Einstein’s 1905 special theory of relativity is based on the idea that the speed of light is constant in all directions regardless of the relative motion of the observer. A consequence of Lorentz invariance, this property was first demonstrated by Albert Michelson and Edward Morley in their famous experiment of 1887.

Michelson and Morley split a beam of light in two and sent the beams off at right angles to two different mirrors. The beams were reflected back and recombined to form an interferometer. If the beams travelled at different speeds in the two directions -- as they would if they were passing through a stationary aether through which the Earth was moving -- then the two beams would be out of phase when they were recombined, leading to an interference pattern. No such pattern was revealed -- ruling out the existence of an aether -- and over the past 120 years the Michelson-Morley experiment has been refined and repeated to confirm that the speed of light is constant to one part in 1016.

However, there is one lingering doubt surrounding the Michelson-Morley experiment, according to Holger Mueller, a physicist at California’s Stanford University. Any Michelson-Morley experiment is also sensitive to possible changes to the length travelled by the light, and to changes in the refractive index of the medium that the light travels through.

Such physical changes could be caused by violations to Einstein’s theory and could prevent the Michelson-Morley experiment from detecting changes in the speed of light. For example, if both the speed of light and the length travelled by a light beam changed by the same factor, the changes would cancel each other out. A general theoretical framework for describing the violation of Lorentz invariance is defined in the standard model extension (SME).

In the past, most physicists simply assumed that the physical properties of Michelson-Morley experiments do not change, and have interpreted the results as proof of the Lorentz invariance of light. Now, however, Mueller and colleagues in Australia, Germany and France have worked out a way to separate possible changes in the speed of light from the variations in the physical properties of the apparatus.

The team performed two different Michelson-Morley experiments – one in Berlin involving infrared light in optical cavities and the other in Perth, employing microwave radiation in a pair of resonating cavities. The research will be described in an upcoming issue of Physical Review Letters.

The researchers used the SME to calculate possible changes to physical properties of both experiments as well as to the speed of light. While the SME predicts that the speed of light in both experiments should change by the same factor, the theory says that changes in the physical properties of the two experiments will change by different factors. By doing two experiments, Mueller obtained sets of equations that can be solved to separate the possible changes to the speed of light from physical changes.

Mueller told Physics Web that the experiments were both run for over a year, which meant his team could measure Lorentz violations that become evident only by a modulation of the experiment's rotation relative to an inertial frame, such as the modulation provided by the Earth's orbit. Making the measurements in different geographical locations meant that the experiments were sensitive to different combinations of Lorentz violations, which would boost their ability to separate the changes.

In all, the team was able to say that Lorentz invariance is not violated in 14 parameters associated with SME to an accuracy of around one part in 1016. While the team were able to boost the accuracy of some parameters by a factor of 50 over previous experiments, Mueller believes the real significance of the team’s work is the ability to simultaneously confirm the Lorentz invariance of light and matter without assuming the Lorentz invariance of the physical properties of the other.