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Physicists have found the strongest evidence yet for no violation of Lorentz symmetry, one of the fundamental symmetries of relativity. Lorentz symmetry states that the outcome of an experiment does not depend on certain aspects of its surroundings, namely the velocity and the direction of its moving reference frame—properties that become relevant when studying astronomical objects and launching satellites, for instance, as well as for unifying quantum mechanics and general relativity.

"We know that general relativity and the Standard Model of particle physics are not the ultimate theories," coauthor Marie-Christine Angonin at the Paris Observatory told Phys.org. "Furthermore, so far, it has been impossible to conciliate in one common theory these two aspects of physics. To succeed in this quest, almost all unification theories predict a breaking of Lorentz symmetry."

To perform the improved test of Lorentz symmetry, the team of physicists from the Paris Observatory and the University of California, Los Angeles, analyzed 44 years of data from lunar laser ranging (LLR) observations.

LLR involves sending laser pulses between a station on the Earth to a reflector on the Moon and back, and measuring the time it takes for the light to complete the round trip, which is roughly 2.5 seconds. Modern LLR experiments can determine the distance between the Earth and Moon to within less than a centimeter.

In the new study, the researchers analyzed data from more than 20,000 reflected laser beams sent between 1969 and 2013 by five LLR stations located at different places on the Earth. The round-trip travel time of the light is influenced by numerous factors, from the location of the Moon in the sky, to the weather and the tides, as well as relativistic effects—which are especially important for testing Lorentz symmetry.

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