Does gravity cause quantum decoherence?

  • #1
Kinker
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Gravity is a force that exists everywhere and does not act stochastically, but acts continuously forever. Does gravity, this force, cause quantum decoherence?
Does gravity cause quantum decoherence?
In the microscopic world, gravity seems to act weakly, but in the macroscopic world, it seems to act strongly. Is this the boundary between the microscopic world and the macroscopic world?
So a phenomenon like quantum tunneling can occur in the microscopic world, but is it impossible in the macroscopic world?
 
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  • #2
No, gravity does not cause decoherence. Only the interaction with other particles can cause decoherence.

Kinker said:
In the microscopic world, gravity seems to act weakly, but in the macroscopic world, it seems to act strongly.
Its strength is the same at all scales. It is just that at small scales other forces (such as electromagnetic) play a much more important role.
 
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  • #3
The point is that the source of gravitation is everything related to energy (momentum, and stress), and this is always positive and adds up. The coupling constant is very small, in fact the smallest of all forces, but the fact that the source has only one sign means that gravitaty can't be screened in any way, while e.g., the electromagnetic interaction tends to bind systems with net-zero charge together. That's why the matter surrounding us is close to uncharged and thus all that remains is the residual interaction due to polarization, which is much weaker than the large gravitational interaction with the Earth, which is why the gravitational interaction of bodies with the Earth is so dominant compared to electromagnetic interaction.
 
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  • #4
DrClaude said:
No, gravity does not cause decoherence. Only the interaction with other particles can cause decoherence.
This is not accurate. Interaction with massless asymptotic particles and/or fields always creates decoherence, and this includes gravitation (once quantized). But gravitation is not necessary for decoherence; QED alone does the job as well. (QCD is different because of confinement of the massless gluons.)
 
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  • #6
As @Demystifier points out, there are research teams investigating this question currently. For the answer to be "yes", gravity must be a quantum force (in order to allow quantum interactions to occur as mentioned in other comments). There is no satisfying evidence that this is the case.

And there is very strong evidence this is not the case - that the answer is "no" (i.e. gravity cannot cause decoherence). Bell tests have been reported over long distances with no appearance of decoherence due to gravity. That should occur when the separation distance increases (if gravity did cause decoherence). In the reference below, the separation is 144 km. The observed CHSH S value is 2.508±0.037. That is a very strong violation of a Bell inequality, and also a very small margin of error.

https://arxiv.org/abs/quant-ph/0607182
Free-Space distribution of entanglement and single photons over 144 km
 
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  • #7
DrChinese said:
That should occur when the separation distance increases
No, it should occur when the "difference in gravity" increases. Separation distances don't necessarily change "gravity" at all. In the paper you reference, the distance is horizontal, so we would not expect "gravity" to be different between the two measurements. Running a similar test where the measurements occurred at significantly different altitudes would at least be a start at testing whether differences in "gravity" affect quantum coherence.

The only experiment I'm aware of that attempts to directly test any aspect of gravity with regard to QM is the vertically oriented neutron interferometry experiment that shows that gravitational potential ##\phi## acts like any other potential in the Schrodinger Equation. But that, in itself, doesn't test for any quantization of gravity; the potential in the Schrodinger Equation is not quantized, it's just an externally imposed fixed potential function.
 
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  • #8
PeterDonis said:
No, it should occur when the "difference in gravity" increases. Separation distances don't necessarily change "gravity" at all. In the paper you reference, the distance is horizontal, so we would not expect "gravity" to be different between the two measurements. Running a similar test where the measurements occurred at significantly different altitudes would at least be a start at testing whether differences in "gravity" affect quantum coherence.

The only experiment I'm aware of that attempts to directly test any aspect of gravity with regard to QM is the vertically oriented neutron interferometry experiment that shows that gravitational potential ##\phi## acts like any other potential in the Schrodinger Equation. But that, in itself, doesn't test for any quantization of gravity; the potential in the Schrodinger Equation is not quantized, it's just an externally imposed fixed potential function.
You are correct that a difference in gravitational potential should indicate that a particle has been affected by quantum gravity. Conceptually, you might see a small redshift. If you observed such shift, and there was decoherence, that would be strong evidence that gravity is a quantum force.

