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Is Gravity a Quantum Force?

quantum gravity atomic interferometer

Artist’s illustration of an atomic interferometer used in the proposed quantum gravity experiment. (Credit: Simca Bouma/UC Berkeley)

Adapted from a NIST news release.

Einstein’s theory of general relativity, which describes gravity as a curvature of space-time, explains a multitude of gravitational phenomena.

But that theory falls apart within the tiniest of volumes – the center of a black hole or the universe at its explosive birth, when it was smaller than the diameter of an atom. That’s where quantum mechanics ought to dominate – yet over the past eight decades, expert after expert, including Einstein, has been unable to unite quantum theory with gravity.

Now, researchers at Berkeley Lab and the National Institute of Standards and Technology (NIST) have now proposed an experiment that may settle the persistent question: Is gravity truly a quantum force? They recently described their work in the journal Physical Review X Quantum.

The experiment takes advantage of two of the weirdest properties of quantum theory: the superposition principle, which holds that an undisturbed atomic particle can be described as a wave, with some probability of being in two places at once; and entanglement, a phenomenon in which two particles can be so strongly correlated that they behave as a single entity.

The experiment was proposed by Daniel Carney, a physicist in Berkeley Lab’s Physics Division; Holger Müller, an associate professor of physics at UC Berkeley and faculty scientist in Berkeley Lab’s Molecular Biophysics and Integrated Bioimaging Division; and Jake Taylor of NIST’s Joint Quantum Institute. The scheme employs a cold cloud of atoms, trapped inside an atomic interferometer, to test if two massive bodies can indeed become entangled by gravity.

Although the ideal experiment may be a decade or more from being built, a preliminary version could be built in just a few years.

“Trying to merge quantum mechanics and general relativity into a single framework brings up many deep problems,” Carney said. “Our experiment would try to answer the simplest possible question: does the gravitational field itself obey the laws of quantum mechanics at all?”

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