Thursday, November 26, 2015

Anecdote 7: Time travel and the most creative lecture I ever saw

My senior undergrad year, Princeton offered their every-three-years-or-so undergrad general relativity course (AST 301), taught at the time by J. R. Gott III.   Prof. Gott ran a pretty fun class, and he was a droll lecturer with a trace Southern accent and a dry sense of humor.  He was most well known at the time for solving the equations of general relativity for the case of cosmic strings, sort of 1d analogs of black holes.  He'd shown that if you have one cosmic string move past another at speeds approaching the speed of light, you could in principle go back in time.

The lectures were in a small tiered auditorium with the main door in the front, and a back entrance behind the last row.  On one Thursday in the middle of the semester, we were sitting there waiting for class to start, when the front door of the auditorium flies open, and in bursts Gott, with (uncharacteristically) messy hair and dressed (unusually) in some kind of t-shirt.  He dashed in, ran over to the utility closet in the front of the room, tore it open, and threw in a satchel of some kind before slamming the door.  He turned, wild-eyed, to the class, and proclaimed, "Don't be alarmed by anything you may see here today!" before running out the front door.

This was odd.  We looked around at each other, rather mystified.

Two minutes later, right at the official start time for class, the back door of the classroom opened, and in stepped a calm, combed Prof. Gott, wearing a dress shirt, tie, and jacket.  He announced, "I'm really sorry about this, but my NASA program officer is here today on short notice, and I have to meet with him about my grant.  Don't worry, though.  I've arranged a substitute lecturer for today, who should be here any minute.  I'll see you next Tuesday."  He then left by the back door.

Another minute or two goes by.  The front door opens again, and in steps a reasonably composed Prof. Gott, again wearing the t-shirt.  "Good morning everyone.  I'm your substitute lecturer today.  I've come back in time from after next Tuesday's lecture to give this class."

This was met with good-natured laughter by the students.

"Unfortunately," he continued, "I didn't have time to prepare any transparencies for today.  That's fine, though, because I'll just make them after the lecture, and then go back in time to before the lecture, and leave them somewhere for myself.  Ahh - I know!  The closet!"  He walked over to the closet, opened the door, and retrieved the bag with the slides.  There was more laughter and scattered clapping.  "Of course," said Prof. Gott, "now that I have these, I don't have to make them, do I.  I can just take these slides back to before the start of class."  Pause for effect.  "So, if I do that, then where did these come from?" More laughter.

Prof. Gott went on to deliver a class about time travel within general relativity (note to self:  I need to read this book!).

Postscript:  The following Tuesday, Prof. Gott arrived to teach class wearing the t-shirt outfit from the previous Thursday.   We were suitably impressed by this attention to detail.  As he walked by handing back our homework sets, I noticed that his wristwatch had a calendar on it, and I said that we should've checked that last time.  He hesitated, smiled a little grin, and then went on.

Thursday, November 19, 2015

Entanglement + spacetime - can someone clue me in here?

There is a feature article in the current issue of Nature titled "The quantum source of space-time", and I can't decide if I'm just not smart enough to appreciate some brilliant work, or if this is some really bad combination of hype and under-explanation.   

The article talks about the AdS-CFT correspondence - the very pretty mathematical insight that sometimes you can take certain complicated "strong coupling" problems (say gravitational problems) in 3d and map them mathematically to simpler (weakly coupled) problems about variables that live on the 2d boundary of that 3d volume.  I've mentioned this before as a trendy idea that's being applied to some condensed matter problems, though this is not without criticism.

Anyway, the article then says that there is deep high energy theory work going on looking at what happens if you mess with quantum entanglement of the degrees of freedom on that boundary.  The claim appears to be that, in some abstract limit that I confess I don't understand, if you kill entanglement on the boundary, then spacetime itself "falls apart" in the 3d bulk.  First question for my readers:  Can anyone point to a genuinely readable discussion of this stuff (tensor networks, etc.) for the educated non-expert?

