Marvin Cohen, nanotubes, and space elevator

Marvin L. Cohen gave the first Loeb lecture at Harvard, and I liked it. The Loeb lecturers are always very special people - very respected physicists and physicists with exceptional skills to present their subjects. Brian Greene was the Loeb lecturer in the spring.

Cohen is a condensed matter physicist, and he decided to present

• The Standard Model of Solids

As the president-elect of the American Physical Society (who will probably replace Helen Quinn, our fellow particle physicist, if I understand it well), he had to start with two commercials.

• One of them was the World Year of Physics 2005. The next year will be celebrated as the international year of physics because it will be 100 years from Einstein's miraculous April 1905 in which he discovered special relativity, the theory of photoelectric effect, and the theory of the Brownian motion, among other things. Cohen's main goal associated with WYP 2005 is to attract many 10-year old girls to physics.
  
• The second commercial was about his plans to reorganize APS. If I remember well, Cohen divided APS into four sections: old-fashioned physics (atomic, molecular, optics); astro-particle (which probably include things like string theory); condensed matter physics; plasma and non-linear physics

I did not make any notes, but let me reproduce some of his points about APS. The "classical" section of APS (atomic, molecular, optics) is doing very well. A significant fraction of astro-particle is about the search for a TOE, but also about the Big Bang. Plasma physics continues to develop thermonuclear fusion reactors. And condensed matter physics, which is the largest portion of APS, is disorganized because it develops into too many directions.




Many people who know a lot about science as well its sociological structure and visions start to agree: the interdisciplinary topics in condensed matter physics start to dominate. Biophysics and nanotechnology start to be the most important applications of condensed matter physics - which means, in a sense, that the physicists are moving towards biology, and we must live with that. The task is to preserve the exceptional role of physics among sciences - the role of the most rigorous natural science.

Most of his talk was about physics, of course. He quoted Dirac in 1929 who declared something along these lines:

• A great portion of physics and all of chemistry can be now reduced to known fundamental physical laws, and it is just a matter of technical difficulty to obtain any results we want.

Cohen showed himself as a reductionist who is simultaneously an emergent person. ;-) Obviously, unlike many of his condensed matter colleagues, he has no problems with the idea of reductionism, and he showed us many examples of the things that have been understood and "reduced", especially if you use the right approximations (pseudopotentials, and so forth).

Take superconductors. It is still believed that all of them are sort of related to BCS superconductors - they are governed by pairs of electrons. If one electron loses momentum (by a collission with an impurity), his partner is still forced to react in the opposite way, and therefore no energy is lost. OK, there are speculations that there can exist other types of superconductors, but nothing conclusive.

It seems that the experimentalists are much ahead of theorists in condensed matter physics. All major new phenomena - such as superconductivity - were first discovered experimentally.

I can imagine that this will change sometime in the future - particle physics is currently just in the opposite situation, because the experimentalists just cannot find anything that the theorists can't predict. Cohen showed how all possible crystallic and superconductive phases of sillicon can be understood, more or less from the basic theoretical principles. Sillicon as a superconductor had a simple, roughly spherical Fermi surface, and therefore it was the first one that was understood. (The first observed superconductor was Mercury, however.) Nevertheless he showed another superconductor, something like MgB_2. Its Fermi surface is composed of four sheets. Nevertheless, today it is probably understood even better than Si, and the pictures looked convincing!

Most of the talk was about nanotubes and their applications, and they were very interesting. Because I must also do other things, let me be sketchy and choose just a couple of examples:

• Nanotubes are very thin tubes whose thickness is as small as tens or hundreds of nanometers
• Nanotubes have been seriously proposed as a material to build a "space elevator" - a kind of rope that will be hanging from the skies. You will be able to climb to the outer space, if you wish ;-)
• Four wine bottles filled with nanotubes, if properly organized, will have greater memory than all human brains in this world altogether - an interesting research direction is to construct computers and memory chips based on these objects
  
• Nanotubes may be inserted into one another, and you can build a nanomotor - a motor that rotates whose size is smaller than the radius of a typical virus
• Nanotubes, if inserted into each other, exhibit friction - the question what does the energy dissipates into is sort of entertaining and possibly deep because there are not too many available degrees of freedom

Marvin Cohen will continue on Wednesday and Thursday, but I am afraid that there are too many other things I must do and attend.