## Monday, May 18, 2015

### Book recommendations: Stuff Matters and The Disappearing Spoon

I've lamented the lack of good popularizations of condensed matter/solid state physics.  I do, however, have recommendations for two relatively recent books about materials and chemistry, which is pretty close.

The Disappearing Spoon, by Sam Kean, is a fun, engaging stroll across the periodic table, exploring the properties of the various chemical elements through the usually fascinating, sometimes funny, occasionally macabre histories of their discoveries and uses.  The title references joke spoons made from gallium that would melt (and fall to the bottom of the cup) when used to stir tea.  The tone is light and anecdotal, and the history is obscure enough that you haven't heard all the stories before.  Very fun.

Stuff Matters, by Mark Miodownik, is similar in spirit, though not quite so historical and containing more physics and materials science.  The author is a materials scientist who happens to be a gifted author and popularizer as well.  He's done a BBC three-episode series about materials (available here), another BBC series about modern technologies, and a TED lesson about why glass is transparent.

## Wednesday, May 13, 2015

### A matter of gravity

Gravity remains an enduring challenge in physics.  Newton had the insight that he could understand many phenomena (e.g., the falling of an apple, the orbit of Halley's comet) if the gravitational interaction between two objects is an attractive force proportional to the product of the objects' masses, and inversely proportional to the square of the distance between them ( $F = - G M_{1}M_{2}/r^{2}$ ), and acts along the line between the objects.  The constant of proportionality, $G$, is Newton's gravitational constant.   About 225 years later, Einstein had the insight that in more generality one should think of gravity as actually distorting space-time; what looks like a force is really a case of freely falling objects moving in the (locally) straightest trajectories that they can.  (Obligatory rubber sheet analogy here.)  In that theory, general relativity (GR), Newton's constant $G$ again appears as a constant of proportionality that basically sets the scale for the amount of space-time distortion produced by a certain amount of stress-energy (rather than just good old-fashioned mass).  GR has been very successful so far, though we have reasons to believe that it is the classical limit of some still unknown quantum theory of gravity.  Whatever that quantum theory is, $G$ must still show up to set the scale for the gravitational interaction.

It makes sense that we would like to know the numerical value of $G$ as accurately and precisely as possible - seems like the first thing you'd like to understand, right?  The challenge is, as I've explained before, gravity is actually an incredibly weak force.  To measure it well in absolute numbers, you need an apparatus that can measure small forces while not being influenced by other, faaaaaar stronger forces like electromagnetism, and you need to do something like measure the force (or the counter-force that you need to apply to null out the gravitational force) as a function of different configurations of test masses (such as tungsten spheres).

I'm revisiting this because of a couple (1, 2) of interesting papers that came out recently.  As I'd said in that 2010 post, the challenge in measuring $G$ is so difficult that different groups have obtained nominally high precision measurements (precise out to the fourth decimal place, such as $G = 6.6730 \pm 0.00029 \times 10^{-11}$ Nm2/kg2) that are mutually inconsistent with each other.  See this plot (Fig. 1 from arxiv:1505.01774).  The various symbols correspond to different published measurements of $G$ over the last 35 years (!).  The distressing thing is that there does not seem to be much sign of convergence.  The recent papers are looking to see whether there is actually some periodicity to the results (as hinted by the sinusoid on the plot).  To be clear:  The authors are not suggesting that $G$ really varies with a several year period - rather, they're exploring the possibility that there might be some unknown systematic effect that is skewing the results of some or all of the various measurement approaches.  As both teams of authors say, the best solution would be to come up with a very clean experimental scheme and run it, undisturbed, continuously for years at a time.  That's not easy or cheap.  It's important to note that this is what real, careful measurement science looks like, not some of the stuff that has made web headlines lately.

## Wednesday, May 06, 2015

### People you should've heard about: John Bardeen

If you ask the average person to name a physicist, chances are they'll mention Einstein, Hawking, and possibly Sheldon Cooper.  Maybe Richard Feynman, Brian Greene or (*sigh*) Michio Kaku.  I'd like to have an occasional series of posts pointing out people that should be well-known, but for some reason are not.  High up on that list:  John Bardeen, who is the only person one of only two people to win two Nobel prizes in the same field.

