Lorentz symmetry says the laws of physics remain the same for any two objects traveling at a constant velocity relative to one another. For example, an apple dropped from the height of 1 m in a moving train will take the same time to hit the floor as an apple dropped 1 m at the side of the tracks.

Physicists in the US have proposed a new way of testing^[1] the veracity of Lorentz symmetry, a fundamental tenet of Einstein's theory of relativity. They believe that careful observation could reveal tiny differences in how a body falls to Earth, depending on the time of year the measurement is made. If verified experimentally, such observations would indicate breakdowns in Einstein's theory and provide important clues in the search for a theory that unifies quantum mechanics and gravity.

Previous tests include "clocking the direction of space"^[2]. The breaking of Lorentz symmetry can cause different points in space–time to have a preferred direction (red arrows). If the direction of space is the same at all points in the Earth's orbit, experiments on board the International Space Station (ISS) should be able to measure the magnitude of the Lorentz-breaking effect. For instance, the orientation of atoms in an atomic clock on board the ISS (blue arrow) will change with respect to the red arrows as the ISS orbits the Earth. This would modify the energy levels of the atoms and therefore cause the clock rate to change cyclically. So far, no noticeable effect has been found by such tests - hence the new proposal noted above.

I must make it clear that Lorentz symmetry does not hold over extended areas in the presence of normal gravitational fields, where it only holds over infinitesimal areas, usually stated as "the laws of physics are locally Lorentz". To understand this better, consider a hypothetical train running at an extreme 5 km/s along a perfectly "straight and level" track, i.e., following a great circle on Earth's surface. Clearly, there will be a centrifugal force inside the train working against gravity,^[3] resulting in the apple taking longer to fall the 1 m relative to the train and hence Lorentz symmetry is violated.

The "clocking the direction of space" test mentioned above is also an "extended area" test, but the periodic deviations due to the slight ellipticity of Earth's orbit are easy to compensate for. Besides that, nothing has been detected so far.

Scientist Kostelecky now talks^[1] about dropping masses from towers or balloons in different seasons. I'm not sure that these would qualify as "infinitesimally small, locally Lorentz" experiments, though. I suppose one can correct the results for all the standard violations of symmetry in a gravitational field and see if there are any residual discrepancies left. Those might then be the effects that Kostelecky and his team are looking for.

I won't put much money on them finding anything, but who knows?

Notes:

[1] http://physicsworld.com/cws/article/news/37295 (to read may require a free subscription to physicsworld.com)

[2] A more technical (in-depth) article on Lorentz symmetry at http://physicsworld.com/cws/article/print/19076

[3] The centrifugal acceleration will be ~ 0.4 g, leaving a net downward acceleration of ~ 0.6 g relative to the train.

Jorrie