Tuesday, 12 October 2010


Via physicsworld.com

Sunday, 10 October 2010

About Testing String Theory by Analogy

I like string theory, but as sad as it may seem we have to face that there is still no experimental test of it. And again, as desperate some people may be not to have wasted their lives (which actually is an unjustifiable fear), if string theory turns out to be not falsifiable, it is not science, but just a book keeping device. That's true. Without any falsifiable prediction, string theory becomes an extremely elegant and compact way to express our nature's knowledge up to date. If you are fine with that, no problem, but sincerely I prefer not to be sure that there will be no more experiments with explanations requiring new physics to be done even in principle. But I can be wrong.

But what this post is really about is alleged tests of string theory based on mathematical analogies. I can't deny that supersymmetry is a non-trivial prediction. If it is true, point to string theory. But other theories can be supersymmetric too. Another day, I heard about a paper using string theory to quantum computing, and these days I have heard a lot about holographic superconductors and AdS/CFT applied to condensed matter.

However, people must remember that applying the methods developed in one theory to other does not provide a proof of the former. The fact that you can use Feynman graphs in condensed matter and it works for explaining superconductivity does not mean that QFT is proved by it, experiments do. There is a difference between the mathematical methods developed to deal with a theory and the theory itself. Some people will say that there isn't, but that is dead wrong! The power of mathematics comes from abstraction and this allows for the use of the same tools to different problems. But physics is not only mathematics and depend on principles that are derived from and tested by experiments.

I am not saying that string theory is not science. On the contrary. It is a possible hypothesis which is being explored. However, it is not a proved theory no matter what the most intelligent people in the world say. Nature usually cares very little about what intelligent people think. There are many examples of it in human history. And the bottom line is that, even if the mathematics of string theory helps other theories, that does not count as a verification of string theory.

Saturday, 9 October 2010

Nobel Prize of Physics for Graphene

I know news run fast through the web and everyone knows by now that the Nobel of physics this year went to Andre Geim and Konstantin Novoselov, from the University of Manchester here in the UK, for the discovery/invention of graphene. As usual, it was a busy week and the only thing I had time to do about it was to put together a gallery of graphene pictures on my other blog Sciencescapes. And probably everyone also knows by now that Andre Geim won the IgNobel prize of physics in 2000 for levitating a frog over a superconducting magnet. The frog paper is free to read: Of Flying Frogs and Levitrons,  by M.V. Berry and A.K. Geim, European Journal of Physics 18, 307 (1997) .

Graphene is a very interesting material. It is the closest you can get to a two-dimensional sheet  for it is a carbon sheet just one atom thick. The picture above is an artistic rendering you can find on Wikipedia. It shows that graphene forms what we call a regular hexagonal lattice. I should have written in this blog about that before, because I always thought these guys would win a Nobel soon, but now I cannot prove it. It was somewhat logical to assume it as if you check the condensed matter part of arXiv daily, you will see that it is hard to find a day without a paper about graphene. 

Due to the fact that it is practically two-dimensional, graphene has many interesting physical properties. In particular, at least for physicists, you can find an anomalous quantum Hall effect. Also, being 2D, graphene can support anyonic quasi-particles, elementary excitations that have statistics which are neither bosonic nor fermionic (see the previous post Anyons). As an extra bonus, graphene appears to be one of the strongest materials that exists, with a breaking strength 200 times greater than steel.

Geim, Novoselov and others wrote a nice review on graphene: The electronic properties of graphene, Neto et al., Reviews of Modern Physics 81, 109 (2009).  There is also this other paper by Peres: The transport properties of graphene: An introduction, Peres, Reviews of Modern Physics 82, 2673 (2010). Unfortunately, you need a subscription to access them. 

As I lost the opportunity to predict the graphene Nobel, this time I will take the risk of making the prediction (which is again fairly obvious) that soon the Nobel will be given to the guys who discovered that the universe expansion is accelerating. They were called the High-z Supernova Search Team, and the discovery came on 1998. Adam Riess was the leader of the team, so he is probably one of the guys who will win the prize. That discovery was completely a surprise at the time as everyone were expecting a decelerating universe. This also led to many famous hypothesis to try to explain it, like dark energy and quintessence.

Tuesday, 5 October 2010


Biological Physics (Updated Edition)
I have just started reading Chapter 5 of the book Biological Physics by Philip Nelson, which is called Life in the Slow Lane: The Low Reynolds-Number World. The book is an undergraduate introduction to biophysics which is extremely well written and very pedagogic. The undergraduate word however just means that the mathematics of the book is not very advanced, for there are a lot of physical insights that are extremely interesting and valuable for any physicist.

In this chapter, Nelson is writing about the difference in the relative viscosity for macroscopic and microscopic objects and the effect of it to the world of cells. In the very beginning, he explains the experiment in the video above, the only difference being that in the book you only have one coloured drop.

In the experiment, the container is composed by two concentric cylinders with corn syrup, a very viscous fluid, filling the space between them. As you then can see, drops of coloured syrup are put in this space. Note how the fluid is viscous by the fact that the drops don't even move once they are there. Then, the handle is turned and the internal cylinder is rotated a number of times. The fluid is dragged by the rotation and the drops apparently mix. The magic happens when the cylinder is rotated in the opposite direction and, miraculously, the drops unmix and reappears almost intact.

The explanation of how this can happen is quite interesting and is given in Nelson's book. What happens is that the drops never really get mixed, because the fluid is so viscous that there is no turbulence. Without turbulence, there is only a very organised laminar movement of the fluids and not the disordered wandering of molecules that causes mixing. The molecules actually stop moving (at least almost) when the rotation stops. When the rotation is realised in the opposite direction, the molecules simply retrace their previous steps and come back to the place where they were in the beginning. Of course, that's not perfect and you can see the drops had fuzzy boundaries where some diffusion and mixing did happen, but that is negligible.

The most interesting part of the discussion in Nelson's book comes afterwards where he explains that water is very viscous from the point of view of bacteria and this kind of effect happen in the microworld. This brings problem for them to move as, if they just swing upwards and backwards some kind of structure, they will never move because the fluid will just trace back the previous movement. I stopped somewhere around there.  If you are interested, I highly recommend Nelson's book.