Bardeen, like many of his contemporaries, followed what would now be considered a meandering, unconventional trajectory into physics, starting out as an undergrad engineer at Wisconsin, working as a geophysicist, enrolling as a math grad student at Princeton, and eventually doing a doctoral thesis with Wigner worrying about electron-electron interactions in metals (resulting in these two papers about how much energy it takes to remove an electron from a metal, and how that can be strongly affected by the very last layer of atoms at the surface - in the 1980s this would be called "surface science" and now it would be called "nanoscience").

Bardeen was a quiet, brilliant person.  After WWII (during which he worked for the Navy), he went to Bell Labs, where he worked with Walter Brattain to invent the point contact transistor (and much more disagreeably with William Shockley), explaining the critical importance of "surface states" (special levels for the electrons in a semiconductor that exist at the surface, where the periodic potential of the lattice is terminated).  Shockley is viewed in hindsight as famously unpleasant as a co-worker/boss - Bardeen left Bell Labs in large part because of this and ended up at Illinois, where seven years later he worked with Bob Schrieffer and Leon Cooper to produce the brilliant BCS theory of superconductivity, earning his second Nobel.  (Shockley's borderline abusive management style is also responsible for the creation of modern Silicon Valley, but that's another story.)

During and after this period, Bardeen helped build the physics department of UIUC into a condensed matter physics powerhouse, a position it continues to hold.  He was very interested in the theory of charge density waves (special states where the electrons in a solid spontaneously take on a spatially periodic density), though according to Lillian Hoddeson's excellent book (see here, too) he had lost the intellectual flexibility of his youth by this time.

Bardeen contributed greatly to our understanding and advancement of two whole classes of technologies that have reshaped the world (transistors and superconductors).  He was not a flamboyant personality like Feynman (after all, he was from the Midwest :-) ), and he was not a self-promoter (like Feynman), but he absolutely deserves greater notoriety and appreciation from the general public.

## Thursday, April 30, 2015

No, NASA has not discovered warp drive.  There is a huge amount of media attention (here, here, here, for examples of relatively mainstream media) being given to a claim that a NASA team has successfully tested a gadget called the EmDrive.  The claim is that one can take a conical microwave resonator (picture the cavity that is your microwave oven, only shaped like a truncated cone rather than a rectangular box), fire up microwaves to drive the resonant modes, and the cone will experience a steady thrust in one direction (the direction of the fat end of the cavity).  There are multiple alleged explanations for this, ranging from botched thinking about special relativity to really bizarre word-salad about virtual particles, the quantum vacuum, and "warp fields".

Let me explain why this is bad science, terrible science journalism, and highly problematic.

First, the science.  Our theory of electricity and magnetism is arguably the best understood, most precisely tested theory we have, both in its classical limit (the limit relevant for your microwave oven) and in its quantum limit (the limit relevant for things like calculating the magnetic moment of the electron, something that we can do to more than 14 decimal places!  According to that theory, a closed microwave resonator does not generate thrust (surprise surprise).  Given over 100 years of tests of classical E&M, it's going to take more than one poorly documented experiment, not published, to convince scientists that something exotic is going on.  Extraordinary claims require extraordinary evidence, and this just isn't it.  Moreover, claims that exotic quantum vacuum effects or "warp fields" are somehow relevant here are just on their face absurd!  The energy densities, the materials involved, none of this couples to exotic quantum vacuum physics any more than my microwave oven does.  This is like arguing that by accelerating a simple dielectric like a piece of plastic, I should see electron-positron pair production and warped spacetime.  It's nonsense.

What would it take to convince me?  How about a thoroughly documented experiment done by someone with credibility in precision measurement, for a start.

