Thursday, May 31, 2018

Coming attractions and short items

Here are a few items of interest. 

I am planning to write a couple of posts about why solids are rigid, and in the course of thinking about this, I made a couple of discoveries:

  • When you google "why are solids rigid?", you find a large number of websites that all have exactly the same wording:  "Solids are rigid because the intermolecular forces of attraction that are present in solids are very strong. The constituent particles of solids cannot move from their positions they can only vibrate from their mean positions."  Note that this is (1) not correct, and (2) also not much of an answer.  It seems that the wording is popular because it's an answer that has appeared on the IIT entrance examinations in India.
  • I came across an absolutely wonderful paper by Victor Weisskopf, "Of Atoms, Mountains, and Stars:  A Study in Qualitative Physics", Science 187, 605-612 (1975).  Here is the only link I could find that might be reachable without a subscription.  It is a great example of "thinking like a physicist", showing how far one can get by starting from simple ideas and using order-of-magnitude estimates.  This seems like something that should be required reading of most undergrad physics majors, and more besides.
In politics-of-science news:

  • There is an amendment pending in the US Congress on the big annual defense bill that has the potential to penalize US researchers who have received any (presently not well-defined) resources from Chinese talent recruitment efforts.  (Russia, Iran, and North Korea are also mentioned, but they're irrelevant here, since they are not running such programs.)  The amendment would allow the DOD to deny these folks research funding.  The idea seems to be that such people are perceived by some as a risk in terms of taking DOD-relevant knowledge and giving China an economic or strategic benefit.  Many major US research universities have been encouraging closer ties with China and Chinese universities in the last 15 years.  Makes you wonder how many people would be affected.
  • The present US administration, according to AP, is apparently about to put in place (June 11?) new limitations on Chinese graduate student visas, for those working in STEM (and especially in fields mentioned explicitly in the Chinese government's big economic plan).   It would make relevant student visas one year in duration.  Given that the current visa renewal process can already barely keep up with the demand, it seems like this could become an enormous headache.  I could go on at length about why I think this is a bad idea.  Given that it's just AP that is reporting this so far, perhaps it won't happen or will be more narrowly construed.  We'll see.


SM said...

Thanks! Victor Weisskopf's article is fantastic. However, there is something I don't understand:
Equation (4) says that Kinetic energy scales as N^(2/3). However, we know that for n particles in a box, energy would scale as N^3. Where is this discrepancy coming from?

Douglas Natelson said...

SM, the N^(2/3) argument comes from a degenerate Fermi gas. The Fermi energy (kinetic energy of the highest occupied single-particle state at T=0) in 3d for a degenerate Fermi gas scales like (N/V)^(2/3), and the average energy per particle is 3/5 of that. The energy density of the Fermi gas then goes like the number density times the average energy per particle, or (N/V)^(5/3), as in Eq. 4. For a derivation, see here: wiki link.

julien Bobroff said...

first thanks for your great blog !
Weisskopf wrote many of these amazing articles from very rough arguments, being able to discuss various topics: energy of solids, thermal expansion, and even cooper pair formation in superconductivity.

You can find most of them gathered here :
If you don't know them, have a look, they are trully amazing.

Julien Bobroff (Univ. Paris-Sud,

Douglas Natelson said...

Julien - thanks! I hadn’t seen those great articles before, and will recommend them to students.