New Theory Unites Einstein’s Gravity With the Quantum World

In a new research reported by University College London (UCL), physicists may have just cracked the code to unite Einstein’s gravity theory with the peculiar rules of quantum mechanics. If their theory holds up, it might untangle a mystery that has puzzled scientists for decades, shedding light on how the universe works and what matter is made of.

Modern physics leans on two key ideas: quantum theory, dealing with the tiniest building blocks of the cosmos, and Einstein’s general relativity, explaining gravity by picturing the bending of spacetime.

However, these theories have been at odds for more than a hundred years, and no one has managed to reconcile them yet. The UCL team might have just changed that, as reported by Interesting Engineering.

Spacetime may not be governed by quantum theory

Many scientists think that Einstein’s gravity theory needs a makeover to match up with quantum theory. The top choices for this quantum gravity makeover are “string theory” and “loop quantum gravity.”

However, Professor Jonathan Oppenheim from UCL’s Physics & Astronomy department has a different take. In a new article in Physical Review X (PRX), he challenges the popular belief that spacetime follows the rules of quantum theory.

Oppenheim suggests an alternative idea in his paper – that spacetime might be classical instead of quantum, shaking up what many currently think is true.

“A postquantum theory of classical gravity” will be out today in @PhysRevX! And the experimental proposal with Carlo Sparaciari, Barbara Šoda and Zach Weller-Davies, to test the quantum nature of spacetime via the “decoherence vs diffusion trade-off", is out in @NatureComms. 1/ pic.twitter.com/sR1H4KW6WB

— Jonathan Oppenheim (@postquantum) December 4, 2023

“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,” explained Professor Oppenheim.

So, instead of changing spacetime, this theory, labeled as a “postquantum theory of classical gravity,” changes quantum theory itself. It forecasts a natural breakdown in predictability caused by spacetime.

This results in unexpected shifts in spacetime, even bigger than what quantum theory anticipated. Consequently, trying to measure the exact weight of things becomes a bit of a guessing game.

Challenges our understanding of the fundamental nature of gravity

The finding poses a challenge to how we comprehend the basic principles of gravity and provides opportunities to investigate whether it might have a quantum aspect, according to Zach Weller-Davies, a co-author and former Ph.D. student at UCL.

The team demonstrated that if spacetime lacks a quantum nature, there must be unpredictable variations in its curvature. These variations carry a distinct mark that can be confirmed through experiments, Weller-Davies further explained.