Quantum Measurements with Gravitational Waves

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Summary:

Would using gravitational waves to measure position and momentum of an electron disprove HUP since grav. waves are not "made of" particles?
Would using gravitational waves to measure (it's obviously a gedankenexperiment!) position and momentum of, say, an electron in a specific state, disprove HUP since the quantum of energy of grav. waves does not exist? Would it be possibile to have an arbitrarily small uncertainty in position measurement, and in momentum measurement (e. g. with arbitrarily small wavelenght and arbitrarily small amplitude of the wave)?

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  • #2
PeroK
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What makes you think a electron gives off gravitational waves? It creates an EM field that theoretically reveals its exact position and momentum. But, that's classical EM, which experimentally QM trumps.

QM (not GR) describes the dynamics of an electron. If anything the HUP disproves GR - if you want to put it like that.
 
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  • #3
Vanadium 50
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What makes you think a electron gives off gravitational waves?
We can take a step back from even this. What makes you think QM forbids measuring both position and momentum?
 
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  • #4
PeterDonis
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Would using gravitational waves to measure (it's obviously a gedankenexperiment!) position and momentum of, say, an electron in a specific state, disprove HUP since the quantum of energy of grav. waves does not exist?
Why do you think the quantum of energy of gravitational waves does not exist?
 
  • #5
DaveE
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The relationship between QM and GR is maybe the biggest unknown in physics now. I certainly don't know the answer, and I have a strong suspicion that no one else does either; at least that's what everyone tells me.

If you want to make progress in this area, you'll want to clean up your question. For example, why do you presuppose that an electron even has "a position"? It's my understanding that the HUP doesn't say that an electron has a unique position and a unique momentum, but we can't measure it. I believe it says that the concept of a unique position and momentum are incompatible in a very fundamental way.

OTOH, physics has been revolutionized in the past with dramatic new theories that contradicted what everyone thought. So, I guess we'll see what develops.
 
  • #6
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What makes you think a electron gives off gravitational waves? It creates an EM field that theoretically reveals its exact position and momentum. But, that's classical EM, which experimentally QM trumps.

QM (not GR) describes the dynamics of an electron. If anything the HUP disproves GR - if you want to put it like that.
No, I don't want to disprove GR, or HUP, or QM or anything else. I only would like to know if what I wrote could be seriously taken as strong clue of the existence of gravitons.
Concerning the fact an electron gives off g. waves, I am not sure; indeed, if I remember well, a dipole oscillation of mass can't generate g. w. Is this the case?

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  • #7
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We can take a step back from even this. What makes you think QM forbids measuring both position and momentum?
If this question is referred to me: I haven't written that.

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  • #8
PeterDonis
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I only would like to know if what I wrote could be seriously taken as strong clue of the existence of gravitons.
If we could actually make sensitive enough measurements involving gravitational waves for quantum aspects of such waves to be testable, then yes, we could test experimentally to see if such quantum aspects were present.

However, we are many, many orders of magnitude away from being able to make such measurements.

Theoretically, most physicists believe that gravity should have quantum aspects because everything else does. That is why one of the main theoretical efforts ongoing in fundamental physics is trying to find a theory of quantum gravity that works.

a dipole oscillation of mass can't generate g. w. Is this the case?
Yes. You need at least quadrupole oscillations. More precisely, you need a nonzero third time derivative of the quadrupole moment.
 
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  • #9
PeterDonis
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If this question is referred to me: I haven't written that.
You might not have intended to, but you weren't very clear about what question you were asking until post #6. Prior to that, it certainly looked as though you were saying that measurements involving gravitational waves could violate the HUP.
 
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  • #10
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You might not have intended to, but you weren't very clear about what question you were asking until post #6. Prior to that, it certainly looked as though you were saying that measurements involving gravitational waves could violate the HUP.
Ok, it was a mistake I made in order to express my question.
Then, could, in theory a g. w. be scattered off an electron in such a way to theoretically measure the electron's position and momentum similarly to what we can do with an em wave?
Thanks.

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  • #11
PeterDonis
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Then, could, in theory a g. w. be scattered off an electron in such a way to theoretically measure the electron's position and momentum similarly to what we can do with an em wave?
I don't see why not, in principle. And in principle we would expect such a measurement to have the same quantum properties as an EM wave measurement does.

Of course in practice it is going to be a long, long time before we can do anything like this.
 
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  • #12
Vanadium 50
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No, I don't want to disprove GR, or HUP, or QM or anything else.
Then why did you title this thread "Disproving Heisenberg principle with Gravitational Waves"?
 
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  • #13
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As I wrote to PeterDonis, it was my mistake. I just wanted to focus the attention on grav. waves' presumed quantization using HUP: either g. w. are quantized or they cannot be used in measuring a particle's non-commuting observables. The concept should be: since HUP obviously holds (I never had doubts about it), does this imply that g. w. have to be quantized (without the need to make the experiment in reality)?

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  • #14
PeterDonis
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I just wanted to focus the attention on grav. waves' presumed quantization using HUP
Then the thread title needs to be changed. Done.

since HUP obviously holds (I never had doubts about it), does this imply that g. w. have to be quantized
This is one of the reasons why most physicists believe we will need a quantum theory of gravity, yes.
 
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  • #15
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Then the thread title needs to be changed. Done.

