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When it comes to a marriage with quantum theory, gravity is the lone holdout among the four fundamental forces in nature. The three others—the electromagnetic force, the weak force, which is responsible for radioactive decay, and the strong force, which binds neutrons and protons together within the atomic nucleus—have all merged with quantum theory to successfully describe the universe on the tiniest of scales, where the laws of quantum mechanics must play a leading role.

Although Einstein's theory of general relativity, which describes gravity as a curvature of space-time, explains a multitude of gravitational phenomena, it fails within the tiniest of volumes—the center of a black hole or the universe at its explosive birth, when it was less than an atomic diameter in size. That's where quantum mechanics ought to dominate.

Yet over the past eight decades, expert after expert, including Einstein, have been unable to unite quantum theory with gravity. So, is gravity truly a quantum force?

Researchers at the National Institute of Standards and Technology (NIST) and their colleagues have now proposed an experiment that may help settle the question.

The experiment takes advantage of two of the weirdest properties of quantum theory. One is the , which holds that an undisturbed atomic particle can be described as a wave, with some probability of being in two places at once. For instance, an undisturbed atom traveling through a region with two slits, passes through not one or the other of the slits but both.

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