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.
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.