Physicists have reported the most accurate test to date of Lorentz symmetry for photons by looking for variations in the speed of light, but found no violations. The study, by Moritz Nagel at the Humboldt-University of Berlin, Stephen Parker at The University of Western Australia, and their co-authors, is published in a recent issue of Nature Communications.
Two sapphire cylinders were loaded in a dual-cavity mount with their crystal axes (black arrows) aligned orthogonally. This was housed within two vacuum cans and cooled to 4 K in a liquid helium dewar. A whispering gallery mode resonance was excited in each sapphire, the Poynting vector is shown by the dashed magenta arrow.
Microwave and DC electronics were used to create two loop oscillators, each locked to the resonance of a sapphire. The two oscillators were beat against each other and the difference frequency was recorded. The apparatus was continuously rotated with a 100 s period on a tilt-controlled air-bearing turntable. Comparison with the original Michelson–Morley arrangement is presented in the top right to demonstrate experimental concept.
"If this were to change depending on the direction it was facing it would indicate that Lorentz symmetry had been violated," Stephen Parker, a quantum physicist at Wester Australia, said in a press release. "But the frequencies didn't even change down to the 18th digit (the smallest part of the measurement of frequency), which is remarkable that this symmetry of nature still holds true at such tiny levels."
Researchers are keen to find Lorentz invariance, as a number of theories that attempt to unify the two models of physics, general relativity and the standard model of particle physics rely upon the violation of Lorentz symmetry.