A quantum origin for spacetime


EXCERPT: . . . “Spacetime and gravity must ultimately emerge from something else,” writes physicist Brian Swingle in the 2018 Annual Review of Condensed Matter Physics. Otherwise it’s hard to see how Einstein’s gravity and the math of quantum mechanics can reconcile their longstanding incompatibility. Einstein’s view of gravity as the manifestation of spacetime geometry has been enormously successful. But so also has been quantum mechanics, which describes the machinations of matter and energy on the atomic scale with unerring accuracy. Attempts to find coherent math that accommodates quantum weirdness with geometric gravity, though, have met formidable technical and conceptual roadblocks.

[...] In any case, investigations along these lines have revealed a surprising possibility: Spacetime itself may be generated by quantum physics, specifically by the baffling phenomenon known as quantum entanglement.

As popularly explained, entanglement is a spooky connection linking particles separated even by great distances. If emitted from a common source, such particles remain entangled no matter how far they fly away from each other. If you measure a property [...] for one of them, you then know what the result of the same measurement would be for the other. But before the measurement, those properties are not already determined, a counterintuitive fact verified by many experiments. It seems like the measurement at one place determines what the measurement will be at another distant location.

That sounds like entangled particles must be able to communicate faster than light. Otherwise it’s impossible to imagine how one of them could know what was happening to the other across a vast spacetime expanse. But they actually don’t send any message at all. So how do entangled particles transcend the spacetime gulf separating them? Perhaps the answer is they don’t have to — because entanglement doesn’t happen in spacetime. Entanglement creates spacetime.

At least that’s the proposal that current research in toy universes[PDF] has inspired. “The emergence of spacetime and gravity is a mysterious phenomenon of quantum many-body physics that we would like to understand,” Swingle suggests in his Annual Review paper. Vigorous effort by several top-flight physicists has produced theoretical evidence that networks of entangled quantum states weave the spacetime fabric. These quantum states are often described as “qubits” — bits of quantum information (like ordinary computer bits, but existing in a mix of 1 and 0, not simply either 1 or 0). Entangled qubits create networks with geometry in space with an extra dimension beyond the number of dimensions that the qubits live in. So the quantum physics of qubits can then be equated to the geometry of a space with an extra dimension. Best of all, the geometry created by the entangled qubits may very well obey the equations from Einstein’s general relativity that describe motion due to gravity — at least the latest research points in that direction. “Apparently, a geometry with the right properties built from entanglement has to obey the gravitational equations of motion,” Swingle writes. “This result further justifies the claim that spacetime arises from entanglement.”

Still, it remains to be shown that the clues found in toy universes with extra dimensions will lead to the true story for the ordinary spacetime in which real physicists strut and fret. (MORE - details)
Thinking QM is the ultimate fundamental is the problem. As with many such things, what unifies seemingly disparate phenomena is often a third, common cause, instead of one mysteriously being the unrecognized cause of the other. Considering we can't even manage to agree on a single interpretation for QM, it seems something more fundamental is needed to narrow the possibilities. But scientists who have to promote their own field to secure funding are not likely to do anything truly new too far afield.

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