
Gravity Kills Schrödinger's Cat - chippy
http://www.scientificamerican.com/article/gravity-kills-schroedinger-s-cat/
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lisper
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?

~~~
Strilanc
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.

~~~
lisper
> 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.

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4ad
Luboš Motl wrote about this: [http://motls.blogspot.co.at/2015/06/sitting-and-
experiencing...](http://motls.blogspot.co.at/2015/06/sitting-and-experiencing-
gravitational.html)

~~~
DonGateley
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.

~~~
4ad
Works fine here, Firefox 38.0.5 on OS X, no extensions.

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tjradcliffe
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.

~~~
jstanley
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?

~~~
marchelzo
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.

~~~
simonh
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.

~~~
JadeNB
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.

~~~
simonh
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.

~~~
JadeNB
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](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_...](https://en.wikipedia.org/wiki/Inflation_\(cosmology\)#Horizon_problem)).

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fragsworth
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.

