Starship Asterisk*

A radical theory that consistently unifies gravity and quantum mechanics while preserving Einstein’s classical concept of spacetime is announced today in two papers published simultaneously by UCL physicists.

An artistic version of Figure 1 in the PRX paper. It depicts 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 actual experiment is typically performed using Carbon-60, one of the largest known
molecules. The UCL calculation indicates that the experiment should also be performed
using higher density atoms such as gold. (Credit: Isaac Young)

Modern physics is founded upon two pillars: quantum theory on the one hand, which governs the smallest particles in the universe, and Einstein’s theory of general relativity on the other, which explains gravity through the bending of spacetime. But these two theories are in contradiction with each other and a reconciliation has remained elusive for over a century.

The prevailing assumption has been that Einstein’s theory of gravity must be modified, or “quantised”, 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 new paper in Physical Review X (PRX), 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 PhD 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. ...

• Physical Review X 13(04):1040 (04 Dec 2023) DOI: 10.1103/PhysRevX.13.041040
• arXiv > hep-th > arXiv:1811.03116 > 07 Nov 2018 (v1), 27 Nov 2023 (v3)

• Nature Communications 14(2023):7910 (04 Dec 2023) DOI: 10.1038/s41467-023-43348-2
• arXiv > quant-ph > arXiv:2203.01982 > 03 Mar 2022