[visionscaper]

> Instead of modifying spacetime, the theory – dubbed a “postquantum theory of classical gravity” – modifies quantum theory and predicts an intrinsic breakdown in predictability that is mediated by spacetime itself. This results in random and violent fluctuations in spacetime that are larger than envisaged under quantum theory, rendering the apparent weight of objects unpredictable if measured precisely enough.

I have so many questions about this paragraph; how did they change quantum theory? Are they saying that breakdown in predictability at the quantum level is actually caused by space-time? Why does this result in violent fluctuations in space-time? And why are the space-time fluctuations larger than in the case of quantum gravity?

[eigenket]

So the fundamental problem in quantum gravity is something like this (arguably).

Particles with mass distort space-time (according to general relativity). Particles can be in superpositions where different parts of the superposition are in different physical positions (according to quantum mechanics). We don't know how to make gravity quantum and it seems to be difficult. So we don't have any damn clue what happens to space-time when an object with mass is in a superposition of different positions.

The suggestion by Oppenheim and friends is that space-time is fundamentally classical and what happens when you put an object with mass in superposition is that random fluctuations in the space-time force the object out of superposition and into a classical random state.

I.e. where quantum mechanics says you'd have a particle in a superposition of being in place A and place B, this new theory says you'd end up with a random choice being made between A and B, and it ends up in one based on something like a coin-flip.

Disclaimer: I don't work on this stuff directly. My expertise is entirely that I attended a talk Jonathan Oppenheim gave at a conference last year.

[tbrownaw]

Would this put a limit on how well a quantum computer (or any other large entangled system) can be isolated from its environment (how long it can be kept from decohering)? Or was a similar limit already there and this just describes it differently?

[eigenket]

Probably for some models of quantum computers yes, but there are many different ways to build one and not all will be limited in this way. For example the polarisation degree of freedom of a photon doesn't appear to interact with gravity at all, so that wouldn't be impacted by this.

[bbor]

Wow that’s a beautiful explanation, and gives me hope - thanks for taking the time to explain such things to a layman :)

So the overall “quantum properties are only visible at that tiny scale” issue could be explained by “particles are ‘forced into a classical state” by a chaotic background space time force”, if I’m understanding correctly? Like super sensitive special electrical chips that are buffeted about/‘forced into a new state’ by cosmic background radiation?