Physicists are delving deeper into the realm of post-quantum theory, unveiling a reality that exists at a level even more perplexing than the already bewildering quantum theory.

In the 1920s, physicists developed vital theories that explained fundamental workings of the universe, yet they continuously encountered phenomena where these theories fell short. This spurred them to glimpse into a more profound layer of reality: the quantum realm. Today, physicists find themselves revisiting this experience. While quantum theory accurately describes many phenomena, it leaves significant gaps when it comes to large cosmic structures influenced by gravity. What kind of post-quantum reality will manifest through these gaps?

James Hefford from the National Research and Development Agency, along with Matt Wilson from the University of Paris-Saclay, has created a mathematical framework outlining a potential post-quantum world—perhaps the deepest layer of reality.

“Quantum theory does not encompass the entirety of the universe,” Hefford remarks. “A significant challenge in physics is developing a quantum gravity theory that reconciles quantum mechanics and gravity. This theory must surpass traditional quantum descriptions.”

Multiple propositions exist for developing a quantum gravity theory, but Wilson and Hefford found their inspiration in the interplay between quantum and classical physics. Everyday experiences shield us from peculiar quantum effects, attributed to a phenomenon known as decoherence, which eliminates the quantum characteristics of most objects. Decoherence brings forth our tangible, rational world from the quantum domain, where the paradoxical states of cats exist and particles can seemingly disappear through barriers. They propose that quantum theory could arise from post-quantum theory through a similar mechanism called “hyperdecoherence.”

This concept isn’t entirely new; a specific theorem established in 2018 suggests that creating a coherent hyperdecoherence process that accurately reproduces quantum theory is mathematically infeasible. However, Hefford and Wilson scrutinized the underlying assumptions of this theorem and devised an innovative approach. The outcome? They entered a remarkably unconventional post-quantum landscape defined by a theory called QBox.

A fascinating aspect of QBox is its redefined conception of causality. Traditionally, causality operates on a clear sequence (event A causes event B or vice versa), but QBox permits a blend of both where causation is ambiguous.

“This introduces causal uncertainty, a critical aspect when pursuing a quantum gravity theory,” notes Carlo Maria Scandoro from the University of Calgary, who was not a part of this project. This uncertainty arises because Einstein’s theory of general relativity enforces varying causal orders across different spacetime points.

This is evident in thought experiments where observers traveling in different spaceships witness the same events but disagree on the chronological order of occurrences.

The researchers also ensured that hyperdecoherence adequately transitions QBox back into quantum theory, stipulating that objects described roughly within the QBox don’t gain precise clarity after hyperdecoherence. Wilson describes this hyperdecoherence as a dimension accessible to entities within the QBox realm—those capable of interacting within its confines—yet obscured from us in the classical or quantum realms.

Currently, the researchers are still clarifying how to conceptualize these dimensions and the experiences of agents operating within them. Preliminary indications suggest that the inaccessible dimensions are temporal rather than spatial—hyperdecoherence selectively concealing past processes while leaving future interactions untouched.

“Previously, there had been speculative models supporting concepts like indeterminate causal order, but formulating comprehensive quantum mechanics proved challenging, with no successful conclusions,” states Ciaran Gilligan Lee, involved in Spotify’s Causal Inference Lab and a co-author of the 2018 theorem opposing hyperdecoherence. He points out that the true merit of this new research lies in its concrete theoretical foundation and its mathematical simplicity. Notably, QBox does not necessitate hypothesizing entirely new constructs like cosmic strings for quantum gravity.

Beyond demonstrating the feasibility of hyperdecoherence as a mathematical function, the subsequent step involves elucidating its physical implications, contends John Selby from the University of Gdańsk, another co-author of the 2018 theorem. “A narrative is essential to clarify why these phenomena arise in our empirical universe.” In his opinion, the mathematical exploration by Hefford and Wilson is a promising foundation, regardless of whether QBox accurately represents the post-quantum layer of reality.

Gilligan-Lee and Selby have also formulated a new theorem, not yet explored by contemporaneous physicists, which may impose stricter criteria on a theory like QBox for it to meaningfully differentiate from quantum theory.

This challenge is welcomed by Wilson, even if it means QBox evolves into a precursor for a more refined vision of post-quantum theory. Notably, this theory may have tangible implications for specific experiments involving overlapping quantum waves, potentially facilitating experimental validation of the QBox concept.

If QBox successfully navigates forthcoming mathematical and experimental hurdles, even more intriguing inquiries will arise. “Can entire frameworks of theory be similarly disentangled?” Hefford speculates. Ultimately, unearthing the deepest realities might necessitate further mathematical exploration.