Physicists often look for the exception that might make a new rule. A prime example is Lorentz invariance, a fundamental principle in relativity, whose exceptions—if there are any—would imply the need for an alternative theory of gravity. A new comprehensive study of atmospheric neutrinos finds no violation of Lorentz invariance. The results place some of the toughest constraints to date on Lorentz-violating theories.
Lorentz invariance assumes that measurements shouldn’t depend on the speed or the orientation of the laboratory frame. As straightforward as that sounds, certain gravity models, such as string theory, predict this rule is broken to a small degree. Lorentz violation (LV) could show up in different ways, such as a directional dependence in the speed of light, but no evidence has so far been collected.
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