Physicists in Germany have performed the most precise Michelson-Morley experiment to date, confirming that the speed of light is the same in all directions. The experiment, which involves rotating two optical cavities, is about 10 times more precise than previous experiments – and a hundred million times more precise than Michelson and Morley's 1887 measurement.

The laws of physics appear to be the same for all processes occurring in laboratories moving at constant speed and for any orientation – a fundamental concept known as Lorentz symmetry. It takes its name from the Dutch physicist Hendrik Antoon Lorentz, who was attempting to explain the null result of Albert Michelson and Edward Morley's famous experiment. Then in 1905, Albert Einstein used Lorentz symmetry as a postulate of his special theory of relativity.

Lorentz symmetry has so far withstood the tests of time, but in recent years physicists have begun to question whether it is indeed an exact symmetry of nature. They are motivated primarily by the development of string and loop quantum gravity theories, which try to make gravity compatible with quantum physics and allow for the possibility that Lorentz symmetry might not hold exactly.

In order to develop these and other theories, physicists need to know if and when the speed of light is different in different directions. Michelson and Morley tackled this problem by splitting light into two beams that travel at right angles to each other, are reflected by mirrors and then recombined with each other to produce an interference pattern, which depends on different lengths of the two paths. A change in this pattern as the interferometer is rotated would suggest that the speed of light is different in different directions.

Floating on air

In the past 120 years physicists have improved the Michelson-Morley experiment – and its latest incarnation can be found in Stephan Schiller's lab at the Heinrich-Heine University in Düsseldorf. The apparatus floats on a thin cushion of air above a 1.3 tonne granite table. It comprises two optical cavities – essentially pairs of mirrors that reflect light back and forth – that are both about 8.4 cm long and at right angles to each other. Because the cavities are slightly different in length, they have slightly different resonant frequencies.

In the experiment, a laser beam is split into two beams, one for each cavity. The frequencies of the beams are then tuned to that of their respective cavities using "acousto-optic modulators". The two beams – which now have different frequencies – are then recombined to produce a beat signal. If the speed of light were different in different directions, it would affect the resonant frequencies of the two cavities in an out-of-step manner, which could then be detected as a shift in the beat frequency as the apparatus is rotated.

Schiller and colleagues Christian Eisele and Alexander Nevsky gathered data as they rotated their experiment about 175,000 times over about 13 months, with each rotation taking 90 seconds. To investigate whether Lorentz symmetry had been violated, the team analysed their time series of beat frequency measurements in terms a simplified version of the Standard Model Extension (SME) – a mathematical framework that describes violations to Lorentz symmetry in terms of 19 measurable parameters.

100 million times better

Schiller's experiment is sensitive to eight of these parameters and the team was able to show that four are zero to about two parts in 1017; one is zero to about one part in 1016; and three are zero to about two parts in 1013. According to Schiller, this represents a factor of more than 10 improvement over previous measurements of these parameters and a factor of about 100 million better than Michelson and Morley's original experiment.

Ben Varcoe at the University of Leeds in the UK told physicsworld.com that Schiller's experiment appears to be the most precise Michelson-Morley experiment to date. He also pointed out that if Schiller and colleagues were able to boost the precision of their experiment by a few more orders of magnitude, it could become sensitive to the effects of dark energy on the propagation of light.

The idea is that if the Earth is moving in a specific direction through stationary dark energy, the latter could be detected as a Lorentz violation. (Michelson and Morley were looking for a similar violation due to luminiferous aether, which we now know does not exist.)

Sensitivity boost

According to Schiller, it should be feasible to boost the sensitivity of the experiment by as much as a factor of 1000 over the next 10 years by introducing major improvements to the apparatus.

However, most current theories of quantum gravity leqad one to expect Lorentz violations at levels of about 10–30 – a precision that has already been reached in some astrophysical measurements of other SME parameters. How to reach such levels with Michelson-Morley experiments "is a tremendous challenge for the future," said Schiller.

The work is described in Physical Review Letters.