Then things really go off the deep end, with claims that entanglement between particles is equivalent to an Einstein-Rosen wormhole connecting the particles.  Now, I'm prepared to believe that maybe there is some wild many-coordinate-transformations way of making the math describing entanglement look like the math describing some wormhole.  However, the theorists quoted here say things that sound stronger than that, and that's completely crazy.  I can create entangled photons in a lab with a low-power laser and a nonlinear crystal, and there is no way that this is physically equivalent to creating highly curved regions of spacetime and nontrivially altering the topology of spacetime.    Can someone explain to me whether the theoretical claims are like the former (there is some formal mathematical similarity between entangled particles and wormholes) or the much more extreme statement?

Tuesday, November 17, 2015

Guide to faculty searches, 2015 edition

As I did four years ago and four years before that, I wanted to re-post my primer on how faculty searches work in physics.  I know the old posts are out there and available via google, but I feel like it never hurts to revisit career-related topics at some rate.  For added complementary info, here is a link to a Physics Today article from 2001 about this topic.

Here are the steps in the typical faculty search process:
  • The search gets authorized. This is a big step - it determines what the position is, exactly: junior vs. junior or senior; a new faculty line vs. a replacement vs. a bridging position (i.e. we'll hire now, and when X retires in three years, we won't look for a replacement then). The main challenges are two-fold: (1) Ideally the department has some strategic plan in place to determine the area that they'd like to fill. Note that not all departments do this - occasionally you'll see a very general ad out there that basically says, "ABC University Dept. of Physics is authorized to search for a tenure-track position in, umm, physics. We want to hire the smartest person that we can, regardless of subject area." The challenge with this is that there may actually be divisions within the department about where the position should go, and these divisions can play out in a process where different factions within the department veto each other. This is pretty rare, but not unheard of. (2) The university needs to have the resources in place to make a hire.  In tight financial times, this can become more challenging. I know of public universities having to cancel searches in 2008/2009 even after the authorization if the budget cuts get too severe. A well-run university will be able to make these judgments with some leadtime and not have to back-track.
  • The search committee gets put together. In my dept., the chair asks people to serve. If the search is in condensed matter, for example, there will be several condensed matter people on the committee, as well as representation from the other major groups in the department, and one knowledgeable person from outside the department (in chemistry or ECE, for example). The chairperson or chairpeople of the committee meet with the committee or at least those in the focus area, and come up with draft text for the ad.  In cross-departmental searches (sometimes there will be a search in an interdisciplinary area like "energy"), a dean would likely put together the committee.
  • The ad gets placed, and canvassing begins of lots of people who might know promising candidates. A special effort is made to make sure that all qualified women and underrepresented minority candidates know about the position and are asked to apply (the APS has mailing lists to help with this, and direct recommendations are always appreciated - this is in the search plan). Generally, the ad really does list what the department is interested in. It's a huge waste of everyone's time to have an ad that draws a large number of inappropriate (i.e. don't fit the dept.'s needs) applicants. The exception to this is the generic ad like the type I mentioned above. Back when I was applying for jobs, MIT and Berkeley had run the same ad every year, grazing for talent. They seem to do just fine. The other exception is when a university already knows who they want to get for a senior position, and writes an ad so narrow that only one person is really qualified. I've never seen this personally, but I've heard anecdotes.
  • In the meantime, a search plan is formulated and approved by the dean. The plan details how the search will work, what the timeline is, etc. This plan is largely a checklist to make sure that we follow all the right procedures and don't screw anything up. It also brings to the fore the importance of "beating the bushes" - see above. A couple of people on the search committee will be particularly in charge of oversight on affirmative action/equal opportunity issues.
  • The dean usually meets with the committee and we go over the plan, including a refresher for everyone on what is or is not appropriate for discussion in an interview (for an obvious example, you can't ask about someone's religion, or their marital status).
  • Applications come in.  Everyone does this electronically now, which is generally a big time-saver.  (Some online systems can be clunky, since occasionally universities try to use the same system to hire faculty as they do to hire groundskeepers, but generally things go smoothly.)  Every year when I post this, someone argues that it's ridiculous to make references write letters, and that the committee should do a sort first and ask for letters later.  I understand this perspective, but I largely disagree. Letters can contain an enormous amount of information, and sometimes it is possible to identify outstanding candidates due to input from the letters that might otherwise be missed. (For example, suppose someone's got an incredible piece of postdoctoral work about to come out that hasn't been published yet. It carries more weight for letters to highlight this, since the candidate isn't exactly unbiased about their own forthcoming publications.)  
  • The committee begins to review the applications. Generally the members of the committee who are from the target discipline do a first pass, to at least weed out the inevitable applications from people who are not qualified according to the ad (i.e. no PhD; senior people wanting a senior position even though the ad is explicitly for a junior slot; people with research interests or expertise in the wrong area). Applications are roughly rated by everyone into a top, middle, and bottom category. Each committee member comes up with their own ratings, so there is naturally some variability from person to person. Some people are "harsh graders". Some value high impact publications more than numbers of papers. Others place more of an emphasis on the research plan, the teaching statement, or the rec letters. Yes, people do value the teaching statement - we wouldn't waste everyone's time with it if we didn't care. Interestingly, often (not always) the people who are the strongest researchers also have very good ideas and actually care about teaching. This shouldn't be that surprising. Creative people can want to express their creativity in the classroom as well as the lab.  "Type A" organized people often bring that intensity to teaching as well.
  • Once all the folders have been reviewed and rated, a relatively short list (say 20-25 or so out of 120 applications) is formed, and the committee meets to hash that down to, in the end, four or five to invite for interviews. In my experience, this happens by consensus, with the target discipline members having a bit more sway in practice since they know the area and can appreciate subtleties - the feasibility and originality of the proposed research, the calibration of the letter writers (are they first-rate folks? Do they always claim every candidate is the best postdoc they've ever seen?). I'm not kidding about consensus; I can't recall a case where there really was a big, hard argument within a committee on which I've served. I know I've been lucky in this respect, and that other institutions can be much more fiesty. The best, meaning most useful, letters, by the way, are the ones who say things like "This candidate is very much like CCC and DDD were at this stage in their careers." Real comparisons like that are much more helpful than "The candidate is bright, creative, and a good communicator." Regarding research plans, the best ones (for me, anyway) give a good sense of near-term plans, medium-term ideas, and the long-term big picture, all while being relatively brief and written so that a general committee member can understand much of it (why the work is important, what is new) without being an expert in the target field. It's also good to know that, at least at my university, if we come across an applicant that doesn't really fit our needs, but meshes well with an open search in another department, we send over the file. This, like the consensus stuff above, is a benefit of good, nonpathological communication within the department and between departments.
That's pretty much it up to the interview stage. No big secrets. No automated ranking schemes based exclusively on h numbers or citation counts.

