New test using cryogenic sapphire oscillators confirms speed of light

Researchers from the University of Western Australia and Humboldt University of Berlin used cryogenic temperatures to optimize frequency stability in a test designed to measure the spatial consistency of the speed of light with a precision ten times greater than ever before. The experiment compared the extremely pure microwave frequency signals from two cryogenic sapphire oscillators against each other over the course of a year.

“The cavity crystal axes were aligned perpendicular to each other such that the Poynting vectors and thus path of light propagation for the resonant modes were orthogonal to each other,” according to the research published this month in Nature Communications. The oscillators were then rotated on a turntable once every 100 seconds. “A fractional change in the speed of light would induce a proportional fractional change in the beat note frequency of the two oscillators.”

The stringent testing also confirmed a core component of Einstein’s theory of relativity known as Lorentz symmetry, which predicts that the speed of light is the same in all directions. “The frequency of the microwave signals directly linked to the speed of light,” said Dr. Stephen Parker, a UWA researcher from the Frequency and Quantum Metrology Research Group. “If this were to change depending on the direction it was facing it would indicate that Lorentz symmetry had been violated. 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.”

Parker said the researchers are in the process of upgrading the experiment and incorporating new optical light sources that would open up possibilities for future research. “This will allow us to improve the sensitivity of our work and explore other ways that Lorentz symmetry could be broken,” he said. “Searching for possible violations of Lorentz symmetry will provide valuable clues for a more comprehensive and unified theory.”