Considering that a photon has its own momentum upon creation, independent of gravity: I believe even a horizontal (or completely vertical for that matter) flight path would allow time for gravitational forces to interact with a photon, although obviously only slightly with a single photon. But then, if you look at many photons, there would be more opportunity. We know that distant sources producing light passing by a lensing galaxy would feel gravitational shift as the light is deflected (i.e. its path changes).

So I don't see that change in altitude alone is a requirement for such tests on photons. I would say that a change in observed path *or* wavelength (there is of course a specific relationship between path/wavelength in this case) would be sufficient. There is no particular requirement that altitude must change (holding all other variables fixed).

Put another way: a null result on a Bell test across a distance is certainly a form of evidence that quantum gravity has not affected the photon. On the other hand: there have been earth to satellite Bell tests. I didn't reference any because the fidelity was too low to make a useful statement at this time.
 
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  • #9
DrChinese said:
I believe even a horizontal (or completely vertical for that matter) flight path would allow time for gravitational forces to interact with a photon, although obviously only slightly with a single photon.
Ah, I see what you mean. Yes, any finite travel time is time for some interaction to theoretically take place, even if the probability is only slight.

But then the question is how slight. And that depends on the coupling constant. Our best current estimate for the coupling constant in the case of gravity is that it is extremely tiny--so tiny that we should not expect to see any such interactions for objects of ordinary sizes and energies on the distance and time scales of our experiments. It takes a huge object like a planet. And even then the main effect we should expect to see is the gravitational potential in the Schrodinger Equation, which indeed we have seen. But to see that effect you need to have an altitude change.

DrChinese said:
We know that distant sources producing light passing by a lensing galaxy would feel gravitational shift as the light is deflected (i.e. its path changes)
The "path change" (which we can observe for light passing by the Sun as well--AFAIK we haven't tested it for smaller masses like planets) is due to the underlying spacetime geometry. Yes, if we discover a theory of quantum gravity it will presumably tell us how "spacetime geometry" emerges from the underlying quantum model. But the fact remains that "spacetime geometry" and the "path change" it causes is a classical effect and doesn't tell us anything useful at the quantum level.
 
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  • #11
Demystifier said:
a) Page-Geilker, 1981? Their abstract: "An experiment gave results inconsistent with the simplest alternative to quantum gravity, the semiclassical Einstein equations. This evidence supports (but does not prove) the hypothesis that a consistent theory of gravity coupled to quantized matter should also have the gravitational field quantized."

Sorry to call out this reference, but surely you realize this is something of a stretch. The concepts of quantum gravity were in their infancy at that point. And the paper contains references to Everett's Relative State Formulation (i.e. MWI). My point is that this experiment is not suitable to support or contradict anything (check citations). There are reasons (such as their assumptions and various interpretational issues) for their specific result, without it saying much one way or another about gravity itself. b) Far more interesting is this 2006 paper (which itself references the Page-Geilker paper):

https://arxiv.org/abs/gr-qc/0611037

:oldbiggrin: :oldbiggrin:c) At the bottom of page 5 in this next reference, a very brief overview of the Page-Geilker paper is provided with a bit of context. Useful because the Page-Geilker paper itself is behind a paywall.

https://arxiv.org/ftp/arxiv/papers/2010/2010.14965.pdfBoth of the above references start with the same basic Semiclassical Gravity approach, and proceed from there. I found things to like in both of these. But make of these as you will, but I don't believe any of these indicate that gravity itself can cause decoherence.
 
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  • #12
I have one question here:

Some of you have said that it's not certain (or even proven to be wrong) that gravity can cause decoherence

However, can't curved spacetime do that? And isn't curved spacetime intrinsically related to gravity?
 
  • #13
Suekdccia said:
I have one question here:

Some of you have said that it's not certain (or even proven to be wrong) that gravity can cause decoherence

However, can't curved spacetime do that? And isn't curved spacetime intrinsically related to gravity?
There are papers on gravity-induced decoherence, and similar, by L.H.Ford et al, which should be easily googlable.

IIRC, a statistical distribution of gravitational interactions with many other particles nearby can make off-diagonal terms in the density matrix decrease rapidly to zero. (However, it's been many years since I looked at this.)
 
