Thursday, April 23, 2015

Anecdote 1: The Qual

A key aspect of a good graduate education is realizing, more than ever, that to be competitive you'll have to raise your game.

My cohort of physics grad students arrived at Stanford in a sunny, dry September of 1993, and we were an interesting bunch.  Four out of the twenty of us were Russian (or from the recently former Soviet Union), and for this story it's important to understand that these folks were incredibly well-prepared in terms of academic physics training.  Growing up in the Soviet system, they basically decided for you when you were something like 14 years old if you were going to be trained as a physicist.  We all got together at a mixer in a crummy graduate apartment, and I still remember a bunch of us standing around the drinks table, chatting about our undergrad schools and what we'd studied.  One person had been a kicker for the Northwestern football team!  One person had been into rock climbing and had done a fun summer program at Los Alamos.  Then one of the Russians said that he'd studied conformal field theory.  For fun.  Kind of set the stage a bit.

At the time the department had a "qualifying exam" that was one of a series of tasks students had to complete in order to (eventually) receive doctoral candidacy.  In this case, the qual was a two-day, six hours each day, written exam with a total of eight problems, basically on advanced Stanford-level undergrad material, administered early in the fall quarter.  Two of the questions were "general physics", meant to test your ability to think on the fly and reason quantitatively as a physicist - these tended to be hard, since they didn't really seem like the kinds of questions you're usually asked in a standard undergrad physics class.  As I later learned from serving as the student rep on the department's qual committee, the point of the test was not to act as a filter to weed out weak students, or some kind of check on admissions.  The intent, at least for the 30% of the faculty who really thought this was a good idea, was that this was an assessment tool.  For example, if you passed overall but did badly on the quantum question, you would be strongly encouraged to think about taking (or grading) the undergrad quantum course.  You had two tries to pass the written exam, and if you were well prepared, you were strongly encouraged to give it a shot as soon as you got started in the program - why wait?  A strong showing on the qual could also ease the process of finding a rotation slot with a would-be thesis adviser.  Still, like any formal exam when the stakes are high, the process was fraught with tension.

Getting a really good qual exam together is very challenging, particularly if you want the problems to be solvable yet not be rehashed from books or other common sources.  This particular year, Bob Laughlin was chairing the qual committee, and he had lost patience with some of his colleagues and decided to put together much of the exam himself.   (Laughlin is a well-known, larger-than-life person who figures in a couple of other stories I may get around to telling.)  The previous year he'd written a question about heat capacity and thermal conductivity involving the cooking of a pot roast.  This problem was sufficiently infamous that he thought it would be funny to write another problem our year about pot roast (though he spelled it "potroast", prompting one Russian to ask, "Vot is this 'po-tro-ast'?").  He wrote a question spoofing "Brilliant Pebbles" (pdf!), a missile defense concept that he found completely ridiculous and impractical.  The exercise was about "brilliant pot roast", with the idea of de-orbiting 2 kg pieces of beef as kinetic kill weapons to take out missiles.   This included giving your opinion and a physics justification of whether the pot roast would splatter on the outside of the missile or punch a cartoonish pot roast-shaped hole through the missile.  He concluded the problem by saying "Don't worry if the numbers you find for this are absurd.  We'll just delete them and replace them with happier numbers.  This is called 'government science'."

We took the test in a big lecture room in one of the buildings ringing Stanford's main quad.   Chalkboards up front, lots of wood, afternoon sunlight slanting through narrow windows near the high ceiling.  The room had somewhat shallow tiered seating and long, curved tables rather than desks, so that everyone taking the exam (probably 30 people or so) could spread out and have plenty of room.   Stanford's honor code meant that the exam was unproctored, but Laughlin was sitting outside doing some reading, in case we had questions about the wording of the test.

Around 5 hours into day 1 (if I recall correctly), Laughlin came into the room, looking somewhat agitated.  "May I have your attention please?  It's been brought to my attention that there is a typographical mistake on the exam."

[groan from frustrated, tired students]

"On the time-dependent quantum problem, these two frequencies \( \omega_{0} \) and \( \omega \) are both supposed to be \( \omega_{0} \).  It may not be analytically solvable as written."

[angry muttering from bitter, aggravated students who had been wasting critical time on this]

"No," says a clear, Russian-accented voice from the back of the room, the same fellow who had studied conformal field theory, "Is difficult, but can be solved.  Have done."

[combination of disbelief, resignation, and semi-desperate laughter from the crowd]

Welcome to physics grad school.