Tips for candidates:

  • Don't wrap your self-worth up in this any more than is unavoidable. It's a game of small numbers, and who gets interviewed where can easily be dominated by factors extrinsic to the candidates - what a department's pressing needs are, what the demographics of a subdiscipline are like, etc. Every candidate takes job searches personally to some degree because of our culture and human nature, but don't feel like this is some evaluation of you as a human being.
  • Don't automatically limit your job search because of geography unless you have some overwhelming personal reasons.  I almost didn't apply to Rice because neither my wife nor I were particularly thrilled about Texas, despite the fact that neither of us had ever actually visited the place. Limiting my search that way would've been a really poor decision - I've now been here 15+ years, and we've enjoyed ourselves (my occasional Texas-centric blog posts aside).
  • Really read the ads carefully and make sure that you don't leave anything out. If a place asks for a teaching statement, put some real thought into what you say - they want to see that you have actually given this some thought, or they wouldn't have asked for it.
  • Research statements are challenging because you need to appeal to both the specialists on the committee and the people who are way outside your area. My own research statement back in the day was around three pages. If you want to write a lot more, I recommend having a brief (2-3 page) summary at the beginning followed by more details for the specialists. It's good to identify near-term, mid-range, and long-term goals - you need to think about those timescales anyway. Don't get bogged down in specific technique details unless they're essential. You need committee members to come away from the proposal knowing "These are the Scientific Questions I'm trying to answer", not just "These are the kinds of techniques I know". I know that some people may think that research statements are more of an issue for experimentalists, since the statements indicate a lot about lab and equipment needs. Believe me - research statements are important for all candidates. Committee members need to know where you're coming from and what you want to do - what kinds of problems interest you and why. The committee also wants to see that you actually plan ahead. These days it's extremely hard to be successful in academia by "winging it" in terms of your research program.
  • Be realistic about what undergrads, grad students, and postdocs are each capable of doing. If you're applying for a job at a four-year college, don't propose to do work that would require $1M in startup and an experienced grad student putting in 60 hours a week.
  • Even if they don't ask for it, you need to think about what resources you'll need to accomplish your research goals. This includes equipment for your lab as well as space and shared facilities. Talk to colleagues and get a sense of what the going rate is for start-up in your area. Remember that four-year colleges do not have the resources of major research universities. Start-up packages at a four-year college are likely to be 1/4 of what they would be at a big research school (though there are occasional exceptions). Don't shave pennies - this is the one prime chance you get to ask for stuff! On the other hand, don't make unreasonable requests. No one is going to give a junior person a start-up package comparable to that of a mid-career scientist.
  • Pick letter-writers intelligently. Actually check with them that they're willing to write you a nice letter - it's polite and it's common sense. (I should point out that truly negative letters are very rare.) Beyond the obvious two (thesis advisor, postdoctoral mentor), it can sometimes be tough finding an additional person who can really say something about your research or teaching abilities. Sometimes you can ask those two for advice about this. Make sure your letter-writers know the deadlines and the addresses. The more you can do to make life easier for your letter writers, the better.
As always, more feedback in the comments is appreciated.

