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Does Quantum Entanglement Distort Gravity?

An unlikely suggestion.
January 11, 2015, 1:00pm

It's ​an unlikely but illustrative suggestion: The phenomenon of quantum entanglement, in which particles are allowed share identities over large distances, has its very own mass. And so entanglement, as suggested in a recent paper, may influence gravity.

First, a step back:

We are who we are, and it's nice to imagine those identities as being distinct and irrevocably unique. Even the droneiest office drone will be different in really all kinds of ways, starting at the bottom with genetic material. We are complicated.

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Particles aren't complicated. Or, rather, they're complicated in other ways. Particles get to be non-distinct, sharing identities and characteristics to the point that they get to, in a sense, become other particles, where whole groups of particles might be described in terms of a single one. But they're not clones; clones can be acted on individually.

Put differently, a group of particles sharing some state cease to be properly individuals. That is, it's no longer possible to fully describe one particle without describing its partners as well. Which is a lot like saying that it's not possible to do something to an individual particle occupying the same state as another, without doing that thing to the second particle. This becomes only slightly less odd when put mathematically.

When particles share the same state, we say they're entangled, a fully bizarre yet omnipresent phenomenon in which the properties of particles become perfectly correlated, such that when one property of one particle is measured, that same property of its entangled partner experiences the same interaction (the measurement). The two could be separated at any distance, and this would still happen instantly.

Einstein wasn't big on the idea, preferring an alternate explanation in which these relationships could be described by hidden variables and not "spooky action at a distance." This more comfortable or at least safe alternative was believing in a fundamental incompleteness to the quantum picture of the world.

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This incompleteness is also suggested by gravity. Gravity is a force so weak that it would seem to be oblivious to the quantum world entirely (or vice versa). ​This is where a new paper, authored by David Bruschi at the Hebrew University of Jerusalem, comes in, offering the suggestion that maybe the phenomenon of entanglement has itself an effect on gravitational fields, just like the spacetime-wrenching motions of black holes or binary star systems but at beyond-diminutive scales.

Bruschi's work was to craft a mathematical solution to how these fields might be perturbed in the presence of entangled particles. A mathematical coupling.

'It has been established that quantum coherence and entanglement in a system can be used to extract energy," Bruschi told me. "In some sense, one can store energy in the coherences. I understand that ​quantum refrigerators use this as a core principle. [This observation] inspired me to ask the question: if, somehow, energy can be extracted from quantum coherences of the system then gravity must be affected by the 'quantumness of the state.' This is to be expected because energy interacts with (i.e., it is a source of) gravity."

The difference is inconceivably small—one part in 1037—but conceivably detectable

The basic idea is that entangled particles just weigh a bit more than they might otherwise as distinct entities. So: the gravitational perturbation becomes a function of both the energy of the particles, as we'd expect, but also the length of time the entanglement persists. The difference is inconceivably small—one part in 1037—but conceivably detectable, at least in theory. It could register.

"For very heavy particles, for particles that are ultra relativistic or for states with a high number of excitations (i.e., N00N states, which have already been employed to greatly enhance estimation of parameters due to their "high" quantum nature), one could hope to increase the above result by several orders of magnitude," Bruschi writes. "This could in principle make the effect measurable."

The effect would be manifested as gravitational waves, in which the motion of massive objects cause space-time ripples to propagate outward as periodic distortions in, yes, time and space itself. These waves have so-far not been directly observed.

Bruschi's idea comes from taking the mathematics of entanglement, how a single particle can result in excitations in a quantum field in two or more places, and putting it in terms of relativity, particularly the creation of gravitational waves.

This coupling isn't guaranteed by any means. It's an assumption, one that seems to lead to things working out. "I am not able yet to say 'where' the energy (read also weight) comes from or 'where is it stored,'" Bruschi said. "For the moment they are vague questions, although of primary importance. Further work will help me understand more and hopefully answer such questions."