Scientists think gravity could be a quantum field

Scientists have proposed a new test to hopefully shed some light on quantum gravity.
Gravity is the only major force in the universe that doesn’t fit into our current quantum framework, driving the push for a grand, unified theory of physics.
The new test wouldn’t prove or fully understand quantum gravity, but it would be a huge step in the right direction – if we can pull it off.

Almost everything we know about the universe comes down to the rules of quantum mechanics. Just about anything can be described as fields (electromagnetismthe strong and weak nuclear forces) and matter, and we have slowly rebuilt our fundamental understanding of the cosmos from these fundamental building blocks.

That is, with the exception of one large, neon-green brick in the uniform, rustic wall of physics: gravity. Gravity, as far as we have been able to prove, is not quantum, even though theoretically it should be so. The world’s greatest scientific minds still don’t have a quantum description of gravity, mostly just ‘quantum gravity.” The best way to describe this at the moment is through general relativity, and the discrepancy between classical gravity and all our known quantum fields is why people continue to search feverishly for a grand, unified theory of physics. We see how gravity and quantum fields work together, so the rules should work together too.

The search for the rules of quantum gravity has been an arduous process, but scientists are pressing ahead anyway because it would change our understanding of… well… really. everything. And this is evident from a recent study published in the journal Physical Assessment Lettersa group of researchers has an idea on how to test the existence of gravity in the quantum realm.

The proposal stems from the idea that, rather than trying to prove pure quantum gravity, we could at least refute pure classical gravity by showing that the measurement of this force follows at least one quantum rule: superposition. The scientists propose to make a very small crystal (we’re talking just nanometers here) and place it in a superposition state: a quantum state in which an object (usually a particle) is in two places at once until it is measured .

Now, superposition has been used to test quantum gravitybut the authors of this new paper argue that observation of the phenomenon is not enough. “Since quantum mechanics is not only defined by the superposition principle, but also requires the unitarity of evolution and the measurement postulate,” the authors wrote. “Witnessing entanglement (…) will imply that gravity is described by quantum mechanics, or by some (unknown) non-classical theory that follows the superposition principle. To know whether gravity is indeed quantum, we need to test other quantum mechanical postulates on gravity.’

So they decided to take it a step further, but they still plan to start with superposition. Once the superposition state is induced, two paths must be followed. In the first path, to set up a kind of control group, the crystal in the superposition state is measured and the final state is recorded. The second path brings another nanocrystal into the mix so close to the first that a very weak gravitational force should exist 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 article in the magazine Naturethat behavior “would be effective”unit of measurement‘ the mass of the first (crystal).” The researchers then measured that first crystal again and looked to see if its final state was different than when it was measured before the second crystal intervention – an effect caused by a phenomenon known as ‘quantum measurement-induced perturbation’, which only happens when quantum effects play a role.

According to Nature, if the state is indeed different, that would be “a first proof of it gravity indeed has a quantum character.” It’s complicated, it’s never been done before, it’s incredibly advanced, and it’s (crucially) not the complete theory of quantum gravity that we hope to eventually decipher. As the authors wrote in the article: “Adding this test to (existing tests of the effects of superposition on gravity) will take us to a more completely demonstration of gravity as a quantum entity.”

But if it works, it would be an excellent next step. Let’s hope success.

Jackie is a writer and editor from Pennsylvania. She especially enjoys writing about space and physics, and enjoys sharing the strange wonders of the universe with anyone who will listen. She is accompanied in her home office by her two cats.