A negative result is not always bad news. In 1887, the physicists Albert Michelson and Edward Morley failed to detect the influence of the mysterious 'ether' ― the medium through which light waves were thought to travel. As they surveyed their apparatus, held in a basement in Cleveland, Ohio, they must have been disappointed. Yet just a few years later, Albert Einstein, inspired in part by Michel-son and Morley's experiment, announced his revolutionary theories of relativity and completely reinvented our notion of space, time and gravity. Today, experimentalists in France and Germany are rerunning Michelson and Morley's experiment with unprecedented precision. But this time they are not looking for the ether. By pushing this test of the constancy of the speed of light to its limits, they hope to find signs of new physics beyond Einstein's equations. If they do, they may be the first to cross the experimental threshold into an exotic new world of 'quantum gravity'. Even a negative result ― essentially one that agrees with Einstein's theories ― might, if it is sufficiently precise, be able to rule out or modify certain ideas about quantum gravity.
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