## Sunday, November 24, 2013

• Here is a (rather over-written) article about how one of the big proponents of massive online open courses (MOOCs) has realized that they are not really a panacea for higher education.
• As Peter Woit reports, there is going to be a 90th birthday bash for Phil Anderson next month.  (No, I was not invited, though he did fall asleep in a seminar by me once.)  More interesting is Anderson's letter to the APS News, where he points out that he was not, in fact, single-handedly responsible for killing the SSC.
• Also in the APS News was a two-part interview (here and here) with Elon Musk.  He's either The Man Who Sold The Moon, or a Bond supervillain.
• This video is a great set of demonstrations about magnetic fields and forces, though the speaker uses some very nonstandard terminology, so it's a bit hard to figure out exactly what's going on.
• Here, Prof. Laithwaite does a further cool demo of a very heavy gyroscope.
• Charles Day was written a thought-provoking essay about the academic job market.

## Monday, November 18, 2013

### Coauthorship

I've been asked by a colleague to write a post about coauthorship.  This topic comes up often in courses on scientific ethics and responsible conduct of research.  Like many of these things, my sense is that good practice prevails in the large majority of circumstances, though not 100% of the time.  I think my views on this are in line with the mainstream, at least in condensed matter physics.  First, to be a coauthor, a person has to have made a real intellectual contribution to the work, somewhere in the planning, execution, analysis, and/or writeup stages.  Simply paying for a person's time, some supplies, or lending a left-handed widget does not alone entitle someone to coauthorship.   It's best to have straightforward, direct conversations with potential coauthors early on, before the paper is written, to make sure that they understand this.  A couple of times I've turned down offers of coauthorship b/c I felt like I didn't really contribute to the paper; once, for example, one of my students did some lithography for a colleague as a favor, while offering advice on sample design.  She rightfully was a coauthor, but I hadn't really done anything beyond say that this was fine with me.

The challenge is, the current culture of h indices and citation metrics rewards coauthorship.  People coming out of large research groups with many-person collaborative projects can end up looking fantastic in some of these metrics, a bias exacerbated if coauthorships are distributed lightly.  Research cultures that have very hierarchical structures can also lead to "courtesy" coauthorships (Does the Big Professor or Group Leader who runs a whole institute or laboratory automatically end up on all the important papers that come out of there, even if they are extremely detached from the work?  I hope not.).

Coauthorship entails responsibilities, and this is where things can get ethically tricky.  As a coauthor, minimally you should contribute to the writing of the manuscript (even if that means reading a draft and offering substantive comments) and actually understand the research.   Just understanding your own little piece and having no clue about the rest is not acceptable.  At the same time, it's not really fair to expect, e.g., the MBE materials grower to know in detail some low-T, rf experimental technique tidbit, but s/he should at least understand the concepts.  A coauthor should know enough to ask salient questions during the analysis and writeup.

Note that all of this gets rather murky when dealing with very large, collaborative projects (e.g., particle physics).  When CERN collaborations produce a paper with 850 coauthors, do I think that each of them really read the manuscript in detail?  No, but they have a representative system with internal committees, etc. for internal review and deciding authorship, and the ones I talk to are aware of the challenges that this represents.

Some topics lend themselves more to a back-an-forth in the comments, and this may be one.  I'm happy to try to answer questions on this.

## Monday, November 11, 2013

### The Orthogonality Catastrophe (!)

Physicists sometimes like to use dramatic terminology to describe phenomena or ideas.  For example, the Ultraviolet Catastrophe was the phrase describing the failure of classical statistical physics to predict the correct form of thermal ("black body") radiation in the high frequency ("UV" at the time) limit.  The obvious disagreement between what seemed like a solid calculation (based on equipartition) and observation (that is, you are not bathed by gamma radiation from every thermal emitter around you) was a herald of the edge of validity of classical physics.

In condensed matter physics, there is another evocative phrase, Anderson's Orthogonality Catastrophe.  (Original paper)   Here's the scenario.  Suppose we have a crystal lattice, within which are the electrons of an ordinary metal.  In regular solid state physics/quantum mechanics, the idea is that the lattice provides a periodic potential energy for the electrons.  We can solve the problem of a single electron in that periodic potential, and we find that the single-particle states (which look a lot like plane waves) form bands.  The many-electron ground state is built up from products of those single-electron states (glossing over irrelevant details).  The important thing to realize is that those single-particle states form a complete basis set - any arrangement of the electrons can be written as some linear combination of those states.  (For students/nonexperts:  This is analogous to Fourier series, where any reasonable function can be written as a linear combination of sines and cosines.  Check out this great post about that.)

Now, imagine reaching in and replacing one of the atoms in the lattice with an impurity, an atom of a different type.  What happens?  Well, intuitively it seems like not much should happen; if there were 1022 atoms in the lattice, it's hard to see how changing one of them could do much of anything.  Indeed, if you compared the solutions to the single-particle problem before and after the change, you would find that the single-particle states are almost identical.  However, "almost" means "not quite".  Suppose the overlap between the new and old single particle states was 0.9999999, where 1 = no change.  For $N$ electrons, that means that the new many-particle ground state's overlap with the old many-particle ground state is goes something like $0.9999999^{N}$.  For a thermodynamically large $N$, that's basically zero.  The new many-particle ground state is therefore orthogonal to the old many-particle ground state.  In other words, in terms of the old basis, it seems like adding one impurity (!) produces an infinite number of electron-hole excitations (!!) (since it takes an infinite number of terms to write the new many-particle ground state in terms of the old).

So, where does this fall apart?  It doesn't!  It's basically correct.  The experimental signature of this ends up being apparent in x-ray absorption spectra, in a variety of meso/nano experiments (pdf), and in cold atoms (pdf).

Any other good physics examples of overly dramatic language?