Monday, November 09, 2015

Aliens, quantum consciousness, and the like

I've seen a number of interesting news items lately.  Here are some that you may have missed.
  • You've probably heard about the recent observations of the star KIC 8462852.  This star, over 1000 ly away, was observed by the Kepler planet-finding mission looking for transit signatures of extrasolar planets.  Short version:  It seems likely that some very weird objects are in orbit around the star, occasionally occulting large percentages (like more than 20%) of the star's light, far more light blocking and much less periodicity than is typically seen in the other 150,000 stars observed by Kepler.   On the one hand, it was suggested that one exotic (though of course unlikely) explanation for this unique phenomenon is megastructures built by an alien civilization.  This was freely admitted to be a long-shot, but generated a lot of attention and excitement.  Now there has been a followup, where observers have pointed the Allen array at the star, and they have looked from 1-3 GHz for unusual radio emissions, finding nothing.   This has generally been handled in the press like it kills the aliens explanation.   Actually, if you read the paper, we'd only be able to detect such emissions (assuming they aren't beamed right at us) if the system was putting out something like a petawatt (1015 Watts) in that frequency range.   The most likely explanation of the Kepler observations is still some natural phenomenon, but the lack of detectable radio signals is hardly conclusive evidence for that.
  • I was previously unaware that the Institute for Quantum Information and Matter at CalTech had a really nice blog, called Quantum Frontiers.  There, I learned from John Preskill that Matthew Fisher has been making serious suggestions that quantum information physics may be relevant to biological systems and neuroscience.   It's important to look hard at these ideas, but I have to admit my own deep-rooted skepticism that either (1) entangled or superposition states survive in biological environments long enough to play a central role in neurological effects; or (2) that there are biological mechanisms for performing quantum information operations on such states.  While nuclear spins are a comparatively isolated quantum system, it's very hard for me to see how they could be entangled and manipulated in some serious way biologically.
  • You may need to sit down for this.  We probably have not found evidence of parallel universes.
  • Nature had a nice Halloween article about six "zombie" ideas in physics that are so-named because they either refuse to die or are "undead".  I've talked about the Big G problem before.

Tuesday, November 03, 2015

Anecdote 6: The Monkey Problem

I should've written about this as a Halloween physics scary story, but better late than never....  I'll try to tell this so that both physicists and non-technical readers can get some appreciation for the horror that was The Monkey Problem [dramatic chord!].

Back in my first year of grad school, along with about 20 of my cohort, I was taking the graduate-level course on classical mechanics at Stanford.  How hard can mechanics really get, assuming you don't try to do complicated chaos and nonlinear dynamics stuff?  I mean, it's just basic stuff  - blocks sliding down inclined planes, spinning tops, etc., right?  Right?

