Physicists propose tabletop experiment to test gravity’s quantumness Premium

Scientists propose a relatively simple yet still complicated tabletop experiment to test the quantumness of gravity, aiming to reconcile general relativity and quantum mechanics.

General relativity and quantum mechanics are two highly successful theories. The former explains gravity and the latter teams up with special relativity to describe the other three forces of nature: electromagnetic, strong nuclear, and weak nuclear forces.

However, scientists don’t know how gravity fits into quantum mechanics. In fact, they have been proposing experiments that can test the quantumness of gravity. On October 29, 2024, one such proposal appeared in the journalPhysical Review Letters.

With concepts like superposition, illustrated by the Schrödinger’s cat thought experiment, and entanglement, quantum mechanics defies classical intuition. Quantum mechanics also allows seemingly absurd phenomena, e.g. the measurement of a quantum system (like a particle) can cause the system to instantaneously ‘collapse’ into one possible state that described the system before the measurement.

In fact, if a system undergoes measurement-induced collapse, it’s said to live by the rules of quantum mechanics. Classical systems like planets orbiting stars, cricket balls flying in the air, and cars on the road don’t do this.

Multiple experiments have found that both quantum mechanics and general relativity are legitimate theories of nature — yet they remain incompatible with each other. This has encouraged physicists to try and come up with a larger theory that can accommodate both. One strong contender is string theory, another is loop quantum gravity. Both of them predict deviations from quantum mechanics and general relativity either at the beginning of the universe or inside black holes, meaning they’re nearly impossible to test.

“So far, experimental tests are extremely difficult — the situation looks very bleak — it is not clear if it can be done at all,” Dipankar Home of Bose Institute, Kolkata, and one of the authors of the new paper, said.

To check whether gravity is quantum mechanical, scientists need precise tests that rule out alternative possibilities.