Monday, April 20, 2015

Anecdotes from grad school and beyond

I've been thinking about what a more general audience likes to read in terms of science writing beyond descriptions of cool science.  Interesting personalities definitely have appeal.  Sure, he was a Nobel Laureate, but my guess is that much of Feyman's popularity originates from the fact that he really was a "curious character" and a great story-teller.  I'm not remotely in the same league, but in my scientific career, going back to grad school, I've been ridiculously fortunate to have had the chance to meet and interact with many interesting people.  Some of the stories might give a better slice-of-life feel for graduate science education and a scientific career than you'd get from The Big Bang Theory.  I'm going to start trying to write up some of these anecdotes - my apologies to friends who have heard some of these before....

Wednesday, April 15, 2015

Several items - SpaceX, dark matter, Dyson spheres, Bell Labs, and some condensed matter articles

There are a number of interesting physicsy science stories out there right now:

  • SpaceX came very very close to successfully landing and recovering the first stage of their Falcon 9 rocket yesterday.  It goes almost without saying that they are doing this because they want to reuse the booster and want to avoid ruining the engines by having them end up in salt water.  I've seen a number of well-intentioned people online ask, why don't they just use a parachute, or set up a big net to catch it if it falls sideways, etc.  To answer the first question:  The booster is designed to be mechanically happy in compression, when the weight of the rocket is pushing down on the lower parts as it sits on the pad, and when the acceleration due to the engines is pushing it along its long axis.  Adding structure to make the booster strong in tension as well (as when it gets yanked on from above by parachute drogue lines) would be a major redesign and would add mass (that takes away from payload).  For the second question:  The nearly empty booster is basically a thin-walled metal tube.  If it's supported unevenly from the side, it will buckle under accelerations (like hitting a net).  Good luck to them!
  • It would appear that there is observational evidence that dark matter might interact with itself through forces that are not just gravitational.  That would be very interesting indeed.  Many "simple" ideas about dark matter (say photinos) are not charged, so real dark-dark interactions beyond gravity could limit the candidates to consider.  I'm sure there will be papers on the high energy part of the arXiv within days claiming that string theory predicts exactly this, regardless of what "this" is.
  • A Penn State group did a study based on WISE data, and concluded after surveying 100000 distant galaxies that there are only about 50 that seem to emit "too much" in the infrared relative to expectations.   Why look for this?  Well, if there were galaxy-spanning civilizations capable of stellar-scale engineering projects, and if they decided to use that capability to build Dyson spheres to try to capture more than 10% of the star-radiated power in the galaxy, and if those civilizations liked temperature ranges near ours, then you would expect to see an excess of infrared.  So.  Seems like galaxy-spanning civilizations that like to do massive building of Dyson spheres and similar structures are very rare.  I can't say that I'm surprised, but I am glad that creative people are doing searches like this.
  • Alcatel-Lucent, including Bell Labs, is being purchased by Nokia.  If anyone knows what this means for Bell Labs research at the combined company, please feel free to post below.  
  • One interesting article I noticed in Nature Physics (sorry for the paywall) shows remarkably nice, clean fractional quantum Hall effect (FQHE) physics in ZnMgO/ZnO heterostructures.  The FQHE tends to be "fragile" - the 2d electron system has to be in a material environment so clean and perfect that not only can an electron make many cyclotron orbits before it scatters off any impurities or defects, but that kind of disorder has to be weak compared to some finicky electron-electron interactions that are at milliKelvin scales.   The new data shows FQHE signatures at "filling fractions" (ratios of magnetic field to electron density) that correspond to some comparatively exotic collective states.  Neat.
  • There is a special issue of Physica C coming out in honor of the remarkable (and very nice guy) Ted Geballe, a pioneer in superconductivity research.  I really don't like Elsevier as a publisher, so I am not going to link to their journal.  However, I will link to the arXiv versions of all the articles I've found from that issue:  "What Tc Tells", "Unconventional superconductivity in electron-doped layered metal nitride halides", "Superconductivity of magnesium diboride", "Superconducting doped topological materials", "Hole-doped cuprate high temperature superconductors", "Superconductivity in the elements, alloys, and simple compounds", "Epilogue:  Superconducting materials, past, present, and future", and "Superconducting materials classes:  Introduction and overview".  Good stuff by some of the big names in the field.