The course was taught by Michael Peskin, a brilliant and friendly person who was unfailingly polite ("Please excuse me!") while demonstrating how much more physics he knew than us.   Prof. Peskin clearly enjoys teaching and is very good at it, though he does have a tendency toward rapid-fire, repeated changes of variables ("Instead of working in terms of \(x\) and \(y\), let's do a transformation and work in terms of \(\xi(x,y)\) and \(\zeta(x,y)\), and then do a transformation and work in terms of their conjugate momenta, \(p_{\xi}\) and \(p_{\zeta}\).") and some unfortunate choices of notation ("Let the initial and final momenta of the particles be \(p\), \(p'\), \(\bar{p}\), and \(\bar{p}'\), respectively.").  For the final exam in the class, a no-time-limit (except for the end of exam period) take-home, Prof. Peskin assigned what has since become known among its victims as The Monkey Problem.

For non-specialists, let me explain a bit about rotational motion.   You can skip this paragraph if you want, but if you're not a physicist, the horror of what is to come might not come across as well.  There are a number of situations in mechanics where we care about extended objects that are rotating.  For example, you may want to be able to describe and understand a ball rolling down a ramp, or a flywheel in some machine.   The standard situation that crops up in high school and college physics courses is the "rigid body", where you know the axis of rotation, and you know how mass is distributed around that axis.   The "rigid" part means that the way the mass is distributed is not changing with time.  If no forces are acting to "spin up" or "spin down" the body (no torques), then we say its "angular momentum" \(\mathbf{L}\) is constant.  In this simple case, \(\mathbf{L}\) is proportional to the rate at which the object is spinning, \(\mathbf{\omega}\), through the relationship \(\mathbf{L} = \tilde{\mathbf{I}}\cdot \mathbf{\omega}\).    Here \(\tilde{\mathbf{I}}\) is called the "inertia tensor" or for simple situations the "moment of inertia", and it is determined by how the mass of the body is arranged around the axis of rotation.  If the mass is far from the axis, \(I\) is large; if the mass is close to the axis, \(I\) is small.  Sometimes even if we relax the "rigid" constraint things can be simple.  For example, when a figure skater pulls in his/her arms (see figure, from here), this reduces \(I\) about the rotational axis, meaning that \(\omega\) must increase to preserve \(L\).

Prof. Peskin posed the following problem:
When you look at the problem as a student, you realize a couple of things.  First, you break out in a cold sweat, because this is a non-rigid body problem.  That is, the inertia tensor of the (cage+monkey) varies with time, \(\tilde{\mathbf{I}}= \tilde{\mathbf{I}}(t)\), and you are expected to come up with \(\mathbf{\omega}(t)\).    However, you realize that there are signs of hope:

1) Thank goodness there is no gravity or other force in the problem, so \(\mathbf{L}\) is constant.  That means all you have to do for part (a) is solve \(\mathbf{L} = \tilde{\mathbf{I}}(t)\cdot \mathbf{\omega}(t)\) for \(\mathbf{\omega}(t)\).  

2) The monkey at least moves "slowly", so you don't have to worry about the possibility that the monkey moves very far during one spin of the cage.  Physicists like this kind of simplification - basically making this a quasi-static problem.

3) Initially the system is rotationally symmetric about \(z\), so \(\mathbf{L}\) and \(\mathbf{\omega}\) both point along \(z\).  That's at least simple.  

4) Moreover, at the end of the problem, the system is again rotationally symmetric about \(z\), meaning that \(\mathbf{\omega}\) at the conclusion of the problem just has to be the same as \(\mathbf{\omega}\) at the beginning.

Unfortunately, that's about it.  The situation isn't so bad while the monkey is crawling along the disk.  However, once the monkey reaches the edge of the disk and starts climbing toward the north pole of the cage, the problem becomes very very messy.  The plane of the disk starts to tilt as the monkey climbs.  The whole system looks a lot like it's tumbling.  While \(\mathbf{L}\) stays pointed along \(z\), the angular velocity \(\mathbf{\omega}\) moves all over the place.  You end up with differential equations describing everything that can only be solved numerically on a computer.

This is a good example of a problem that turned out a wee bit harder than the professor was intending.  Frustration among the students was high.  At the time, I had two apartment-mates who were also in the class.  We couldn't discuss it during the exam week because of the honor code.  We'd see each other on the way to brush teeth or eat dinner, and an unspoken "how's it going?" would be met with an unspoken "ughh."  Prof. Peskin polled the SLAC theory group for their answers, supposedly.  (My answer was apparently at least the same as Prof. Peskin's, so that's something.)  Partial credit was generous.  

