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Author Topic: How would the uncertainty principle apply to gravity?  (Read 7623 times)

Offline jeffreyH

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We assume the centre of gravity of a mass as reference point for gravitational calculations. This assume a well defined directional component for gravitation. Could the uncertainty principle apply to gravity and if not why not?


 

Offline JP

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Re: How would the uncertainty principle apply to gravity?
« Reply #1 on: 09/06/2014 00:27:52 »
The uncertainty principle is a quantum mechanical effect, so presumably in a working theory of quantum gravity it would apply. 

The effect is so small that we can't observe it with current experiments.
 

Offline evan_au

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Re: How would the uncertainty principle apply to gravity?
« Reply #2 on: 09/06/2014 12:26:03 »
The first accurate measurement of gravitational attraction was performed by Cavendish, who used 2" and 12" lead balls, so locating the center of these accurately would be very difficult. The balls were never really at rest, but oscillated with a period of about 20 minutes, so any measurements would need to be averaged over this time.

Measurement of gravitation is quite inaccurate, so any quantum fluctuations would be very hard to measure. It is said that the gravitational constant G is the least accurately known fundamental constant.
 

Offline PmbPhy

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Re: How would the uncertainty principle apply to gravity?
« Reply #3 on: 09/06/2014 14:51:58 »
We assume the centre of gravity of a mass as reference point for gravitational calculations. This assume a well defined directional component for gravitation. Could the uncertainty principle apply to gravity and if not why not?
I'm not clear on what you're asking. What do you mean "apply to gravity"? The uncertainty principle applies to particles, not fields. It applies to an ensemble (i.e. collection) of measurements of quantities such as the position and momentum of a either a free particle or a particle moving in a potential well. The potential well could be caused by an electric field, the strong force (Yukawa potential) or the gravitational field. In this sense it applies to gravity.
 

Offline PmbPhy

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Re: How would the uncertainty principle apply to gravity?
« Reply #4 on: 09/06/2014 14:59:50 »
Quote from: evan_au
Measurement of gravitation is quite inaccurate, ...
You're referring only to the gravitational attraction of small objects. However it's extremely easy to measure the gravitational attraction between me and the earth. My doctor asks me to do it everytime I visit his office.

Quote from: evan_au
..so any quantum fluctuations would be very hard to measure. It is said that the gravitational constant G is the least accurately known fundamental constant.
It's not clear whether he's refering to the uncertainty principle applied to the field itself or particles moving in the field. I'm not even sure that there's an uncertainty principle for the fields themselves. Are you?
 

Offline PmbPhy

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Re: How would the uncertainty principle apply to gravity?
« Reply #5 on: 09/06/2014 15:33:22 »
We assume the centre of gravity of a mass as reference point for gravitational calculations. This assume a well defined directional component for gravitation. Could the uncertainty principle apply to gravity and if not why not?
I spoke to a friend about this who's a textbook author on the subject. In case you were asking about using the HUP with respect to the field and not a particle moving in the field he wrote
Quote
Yes, the uncertainty principle applies to gravitational fields.
Roughly, the uncertainty principle for the metric tensor says that delta (g_munu) = L*/L, where L* is the Planck length, and L is the size of the region in which we measure the metric tensor. If L is of the same order of magnitude as L*, then the metric tensor becomes totally uncertain.
Does that answer your question?
 

Offline JP

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Re: How would the uncertainty principle apply to gravity?
« Reply #6 on: 09/06/2014 16:34:08 »
We assume the centre of gravity of a mass as reference point for gravitational calculations. This assume a well defined directional component for gravitation. Could the uncertainty principle apply to gravity and if not why not?
I spoke to a friend about this who's a textbook author on the subject. In case you were asking about using the HUP with respect to the field and not a particle moving in the field he wrote
Quote
Yes, the uncertainty principle applies to gravitational fields.
Roughly, the uncertainty principle for the metric tensor says that delta (g_munu) = L*/L, where L* is the Planck length, and L is the size of the region in which we measure the metric tensor. If L is of the same order of magnitude as L*, then the metric tensor becomes totally uncertain.
Does that answer your question?

