Post Quantum Gravity: Can Gravity Be Classical in a Quantum World?

Overview

The video examines a provocative idea in theoretical physics: gravity might not need to be quantized to coexist with quantum mechanics. Propounded by Jonathan Oppenheim, the post quantum gravity hypothesis suggests gravity could remain classical, but with an intrinsic randomness that reflects quantum matter.

Key insights

• Two pillars of physics, general relativity and quantum mechanics, resist a straightforward unification because they operate with different mathematical objects.
• Semi classical gravity uses the expectation value of the quantum stress energy tensor to drive a classical spacetime, which leads to odd behavior for quantum superpositions.
• An alternative is to keep gravity classical while allowing multiple classical spacetimes corresponding to quantum configurations, but this clashes with Heisenberg uncertainty unless randomness is introduced.
• Post quantum gravity adds noise to gravity itself, creating a feedback loop that decoheres quantum states and prevents gravity from acting as a perfect measurement device that would violate quantum principles.

In short, the episode discusses whether gravity can be classical with a stochastic twist, and what that would mean for unifying physics and for the fate of quantum information.

Introduction to the Gravity Quandary

The talk centers on a century-old tension between two fundamental theories: general relativity describes the geometry of spacetime and gravity on large scales, while quantum mechanics governs the smallest scales. These theories are exquisitely accurate in their domains, yet they clash at their intersection. The presenter revisits the idea of quantum gravity as the master theory, but with a twist: gravity itself might remain classical while matter is quantum, if randomness is built into the gravitational interaction.

From Semi Classical Gravity to Competing Classical Spacetimes

Two paths are considered for a classical gravity interacting with quantum matter. The first is semi classical gravity, where the spacetime geometry is sourced by the expectation value of the quantum stress energy tensor. In this view, a quantum object in a superposition would produce a spacetime defined by the average configuration, which yields counterintuitive behavior such as intermediate gravitational effects and ambiguous motion. The second option imagines a separate classical spacetime for each mass energy distribution implied by the quantum state, leading to a probabilistic gravitational influence that mirrors quantum randomness but risks violating the uncertainty principle when probed with measurement setups like a double slit experiment.

The Noisy Gravity Proposal

The core proposal, post quantum gravity, introduces true randomness into gravity itself. Rather than gravity providing perfect information about the source positions, gravity fluctuates in a way that encodes the quantum state of the matter that generates it. This noise feeds back on the matter, inducing decoherence and gradually localizing the quantum object into a definite position. The result is a single classical spacetime that remains compatible with quantum dynamics without violating foundational principles, because the gravitational interaction no longer acts as a perfect, deterministic measurement of position.

Consequences for Quantum Information and Experiments

One radical consequence is the possible destruction of quantum information through gravitationally induced decoherence. If gravity truly injects randomness, information can effectively be lost, offering a potential resolution to puzzles like the black hole information paradox. The discussion also addresses how noisy gravity would erase the telltale path information in a double slit experiment, thus preserving quantum behavior of matter while avoiding a classical gravitational measurement that would reveal the path without disturbance.

Outlook

The speaker emphasizes that post quantum gravity is not claimed to be the final answer, but illustrates there are still creative avenues to pursue in the quest to unify relativity and quantum theory. The takeaway is that both quantum gravity and post quantum gravity remain viable directions, and the truth may involve elements of both or an entirely new framework in the future.