Here’s what you’ll learn when you read this story:

• A new theory asks what’s “above” quantum physics in our physical world.
• Decoherence of quantum physics creates classical physics, suggesting that hyperdecoherence also exists.
• The new theory offers a fresh perspective that resolves an idea bottleneck that’s nearly a decade old.

Two physicists at Paris-Saclay University recently suggested a new “meta” physics that could encompass both quantum and classical physics while filling in the remaining gaps. The idea of a grand unifying theory that brings together quantum physics and classical physics is nearly as old as quantum physics itself. Almost as soon as the the rules of the quantum realm were discovered, scientists realized that they had some scenarios where quantum and classical physics were incompatible. Now, in a new paper published in the journal Physical Review A, physicists James Hefford and Matt Wilson suggest that something called a quantum box could contain both types of physics without a conflict. Their conclusions may naturally contradict each other, which, they write, is a good way to keep the discussion going.

The theory involves a concept known as hyperdecoherence, which is a theorized step above regular decoherence (like the integral in calculus). Current wisdom states that our observable classical world is found in pockets where the overall quantum system is too scrambled, leading to things like the arrow of time always moving forward, which we can observe “locally” (in the physics sense). Maybe, then, quantum mechanics is also found in pockets of hyperdecoherence within another, even more encompassing theory.

In 2018, scientists Ciaran M. Lee and John H. Selby wrote about hyperdecoherence in the journal Proceedings of the Royal Society A. They explained the theory and said that any plausible version must not require both causality—the predictable flow of time and events from the present into the future—and purification:

Causality formalizes the statement that information propagates from present to future, and purification [formalizes] that each state of incomplete information arises in an essentially unique way due to lack of information about an environment.

In other words, purification means that any gap in our knowledge about a system can be traced back to one specific missing piece of information about its environment—and Lee and Selby showed mathematically that hyperdecoherence cannot satisfy both of these conditions simultaneously, making their result what's known as a no-go theorem. Their theory proves a negative in a strict mathematical sense—you can set two things equal to each other, or equivalent over a range, and show that it’s not possible to satisfy the criteria when using certain values. Lee and Selby concluded with, “As with all no-go theorems, our result is only as strong as the assumptions which underlie it.”

Hefford and Wilson pick their work up in response to this challenge. “This hyper-decoherence map,” they wrote, “evades the no-go theorem of Lee and Selby by relaxing constraints on signaling to the past and the uniqueness of purifications.”

Their theory, which they call QBox, doesn’t rely on causality. The team suggests that it mostly fits the purification rule, because QBox doesn’t have unique purifications—just common ones for general quantum states. Multiple quantum states can map to one purification, which Hefford and Wilson claim is the wiggle room they need to build a working theory within the idea of hyperdecoherence.

Causality, Hefford and Wilson assert, was able to serve as more of an afterthought in their theory, because purification is the stricter and more relevant rule if we want to prove or disprove any model of hyperdecoherence. In fact, QBox is inherently noncausal, because its inventors define it as “the most natural fragment of higher-order quantum theory for modeling indefinite causal order.”

In a way, this work is like filtering the search results in an online store. Maybe there are 100 pairs of pants, and you see that 50 of them come in your size. But when you try to see only the red pairs, the results drop to 0. Maybe just wanting “red” is too strict, because pants might be listed as magenta, or coral, or something else that’s close enough for your dress code. You just needed a slightly relaxed definition of “red pants,” because you can’t immediately make yourself a different size.

QBox may be the right size pair of pants; and, by excluding only unique purifications, we may just get some shade of red after all.