The odds of the bet Jonathan Oppenheim made in 2023 are generous, even at 5000 to 1. But if he wins, it could transform our view of the universe. The physicist is wagering that space-time, and therefore gravity, isn’t governed by quantum theory.
That swims firmly against mainstream physics, which says gravity must ultimately be quantum, like the rest of the fundamental forces – even if we can’t currently work out how. The wager is playful, but the question is serious, representing perhaps the most intractable puzzle in modern physics. And although Oppenheim is still out on a limb with his “post-quantum” theory of gravity, he is increasingly convinced his peers should take notice – not least because of the complexity of the alternatives.
Accept the model’s perhaps most controversial innovation, and Oppenheim’s calculations show that it might elucidate another great mystery of modern physics: the nature of the invisible dark matter that seems to provide most of the gravitational oomph in the cosmos. Might he just be on to something? How a quantum innovation may quash the idea of the multiverse For decades now, scientists have been trying to reconcile the two pillars of modern physics: quantum theory, which governs the behaviour of particles and forces on small scales, and Albert Einstein’s general theory of relativity, which describes gravity and determines how the universe works on the largest scales.
Each of these pillars has been extensively stress-tested and stood strong. Yet, according to our current understanding, the two are fundamentally incompatible. Quantum theory explains the workings of three fundamental forces – electromagnetism and the strong and weak nuclear forces – through the exchange of quantum particles that bind matter together in different ways.
But according to general relativity, massive objects generate gravity not by exchanging quantum particles, but by warping space and time around them, causing them to “fall in” towards each other. Each theory works spectacularly well in its domain: quantum forces explain how the matter that makes up the chair we are sitting on holds together; gravity explains why our bodies fall to Earth with a bump if we miss the chair when we try to sit. But this coexistence breaks down where large scales meet small ones: when quantum particles cross over the event horizon of a massive black hole, for instance, or in the very first instants of the big bang, when our known universe began as a tiny pinprick of mass and energy.
The vast majority of physicists think that the most likely solution will be some form of quantum gravity, in which gravity is built out of tiny quantum pieces like the other forces. At extraordinarily small scales, the smoothly warping space-time of Einstein’s theory would break down and pixelate, like an image zoomed in too far. There are many different ways to quantise gravity and space-time.
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