An article published in the journal “Physical Review X (PRX)” proposes what is called a postquantum theory of classical gravity. Professor Jonathan Oppenheim of University College London (UCL) offers a different approach to that adopted by most of his colleagues by proposing to modify quantum theory to unify it with relativistic gravity. An article published in the journal “Nature Communications” offers some reflections from Professor Oppenheim’s former Ph.D. students on the consequences of his theory and proposes an experiment to verify it.

The image (Courtesy Isaac Young. All rights reserved) shows an artistic version of Figure 1 in the paper in PRX. 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.

What continues to appear to scientists as an incompatibility between quantum theory and Albert Einstein’s theory of general relativity constitutes one of the major problems in today’s physics. Many scientists have attempted, so to speak, to quantize gravity in order to include it in a finally unified quantum theory. This approach was embraced by Geoff Penington to develop string theory and Carlo Rovelli to develop quantum gravity.

Now Professor Jonathan Oppenheim has taken a practically opposite approach in which it’s the quantum theory that is modified. According to him, gravity and space-time are correctly described by the theory of general relativity while matter obeys quantum mechanics. To work in the interaction between gravity and matter, this combination requires a certain randomness in the behavior of space-time.

Professor Jonathan Oppenheim’s former Ph.D. students explored the possible consequences of this theory. For example, the weight of an object would fluctuate over time. This means that a very precise measurement of that weight could be a test for this theory.

The prototype kilogram at the Bureau international des poids et mesures, France, was mentioned as a possible object to be used for the test: if the fluctuation of the measurements remained below a certain threshold, it would mean that this theory is wrong. There are mathematical formulas behind this experiment that can provide theoretical predictions to compare with very precise measurements.

The consequences of the idea behind this theory also concern black holes and in particular the information paradox. It’s one of the contradictory results between quantum theory and general relativity but Jonathan Oppenheim’s postquantum theory would resolve it by allowing the loss of information when an object enters a black hole.

On paper, the theory is interesting but its real possibilities of working when tested remain to be verified. Geoff Penington and Carlo Rovelli are so skeptical that they accepted a 1:500 odd bet with Jonathan Oppenheim: if this theory turns out to be wrong, Oppenheim will give each of his colleagues an item of their choice but if the theory works, each of the two will give Oppenheim 500 items. The item will be chosen by the winner but the value must not exceed 25 US cents. So the discussions have already begun and all that remains is to wait for Oppenheim or someone else to be able to conduct some tests to understand if this approach makes sense.