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June 10, 2004

Experiments probing quantum gravity

Posted by Urs Schreiber

There still seems to be a problem with the newsserver at Harvard, from which the messages of sci.physics.strings emanate. For some reason whenever I take the time to write a longer message the server seems to decide that it has better things to do than propagating it to other newsservers. I hope we can solve this weird problem in the future (maybe it is just me not using the various programs correctly, somehow?) but for the time being I would like to make these lost messages available here at the Coffee Table, not the least because they are replies to questions that have been asked.

So here is the first one:

Alexander Blessing had asked, more or less, for experiments that could say something about quantum gravity.

There are indeed a couple of interesting ways, in principle at least, to look for new physics without using ever more expensive particle accelerators:

Here is a set of slides by Pisin Chen from SLAC discussing some implications of possible astrophysical observations for new physics, see also this link.

[Update 06/23/04 A relatively new proposal is now getting a lot of attention. Polchinski and others have studied the effect of fundamental and D-strings of cosmic extension. They say the gravitational waves emitted by such ‘cosmic strings’ could be detected by the gravitational wave detector LIGO. See here for more details.]

People are looking in particular at the following effects:

1) Ultra High Energy Cosmic Rays

2) Lorentz violation on small scales

3) Detection of dark matter

4) Search for additional spatial dimensions and violations of the equivalence principle

5) Spacetime granularity

I’ll give some links and comments for all these 5 points:

1) Ultra High Energy Cosmic Rays should on theoretical grounds be cut off at $5x10^{19}$ eV, the so-called Greisen-Zatzepin-Kuzmin (GZK) limit, due to the absorbtion of highly energetic rays by the cosmic microwave background.

But at least one experiment has claimed to have seen particles boyond the GZK limit, for a review see

Glennys R. Farrar, Tsvi Piran:
GZK Violation - a Tempest in a (Magnetic) Teapot?,
Phys.Rev.Lett. 84 (2000) 3527

I have heard that Smolin and some other people believe that they can explain the GZK violation with ‘doubly special relativity’, that this is furthermore somehow a prediction of LQG and that they are hoping that the GLAST experiment in 2006 will say something about this (but I am having
trouble finding more details on this speculation, in hep-th/0401087 Smolin vaguely talks about some such experiments).

Recently Edward Witten commented on this experiment where he said:

It is not clear at the moment that there is anything to explain. One of the two main experimental groups (AGASA) reported as of over a year ago that their high energy data are in accord with the expected GZK cutoff. The other group continues to report a discrepancy. We’ll see what happens as data improve.

(quote from the message at the above link).

2) People are apparently looking for violation of Lorentz invariance at small scales/large energies:

From some theories of quantum gravity one might expect such effects, probably not from string theory.

3) Nature of dark matter.

Several people are trying to determine the presence and maybe the nature of dark matter.

For instance the DAMA collaboration and the UK Dark Matter collaboration as briefly summarized in this message by John Baez.

Of course the true nature of dark matter may be a very important clue for how the true theory of quantum gravity looks like, but current experiments are of course very far from saying anything about this.

4) Search for additional spatial dimenions and violations of the equivalence principle

There are potentially very interesting high-precision measurements of the equivalence principle, most notably by the EotWash group.

Variation of the usual $1/r^2$ force law of gravity on small scales could be related to extra dimensions and or torsion, for instance.

5) Spacetime granularity

From quantum gravity some people expect that on extremely small scales
spacetime will show some sort of foamy structure, maybe being topologically non-trivial. An old idea by Percival and collaborators is that atom interferometry, e.g. the 2-slit experiment with heavy stuff such as Buckminster Fullerenes (Nature Vol. 401, No. 6754, p. 680 (1999).) as done by Zeilinger’s group, may be sensitive to such a spacetime grabularity.

I had mentioned that in the past from time to time.
giving some references.

I am not sure what string theory really says about ‘quantum spacetime foam’ at small scales.

When we discussed Smolin’s paper it was argued by some people that string theory (on Minkowski space, say) predicts smooth spacetime
down to all scales. But this is not correct, as everybody knows and as I have tried to argue in more detail in this and this message once based on the discussion in Fedele Lizzi anmd Richard Szabo, Duality Symmetries and Noncommutative Geometry of String Spacetimes (1997) because as you try to probe the smooth background with highly energetic strings you will eventually see stringy effects and not be able to resolve the smoothness of the classical background at all, so that effectively it is not smooth on small scales.

So: Might stringy physics effect the phase of atoms/molecues used in matter interferometry ever so slightly?

Posted at June 10, 2004 9:36 PM UTC

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