Monday, March 30, 2020

Phil Anderson and the end of an era

Social media spread the word yesterday evening that Phil Anderson, intellectual giant of condensed matter physics, had passed away at the age of 96.

It is hard to overstate the impact that Anderson had on the field.  In terms of pure scientific results, there are others far more skilled than I who can describe his contributions, but I will mention a few that are well known:

  • He developed what is now known as the Anderson model, a theoretical treatment originally intended to capture the essential physics in some transition metal-based magnets.  The model considers comparatively localized d orbitals and includes both hopping to neighboring sites in a lattice as well as the "on-site repulsion" U that makes it energetically expensive to have two electrons (in a spin singlet) on the same site.  This leads to "superexchange" processes, where energetically costly double-occupancy is a virtual intermediate state.  The Anderson model became the basis for many developments - allow coupling between the local sites and delocalized s or p bands, and you get the Kondo model.  Put in coupling to lattice vibrations and you get the Anderson-Holstein model.  Have a lattice and make the on-site repulsion really strong, and you get the Hubbard model famed in correlated electron circles and as the favored treatment of the copper oxide superconductors.
  • Anderson also made defining contributions to the theory of localization.  Electrons in solids are wavelike, and in perfect crystal lattices the ones in the conduction and valence bands propagate right past the ions because the waves themselves account for the periodicity of the lattice.  Anderson showed that even in the absence of interactions (the electron-electron repulsion), disorder can scatter those waves, and interference effects can lead to situations where the final result is waves that are exponentially damped with distance.  This is called Anderson localization, and it applies to light and sound as well as electrons.  With strict conditions, this result implies that (ignoring interactions) infinitesimal amounts of disorder can make a 2D electronic system an insulator.  
  • Here is his Nobel Lecture, by the way, that really focuses on these two topics.
  • In considering superconductivity, Anderson also discovered what is now known as the Higgs mechanism, showing that while the bare excitations of some quantum field theory could be massless, coupling those excitations to some scalar field whose particular value broke an underlying symmetry could lead to an effective mass term (in the sense of how momentum and energy relate to each other) for the originally massless degrees of freedom.  Since Anderson himself wrote about this within the last five years, I have nothing to add.
  • Anderson also worked on superfluidity in 3He, advancing understanding of this first-discovered non-electronic paired superfluid and its funky properties due to p-wave pairing.
  • With the discovery of the copper oxide superconductors, Anderson introduced the resonating valence bond (RVB) model that still shapes discussions of these and exotic spin-liquid systems.
Beyond these and other scientific achievements, Anderson famously articulated a key intellectual selling point of condensed matter physics:  emergent properties from collective actions of large numbers of interacting degrees of freedom can be profound, non-obvious, and contain foundational truths - that reductionism isn't always the path to understanding or "fundamental" insights.  More is different.  He also became a vocal critic about the Superconducting Supercollider.  (For what it's worth, while this certainly didn't help collegiality between high energy and condensed matter physics, there were many factors at play in the demise of the SSC.  Anderson didn't somehow single-handedly kill it.)

Anderson was unquestionably a brilliant person who in many ways defined the modern field of condensed matter physics.  He was intellectually active right up to the end, and he will be missed.  (For one of my own interactions with him, see here.)

Friday, March 20, 2020

(Experimentalist) grad students + postdocs in the time of covid-19

As I write this, a very large fraction of the research universities in the US (and much of the world) are either in a shutdown mode or getting there rapidly.  On-campus work is being limited to "essential" operations.  At my institution (and most of the ones I know about), "essential" means (i) research directly related to diagnosing/treating/understanding covid-19; (ii) minimal efforts necessary to keep experimental animals and cell lines going, as the alternative would be years or decades of lost work; (iii) maintenance of critical equipment that will be damaged otherwise; (iv) support for undergraduates unable to get home.

For people in some disciplines, this may not be that disruptive, but for experimentalists (or field researchers), this is an enormous, unplanned break in practice.  Graduate students face uncertainty (even more than usual), and postdocs doubly so (and I haven't seen anything online discussing their situation.  An eight week hitch in the course of a six year PhD is frustrating, but in a limited-duration postdoc opportunity, it's disproportionately worse.  The economics faced by universities and industry will also complicate the job market for a while.), and are often far from their families.

If we'd experienced something like this before, I could offer time-worn wisdom, but we've never had circumstances like this in the modern (post-WWII) research era.  This whole situation feels surreal to me.  Frankly, focusing and concentrating on science and the routine parts of the job have been a challenge, and I figure it has to be worse for people not as ancient established.  Here are a few thoughts, suggestions, and links as we move to get through this:

  • While we may be physically socially distancing, please talk with your friends, family, and colleagues, by phone, skype, zoom, slack, wechat, whatever.  Try not to get sucked into the cycle of breaking news and the toxic parts of social media.  Please take advantage of your support structure, and if you need to talk to someone professional, please reach out.  We're in this together - you don't have to face everything by yourself.
  • Trying to set up some kind of routine and sticking to it is good.  Faculty I know are trying to come up with ways to keep their folks intellectually engaged - regular group meetings + presentations by zoom; scheduled seminars and discussions via similar video methods across research groups and in some cases even across different universities.  For beginning students, this is a great time to read (really read) the literature and depending on your program, study for your candidacy/qualifier.  Again, you don't have to do this alone; you can team up with partners on this.  For students farther along, data analysis, paper writing, planning the next phase of your research, starting to work on the actual thesis writing, etc. are all possibilities.  For postdocs interested in academia, this is potentially a time to comb the literature and think about what you would like to do as a research program.  Some kind of schedule or plan is the way to divide this into manageable pieces instead of feeling like these are gigantic tasks. 
  • The Virtual March Meeting has continued to add talks.
  • My friend Steve Simon's solid state course lectures are all available.  They go with his book.  They are also just one example of the variety of talks available from Oxford - here are the other physics ones.
  • Here is a set of short pieces about topology in condensed matter from a few years ago.
  • And here is a KITP workshop on this topic from this past fall.
  • These are some very nice lecture notes about scientific computing using python.  Here is something more in-depth on github.  Could be a good time to learn this stuff....
  • On the lighter side, here are both PhD Comics movies for free streaming.
Feel free to leave more suggestions and links in the comments.  I'm sure we could all use them.  Stay safe.

