Testing Local Lorentz Invariance with Laser Tracking of the LAGEOS and LAGEOS II Satellites.

Violations of Lorentz invariance, a cornerstone of modern physics, are predicted by theories of quantum gravity and by extensions of general relativity involving new vector or tensor fields. In the weak-field limit, such a violation would primarily manifest as a nonzero value for the post-Newtonian parameter α_{1}, which is identically zero in general relativity. We present a new test of local Lorentz invariance by searching for this signature in the orbits of the LAGEOS and LAGEOS II satellites. By applying a phase sensitive detection technique to the mean argument of latitude, derived from about 30 years of satellite laser ranging data, we isolate the periodic signal potentially induced by a preferred reference frame aligned with the cosmic microwave background. Our analysis yields a new constraint |α_{1}|∼2×10^{-5}. This result improves upon the previous best limit from lunar laser ranging and provides the most stringent constraint to date on preferred-frame effects in Earth's gravity.