**New theory may finally unite Einstein’s gravity with the quantum world**

**In two papers published simultaneously by Univesity College London (UCL), physicists may have finally devised a theory that could unify Einstein’s theory of gravity with the strange world of quantum mechanics. If the theory is correct, it could solve one of the last centuries’ trickiest problems regarding our understanding of the universe’s workings and the nature of matter.**

**A century-long problem solved?**

**The foundation of modern physics rests upon two fundamental theories: quantum theory and Einstein’s theory of general relativity. Quantum theory governs the behavior of the smallest particles in the universe, while the theory of general relativity explains gravity through the curvature of spacetime. However, these two theories have conflicted for over a century, and a resolution to their contradiction has yet to be found.**

**Many scientists believe that Einstein’s theory of gravity needs to be modified or “quantized” to fit into the framework of quantum theory. Two of the most prominent contenders for a quantum theory of gravity are “string theory” and “loop quantum gravity.”**

**However, according to a new paper in Physical Review X (PRX) by Professor Jonathan Oppenheim from UCL’s Physics & Astronomy department, the widely accepted idea that spacetime is governed by quantum theory may not be accurate. Oppenheim proposes an alternative approach in his paper, suggesting that spacetime may be classical instead of quantum, thereby challenging the current consensus.**

**“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.**

**To this end, instead of modifying spacetime, the theory – dubbed a “postquantum theory of classical gravity” – modifies quantum theory and predicts an intrinsic breakdown in predictability mediated by spacetime itself. This leads to unpredictable fluctuations in spacetime, larger than those predicted by quantum theory. Thus, the weight of objects becomes unpredictable with precise measurements.**

**“This discovery challenges our understanding of the fundamental nature of gravity but also offers avenues to probe its potential quantum nature,” explained 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.**

**“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,” he added.**

**“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,” Weller-Davies explained.**

**The professor himself wrote the first paper, while the second one was written by his former PhD students. The latter paper discusses the implications of the theory and proposes an experiment to test it. The investigation involves measuring a mass with high precision to observe if its weight fluctuates over time.**

**The theory now needs testing**

**For example, the International Bureau of Weights and Measures in France routinely weighs a 1kg mass. The authors propose this could be used as a test case for the theory. If the measurements of this 1kg mass exhibit fluctuations smaller than required for mathematical consistency, the theory can be invalidated.**

**Source: Interesting Engineering**

**Quantum tool opens door to uncharted phenomena**

**New theory may finally unite Einstein’s gravity with the quantum world**