This is one of the reasons why most physicists believe we will need a quantum theory of gravity, yes.
Thanks.

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  • #16
Vanadium 50
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Well, we should be forgiven that we thought you meant that this disproves HUP because a) it was in the title (now changed) and b) in the text (still there).

Now we have another issue - I can't for the life of me figure out what we are trying to show. That electrons do (or do not) interact with gravity waves? That gravity waves do (or do not) obey the HUP. Something else? Message #10 did not clarify it. Why don't you think about it and post exactly what you want to know.
 
  • #17
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Well, we should be forgiven that we thought you meant that this disproves HUP because a) it was in the title (now changed) and b) in the text (still there).

Now we have another issue - I can't for the life of me figure out what we are trying to show. That electrons do (or do not) interact with gravity waves? That gravity waves do (or do not) obey the HUP. Something else? Message #10 did not clarify it. Why don't you think about it and post exactly what you want to know.
In a discussion with a friend, he claimed that gravitons = quantum of g.w. have to exist, because if they didn't, it would be possibile to measure, e. g., an electron's position and momentum in some state with ∆x and ∆p (standard deviations) arbitrarily small, disproving HUP, which is impossible.
Actually I didn't believe this simple consideration could prove that g.w. have to be quantized, even if I know very little on the subject, so I asked here to have more... context and PeterDonis answered clearly that... I was wrong.

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  • #18
vanhees71
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Nobody knows what gravitons are, because there's no satisfactory quantum (field) theory of gravitation yet. So it's useless to speculate about their properties. Whether or not there is a quantum theory including gravitation is not known either, i.e., it's not clear whether or not the fundamental formulation of QT has to be changed or not. So the question, whether or not the general Heisenberg-Robertson uncertainty relation (which is about state preparation not about the ability or disability to measure simultaneously any pair of observables) still holds in the present form or not in some future quantum theory including the gravitational interaction of not.
 
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  • #19
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Nobody knows what gravitons are, because there's no satisfactory quantum (field) theory of gravitation yet. So it's useless to speculate about their properties. Whether or not there is a quantum theory including gravitation is not known either, i.e., it's not clear whether or not the fundamental formulation of QT has to be changed or not. So the question, whether or not the general Heisenberg-Robertson uncertainty relation (which is about state preparation not about the ability or disability to measure simultaneously any pair of observables) still holds in the present form or not in some future quantum theory including the gravitational interaction of not.
So you're saying that the Gedankenexperiment I described cannot prove at all that g. w. are quantized?

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  • #20
PeterDonis
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So you're saying that the Gedankenexperiment I described cannot prove at all that g. w. are quantized?
No, he's saying that we currently have no theory that predicts what the result of your gedankenexperiment would be. A theory that could do that would have to be a theory that has General Relativity and our current quantum field theory as approximations in appropriate limits (roughly, the limit in which gravity is significant but quantum mechanics can be ignored, and the limit in which QM is significant but gravity can be ignored), and also explains what happens in cases, such as your gedankenexperiment, in which both gravity and quantum mechanics are significant and cannot be ignored.

However, if we were able to actually do your experiment, it wouldn't matter whether we had a theory; the experiment itself would tell us whether gravitational wave measurements obey the HUP or not.
 
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  • #21
Vanadium 50
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So you're saying that the Gedankenexperiment I described
You haven't proposed a Gedankenexperiment. You have neither described the state preparation nor the measurement. When pressed, you say what you are proposing is the opposite of what you have written. Thus far, all we have is there's an electron, a gravitational wave, and then something something something.

I suspect that if you described a proper experiment - defined in enough detail that someone could in principle perform it - you would discover that you are insensitive to the effect that you are interested in, because the electron and whatever generates the gravitational radiation are still subject to quantum mechanics.
 
  • #22
Vanadium 50
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I can only conclude that you don't have an experiment in mind - one defined in enough detail that someone could in principle perform it.
 
  • #23
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However, if we were able to actually do your experiment, it wouldn't matter whether we had a theory; the experiment itself would tell us whether gravitational wave measurements obey the HUP or not.
I don't think we do. I don't think we even have an experiment. But if the idea is to somehow look at the path of the recoil electron, that recoil electron obeys the HUP.
 
  • #24
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Nobody knows what gravitons are, because there's no satisfactory quantum (field) theory of gravitation yet. So it's useless to speculate about their properties. Whether or not there is a quantum theory including gravitation is not known either, i.e., it's not clear whether or not the fundamental formulation of QT has to be changed or not. So the question, whether or not the general Heisenberg-Robertson uncertainty relation (which is about state preparation not about the ability or disability to measure simultaneously any pair of observables) still holds in the present form or not in some future quantum theory including the gravitational interaction of not.
Thanks, vanhees.

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  • #25
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No, he's saying that we currently have no theory that predicts what the result of your gedankenexperiment would be. A theory that could do that would have to be a theory that has General Relativity and our current quantum field theory as approximations in appropriate limits (roughly, the limit in which gravity is significant but quantum mechanics can be ignored, and the limit in which QM is significant but gravity can be ignored), and also explains what happens in cases, such as your gedankenexperiment, in which both gravity and quantum mechanics are significant and cannot be ignored.

However, if we were able to actually do your experiment, it wouldn't matter whether we had a theory; the experiment itself would tell us whether gravitational wave measurements obey the HUP or not.
Tanks, PeterDonis.

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