Lorentz Violations? Not Yet

Lorentz invariance, the idea that the result of a physics experiment should stay the same whether the apparatus is motionless or traveling at some great constant speed relative to a reference point, is taken for granted in the theory of special relativity. Yet in recent years some scientists have come to question this pillar of physics, and to suggest theoretical models (called "standard model extensions," or SMEs ) incorporating Lorentz violations and experimental ways of settling the matter (see Update 578). In these models, the speed of light is not universal but will have extra terms dependent on the speed or orientation of the apparatus (see http://media4.physics.indiana.edu/~kostelec/faq.html ). Even before the advent of Einstein's relativity, the Michelson-Morley experiment tried to perceive (unsuccessfully) a difference in the speed of light when the Earth was traveling in two different directions in space while on opposite sides of its orbit around the sun. Now scientists have to be more subtle in their approach. In one new laboratory experiment, just completed by Stanford physicists (John Lipa, 650-723-4562) microwaves in two resonant cavities (one oriented east-west, the other pointing vertically) are monitored as the Earth sweeps around the sun. Any orientation- or speed-dependent changes in the speed of light would alter the resonant conditions of the cavities in a measurable way. The geometry of the experiment gives it optimal sensitivity to a number of coefficients in a generalized SME. The Stanford group sees no such anisotropy at the level of 10^-13 for velocity-independent terms, and at the 10^-9 level for velocity-dependent terms. (Lipa et al., Physical Review Letters, 14 February 2003.)