Wednesday, June 16, 2010

Quick note: Two proposals in breaking of the equivalence principle

Until Friday it is very unlikely that I have time to properly read the two articles that I am going to link here. But I find that they are challenging enough to leve a note abut them.

The first article was cited yesterday in the arxiv blog: New Quantum Theory Separates Gravitational and Inertial Mass (if you are Spanish and prefer reading it in that language there is a translated version by kanijo: Nueva teoría cuántica separa las masas gravitatoria e inercial.

Those entries make reference to the following arxiv paper: Inertial And Gravitational Mass In Quantum Mechanics

I haven't read the actual paper, as I said I have not too much free time this week and to be honest reading the Tech Review entry I was not too inclined to do so. But somewhat amazingly (how many probabilities are there of having such a provocative proposal two consecutive days from independent sources) the first article in arxiv hep-th of today makes a similar claim based, this time, in string theory:The influence of D-branes' backreaction upon gravitational interactions between open strings

This is the abstract:

We argue that gravitational interactions between open strings ending on D3-branes are largely shaped by the D3-branes' backreaction. To this end we consider classical open strings coupled to general relativity in Poincare AdS5 backgrounds. We compute the linear gravitational backreaction of a static string extending up to the Poincare horizon, and deduce the potential energy between two such strings. If spacetime is non-compact, we find that the gravitational potential energy between parallel open strings is independent of the strings' inertial masses and goes like 1/r at large distance r. If the space transverse to the D3-branes is suitably compactified, a collective mode of the graviton propagates usual four-dimensional gravity. In that case the backreaction of the D3-branes induces a correction to the Newtonian potential energy that violates the equivalence principle. The observed enhancement of the gravitational attraction is specific to string theory; there is no similar effect for point-particles.

It is curious how physicist use the same majestatic plural as the pope in their papers because the "we" correspond to a single author, Raphael Benichou. He has seven published articles in arxiv -being this the first one in solitary-, in different topics of string theory.

As one can suspect the violation of the principle of equivalence (PE) is only for the string living in the brane (yes, the article treats about a braneworld scenario) and it is conserved in the bulk. Well, I must say that now -reading the discussion of the article- I have just learned that this is not the first stringy paper that raises the question of the violation of the PE. Apparently the novelty is that in the previous results the rupture was based in generic effects and it applied to point particles. The result of the article is purely stringy, it resorts in features of open strings in the brane. Well, fine, but a braneworld scenario is itself very stringy in its nature and I barely can guess a good reason to have such a world if it is not in string theory. Still this effect would be doubly stringy motivated.

The set up of the paper is a toy model using D-brane models that are far away from realistic D-brane phenomenology. I didn't read the article in detail so I don't now how much the separation of the conventional gravitational potential depends on the details. Also I must say that I haven't seen an explicit mention on how weak the effect is and if there is some possibility of seeing it in an actual experiment. The fact is that what is predicted is a slower decay of the gravitational attraction at large distances (in that respect it would be something like a MOND, modified Newtonian dynamics, of the kind used to explain rotations of galaxies without the need of dark matter. I am not sure why the author doesn't say anything about this point in the discussion *). This is different from the usual result in braneword gravitational phenomenology where one looks for an increase of gravity at very small distances due to the fact that the graviton propagates in the bulk a certain very small distance.

Well, my provisional conclusion is that the first article, discussed in arxiv blog, seems to be far from being the first proposal based on "quantum gravity" of a violation of the equivalence principle. It's distinction would be that it makes not use at al of string theory, which depending on tastes could be a pus or a minus. Obviously both approaches are totally different the one of the other. I have just made a very quick view of the non stringy article and I can't comment anything about it. Probably it is easies to read for people with few or null basic in string theory. Inparticular I guess that everyone with a solid basic in QFT and GR could read it safely and take his own conclusions.

I thin that maybe Lubos will say something more about the paper later today. But there are another interesting papers in arxiv so maybe not (I seriously doubt that Distler would say anything). It is a pity that there are not actually more very active blogs by string theorist out there. I believe hat my understanding of string theory has grown enough so that what I say could have some credibility, but still it is sure that there is a lot people out there that could make a better job than me and everyone in the field would beneficiate if some of them would blog in a regular basic.

*UPDATE: Ok, seemingly my knowledge of MONDS was very outdated. Consulting wikipedia entry on MOND I see that the modern form doesn't relay on modifications of newtonian potential. In the same page it is cited an arxiv paper by bekenstein reviewing MOND that could be an interesting reading for anyone wh is curious abut MOND theories. Advise: There are many good reasons to doubt on the validity of MONDs, of course. But at least it would be good t know with certainty what it says and so avoid trivial mistakes ;).

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