Science: A proposed theory to unify classical and quantum physics

This has the feel of something that could be a very big deal in science. The Brighter Side News reports:

For over a century, quantum mechanics and Einstein’s general relativity have stood as the cornerstones of modern physics, yet their unification remains one of science’s greatest challenges.

Now, researchers at University College London (UCL) have introduced a groundbreaking theory that challenges conventional approaches to this problem.

Quantum gravity seeks to bridge the gap between the microscopic world, where quantum mechanics governs particle behavior, and the macroscopic realm, where gravity shapes spacetime.

Traditionally, physicists have assumed that Einstein’s theory must be modified to fit within the quantum framework. However, UCL researchers propose a striking alternative: a "postquantum theory of classical gravity" that reexamines the fundamental relationship between these two domains.

This latest proposal challenges conventional wisdom, suggesting that instead of forcing gravity into a quantum framework, researchers should explore a new perspective—one where classical gravity interacts with quantum systems in ways previously unexplored. The implications of this theory could reshape our understanding of the universe, offering a fresh path toward reconciling two of physics' most successful yet conflicting models.

Enter Professor Jonathan Oppenheim and his team at UCL, who have challenged the status quo with their groundbreaking theory. In two parallel papers published simultaneously, they propose a novel perspective that suggests spacetime may remain classical and unaffected by quantum mechanics.

This theory, as described in a paper published in Physical Review X (PRX), refrains from modifying spacetime itself and instead modifies quantum theory.

An experiment in which heavy "particles" (illustrated as the moon), cause an interference pattern (a quantum effect), while also bending spacetime. The hanging pendulums depict the measurement of spacetime

The core tenet of this theory is that spacetime remains classical, not subject to the constraints of quantum theory. Instead, quantum theory is tweaked to account for intrinsic unpredictability mediated by spacetime. The consequence? Spacetime experiences random and violent fluctuations that exceed the expectations set by quantum theory. These fluctuations, if measured precisely enough, render the apparent weight of objects unpredictable.

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Until now, physicists including Einstein tried to unify classical and quantum physics by trying to fit  classical with quantum physics. That failed. This new theory reverses it. It proposes two possibilities. One is that classical physics (Einstein's relativity and spacetime) is not quantum at all, but instead quantum phenomena arise from properties inherent in spacetime. Here, quantum theory gets modified to account for a proposed intrinsic unpredictability in spacetime. The hypothesis is that spacetime is subject to random and "violent" fluctuations, which are more than quantum physics predicts. 

Researchers propose testing this first theory by detecting changes in the mass of an object over time due to fluctuations in spacetime. The theory precisely predicts exact changes in mass that would be expected. If the measurements of a chosen mass are fluctuations less than those required for mathematical consistency with the new theory, that would contradict the new theory. But if the predicted changes in mass are observed, that would constitute strong evidence that this hypothesis is correct.

A second theory is a hybrid model that postulates that classical spacetime interacts with quantum fields. To test that possibility, researchers propose using gravity to see if it influences quantum entanglement of subatomic particles, atoms, or more likely large masses like 1 mg. If spacetime is classical, entanglement would behave in a quantum way, but if spacetime is quantum, entanglement would behave as it has been observed until now. The new variable here is looking for effects of gravity on quantum entanglement. That has never been done before.

Researchers estimate that it will take about 20 years to test these hypotheses. The reason is that new, far more accurate technology to measure time and gravity strength are needed. The most accurate atomic clock available now can measure increments of time in increments of 8.1 x 10^-19 seconds, an accuracy level of 1 sec. in 30 billion years. That is not nearly accurate enough. Also, current devices to detect gravity fields are not sensitive enough. Gravity field detectors will require developing something like quantum gravity gradiometers using cold atom interferometry. That technology might potentially measure Earth's gravitational field with enough precision to do the experiment.

In addition, ways to prepare and maintain quantum states of heavy objects, like a milligram mass, in superposition or entanglement is necessary. This would require (i) cooling systems to near absolute zero to minimize thermal noise and decoherence, and (ii) isolating the instrument from environmental disturbances such as electromagnetic fields, thermal fluctuations, and mechanical vibrations. That probably would require ultra-high vacuum conditions and advanced vibration and radiation isolation techniques. Some of this might require doing experiments in a quiet place in space, like L2 where the James Webb Space Telescope is currently parked and doing its experiments.