QBox theory may offer glimpse of reality deeper than quantum realm

Physicists are making new forays into the world of post-quantum theories, discovering what reality may be like on a deeper, stranger level than the already infamously strange quantum theory.

In the 1920s, physicists had several extremely useful theories about how the world worked, but they kept discovering situations in which those theories didn’t work. Through these holes in so-called classical physics, they glimpsed a deeper layer of the world that is the basis of everything: the quantum domain. Today, physicists have a feeling of déjà vu. Quantum theory works incredibly well, but it also has gaping holes when confronted with cosmically large objects controlled by gravity. What kind of post-quantum world might reveal itself through this hole?

James Hefford of the National Institute for Research in Digital Sciences and Technologies and Matt Wilson of the University of Paris-Saclay, both in France, have developed a mathematical sketch of a plausible post-quantum world, perhaps the deepest layer of reality yet.

“Quantum theory does not describe the entire universe,” explains Hefford. “One of the biggest problems in physics is arriving at a theory of quantum gravity, a theory that describes both quantum theory and gravity. This thing would have 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 don’t encounter strange quantum effects in everyday life due to a process called decoherence, which destroys the quantum character of most objects. It is through decoherence that our very reasonable and tangible world emerges from the quantum world in which cats can apparently be simultaneously dead and alive, and particles can pass through walls like ghosts. The researchers postulated that an analogous process of “hyperdecoherence” should cause quantum theory to emerge from a post-quantum theory.

The idea had been studied before, but a specific theorem from 2018 had proven that it was mathematically impossible to propose a sensible, internally consistent hyperdecoherence process that would correctly reproduce quantum theory. Hefford and Wilson carefully studied the assumptions behind this theorem and devised a workaround. The price they paid was to find themselves in a very strange post-quantum realm: a theory called QBox.

One of its main characteristics is that it goes against the conventional idea of ​​causality. Conventionally, either event A causes event B or vice versa, but in QBox it is allowed to have mixtures of “A causes B” and “B causes A” where it is impossible to say which one is unambiguously correct.

“It’s a causal indeterminacy. We have to worry about it if we want to pursue a theory of quantum gravity,” says Carlo Maria Scandolo of the University of Calgary in Canada, who did not work on the project. Indeed, our best theory of gravity – Albert Einstein’s general relativity – imposes different orders of cause and effect at different points in space-time, he explains.

This manifests itself, for example, in thought experiments in which people traveling in different spaceships and observing the same set of events cannot agree on the order in which they occurred.

The two physicists also had to ensure that hyperdecoherence correctly reduced QBox to quantum theory. For example, they had to ensure that a given object, which we can only approximately know in QBox, does not become more clearly known after hyperdecoherence. Wilson says the process of hyperdecoherence is akin to the fact that there are dimensions that an agent operating in the QBox domain – someone who can interact with objects within – can access, but which become hidden for those of us who can only access the quantum or classical domains.

Both researchers are still working on how to think about these dimensions and what exactly the agent would experience, but the dimensions that become inaccessible appear to be temporal rather than spatial, with hyperdecoherence cutting off access to processes that occur backwards, going into the past rather than the future.

“There is [previously] “There have been a few toy theories that would support notions of indefinite causal order and so on, but getting them to then replicate the whole of quantum mechanics was a challenge that no one seemed to have really achieved,” says Ciarán Gilligan-Lee of Spotify’s Causal Inference Research Lab, co-author of the 2018 theorem arguing against hyperdecoherence. Mathematically minimal. Despite some of its quirks, QBox doesn’t need to create a whole new world of objects, such as cosmic strings, to arrive at a theory of quantum gravity, he says.

An important next step will be to flesh out the physical details of hyperdecoherence, in addition to proving that it can exist as a mathematical function, says John Selby of the University of Gdańsk in Poland, who was the other co-author of the 2018 theorem. “There should be a narrative, a story that explains why, in our current experiments, this is [what is] “, he says. In his opinion, even if QBox turns out not to be exactly what the post-quantum layer of reality is, Hefford and Wilson’s mathematical work provides a promising starting point.

Gilligan-Lee and Selby also wrote a new and different theorem that has not yet been revised by other physicists, but which could place tighter constraints on what it would mean for a theory like QBox to significantly decohere with quantum theory.

Such challenges are welcome, even if they mean QBox ends up being a stepping stone toward a better wish list of what a post-quantum theory should be, Wilson says. It is noteworthy that experimental testing of QBox might eventually be possible, as the theory could have real-world implications for certain experiments in which quantum waves overlap.

And if QBox succeeds in overcoming future mathematical and experimental challenges, an even more tantalizing question will become relevant. “Can you have whole towers of theories decoherent within each other by similar methods? [decoherence] mechanisms? Hefford said. After all, the quest for the deepest layer of reality might involve more mathematical investigations.