Testing Special Relativity and Newtonian Gravity

Lorentz invariance says that the laws of physics are the same for an observer at rest on the Earth or one who is rotated through some angle or traveling at a constant speed relative to the observer at rest. Looking for a crack in the universe in the form of a very faint field pervading the Cosmos, one that exerts a force on electron spin, would mean the end of Lorentz invariance.

An important ingredient in Einstein's theory of special relativity, Lorentz invariance has been borne out in numerous experiments. A new experiment conducted at the University of Washington, in Seattle, has sought such an anomalous field and not found it even at an energy scale of 10^-21 electronvolt. This is the most stringent search yet -- by a factor of 100 -- for Lorentz-invariance-violating effects involving electrons.

The Washington work, described at this week’s American Physical Society's (APS) April Meeting in Dallas by Claire Cramer, is part of an ongoing battery of tests carried out with a flexible and sophisticated torsion-balance apparatus. In this case, a pendulum is made of blocks whose magnetism arises from both the orbital motion of an electron around its nucleus and from the intrinsic spin of the electron itself. Carefully choosing and arranging the blocks, one can create an assembly that has zero magnetization and yet still have an overall nonzero electron spin. Cramer refers to this condition as a "spin dipole," analogous to the case of an electric dipole, an object with zero net charge but which, because of a displaced arrangement of positive and negative charge, possesses a net electric field.

The existence of a preferred-direction, Lorentz-violating spin-related force would have shown up as a subtle mode in the rotation of the pendulum. The conclusion: any such quasi-magnetic field would have to be weaker than about a femtogauss, or 10^-15 gauss.

At the APS meeting, Eric Adelberger, leader of the Washington group, summarized some of the other efforts underway in his lab such as the search for evidence of extra dimensions in the form of departures from Newtonian gravity -- for instance, the inverse-square dependence -- at a size scale of tens of microns. In fact, he said that something strange was happening at a measurement scale of about 70 microns; the most likely explanation of this, he conceded, was an experimental artifact.