
Figure 2: Selected tests of local Lorentz invariance showing the bounds on the parameter
, which
measures the degree of violation of Lorentz invariance in electromagnetism. The Michelson–Morley,
Joos, Brillet–Hall and cavity experiments test the isotropy of the round-trip speed of light. The
centrifuge, two-photon absorption (TPA) and JPL experiments test the isotropy of light speed using
one-way propagation. The most precise experiments test isotropy of atomic energy levels. The limits
assume a speed of Earth of
relative to the mean rest frame of the universe.

