Gravity Kills Schrödinger's Cat | Hacker News Reader

Gravity Kills Schrödinger's Cat

scientificamerican.com

53 pointschippy16 comments

It's a real shame that cats became the canonical subject of macroscopic quantum mechanical thought experiments because they're just too damn complicated. The interesting case from the point of view of GR is not whether gravity causes the cat's state to collapse, but what happens to spacetime when a macroscopic object is in a superposition of position states. So the interesting thought experiment is not Shroedinger's cat, but Shroedinger's neutron star: you get a (really really big!) sealed box with a neutron star inside, and somehow arrange for it to evolve into a quantum superposition of position states. Then outside the box you send a beam of photons past the box. The photons are deflected gravitationally by the neutron star on one of two trajectories. You then recombine the two trajectories (without, of course, measuring the photons en route). The $64,000 question is: do you get an interference pattern?

You'd need the box to block gravity, and have a way of letting only your single photon pass through the box to interact with the system. Otherwise the star's position would have already been measured by past light being deflected.

The two photon paths would not interfere with each other. The injected photon's resulting position is entangled with the star's position, meaning you could tell which leg of the interferometer the photon had taken by later opening the star box, so it acts analogous to putting a detector on one arm of an interferometer and you get no self-interference.

> Otherwise the star's position would have already been measured by past light being deflected

Well, this is the interesting question, isn't it. We don't actually know what happens to spacetime when you get a macroscopic divergence like this because we only have access to this one universe, and no good theory of quantum gravity (yet). It's possible (for example) that there is only one spacetime for the wavefunction, and that what is currently called "dark matter" is really the result of macroscopic superpositions of states.

Yes, that's far less complicated for the layman to understand.

Luboš Motl wrote about this: http://motls.blogspot.co.at/2015/06/sitting-and-experiencing...

Caution that Lumo's site crashes Firefox. Could be due to the 25 or so trackers he employs or one of his plethora of bizarre adds or maybe the Ajax math typesetter he uses for notation.

Works fine here, Firefox 38.0.5 on OS X, no extensions.

This is the relevant quote:

    "But from a deep, fundamental point of view, this is  
    nothing new," he says. A gravitational field is merely 
    another environment to interact with, so invoking it 
    does not explain whether quantum behaviour might lead to 
    classical reality if gravity’s influence were 
    mitigated—for example, by doing the experiment in 
    gravity-free space.

It's an interesting piece of work, but doesn't seem to add anything fundamental to the discussion of gravity and QM. It just shows how gravity can result in decoherence if different parts of the wavefunction experience different clock rates due to gravitational fields.

I don't know the technical details but how would "different parts of the wavefunction experience different clock rates due to gravitational fields" ever not be the case?

Every particle in the universe is acted upon by every other particle, via gravity, so does this not mean that every particle, in some small capacity, "observes" every other particle?

AFAIK, the force of gravity propagates at light-speed, so every particle in the universe is not being acted upon by every other particle in the universe via gravity, because the distance between some particles is increasing faster than that due to the expansion of the universe.

All that means is that movements of particle A and thus changes in its gravitational field now will never reach particle B, which is so far away that the relative expansion rate is c or more. However the historical gravitational field of Particle A, the part of its field that is within particle B's light cone, will always affect particle B.

What if the light cones of A and B don't intersect at all? There are certainly places in the universe so far apart that light not only can't now, but could never, have travelled from one to the other; that is part of why the near-isotropy of background radiation is something that needs to be explained, I thought.

Are there regions of the universe that have never been causally connected? They all originated in the same event, so I'd be surprised if that's true, but I'm no expert. I know cosmic inflation during the big bang was superluminal, but I have to concede I'm not completely familiar with the consequences of that, so good point.

Indeed, the fact that they are moving apart at superluminal speeds is exactly why they have never been causally connected. The fact that such non-causally connected islands still "look the same" is called the horizon problem (https://en.wikipedia.org/wiki/Horizon_problem), and it is one of the issues that Guth's inflation might solve (https://en.wikipedia.org/wiki/Inflation_(cosmology)#Horizon_...).

I think the reason it doesn't count as a measurement is that the the gravitational effects are so weak the no-far-off-particle and yes-far-off-particle outcomes overlap substantially.

You start with a cross-section of where a "measurer" particle might be, and if the far-off "to-be-measured" particle is present its gravity shifts the cross-section of the measurer particle's position very slightly, but there's still a lot of overlap with the original cross-section. A proper measurement would have nearly no overlap, instead of almost complete overlap.

The title of the article makes a claim that we don't know is actually true, as evidenced by the article explaining how we might test this in the future.

It's entirely possible that gravity doesn't reveal information about quantum state, and instead is derived from the superposition of states.

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