The speed of light is a constant. A new test -- the most precise test of a photon's speed -- confirms it.

The results of the test support the concept of Lorentz symmetry, the idea that the laws of physics are constant in all directions.

Researchers from the University of Western Australia and Humboldt University of Berlin measured the spatial consistency of the speed of light using two cryogenic sapphire oscillators. Two pieces of sapphire are vibrated at identical frequencies and supercooled with liquid helium, thus freezing the exact frequency.

Microwave frequencies within the cavities containing the two sapphires were then monitored for two years. The sapphires are aligned at perfect right angles, so that any variance in the speed of light along any axes would be detected, as it would throw off the finely tuned frequencies of the two sapphires.

Researchers say their experiment produced results ten times more precise than previous Lorentz symmetry experiments.

"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."

Parker, who helped lead the new research, published this week in the journal Nature Communications, says his research team is working on incorporating new sources of light into their experimental setup. Their goal is to continue looking for instances of invariance, under various and more precisely measured circumstances.

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.