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.)


  1. #1 A quantum diaries survivor
    January 13, 2007

    Wow, that’s an interesting result! I wish I could read the entire paper. Is there any site with more information which is freely accessible ?


  2. #2 Ahcuah
    January 13, 2007

    arXiv pre-print here.

  3. #3 Uncle Al
    January 13, 2007

    Gravitation may leak into compactified dimensions perceptible at or below their estimated scale lengths of 10^[(30/n)-19] meters,

    Sections 113, 114, and 115
    New Eot-Wash pubs URL for foregoing pdfs

    The next stop is three compactified dimensions with a contingent threshold-of-action diameter of 20 angstroms or less. Gravitational, diastereotopic chiral vacuum insertion, and Equivalence Principle parity anomalies are then fully developed at crystal unit cell dimensions. Mass distribution chirality emerges within the following sphere diameters: 3.23 A for alpha-quartz, 4.65 A for benzil, 6.99 A for Te. Well within threshhold! It’s a two-day experiment in commercial hardware, folks,

    Somebody should look.

  4. #4 Brad Holden
    January 13, 2007

    Go Eric, go!

    Tom Murphy, now at UCSD, and Eric Adelberger (along with collaborators) are also testing GR using lasers, the Apache Point 3.5m telescope and corner reflectors left by astronauts on the moon. Any science that involves firing laser beams at the moon is by definition cool.

    These guys just keep thinking up great ideas for testing gravity.

  5. #5 Matt
    January 13, 2007

    Deadline for grad apps to U-Wash was today, and I applied there for physics primarily because of the Eot-Wash group. I find it absolutely amazing that they are measuring gravitation attraction from such short distances using a method pioneered in the early 19th century. Gotta love gravity tests that can be done in-house.

  6. #6 Rob Knop
    January 14, 2007

    I’ll have to read this. Good stuff.

    One of the neat things about GR is the fact that is at it gets tested harder and harder, it really just seems to keep working.

    On my iconoclastic days, I wonder if we’re barking up the wrong tree when we assume that it is GR, rather than quantum field theory, that will have to be modified in order to solve the classic GR/QM breakdown at very high densities. In many ways, GR is a much more beautiful theory.

    I have a friend, Tom Murphy, at UCSD, who’s working on the APOLLO project. He’s using the retroreflecters left behind on the Moon to measure the distance to the moon to mm accuracy– to look for tiny deviations from the Moon’s predicted orbit due to GR.


  7. #7 A Babe in the Universe
    January 14, 2007

    Maybe the extra dimensions don’t exist. How small would we have to look to show that?

  8. #8 Uncle Al
    January 14, 2007

    Gravitation is metric (Einstein) or affine-teleparallel (Cartan, Weitzenböck). Their predictions are only disjoint for physical spin, quantum spin, spin-orbit coupling, and mass distribution parity divergence.

    Neutron star cores may be strange matter, pion condensate, lambda hyperon, delta isobar, or free quark matter. Gravitationally hyper-bound (~30% of rest mass), hyper-spinning (~20% lightspeed at equator), hyper-magnetized (10^8 tesla), hyper-dense (4-9×10^14 g/cm^3), superconducting neutronium binary pulsars’ orbit and orbital decay (gravitational radiation) obey GR. No parity test has ever been attempted.

    Newton and Einstein postulate isotropic vacuum. Affine-teleparallel theories allow a chiral pseudeoscalar vacuum background. Left and right shoes insert into chiral vacuum with different energies. They would locally vacuum free fall along non-parallel minimum action trajectories. An 8% (wow!) parity anomaly is consistent with all prior observations in all venues. Somebody should look.

  9. #9 Matt
    January 14, 2007

    In the article, the authors state that for one extra space dimensions having a simple Yukawa potential, the size would have to be less than 44 microns. Two extra dimensions seems to be tightly restricted as well but I am missing the approximate size if it is in the paper.

    These constraints are only from the the torsion pendulum experiments. Cosmological and particle physics measurements constrain these various models in different areas.

  10. #10 Aaron Bergman
    January 14, 2007

    On my iconoclastic days, I wonder if we’re barking up the wrong tree when we assume that it is GR, rather than quantum field theory, that will have to be modified in order to solve the classic GR/QM breakdown at very high densities.

    Well, if one believes in string theory, they both get modified.