As for science journalism:  The number of outlets who uncritically pass along something like this is appalling.  What's worse, they distort it even more - the third link up top not only claims that this is a reactionless drive, but that it will allow faster-than-light travel.  What the hell?  (Yes, I know that the Daily Fail is third-rate fish-wrap.)  I fully expect to see a CNN story about this, and it will be terrible.  This will propagate in the media for several days, and they will portray it as some underdog inventors showing that the Scientific Establishment is wrong, or they'll present this as an actual scientific controversy, when in fact the burden is all on the experimenters to show that their work (which flies in the face of decades of contrary evidence) is right.  Hey, IFLS:  You should be ashamed of yourselves for your coverage of this.  Good grief - I thought part of your message was that people should, I don't know, think critically!

Why is this problematic?  It's an issue because people don't trust science, in part because they end up reading uncritical bull like this and come away thinking that science is either a dodge, a scam, or entirely a matter of opinion, when in fact it's an approach to thinking critically about the world that has made possible all of modern technology and medicine.

## Wednesday, April 29, 2015

### Anecdote 2: Life in a lab - the Demon Liquefier From Hell

I know this will come as a shock to many of you (ahem), but when I was a kid I watched a lot of Star Trek reruns.  Even in middle school one story-telling trope that seemed phony to me was the way Scotty (and Kirk) could tell just from the sound and feel of the ship whether something was wrong with the engines or environmental controls.   Years later, as a grad student in the Osheroff lab, I realized that this was actually one of the more realistic bits of writing and characterization in the show.

Our lab focused on ultralow temperature physics.  We ran experiments using dilution refrigerators (also see here), and these each required multiple vacuum pumps running continuously (in our case, each fridge needed a helium-leak-tight, sealed, mechanical "roughing" pump, a big conventional mechanical pump (for the "1K pot"), and a large diffusion pump as a "booster").   The mechanical pumps were housed in a cabinet in a room one floor below the main lab, and even with that kind of distance and insulation they provided a continuous background hum to the room.  That basement room also contained our group's helium liquefier, an ancient beast of a machine (a twin is shown here) that took in recycled helium gas from our experiments, cooled it by using pistons to drive a big flywheel, and then liquefied it by squirting it through a tiny, cold orifice.  The liquefier provided something between a wheeze and a heartbeat to the lab, a steady state "pachooka pachooka" sound with a repetition period of around one second when it was working well.  The muffled version of this noise also permeated the lab.  After being in the group for a few months, I understood completely where Scotty was coming from.  It was deeply disturbing to walk into the lab and realize that something, somewhere was amiss because the sound or extremely subtle floor vibrations felt "off".

The liquefier (officially the Demon Liquefier From Hell [DLFH], or The Liquef--ker) was a formative part of our lab's grad school experience.  Running the system, which predated any serious automated controls, required some amount of fiddling in the best of times, interpreting half a dozen cryptic gauges ("inches of water" as a pressure unit?  Really?), with the only useful diagnostic being whether the liquid level in the big helium storage dewar is increasing or not.  A period preventative maintenance every few months meant replacing press-fit bearings, cleaning amazingly stinky phenolic parts, and worrying that we would bend a cam "wrist" and be out hundreds of dollars for a spare as well as having the system be down for a week.  Even before helium prices rose dramatically, recycling helium was a good idea if you could do it.   One of the most depressing calculations you could do as a student in our lab, as you were listening to the intake purifier blow moisture like a sad sneeze and wondering why the hell the DLFH wasn't making liquid, was to compare the cost of your time, recycled helium, and externally purchased helium, and realize that it was clearly financially smart for your adviser to use you to maintain the system.

The DLFH was certainly educational.  I learned a lot about engines and big mechanical systems.  I learned that it is only marginally cheaper to build a heavy crate and ship via an express carrier than it is just to buy a plane ticket for a 130 kg flywheel.  I learned what it feels like to take a jolt of 208 V (not recommended) and that yelped curses from that room could still be heard up in the lab.  To this day I still reflexively shudder a bit when I hear that "pachooka" sound when I visit a place with a similar gadget.

## 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.

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]