For more than a century, physics has been caught between two incompatible descriptions of reality. General relativity describes gravity as the curvature of spacetime, smooth, continuous, classical. Quantum mechanics describes everything else as probabilistic, discrete, and fundamentally uncertain.
Marrying them into a theory of quantum gravity has been the holy grail of theoretical physics for decades. String theory proposes extra spatial dimensions. Loop quantum gravity quantizes spacetime into discrete spin networks. Both approaches quantize gravity, that is, they make gravity quantum. But a third path has been quietly gaining traction, and it takes the opposite approach entirely.
Jonathan Oppenheim, a theoretical physicist at University College London, has proposed a theory that keeps gravity classical and instead introduces fundamental randomness into the quantum world. His “post-quantum theory of classical gravity”, published in Physical Review X in 2023, has now generated enough follow-up work, critical response, and experimental proposals that it warrants the attention of anyone following the quest for a unified theory.
The core idea
Oppenheim’s key insight is deceptively simple: instead of quantizing gravity to make it fit quantum theory, modify quantum theory to accommodate classical gravity.
In standard quantum mechanics, the Schrödinger equation deterministically evolves a system’s wavefunction until measurement collapses it probabilistically. Oppenheim replaces the deterministic evolution with a fundamentally stochastic one. The dynamics of a quantum system immersed in a classical gravitational field become unpredictable at a fundamental level, not because of measurement, but because spacetime itself introduces random fluctuations into the system’s evolution.
This is not the same as the Copenhagen interpretation’s “collapse upon measurement.” In post-quantum gravity, randomness is intrinsic and continuous. The Born rule, the calculational tool that gives probabilities in standard quantum mechanics, emerges naturally from the mathematics rather than being imposed as an axiom.
Mathematically, Oppenheim derived a rigorous trade-off relationship: the more a system decoheres (loses quantum coherence due to interactions with spacetime), the more it diffuses (experiences random fluctuations from spacetime). This trade-off, Oppenheim showed, is unavoidable in any theory that couples a quantum system to a classical one.
Testable predictions
What makes post-quantum gravity unusual among unified theories is that it makes concrete, falsifiable predictions that current experiments can test.
First, time wobbles: if spacetime is fundamentally classical and stochastic, the rate at which time passes should undergo tiny random fluctuations. Precision atomic clocks, which already measure time with extraordinary accuracy, could detect this jitter.
Second, mass fluctuations: a standard mass placed on an ultra-sensitive scale should exhibit random weight fluctuations on the order of 10⁻¹⁵ g. The International Bureau of Weights and Measures’ 1 kg prototype and similar high-precision mass standards are, in principle, capable of detecting this signal.
Third, a minimal noise bound: In a March 2026 preprint, Fabiano et al. derived a lower bound on the stochastic noise that any classical gravity theory must produce. Measuring fluctuations below this threshold would definitively prove that gravity is quantum, a clean experimental falsification.
Oppenheim himself has acknowledged the slim odds of his theory being correct. When challenged, he made a remarkable 5000:1 bet, against his own theory, with Geoff Penington (a string theorist at Stanford) and Carlo Rovelli (a loop quantum gravity pioneer at Perimeter Institute). If experiments detect the predicted spacetime fluctuations, Oppenheim wins. If they don’t, he pays out.
The critique
The theory has not gone unchallenged. Sabine Hossenfelder, a physicist and critic of fashionable theories, has argued that while the framework is compelling, Oppenheim’s claim that it reproduces Modified Newtonian Dynamics (MOND) is flawed, the correction is linear in the gravitational potential, while MOND requires a non-linear regime.
More formally, Mark Hertzberg (Tufts/MIT) and Avi Loeb (Harvard) published a 2024 critique in Journal of Cosmology and Astroparticle Physics identifying four specific problems: the theory predicts the wrong force law, its MOND-like behavior is not actually MONDian, the predicted fluctuation spectrum does not match observations, and there are theoretical inconsistencies in the mathematical framework.
Oppenheim responds that some of these criticisms apply to an earlier version of the theory and that the framework is still evolving. He describes his own work as “more of a research program than a finished theory”, an attempt to show that the post-quantum approach is mathematically consistent and experimentally accessible, not a final answer.
Why it matters
Whether or not post-quantum gravity turns out to be correct, its significance lies in expanding the landscape of what a unified theory could look like. For decades, the assumption has been that gravity must be quantized, it has seemed the only self-consistent path. Oppenheim has shown that a classical gravity + stochastic quantum theory is mathematically viable, opening experimental avenues that the quantization-only approach never considered.
Nature will decide which path is correct. But for the first time in a generation, there is a concrete experiment that could rule out an entire class of theories, and that, regardless of the outcome, is progress.
Source
Oppenheim, J. “A Postquantum Theory of Classical Gravity?” Physical Review X 13, 041040 (2023). DOI: 10.1103/PhysRevX.13.041040
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