The null result of the celebrated 1887 Michelson–Morley experiment was surprising and difficult to explain in terms of then prevalent physics concepts. It required a fundamental change in the notions of space and time and was finally explained, almost 20 years later, by Albert Einstein’s special theory of relativity. (See the May 1987 special issue of Physics Today devoted to the centennial of the experiment.) Special relativity postulates that all laws of physics are invariant under Lorentz transformations, which include ordinary rotations and changes in the velocity of a reference frame. Subsequently, quantum field theories all incorporated Lorentz invariance in their basic structure. General relativity includes the invariance through Einstein’s equivalence principle, which implies that any experiment conducted in a small, freely falling laboratory is invariant under Lorentz transformations. That result is known as local Lorentz invariance.
Experimental techniques introduced throughout the 20th century led to continued improvements in tests...
