them up to a fair fraction of the speed of light, and slamming them into something else, to see what comes out. It's been compared to trying to figure out how a clock works by throwing it off a building and looking at the pieces that come out when it hits the ground.

On the theoretical side, it consists of some of the weirdest stuff you can imagine. In the regimes where these people work, even the fundamental forces are described in terms of the collisions and interactions of particles. All the fundamental forces start to merge together, the world actually exists in twenty-seven dimensions (give or take a few), and nothing much resembles reality as we know it. This is the domain of people who breathe the rarefied air of string theory (Aaron Bergman is a string theorist, and takes a whack at explaining some of it on his weblog), and judge theories as much by their mathematical elegance as by their correspondence to reality, simply because the experiments are too difficult to manage.

It may seem strange to list this field immediately after Cosmology (which deals with the study of Whopping Huge Things), but actually, the unimaginably small and the unimaginably large start to wrap into one another. If we can understand why the fundamental particles behave in the ways they do, that provides crucial information about how the Universe got here, and if we can figure out where the Universe came from and how the stuff we see formed after the Big Bang, we can maybe shed some light on why the particle zoo behaves the way it does. Think of this field as a sort of Adjunct Department of Big Questions (About Very Small Things).

This is also the Division of Very Large Collaborations. Since accelerators are so incredibly expensive, particle physicists have to team up into very large groups in order to amass the necessary funding. In idle moments, we used to check the author lists of particle physics papers to see if we could find ones with an author name for every letter of the alphabet ("X" and "Q" aren't as rare as you'd think, given the large numbers of Chinese physicists in these collaborations).

It's also the domain of the incredibly arrogant. Among scientists in general, physicists are regarded as arrogant, sneering at chemists and biologists because physics is more fundamental than those disciplines (Ernest Rutherford famously remarked "In science there is only physics, all the rest is stamp collecting."). Among physicists, the elementary particle crowd is regarded in much the same way-- they win all the "my work is more fundamental than yours" contests, and as a corollary, tend to regard their work as more important than anybody else's. Which leads to asking for six hundred gajillion dollars to build a new accelerator, and also to some foot-stamping hissy fits when they don't get the funding.

Nuclear Physics: A sort of intermediate regime between particle and atomic physics. They have to think about quarks and the like, but deal with them assembled into protons and neutrons. They deal with protons and neutrons assembled into nuclei, but don't worry about the rest of the atom, or interaction with other atoms. Experimental work in this field still involves accelerating things to very high speeds and slamming them into other things, but the collisions aren't quite as violent, and the accelerators aren't quite as expensive. The cutting-edge work in the field still involves large collaborations, though not quite as large.

In some ways, this is almost a forgotten field. It's far enough removed from everyday reality that it doesn't get press for producing useful things, but it's not as concerned with Big Questions as elementary particle physics, so it doesn't get press for answering Deep Questions. Other than the sub-set of nuclear physicists who do build bombs, you're unlikely to see these people in the New York Times.

Condensed Matter Physics: If you see a physicist wearing a white lab coat, he's either appearing in a movie, or he's a condensed matter physicist. This may be the largest single field of physics, and it's probably the most important in everyday terms.

Condensed matter physics involves the study of things which are, well, condensed: solids and liquids. It turns out to be easy to describe gases-- describe the properties of the individual particles, and make some statistical statements about the gas as a whole, and you're pretty much done. Solids and liquids, on the other hand, are much more difficult to describe. The properties of the individual atoms are still important, but they're greatly modified by having all those other atoms around, and every atom in the sample interacts quite strongly with its neighbors, all the time. This is a difficult situation to describe, and condensed matter physics has developed a wealth of techniques for dealing with these sorts of problems.

This is important, and profitable, because solids and liquids are the basis of much of modern technology. Condensed matter physicists study the properties of semiconductors (which include computer chips) and superconductors (which may be crucial for future technology), as well as materials science in general (building stronger materials, more flexible materials, lighter materials, or whatever).

Plasma Physics: Another regime associated with difficult problems, starting with "what is a plasma?" (Ask a dozen plasma physicists to define a plasma, you're likely to get fifteen different answers...) Very roughly speaking, a plasma is a gas of ionized particles. The individual components are free to move around as they like (as opposed to being bound into a solid or liquid), but they interact quite strongly with one another (meaning that the problem can't be cleanly separated into individual properties and statistical properties of the whole gas).

Plasma physics covers a wide range of topics, from plasma etching, to fluorescent light tubes, to interstellar gas clouds, to the composition and behavior of stars. Of course, the best-known application of plasma physics is the classic light bulb in a microwave, but only slightly less well known is the pursuit of fusion power generation. Electricity production by commercial fusion plants is no more than twenty years off, and expected to remain that way for the forseeable future.

Biophysics: This is more than just applying the equations of physics to determine exactly how much damage you're going to do to yourself when you fall from a high place. Biophysics is the study of the physical properties of biological systems-- how biological molecules arrange themselves, how electrical signals are transmitted between cells, how various life processes proceed on the atomic or molecular level.

This tends to shade into chemistry and biology (unsurprisingly), but it's a significant and growing field. Other biophysics type activities include developing new techniques for looking inside living things-- (N)MRI systems come from the medical side of biophysics research.

Atomic, Molecular, and Optical Physics: This is where I work. It's a very broad field, covering everything from laser development to quantum information processing, and from atomic spectroscopy to quantum state engineering. It's a fascinating field, but then I would say that, wouldn't I?

AMO Physics (as it's usually abbreviated) exists in a realm somewhere between plasma, condensed matter, and nuclear physics. We're aware of the nucleus of the atom, and it's important, but we're not overly concerned with the details. We sometimes deal with ions, but not too many of them, and we're generally happier if the atom has all the electrons it's supposed to. We deal with interactions between atoms, but not really large numbers of them at the same time (a well-known AMO joke (which may well have originated with Art Schawlow) runs "A diatomic molecule is a molecule with one atom too many.").

The systems we study are large enough to retain a clear connection to everyday reality, but small enough that quantum mechanics dominates their behavior-- another term which fairly accurately describes a lot of what goes on in AMO physics these days is "Quantum Optics," meaning that you need to treat both the atoms and the light field quantum-mechanically (the atoms behave like quantum-mechanical waves, the light waves are made up of particle-like photons). I've sometime jokingly referred to myself as a "quantum mechanic," which isn't too far off. The study of the physics of individual atoms is, after all, what gave rise to quantum mechanics in the first place.

This is also, generally, table-top physics. Experiments are conducted in smallish groups-- an AMO paper with more than five or six co-authors is rare-- and are cheap enough to be done in small labs on college campuses or the like (even though the cost of setting up a lab can be eye-popping-- I've spent in excess of $15,000 in the past three weeks-- this is small change on the scale of major science funding). It's also a fairly close community, at least on the "atomic" side (there are thousands of laser people)-- while the field itself covers a broad range of topics, it's not hard to keep track of the major players, and conferences in the field are large enough to be interesting but not so big as to be overwhelming.

I'm leaving out a number of categories, I'm sure (non-linear dynamics and statistical mechanics chief among them), but this is running rather long as it is, and the above will serve as a rough guide to the various classes of physics research (and the disparaging comments I make about other fields). So we'll stop here, and pick up at a later date with what, exactly, I do within the field of atomic physics.

(If you'd like to see the original post, in its original form, it's here.)