Hughes–Drever experiment

As in Michelson–Morley experiments, the existence of a preferred frame of reference or other deviations from Lorentz invariance can be tested, which also affects the validity of the equivalence principle.

[A 1][A 2][A 3][A 4][A 5][A 6] Giuseppe Cocconi and Edwin Ernest Salpeter (1958) theorized that inertia depends on the surrounding masses according to Mach's principle.

Heuristic arguments led them to believe that any inertial anisotropy, if one existed, would be dominated by mass contributions from the center of our Milky Way galaxy.

[2] Vernon W. Hughes et al. (1960)[3] and Ronald Drever (1961)[4] independently conducted similar spectroscopic experiments to test Mach's principle.

If mass isotropy is satisfied, each transition between a pair of adjacent levels should emit a photon of equal frequency, resulting in a single, sharp spectral line.

[A 7] Neither Hughes nor Drever observed any frequency shift of the energy levels, and due to their experiments' high precision, the maximal anisotropy could be limited to 0.04 Hz = 10−25 GeV.

According to this model, Lorentz violations in the presence of preferred frames can lead to differences between the maximal attainable velocity of massive particles and the speed of light.