Lorentz Invariance Still Stands

Lorentz invariance expresses the proposition that the laws of physics are the same for different observers, for example, an observer at rest on Earth or one who is rotated through some angle, or traveling at a constant speed relative to the observer at rest. It is the pillar of Einstein’s theory of special relativity, and every experiment conducted to date has verified it. But if new, far more sensitive experiments could detect a very faint field pervading the cosmos, one that exerts a force on electron spin, that would topple Lorentz invariance.

Fortunately for fans of Einstein and relativity, a new experiment at the University of Washington sought just such an anomalous field and came up empty-handed, even at an unprecedented energy scale of 10^-21, according to results presented at the APS April Meeting in Dallas. This is the most stringent search to date (by a factor of 100) for violations of Lorentz invariance involving electrons.

Eric Adelberger’s UW group is conducting an ongoing battery of tests carried out with a flexible and sophisticated torsion-balance apparatus. In 2000, they were one of three separate research groups to measure the gravitational constant (“Big G”) to the greatest precision to date, although the various measurements didn’t agree with each other.

Most recently, they set out to test Lorentz invariance with a torsion pendulum, in hopes that even a slight departure from expected behavior in spacetime might signal an interesting new phase in our understanding of the universe. According to Clare Cramer, a member of Adelberger’s research group, in this particular experiment, the apparatus involved a pendulum 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.

By carefully choosing and arranging the blocks, they can create an assembly that has zero magnetization and yet still has an overall nonzero electron spin. Cramer calls this condition a “spin dipole,” and likens it to the case of an electric dipole, which has zero net charge, yet possesses a net electric field because of a displaced arrangement of positive and negative charge. 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 femto-gauss.

They are also searching for evidence of extra dimensions in the form of departures from Newtonian gravity–such as the inverse-square dependence–at size scales of tens of microns. While the group did find some strange results at a measurement scale of about 70 microns, Adelberger conceded this was most likely due to an experimental artifact.

A portion of the above article appeared in Physics News Update Number 775.