This does not hold when local tensor fields get vacuum expectation values.
This statement was tested at SYRTE in a series of experiments using a cryogenic sapphire oscillator and a hydrogen maser. Studying the absence of this can be aided by a Bumblebee model. That approach was co-advanced by Kostelecky. Another test of this looked at spin precession in Lithium 7. That experiment is known as the Hughes-Drever experiment. This does not hold when local tensor fields get vacuum expectation values. The Standard Model Extension includes operators that do not obey this. As Maxwell's field equations can be put in tensor form in 4-space, those equations have this property. A violation of CPT symmetry leads to a violation of this. If this holds, then the speed of light in a vacuum is always the same. FTP, name this invariance of the laws of physics under its namesake's transformations.
This statement was tested at SYRTE in a series of experiments using a cryogenic sapphire oscillator and a hydrogen maser.
{Lorentz} invariance [or {Lorentz} symmetry; prompt on "{isotropy} of the {speed} of {light}"]