Sunday, April 12, 2015

The Leidenfrost Effect, or how I didn't burn myself in the kitchen

The transfer of heat, the energy content of materials tied to the disorganized motion of their constituents, is big business.  A typical car engine is cooled by conducting heat to a flowing mixture of water and glycol, and that mixture is cooled by transferring that heat to gas molecules that get blown past a radiator by a fan.  Without this transfer of heat, your engine would overheat and fail.  Likewise, the processor in your desktop computer generates about 100 W of thermal power, and that's carried away by either a fancy heat-sink with air blown across it by a fan, or through a liquid cooling system if you have a really fancy gaming machine.

Heat transfer is described quantitatively by a couple of different parameters.  The simplest one to think about is the thermal conductivity \(\kappa_{T}\).  If you have a hunk of material with cross-sectional area \(A\) and length \(L\), and the temperature difference between the hot side and the cold side is \(\Delta T\), the thermal conductivity (units of W/m-K in SI) tells you the rate (\(\dot{q}\), units of Watts) at which thermal energy is transferred across the material:  \( \dot{q} = \kappa_{T} A \Delta T/L\).

Where things can get tricky is that \(\kappa_{T}\) isn't necessarily just some material-specific number - the transport of heat can depend on lots of details.  For example, you could have heat being transferred from the bottom of a hot pot into water that's boiling.  Some of the energy from the solid is going into the kinetic energy of the liquid water molecules; some of that energy is going into popping molecules from the liquid and into the gas phase.  The motion of the liquid and the vapor is complicated, and made all the more so because \(\kappa_{T}\) for the liquid is \(>> \kappa_{T}\) for the vapor.  (There is a generalized quantity, the heat transfer coefficient, that is defined similarly to \(\kappa_{T}\) but is meant to encompass all this sort of mess.)  If you think about \(\dot{q}\) as the variable you control (for example, by cranking up the knob on your gas burner), you can have different regimes, as shown in the graph to the right (from this nice wikipedia entry).  

At the highest heat flux, the water right next to the pan flashes into a layer of vapor, and because that vapor is a relatively poor thermal conductor, the liquid water remains relatively cool (that is, because \(\kappa_{T}\) is low, \(\Delta T\) is comparatively large for a fixed \(\dot{q}\)).    This regime is called film boiling, and you have seen it if you've ever watched a droplet of water skitter over a hot pan, or watched a blob of liquid nitrogen skate across a lab floor.  The fact that the liquid stays comparatively cool is called the Leidenfrost Effect.   This comparatively thermal insulating property of the vapor layer can be very dramatic, as shown in this Mythbusters video, where they show that having wet hands allows you to momentarily dip your hand in molten lead (!) without being injured. Note that this demo was most famously performed by Prof. Jearl Walker, author of the Flying Circus of Physics, former Amateur Scientist columnist for SciAm, and inheritor of the mantle of Halliday and Resnick.  The Leidenfrost Effect is also the reason that I did not actually burn my (wet) hand on the handle of a hot roasting pan last weekend.

This heat transfer example is actually a particular instance of a more general phenomenon.  When some property of a material (here \(\kappa_{T}\)) is dramatically dependent on the phase of that material (here liquid vs vapor), and that property can help determine dynamically which phase the material is in, you can get very rich behavior, including oscillations.  This can be seen in boiling liquids, as well as electronic systems with a phase change (pdf example with a metal-insulator transition, link to a review of examples with superconductor-normal metal transitions ).  

Friday, April 10, 2015

submerged due to grant deadline

Fear not, a new post is coming soon, but for now I'm trying to finish off a proposal.

Wednesday, April 01, 2015

America's "obsession with STEM education" is neither an obsession, nor is it dangerous

I'm late to the party about Fareed Zakaria's piece in the Washington Post titled "Why America's Obsession with STEM Education is Dangerous".  Zakaria is a smart guy, and I recognize that he has a book to sell, but this article is rhetorically frustrating:  He demolishes a serious straw man.  He wants people to be aware of the importance of a broad-based education, and he is apparently worried (or claiming to be for the sake of getting attention) that the US is culturally too focused on STEM and not enough on the other things, like creativity, the arts, and teaching people how to write well.

He is absolutely right that a broad-based education is generally a good idea, and that teaching people actual critical thinking and writing skills and an appreciation for things beyond math and science is also good.  However, I don't think you'll find any reasonable person advocating for purely technical educations with no cultural appreciation and ignoring teaching people how to communicate.   It's easy to demolish an argument that no one is making.  I could write 500 words about how it's crazy for people to drive themselves into crushing debt to get degrees that fail to teach them anything beyond rudimentary writing skills, but that would not be an assault on liberal education.

