GRAVITY HAS BEEN MEASURED AT THE SUB-MILLIMETER SCALE for the first time. Gravity has of course long been studied over planetary distances but is more difficult to gauge at the terrestrial scale, where intrusive electric and magnetic fields, many orders of magnitude stronger than gravity fields, can be overwhelming. Nevertheless, Eric Adelberger and his colleagues at the University of Washington have managed to measure the force of gravity over distances as small as 150 microns using a disk-shaped pendulum carefully suspended above another disk, with a copper membrane stretched between them to help isolate electrical forces. (This experiment should not be confused with another University of Washington effort in which the gravitational constant is measured with higher precision—see Update 482).
Adelberger (206-543-4294, eric@gluon.npl.washington.edu) presented one of several talks at this week's APS meeting in Long Beach, California devoted to short-range gravity, a subject which has suddenly attracted much theoretical and experimental interest owing to a relatively new model which supposes the existence of extra spatial dimensions in which gravity, but not other forces, might be operating. According to Nima Arkani-Hamed of LBL (arkani@thsrv.lbl.gov, 510-486-4665) this is why gravity is so weak: it dilutes itself in the extra dimensions. In other words, ordinary particles are tethered to our conventional spacetime, or "brane," while gravitons are free to roam into otherwise unseeable dimensions.
One implication of the model, testable with tabletop experiments such as Adelberger's, is that the gravitational force might depart from Newton's inverse square law (gravity inversely proportional to the square of the distance between two objects) at close range. Adelberger did not observe such a departure at distances down to tenths of a millimeter and will continue to explore even shorter distances. For a list of tabletop experiments underway, see http://gravity.phys.psu.edu/mog/mog15/node12.html.
Another interesting implication of the model introduced by Arkani-Hamed (and others; see preprint hep-th 9803315) two years ago is that the unification of the four known forces would not necessarily occur at energies as high as 1019 GeV but possibly at energies as low as 104 GeV, an energy scale within reach of the Large Hadron Collider under construction at CERN. Extra dimensions could, for example, manifest themselves in proton-proton smashups as an apparent disappearance of energy, implying that some of the collision energy had been converted into gravitons (the particle form of gravity) which then disappear into the extra dimensions. The gravitons produced in this way might come back into our conventional world of 3 spatial dimensions and decay into two photons.
Physicists have already looked for this kind of event. Gregory Landsberg of Brown University (401-863-1464; landsberg@hep.brown.edu) reported that at the D0 experiment at Fermilab some energetic two-photon events have been observed (including one in which the energy of the photons added up to 574 GeV, representing the highest composite mass ever seen in the D0 experiment) but not often enough to constitute evidence for extra dimensions. In fact this shortage of events has been translated into a lower limit of 1300 GeV for the energy at which a prospective unification of the forces could take place.
