Physicists are making new inroads into the world of post-quantum theories, uncovering what reality may look like on a level deeper and stranger than the already infamously odd quantum theory. In the 1920s, physicists had several extremely useful theories for how the world works, yet they kept uncovering situations where those theories didn’t work. Through these holes in so-called classical physics, they glimpsed a deeper layer of the world that underlies everything – the quantum realm. Now, physicists are having déjà vu. Quantum theory works incredibly well, but it also has gaping holes when confronted with cosmically large objects controlled by gravity. What sort of post-quantum world could reveal itself through that hole? James Hefford at the National Institute for Research in Digital Science and Technology and Matt Wilson at Paris-Saclay University, both in France, have now developed a mathematical sketch of one plausible post-quantum world, possibly the deepest layer of reality yet. “Quantum theory doesn’t describe the entire universe,” says Hefford. “One of the greatest problems in physics is to come up with a theory of quantum gravity, a theory that describes both quantum theory and gravity. That thing ought to somehow go beyond just quantum theory.” There are many proposals for how to develop a theory of quantum gravity, but Wilson and Hefford took inspiration from the relationship between quantum and classical physics. Specifically, we do not encounter strange quantum effects in everyday life because of a process called decoherence, which destroys most objects’ quantumness. It is thanks to decoherence that our very reasonable and tangible world emerges from the quantum world in which cats can seemingly be simultaneously dead and alive, and particles can tunnel through walls like ghosts. The researchers posited that an analogous process of “hyperdecoherence” ought to make quantum theory emerge from a post-quantum