##### In Brief

A study using the new generation of super accurate strontium clocks supports physicists' theories about time dilatation and Lorentz invariance.A new generation of super accurate clocks are proving that Einstein’s idea that time is not absolute is correct. This is not great news for anyone looking for a unifying theory for everything from Einstein’s work to Quantum Mechanics, but it’s decent news for the rest of us who are relying on precision timepieces.

There is a highly regarded theory in physics that any two observers moving at a constant speed relative to each other will experience the same exact laws of physics. This symmetry of special relativity is called Lorentz invariance. Each of the symmetrical speeders would see themselves as stationary, but they would look over and see the other person’s clock running slowly. This is the time dilation effect, and Einstein’s theory of General Relativity adds gas to the fire by saying that if the speeders experience different gravitational forces, their clocks will run differently.

So far, we have confirmed these ideas by comparing atomic clocks on GPS satellites with their Earthbound cousins. However, it has always been clear that deviations from relativity would be minuscule, and so it remained possible that we simply lacked tools with the sufficient precision to detect any deviations.

## Time Matters

Enter the next-generation strontium clocks, which are at least three times more precise than the caesium-133 models, neither losing nor gaining more than a second over the course of 15 billion years. Pacôme Delva of the Paris Observatory and his team used fiberoptic links to test time dilation between these strontium clocks in London, Paris, and Braunschweig, Germany. Because of their positions on the Earth’s surface, the clocks would tick at slightly different rates. If the theory of relativity was accurate, it would correctly predict those differences.

The team took their measurements and then calculated a parameter called alpha. No violation of the Lorentz invariance would produce an alpha of zero, and the results produced a near-zero alpha of less than 10^{-8}. This result supports the Lorentz invariance and is twice as accurate as the best limit from previous work, and two orders of magnitude better than previous caesium clock results.

As far as strontium clocks go, things are looking good for relativity. Things are also looking good for advanced technologies that demand precision time measurement, such as GPS systems and autonomous vehicles.

Still, fans of the Lorentz invariance shouldn’t get too comfortable. Although there is not yet any evidence of a violation, physicists all over the world are going to keep looking. A confirmed violation of the Lorentz invariance would have tremendous implications for quantizing gravity and our understanding of the nature of Dark Energy and Dark Matter.