In two key respects, Zakaria has missed the boat.  First, while there is basically zero chance that we are going to abandon broad-based education in the US, it does seem like there is a far more real danger that we are trending away from science and rationality (c.f. vaccines, evolution, climate science).  Second, and here he was much closer to right, there is a danger in viewing absolutely all public investment in people (via education) and research purely in terms of short-term economic benefit - essentially eschewing basic research or basic education in favor purely of applied research and vocational training of obvious economic benefit to the country.   Frankly, there are people out there who truly do not believe in public education, period, and that's much scarier to me than an imagined attack on the value of the humanities as a component of an education.

Monday, March 30, 2015

The physics of drying your hands

We've all been there:  You wash your hands after using the restroom facilities, and turn away from the sink only to find one of those sad, completely ineffectual, old-style hot-air hand dryers bolted to the wall.  You know, the kind with the info graphic shown to the right (image credit:  nyulocal.com).  Why do these things work so poorly compared to paper towels?  What insight did Excel and Dyson have that makes their systems so much better?

It all comes down to the physics of trying to dry your hands.  At a rough estimate, the surface area of your hands is around 430 cm2.   If your hands, when wet, are coated on average by a layer of water 100 microns thick (seems not crazy), that's a total volume of water of 4.3 cm3.  How can you get that water off of you?  One approach, apparently the one pursued by the original hot air dryers, is to convert that water into vapor.  Clearly the idea is not to do this by raising the temperature of your hands to the boiling point of water.  Rather, the idea is to flow hot, dry air over your hands, with the idea that the water molecules in question will acquire the necessary latent heat of vaporization (the energy input required to pull water molecules out of the condensed (liquid) phase and into the vapor phase) from their surroundings - the dry air, your hands, etc.  This "borrowing" of energy is the principle behind evaporative cooling, why you feel cold when you step out of the shower.

[A digression in fancy thermodynamic language:  When liquid water is in contact with dry air, the chemical potential for the water molecules is much higher in the liquid than in the air.  While the water molecules are attracted to each other via hydrogen bonds and polar interactions, there are so many many more ways that the water molecules could be arranged if they were diluted out into vapor in the air that they will tend to leave the liquid, provided each molecule can, through a thermal fluctuation of some sort, acquire enough energy to sever its bonds from the liquid.  The departing molecules leave behind a liquid with a lower average total energy, cooling it.  Note that water molecules can come from the vapor phase and land in the liquid, too, depositing that same latent heat per molecule back into the liquid.  When the departure and arrival processes balance, the vapor is said to be at the "saturated vapor pressure", and evaporative cooling stops.  This is why sweating a whole bunch on a super humid day does not cool you off.]

Back to your hands.  Converting 4.3 cm3 of water into vapor requires about 9700 Joules of energy.  If you wanted to do this with the heat supplied by the hot air dryer, and to do it in about a minute (which is far longer than most people are willing to stand there rubbing their hands as some feeble fan wheezes along), the dryer would have to be imparting about 160 W of power into the water.  Clearly that's not happening - you just can't get that much power into the water without cooking your hands!  Instead, you give up in disgust and wipe your hands discreetly on your pants.

In contrast, paper towels use thermodynamics much more effectively.  Rather than trying to convert the water to vapor, paper towels take great advantage of (1) the very large surface area of paper towels, and (2) capillary forces, the fact that the liquid-solid surface interaction between water and paper towel fibers is so attractive that it's energetically favorable for the water to spread out (even at the cost of increasing more liquid-vapor interface) and coat the fibers, soaking into the towel.  [Bonus physics lesson:  the wet paper towel looks darker because the optical properties of the water layer disfavor the scattering processes on micron-scale bits of fluff that tend to make the towel look white-ish.]  Yes, it takes energy to make paper towels, and yes, they must then be disposed.  However, they actually get your hands dry!

What about Excel and Dyson?  They realized very clearly that trying to vaporize the water on your hands is a fool's errand.  Instead, they try to use actual momentum transfer from the air to the water to blow the water off your hands.  Basically they accelerate a stream of air up to relatively high velocity (400 miles per hour, allegedly, though that sounds high to me).  That air, through its viscosity, transfers momentum to the water and that shear force drives the water off your hands.  They seem to have found a happy regime where they can blow the water off your hands in 10-15 seconds without the force from the air hurting you.    The awesome spectacle of those good dryers just shows how sad and lame the bad ones are by comparison.