Radical theory unites gravity, spacetime, and the quantum realm

Physicists have proposed a radical approach that questions decades of belief about how gravity, spacetime, and quantum mechanics might fit together.

They have introduced a theory that keeps the classical concept of spacetime as envisioned by Einstein, even as it addresses a long-standing rift in modern physics.

For years, quantum theory and general relativity have each held important places in science, yet they have never fit snugly alongside one another.

This mismatch has led many to ask if space and time should be broken down into quantum bits, or if something else might be going on.

Published in two respected journals, this line of thought challenges the standard assumption that Einstein’s theory of gravity must be fully recast in quantum terms.

Researchers note that established efforts, including string theory and loop quantum gravity, are anchored in the idea that spacetime itself should become quantum. But the new direction suggests that quantum theory might need rethinking.

Observers say this could affect how precisely we can measure an object’s weight or how long certain atoms can remain in a strange two-place-at-once condition.

Radical postquantum proposal

In work spearheaded by Professor Jonathan Oppenheim of University College London (UCL) Physics & Astronomy, this theory reframes spacetime as something that might never have been quantum at all.

“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?” Oppenheimer posits.

“Now that we have a consistent fundamental theory in which spacetime does not get quantized, it’s anybody’s guess.”

Theory revises quantum spacetime rules

This perspective replaces the usual search for a quantum version of gravity with what is described as a postquantum theory of classical gravity.

In this model, spacetime remains classical yet experiences unpredictable shifts.

Instead of the subtle effects typically predicted by standard quantum calculations, these fluctuations could be more pronounced, making precise weight measurements trickier.

Meanwhile, a research group led by Professor Oppenheim’s former PhD students has outlined a method to see if spacetime is truly quantum. They propose measuring a mass with extreme accuracy to detect unusual variations.

Experimental possibilities

The second paper points to an experiment that examines whether a precisely defined weight, such as the 2.2 lb standard from the International Bureau of Weights and Measures in France, shows any unexpected changes over time.

Another test involves placing a heavy atom in two spots at once to see if a classical spacetime might force it to pick one spot faster than expected.

According to Zach Weller-Davies, a key collaborator, “If spacetime doesn’t have a quantum nature, then there must be random fluctuations in the curvature of spacetime with a particular signature that can be verified experimentally.”

Testing time and weight

Potential tests also look at whether time itself might be quantum or classical.

“Because gravity is made manifest through the bending of space and time, we can think of the question in terms of whether the rate at which time flows has a quantum nature, or classical nature,” Dr. Barbara Šoda from the Perimeter Institute remarked.

Testing this is almost as simple as testing whether the weight of a mass is constant, or appears to fluctuate in a particular way.

“While the experimental concept is simple, the weighing of the object needs to be carried out with extreme precision,” Dr. Carlo Sparaciari of UCL pointed out.

“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.”

Why does any of this matter?

Professor Oppenheim, Professor Carlo Rovelli, and Dr. Geoff Penington have placed 5000:1 odds on the outcome of such tests, reflecting the intense interest in whether spacetime is genuinely quantum or classical.

Observers in the field say those odds might shift if fresh data confirms one viewpoint or the other.

This framework traces back to work on the black hole information problem, a puzzle that usually presupposes information is never lost.

Here, it may vanish, fitting with general relativity’s suggestion that black holes can destroy information.

Spacetime and the quantum realm

This postquantum picture bypasses the standard measurement principle in quantum theory and could explain why superpositions come to an end without extra assumptions.

The research team indicates it emerged from analyzing black hole scenarios, where standard quantum beliefs clash with Einstein’s equations.

Professor Sougato Bose of UCL Physics & Astronomy, who was not part of these findings, commented, “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.”

Such ideas could settle a major divide between quantum laws and gravity, hinting at a more cohesive picture of the cosmos.

The full studies were published in the journals Nature Communications and  Physical Review X (PRX).

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