There was also a nice sarcastic response on the part of the students, in the form of "alternate solutions".  Examples included "Heisenberg's Monkey" (the uncertainty principle makes it impossible to know \(\mathbf{I}\)....); "Feynman's Monkey" (the monkey picks the lock on the cage and escapes); and the "Gravity Probe B Monkey" (I need 30 years and $143M to come to an approximate solution).  

Now I feel like I've done my cultural duty, spreading the tale of the mechanics problem so hard that we all remember it 22+ years later.  Professors, take this as a cautionary tale of how easy it can be to write really difficult problems.  To my students, at least I've never done anything like this to you on an exam....

Thursday, October 29, 2015

Phase transitions: Hiding in plain sight

Phase transitions are amazing things.  As some knob is turned (temperature, pressure, magnetic field), a physical system undergoes a sudden, seemingly discontinuous change in material properties.  This doesn't happen when looking only at one or two of the constituents of the system - only when we have a big ensemble.  In our daily experience, we are used to the control parameter being temperature, and we take particular notice of phases that have dramatically different, obvious-to-the-naked-eye properties.  Solids and liquids have completely different responses to shear (or rather, liquids lack rigidity).  Liquids and gases have vastly different densities.

It turns out that there are many more phases out there, distinguished in ways that are more subtle and harder to see.  Gadolinium is a magnet below room temperature, and a nonmagnetic metal above room temperature, but to the naked eye looks the same in both phases.  We only know that the transition is there because it has measurable consequences (e.g., you could actually see magnetic forces from a hunk of cold gadolinium).

This week, there was some media attention paid to work from David Hsieh's group at Cal Tech, where they discovered an example of a particularly subtle transition in strontium iridate (Sr2IrO4).  In that stuff, similar in some ways to the copper oxide superconductors (based on CuO4 motifs), there are unpaired electrons (and therefore unpaired spins) on the iridium atoms.  Below a critical temperature (near 200 K, or about -70 Celsius), these spins somehow spontaneously arrange themselves in a subtle way that picks out special directions in the crystal lattice and breaks mirror symmetry, but is not some comparatively well-known kind of magnetic ordering.  They are only able to identify this weird "hidden" ordered phase via a particular optical measurement, since the broken symmetry of the ordered state "turns on" some optical processes that would otherwise be forbidden.  The interest in this system stems from whether similar things may be happening in the cuprate superconductors, and whether the iridates could be tweaked and manipulated like the cuprates.  Neat stuff, and a reminder that sometimes nature can be subtle:  There can be a major change in symmetry properties (i.e., above the transition temperature, the x and y directions in the crystal are equivalent, but below the transition they aren't anymore) that shows up spontaneously, but is still hard to detect.

Tuesday, October 27, 2015

No, dark matter probably did not do in the dinosaurs.

Resurfacing briefly in the midst of proposal writing, I feel compelled to comment on recent media coverage of Lisa Randall's new book.  The science story is an interesting one, as recounted here.   In brief:  There is some evidence of periodicity in mass extinctions due to impacts, though there are at least three hidden assumptions even in that statement.  One possible source of periodicity could be associated with the motion of the solar system (and by extension the earth) in and out of the galactic plane as the sun orbits the center of mass of the Milky Way.  Lisa Randall and Matthew Reece argued that passage through a comparatively thin disk of dark matter on the galactic plane could lead to gravitational perturbations that could lead to Oort Cloud comets getting dinked toward the inner solar system.  A neat idea, though pretty hard to test except through indirect means.  

So, could this have happened?  Sure.  Is it likely?  Well, that's much trickier to evaluate.  There are plenty of other sources of gravitational perturbations (e.g., the passage of nearby stars) that we know for sure take place.  There is no strong observational evidence right now that the would-be disk of dark matter exists, let alone whether it has the properties needed to provide a significant uptick in cometary impacts.  Lisa Randall is undoubtedly a gifted writer, and there is real science here (witness the published paper that discusses ways to test the idea), but the breathless media reaction is somewhat disappointing.