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Comment by maho | Hacker News Reader

After skimming the second paper, I still don't understand how precision mass measurements come into play here. They mention Cavendish-type measurements, but they are used for measuring the gravitational constant. Of course, you can turn the formula around, plug an unknown mass into the apparatus and then call it a mass measurement, but it's going to be a very imprecise measurement. A Penning trap can give you 11 to 12 significant digits -- a Canvendish-type measurement could give you maybe 5 or so, I think.

Or is it because the Penning trap measures "inertial mass" but they really want a measurement of "gravitational mass"? But wouldn't inertial mass fluctuate the same way?

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(I went to a talk by Oppenheim author a couple weeks ago on this topic.) The idea is that gravity, as a force, only operates classically. More precisely: there is a classical state describing the curvature of space time, and then a quantum state describing the configuration of particles on that spacetime. But then, that quantum state needs to affect the classical state again (mass bends space), which would usually lead to the classical half becoming quantum and entangled with the other half.

You can keep the classical half (the shape of spacetime) classical, if the effect of the quantum part is partially stochastic. There's a minimum amount of random noise you need for it to be mathematically consistent. So, you set up an experiment where a particle is acting on another via gravity. There's a quantity of noise you should expect to see in the gravitational force.

"Inertial Mass=Gravitational Mass" now only holds on average. The gravitational mass will effectively have a Brownian noise term added in.

If this hypothesis is true, would it give us a way to distinguish many-worlds vs. wave function collapse? If the many "worlds" are all interacting with a single classical spacetime, we should be able to measure the gravity of other worlds, right? I'm not a physicist, but that sounds a lot like dark matter to me.

There is no implication that the classical spacetime wouldn’t still split into branches, possibly with different variations in the stochastic effect from each other.

Answering my own question based on Oppenheim's lecture[1]: In his theory, the stochastic interaction between quantum and classical states necessarily causes the wave function to collapse, so it is not compatible with many-worlds. Being able to measure the gravity of other quantum worlds is something predicted by semiclassical gravity, which Oppenheim calls "complete nonsense".

[1] https://www.youtube.com/watch?v=sde7k3jJp5E

So if I am understanding this correctly:

Quantum particles can effect change (curve) spacetime without direct quantum action if you sprinkle a bit of randomness into the (quantum acting on spacetime) effect?

Wouldn't it be enough to measure the (fluctuation of the) total gravitation force ( = gravitational constant times mass) exerted on the second particle, in order to draw conclusions about the nature of the gravitational force at small scales?

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