Oppenheim was originally a string theorist, and string theorists believe in the primacy of quantum mechanics. In both theories, our familiar, classical world somehow emerges from these fundamentally quantum building blocks. A smaller faction looked to loop quantum gravity, which replaces the smooth space-time of Einstein’s general relativity with a network of interlocked loops. To deal with this deep conceptual conflict, most theorists turned to string theory, which imagines that matter and space-time emerge from tiny, vibrating strings. But that doesn’t work, because quantum theory only makes sense against a classical space-time background - you can’t add and then evolve quantum states on top of an uncertain foundation. So if gravity is quantized, that means space-time is also quantized.

The problems arise because gravity is a result of space-time itself, rather than something that acts on top of it.

Over the 20th century, physicists gradually made sense of electromagnetism and other forces using this framework.īut when they tried to quantize gravity, they ran into unnatural infinities that had to be sidestepped with clumsy mathematical tricks. What’s more, the more certain you are about a particle’s location, the less certain you are about its momentum. For example, when you measure a quantum particle, you can’t predict exactly where you will find it, but you can predict the likelihood that it will be found in a particular place. Quantum theories are based on probabilities rather than certainties. Maybe, he argues, our presumption that it must be quantized is wrong. Jonathan Oppenheim, who runs a program exploring post-quantum alternatives at University College London, suspects that’s because gravity simply can’t be squeezed into a quantum box. So for almost a century, theorists have tried to create a unified theory by quantizing gravity, or sculpting it according to the rules of quantum mechanics. But in those settings, quantum mechanics collides with classical gravity in a resolutely incompatible way.

Most physicists expect that when we zoom in on the fabric of reality, the unintuitive weirdness of quantum mechanics persists down to the very smallest scales.

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