Thursday, March 12, 2020

Exponentials, extrapolation, and prudence

It's been a remarkable week.  There seems to be a consensus among US universities, based in part on CDC guidelines, and in part on the logistically and legally terrifying possibility of having to deal with dormitories full of quarantined undergraduates, that the rest of the 2019-2020 academic year will be conducted via online methods.  This will be rough, but could well be a watershed moment for distance education techniques.  The companies that make the major software platforms (e.g. zoom, canvas) and their web storage are facing a remarkable trial by fire when the nation's large universities all come back from break and hundreds of thousands of students all try to use these tools at once.

At the same time that all this is going on, many doctoral programs around the country (including ours) that had not already done their graduate recruiting visitations were canceling open houses and trying to put together virtual experiences to do the job.  

There is a lot to unpack here, but it's worth asking:  Are people over-reacting?  I don't think so, and over-reacting would be better than the alternative, anyway.  Different estimates give a range of values, but it would appear that the age-averaged mortality rate of covid-19 is somewhere between 0.7% and 3%.  (The current number in the US is something like 2.9%, but that's probably an overestimate due to appallingly too little testing; in the non-Wuhan parts of China it's like 0.6%, but in Italy it's over 3%.)  The disease seems comparable in transmission to the annual influenza, which in the US is estimated to infect 35-40M people every year, and with a mortality rate of around 0.1% leads to something like 35-40K deaths per year.  Given this, it's not unreasonable to think that, unchecked, there could be between 250K and 1.2M deaths from this in the US alone.  A key issue in Italy stems from the hospitalization rate of around 10-15%.  If the cases come too rapidly in time, there just aren't enough hospital beds.  This is why flattening the curve is so important.

It annoys me to see some people whom I generally respect scientifically seem to throw their numerical literacy out the window on this.  We shouldn't freak out and panic, but we should understand the underlying math and assumptions and take this appropriately seriously.

Update:  Notes from a meeting at UCSF (web archive version of link) hosted by, among others, Joe DeRisi.  I first met Joe when we became Packard Fellows back in 2003.  He's a brilliant and very nice guy, who with colleagues created the viral phylogeny chip that identified SARS as a previously unknown coronavirus and pinpointed its closest relatives.

Friday, March 06, 2020

More about the APS meeting(s) and covid-19

Just to follow up:

  • The APS is partnering with the Virtual March Meeting, as well as collecting talks and slides and linking them to the online meeting program.  
  • There is going to be a Virtual Science Forum this afternoon (Eastern Standard Time, Friday, March 6) using zoom as a meeting platform, featuring what would have been March Meeting invited talks by Florian Marquardt, Eun-Ah Kim, and Steve Girvin.
  • The APS is working on refunds.  All told, the society is going to lose millions of dollars on this.
  • I am very surprised that the webpage for the APS April Meeting does not, as of this writing, have anything on it at all about this issue.  I've already passed on my strong suggestion that they at least put up a notice that says "We are closely monitoring the situation and will make a firm decision about the status of the meeting by [date]."  
  • The ACS has a notice on their page about their national meeting scheduled for Philadelphia on March 22-26.  I'm rather surprised that they are still going ahead. Update:  ACS has now cancelled their spring meeting.
  • The MRS seems to have nothing up yet regarding their April meeting.
People tend to have poor intuition about exponential functions.  I'm not an alarmist, but it's important to consider:  total US cases of covid-19 today are the level Wuhan was seven weeks ago. Hopefully measures people are taking (social distancing, hand washing, dropping non-critical travel) plus seasonality of illness plus lower population density plus fewer smokers will help keep things comparatively manageable. The US government realistically will not take some of the steps taken by the Chinese government (e.g., wholesale travel restrictions, military-enforced quarantines).

Tuesday, March 03, 2020

Virtual March Meeting

In the wake of the cancellation of the 2020 APS March Meeting due to concerns about COVID-19, an effort has sprung up, the Virtual March Meeting, with the idea of having would-be speakers record and upload their presentations.   (I believe that this was spearheaded by q-ctrl, but I'm not certain.  If someone knowledgeable about this would like to explain in the comments, that would be very helpful.)

In general, this is a great idea.  There were a number of talks, particularly some invited sessions, that I was very much hoping to see at the meeting, and if this is a way of providing access to at least some of that content, I'm all in favor.

There are some downsides.  No interactive Q&A.  Some people are willing to be speculative and show a couple of in-progress/not-yet-submitted slides in their talks, but they are unlikely to want their pre-publication ideas out there on the internet forever.  It seems unlikely that there will be large-scale participation, particularly by the generally busy folks who are giving the longer invited talks and prize talks.  Still, some effort to accommodate limited travel is better than nothing.

I've attempted to upload my own contributed talk, though it doesn't seem to have materialized yet on their siteHere it is.  If you really want to get the March Meeting experience, you should watch this from the back of a small, uncomfortably crowded room with dodgy air temperature and unreliable audio.  Also, you should pretend that the session chair stands up and starts glowering at me on slide 18.   (This is in the spirit of a comment made by a friend who once said that he couldn't make it to Princeton reunions, so instead he was going to simulate the experience by pouring beer and mud in his shoes and squishing around in the humidity.)