Is Special Relativity Wrong?

The centennial of Albert Einstein’s miracle year of 1905 has arrived and so it is pertinent to ask how one of his most famous theories is doing. Physicists don’t necessarily believe that Einstein’s rules about the nature of spacetime are mistaken, but as part of the continual scientific effort to extend what is known about the universe physicists search for subtle hints of a departure from expected behavior.

Special relativity predicts that clocks traveling in various directions and with various fixed speeds relative to each other will tell time differently, but in such a way that spacetime has no preferred or distinguishable direction, a proposition known as Lorentz invariance.

Physicists, always on the lookout for departures from received opinion, and also motivated by theoretical suggestions that such effects might be expected, take this as an invitation precisely to search for such a special direction or to find that the variation of clock rates does not adhere to Einstein's equations.

Such effects are described by the “Standard-Model Extension” (SME) and they can come in several forms. One disproof of special relativity would be the finding that matter and antimatter behaved differently. Another would be a birefringence violation: observing that light with different polarizations travels at different velocities through vacuum.

Still another disruption of the Einsteinian view would occur if the universe were pervaded by an underlying oriented energy field, one that interacted weakly with known particles so as to favor one direction over another.

A new experiment puts this latter violation to its most stringent test yet. As so often happens when searching for extremely subtle effects, no departure from known physics was found but a new upper bound could be established.

Ronald Walsworth and his Harvard-Smithsonian colleagues, in conjunction with theorist Alan Kostelecky at Indiana University, look at how atoms prepared in special magnetic states (the precision of their light emissions allow them to serve as “clocks”) vary in their timekeeping when moving at certain velocities (or "boosts") relative to the hypothetical Lorentz-symmetry-violating fields that may permeate the universe.

In this case the two clocks consist of a sample of helium-3 atoms and a sample of xenon-129 atoms held in a container within a fixed magnetic field. The clock rate in each case is the rate at which the atomic nuclei precess in the magnetic field. The emissions from one atomic species were fed into a feedback mechanism for controlling the magnetic field, so in effect the one set of atoms (or, to be more precise, their nuclear spins) acted as a reference clock while the other species served as the test clock.

The whole apparatus, and the absolute orientation of the applied magnetic field in spacetime (and along with it the orientation of the atoms and their emissions) change as the Earth rotates daily and as the Earth takes its annual course around the sun.

Furthermore, to achieve the necessary level of precision (based on the light let loose by the atoms), the Harvard researchers achieved the difficult experimental feat of having the two atom samples operate in a maser mode (that is, they performed like a laser) within the same container. The existence of a Lorentz-violating field, one that like a magnetic field favors a particular orientation in an otherwise isotropic spacetime, could cause the two clocks to become more out of synch as they move relative to the Lorentz-violating field.

The main result of the experiment was to put a stringent new limit on a coupling of material particles (primarily the neutron) to such fields. The upshot: no Lorentz “boost” violations are seen at a level of one part in 10²⁷. (Cane et al., Physical Review Letters, 3 December 2004; previous relativity test summarized at http://www.aip.org/pnu/2003/split/623-2.html; contact Ron Walsworth at 617-495-7274, rwalsworth@cfa.harvard.edu; background articles in Physics Today, July 2004, Scientific American, Sept 04; Harvard website at http://cfa-www.harvard.edu/Walsworth/Activities/DNGM/DNGM2.html; Kosetlecky site, http://www.physics.indiana.edu/~kostelec/faq.html#30