Far out cosmology: new theory to join classical and quantum (Introduction)

by David Turell , Monday, December 04, 2023, 20:41 (353 days ago) @ David Turell

And it may be experimentally verified:

https://phys.org/news/2023-12-theory-einstein-gravity-quantum-mechanics.html

"The prevailing assumption has been that Einstein's theory of gravity must be modified, or "quantized," in order to fit within quantum theory. This is the approach of two leading candidates for a quantum theory of gravity, string theory and loop quantum gravity.

"But a new theory, developed by Professor Jonathan Oppenheim (UCL Physics & Astronomy) and laid out in a paper in Physical Review X, challenges that consensus and takes an alternative approach by suggesting that spacetime may be classical—that is, not governed by quantum theory at all.

"Instead of modifying spacetime, the theory—dubbed a "postquantum theory of classical gravity"—modifies quantum theory and predicts an intrinsic breakdown in predictability that is mediated by spacetime itself. This results in random and violent fluctuations in spacetime that are larger than envisaged under quantum theory, rendering the apparent weight of objects unpredictable if measured precisely enough.

"A second paper, published simultaneously in Nature Communications and led by Professor Oppenheim's former Ph.D. students, looks at some of the consequences of the theory, and proposes an experiment to test it: to measure a mass very precisely to see if its weight appears to fluctuate over time.

"For example, the International Bureau of Weights and Measures in France routinely weigh a 1kg mass which used to be the 1kg standard. If the fluctuations in measurements of this 1kg mass are smaller than required for mathematical consistency, the theory can be ruled out.

"The outcome of the experiment, or other evidence emerging that would confirm the quantum vs. classical nature of spacetime, is the subject of a 5000:1 odds bet between Professor Oppenheim and Professor Carlo Rovelli and Dr. Geoff Penington—leading proponents of quantum loop gravity and string theory respectively.

***

"Professor Oppenheim said, "Quantum theory and Einstein's theory of general relativity are mathematically incompatible with each other, so it's important to understand how this contradiction is resolved. Should spacetime be quantized, or should we modify quantum theory, or is it something else entirely? Now that we have a consistent fundamental theory in which spacetime does not get quantized, it's anybody's guess."

"Co-author Zach Weller-Davies, who as a Ph.D. student at UCL helped develop the experimental proposal and made key contributions to the theory itself, said, "This discovery challenges our understanding of the fundamental nature of gravity but also offers avenues to probe its potential quantum nature.

"'We have shown that if spacetime doesn't have a quantum nature, then there must be random fluctuations in the curvature of spacetime which have a particular signature that can be verified experimentally.

"'In both quantum gravity and classical gravity, spacetime must be undergoing violent and random fluctuations all around us, but on a scale which we haven't yet been able to detect. But if spacetime is classical, the fluctuations have to be larger than a certain scale, and this scale can be determined by another experiment where we test how long we can put a heavy atom in superposition of being in two different locations."

"Co-authors Dr. Carlo Sparaciari and Dr. Barbara Šoda, whose analytical and numerical calculations helped guide the project, expressed hope that these experiments could determine whether the pursuit of a quantum theory of gravity is the right approach.

***

"'But what I find exciting is that starting from very general assumptions, we can prove a clear relationship between two measurable quantities—the scale of the spacetime fluctuations, and how long objects like atoms or apples can be put in quantum superposition of two different locations. We can then determine these two quantities experimentally."

"Weller-Davies added, "A delicate interplay must exist if quantum particles such as atoms are able to bend classical spacetime. There must be a fundamental trade-off between the wave nature of atoms, and how large the random fluctuations in spacetime need to be."

"The proposal to test whether spacetime is classical by looking for random fluctuations in mass is complementary to another experimental proposal that aims to verify the quantum nature of spacetime by looking for something called "gravitationally mediated entanglement."

"Professor Sougato Bose (UCL Physics & Astronomy), who was not involved with the announcement today, but was among those to first propose the entanglement experiment, said, "Experiments to test the nature of spacetime will take a large-scale effort, but they're of huge importance from the perspective of understanding the fundamental laws of nature. I believe these experiments are within reach—these things are difficult to predict, but perhaps we'll know the answer within the next 20 years.'"

Comment: like MOND an interesting approach, but if solved 20 years from now, only some of us will learn the answers. I'm doubtful about this approach. The universe is obviously based on an underlying quantum background. I can't imagine it being somehow different in spacetime.