Scientists tried to break Einstein’s speed of light rule

Einstein’s claim that the speed of light is constant has survived more than a century of scrutiny—but scientists are still daring to test it. Some theories of quantum gravity suggest light might behave slightly differently at extreme energies. By tracking ultra-powerful gamma rays from distant cosmic sources, researchers searched for tiny timing differences that could reveal new physics. They found none, but their results tighten the limits by a huge margin.