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  • #14
strangerep said:
There are papers on gravity-induced decoherence, and similar, by L.H.Ford et al, which should be easily googlable.

IIRC, a statistical distribution of gravitational interactions with many other particles nearby can make off-diagonal terms in the density matrix decrease rapidly to zero. (However, it's been many years since I looked at this.)
Mmh... Let me explain my point. I understand that there are gravity-induced decoherence models (which may or may not be right), but then there is the concept that curved spacetime may disturb quantum systems (this is one of the assumptions of Hawking radiation, as the intense gravitational field of the black hole, translated into an intense curved spacetime, can modify quantum fluctuations and radiate particles), and this is an assumption that every physicist I've talked to makes.

So that's where I'm confused: How is it that there is much doubt about quantum decoherence being caused by gravity but then it is widely assumed that curved spacetime can disrupt quantum systems?
 
  • #15
Suekdccia said:
it is widely assumed that curved spacetime can disrupt quantum systems?
This is not the case. Curved spacetime changes a quantum system by adding an external gravitation field. It is not a ''disruption'' but the same kind of change as adding an external electromagnetic field would have (a standard procedure in quantum mechanics), except that the gravitational field has spin 2 whereas the electromagnetic field has spin 1.
 
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  • #16
A. Neumaier said:
This is not the case. Curved spacetime changes a quantum system by adding an external gravitation field. It is not a ''disruption'' but the same kind of change as adding an external electromagnetic field would have (a standard procedure in quantum mechanics), except that the gravitational field has spin 2 whereas the electromagnetic field has spin 1.
When I said "disruption" I meant to make a quantum system decohere or a quantum wavefunction collapse. Adding an electromagnetic field to the system will decohere an entangled system or will collapse the wavefunction. So wouldn't curved spacetime (and therefore gravity) do the same?
 
  • #17
Suekdccia said:
When I said "disruption"
It would be better to use terminology that means the same for everyone. Otherwise misunderstandings are almost unavoidable!
Suekdccia said:
I meant to make a quantum system decohere or a quantum wavefunction collapse. Adding an electromagnetic field to the system will decohere an entangled system or will collapse the wavefunction.
No. Adding an external field changes the unitary dynamics to another unitary dynamics, hence does not create decoherence or collapse. The latter is created exclusively by dropping degrees of freedom in the environment, and is in this case unavoidable.
Suekdccia said:
So wouldn't curved spacetime (and therefore gravity) do the same?
Neither electromagnetism nor gravity induces decoherence since unitary dynamics implies no decoherence.

Instead, disregarding the environment (by not modeling its degrees of freedom in the wave function) decoheres a wave function, since only a subset of the actual degrees of freedom are correctly modelled and the remaining ones are accounted for only in the mean, spoiling unitarity.
 
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  • #18
A. Neumaier said:
It would be better to use terminology that means the same for everyone. Otherwise misunderstandings are almost unavoidable!

No. Adding an external field changes the unitary dynamics to another unitary dynamics, hence does not create decoherence or collapse. The latter is created exclusively by dropping degrees of freedom in the environment, and is in this case unavoidable.

Neither electromagnetism nor gravity induces decoherence since unitary dynamics implies no decoherence.

Instead, disregarding the environment (by not modeling its degrees of freedom in the wave function) decoheres a wave function, since only a subset of the actual degrees of freedom are correctly modelled and the remaining ones are accounted for only in the mean, spoiling unitarity.
I think I'm understanding it now. So, if there is an external gravitational field and it is disregarded from the wavefunction as part of the environment then it could cause the system to collapse/decohere?
 
  • #19
Suekdccia said:
if there is an external gravitational field and it is disregarded from the wavefunction as part of the environment then it could cause the system to collapse/decohere?
No. The external field is part of the Hamiltonian, not part of the wave function. Thus your assumption is meaningless.

But if the wave function of your system does not have arguments that model the macroscopic equipment with which the system is handled, there is already enough decoherence to cause collapse. This is the main difficulty that quantum computing faces. The amount of decoherence can be made small by a careful choice of degrees of freedom to study and by minimizing the uncontrolled interactions with the environment, but cannot be eliminated entirely.
 
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