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Scientists have proposed a new test to hopefully help shed some light on quantum gravity.
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Gravity is the only major force in the universe that doesn’t fit into our current quantum framework, driving the pursuit of a grand unified theory of physics.
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The new test would not be proof or a full understanding of quantum gravity, but it would be a huge step in the right direction—if we can pull it off.
Almost all we know about the universe boils down to the rules of quantum mechanics. Just about everything can be described as fields (electromagnetism, the strong and weak nuclear forces) and matter, and we’ve slowly re-built our fundamental understanding of the cosmos out of these fundamental building blocks.
That is, except for one big, neon-green brick in the uniform, rustic wall of physics—gravity. Gravity, as far as we have been able to prove, isn’t quantum, even though it theoretically should be. The greatest scientific minds in the world still don’t have a quantum description of gravity, usually just called “quantum gravity.” The best way to describe it right now is through general relativity, and the disconnect between classical gravity and all of our known quantum fields is the reason people continue to feverishly search for a grand unified theory of physics. We watch gravity and quantum fields work together, so the rules should work together, too.
The search for the rules of quantum gravity has been a slog, but scientists keep at it regardless, as it would truly change our understanding of… well… everything. And according to a recent study, published in the journal Physical Review Letters, one group of researchers has an idea regarding how to test for the existence of gravity in the quantum realm.
The proposal stems from the idea that instead of trying to prove purely quantum gravity, we could at least disprove purely classical gravity by showing that the measurement of this force follows at least one quantum rule: superposition. The scientists suggest creating a teeny-tiny crystal (we’re talking only nanometers in size) and placing it into a state of superposition—a quantum state where an object (usually a particle) is in two places at once until it is measured.
Now, superposition has been used to test for quantum gravity, but the authors of this new paper argue that observation of the phenomenon isn’t enough. “As quantum mechanics is not defined by the superposition principle alone, but also requires the unitarity of evolution and the measurement postulate,” the authors wrote. “Witnessing entanglement […] will imply that gravity is described either by quantum mechanics, or by a (unknown) nonclassical theory that obeys superposition principle. To know whether gravity is indeed quantum, we need to test other quantum mechanical postulates for gravity.”
So, they decided to go a step farther—but they still intend to start with superposition. Once the superposition state has been induced, two paths are to be followed. On the first path, to set up a kind of control group, the crystal in the superposition state is measured and its final state recorded. On the second path, another nanocrystal is brought into the mix close enough to the first that there should be a very weak gravitational force between the two crystals.
The gravity of that second crystal should (in theory) be felt by the first crystal in the superposition. According to a research highlight write-up in the journal Nature, that behavior “would effectively ‘measure’ the mass of the first [crystal].” Then, the researchers would measure that first crystal again and see if its final state was different from when it was measured pre-second-crystal intervention—an effect caused by a phenomenon known as ‘quantum measurement-induced disturbance,’ which only happens if quantum effects are at play.
According to Nature, if the state is in fact different, it would be “a first proof that gravity does indeed have a quantum nature.” It’s complicated, it’s never been done, it’s incredibly cutting-edge, and it is (critically) not the full theory of quantum gravity that we eventually hope to decode. As the authors wrote in the paper, “adding this test to [existing tests of superposition’s effects on gravity] will take us towards a more complete demonstration of gravity as a quantum entity.”
But if it works, it would be an excellent next step. Let’s hope for success.
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