Free-fall experiment could test if gravity is a quantum force

Dr.Wayne WANG

Free-fall experiment could test if gravity is a quantum force

自由落体实验可以测试重力是否是量子力

By Anil Ananthaswamy

                               
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Despite decades of effort, a theory of quantum gravity is still out of grasp. Now a group of physicists have proposed an experimental test of whether gravity is quantum or not, to settle questions about the force’s true nature.

The search for quantum gravity is an effort to reconcile Einstein’s general relativity with quantum mechanics, which is a theory of all the fundamental particles and the forces that act on them – except gravity. Both are needed to explain what happens inside black holes and what happened at the big bang. But the two theories are incompatible, leading to apparent paradoxes and things like singularities, where the theories break down.

If gravity is a quantum mechanical force, adjacent free-falling masses, each of which is in a superposition of being in two places at once, could get entangled by gravity such that measuring the properties of one mass could instantly influence the other. To test this, Sougato Bose of University College London and his colleagues have proposed an experiment.

Branching paths

It starts with a neutrally charged mass weighing about 10-14 kilograms. Embedded within the mass is some material with a property called spin, which can be up or down. This mass falls through a continuously varying magnetic field, which changes the path of the mass depending on its spin. It is like the mass encounters a fork in the road and takes one path if its spin is up, and another if its spin is down.

As it falls, the mass is in a superposition of being on both paths. Next, a series of microwave pulses manipulate the spin at various stages of descent and thus the paths the mass takes. At the bottom, the paths then come together again and the mass is brought to its original state.

To use this set-up to test the quantum nature of gravity, two such masses would be dropped through the magnetic field. Each mass has two possible paths. This results in four possible states for the two masses combined. One of these states represents paths in which the masses come closest together.

This distance should be no less than 200 micrometres to avoid other interactions that can dominate gravity. Once the masses are back to their original state, a test to see if their spin components are entangled should tell us if gravity is indeed a quantum force. The assumption, of course, is that the experiment ensures there are no other ways in which the masses can get entangled – such as via electromagnetic interactions or the Casimir force.

Bose points out, however, that a null result – in which no entanglement is observed – wouldn’t constitute proof that gravity is classical, unless the experiment can definitively rule out all other interactions with the environment that can destroy entanglement, such as collisions with stray photons or molecules.

Quantum roots?

Antoine Tilloy at the Max Planck Institute of Quantum Optics in Germany is impressed. But he points out that a positive result will falsify only some classes of theories of classical gravity. “That said, the class is sufficiently large that I think the result would still be amazing,” he says.

Even a verifiable null result would be exciting because it would mean gravity doesn’t have quantum roots, says Maaneli Derakhshani of Utrecht University in the Netherlands. “This would then raise tough but interesting questions about how and when exactly gravity ‘turns on’ in the quantum-classical transition for ordinary matter,” says Derakhshani. “A null result would be the most surprising and interesting outcome.”

The biggest hurdle to carrying out the experiment for real would be putting such relatively large masses in a superposition. The most massive objects that have been observed to be in two places at once are still orders of magnitude smaller than what is required here. But efforts to go higher are ongoing.

This work is soon to be published in Physical Review Letters.

Reference: arxiv.org/abs/1707.06050

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