Extra Dimensions Get Smaller

One of my favorite experiments in physics has released a new set of results in Physical Review Letters, putting experimental limits on the size of any extra dimensions of the sort predicted by string theory:

We conducted three torsion-balance experiments to test the gravitational inverse-square law at separations between 9.53 mm and 55 µm, probing distances less than the dark-energy length scale λ~85 µm. We find with 95% confidence that the inverse-square law holds (|α|<=1) down to a length scale λ=56 µm and that an extra dimension must have a size R<=44 µm.

You’ll need a subscription and a Ph.D. to read the whole thing (though you may be able to find it for free on the ArXiV), but the upshot is that the Eot-Wash group has completed the analysis of the torsion balance experiments I talked about a couple of summers ago, and confirmed that Newton’s law of gravitation appears to hold even for very short distances between masses. This puts a constraint on the size of hypothetical extra dimensions, because one of the predictions of theories with extra dimensions is that gravity should behave differently at short distances.

This is, unfortunately, essentially a negative result– they haven’t found anything dramatic and new, just boring old classical gravity/ General Relativity. Had they seen a change, one way or another, this would be the hottest news in years, but as it is, they’ve just established an upper limit on extra dimensions that very few people thought would be that big, and while it’s an editor’s pick for PRL, it’s not seen as newsworthy.

It’s a pity, though, because I think these experiments are absolutely phenomenal (which is why I keep talking about them), and I think they deserve more credit than they get. There are precious few experiments that intersect with the world of string theory at all, even in a negative, upper-bound-setting kind of way, and they ought to be applauded for having the creativity to go looking for clever ways to explore theory, rather than twiddling their thumbs and waiting and hoping for results from the LHC.

(Update: Sean has a few more details, and I have a few comments in the comments. And curse the spammers who broke TrackBack, anyway.)