The most careful and respected experimental group in its field which resides at University of Washington - Eric Adelberger et al. - seems to have detected deviations from Newton's gravitational law at distances slightly below 100 microns at the "4 sigma" confidence level. Because they are so careful and the implied assertion would be revolutionary (or, alternatively, looking spectacularly dumb), they intend to increase the effect to "8 sigma" or so and construct different and complementary experiments to test the same effect which could take a year or two (or more...) before the paper is published. You know, there are many things such as the van der Waals forces and other, possibly unexpected, condensed-matter related effects that become important at the multi-micron scales and should be separated from the rest.
Their measured force at these multimicron distances is weaker than expected from Newton's formula. This is unusual because in most models, one expects the force to grow stronger at short distances. For example, in theories with two
(that motivated these experiments) and quantum gravity at 10 TeV, the two large dimensions should be about 100 microns in size. The gravitational law "1/r^2" should switch to "1/r^4" at shorter distances - it should become stronger. One can also add massive scalar fields, but the most typical expectation is that a force obtained from the exchange of a scalar field is also attractive (between objects with the same "scalar charges"); it would therefore strengthen gravity. Of course, a new (massive) spin 1 field would, on the contrary, lead to repulsive interactions. All these scenarios with new intermediate particles are problematic because we can ask: why have not we seen these messengers yet? In most contexts, one would need to make their couplings incredibly small - as weak as gravity - which also suppresses the messenger production. But in this case, in a sense, the new interaction is a manifestation of a new mode of gravity (like its KK modes).
For the latest paper of that group - which could not announce any new effects yet - see
Note that 100 microns is also the scale of the vacuum energy - the cosmological constant. In other words, "1/(100 microns)^4" equals the energy density of the observed vacuum energy. Our experimentalist says that they would like to promote the idea that the gravitational oscillations do not exist below 100 microns. Apparently, the Washington physicists like to think about the theoretical concept of a "fat graviton" being relevant for their current observations. Only virtual particles with the wavelength longer than 100 microns (small enough momenta) should contribute to the vacuum energy because the modes of the graviton do not exist at shorter distances. This also means that gravity becomes weaker at shorter distances. Let me choose, with the help of Ann Nelson, the following references for this kind of ideas:
- Tom Banks' prolegomena 1988
- Silas Beane's sub-centimeter paper 1997
- Raman Sundrum 1997
- Anthony Zee's fat graviton 2003
Yes, I find these ideas about the origin of the cosmological constant more predictive (correlated with the predicted submillimeter deviations), exciting, and likely than the solution via the anthropic landscape. It remains to be seen how a fat graviton like this one may be derived from a more fundamental theory - namely (or "for example") from string/M-theory. No doubt, a fat graviton seems unnatural in all stringy models I know of - especially because the photon and other particles can't be fat (they're definitely smaller than 10^{-17} meters according to our observations). But there could exist a way to put these ideas on firm ground.
My thanks also go to Ann Nelson for her speedy and detailed comments about the history of these ideas.