Strictly speaking, this is an uncertainty relation, not the uncertainty principle.  The relation applies to classical fields (or basically anything that has continuous wavelike properties).  The principle applies to quantized particles.
 

Offline jeffreyH

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Re: How would the uncertainty principle apply to gravity?
« Reply #7 on: 09/06/2014 20:11:59 »
This question was in relation to a hypothetical graviton or gravitons plural.
 

Offline JP

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Re: How would the uncertainty principle apply to gravity?
« Reply #8 on: 09/06/2014 20:15:49 »
This question was in relation to a hypothetical graviton or gravitons plural.

We don't know, since there isn't an accepted theory of gravitons.  Presumably since they're quantum objects the uncertainty principle applies to them.  Of course, some theories of quantum gravity are quite exotic, so in those cases maybe not.
 

Offline PmbPhy

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Re: How would the uncertainty principle apply to gravity?
« Reply #9 on: 10/06/2014 01:43:17 »
This question was in relation to a hypothetical graviton or gravitons plural.

We don't know, since there isn't an accepted theory of gravitons.  Presumably since they're quantum objects the uncertainty principle applies to them.  Of course, some theories of quantum gravity are quite exotic, so in those cases maybe not.
Question: Since gravitons are the mediators of the gravitational field doesn't that make them virtual particles? Does the uncertainty principle hold for virtual particles? After all in order to apply the uncertainty principle you have to be able to detect the gravitons and if they're virtual particles they can't be detected.
 

Offline jeffreyH

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Re: How would the uncertainty principle apply to gravity?
« Reply #10 on: 10/06/2014 19:52:15 »
This question was in relation to a hypothetical graviton or gravitons plural.

We don't know, since there isn't an accepted theory of gravitons.  Presumably since they're quantum objects the uncertainty principle applies to them.  Of course, some theories of quantum gravity are quite exotic, so in those cases maybe not.
Question: Since gravitons are the mediators of the gravitational field doesn't that make them virtual particles? Does the uncertainty principle hold for virtual particles? After all in order to apply the uncertainty principle you have to be able to detect the gravitons and if they're virtual particles they can't be detected.

That's a very good question.One that should be given proper consideration. I would be interested on your thoughts on the subject Pete. It is an area that no one has a good answer to. We don't seem to be able to detect gravitons. I knew something interesting would come from this topic.
 

Offline JP

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Re: How would the uncertainty principle apply to gravity?
« Reply #11 on: 10/06/2014 20:06:31 »
This question was in relation to a hypothetical graviton or gravitons plural.

We don't know, since there isn't an accepted theory of gravitons.  Presumably since they're quantum objects the uncertainty principle applies to them.  Of course, some theories of quantum gravity are quite exotic, so in those cases maybe not.
Question: Since gravitons are the mediators of the gravitational field doesn't that make them virtual particles? Does the uncertainty principle hold for virtual particles? After all in order to apply the uncertainty principle you have to be able to detect the gravitons and if they're virtual particles they can't be detected.

Short answer: Yes, the uncertainty principle holds for virtual particles. 

Long answer:  The uncertainty principle holds for all quantum particles, period.  If they're particles that have wavelike properties, they necessarily obey uncertainty principles.  The distinction between real and virtual particles is fuzzy because real means that a particle exists for all time.  A virtual particle has a finite existence because it is emitted/absorbed at points in time.  But really nothing exists for all time, so all particles are in a sense virtual. 

The reason we distinguish between them is that typically there are interactions that involve very short-lived particles and interactions that involve very long-lived particles without a lot in between, so it makes sense to categorize them as such.  Because of the energy/time uncertainty principle a short-lived particle has a wide spectrum of allowed energies, whereas a long-lived particle has a narrow spectrum of allowed energies.  This is why we say virtual particles can be off-shell (i.e. they can have energies not allowed for free particles):
http://en.wikipedia.org/wiki/On_shell_and_off_shell
http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html
 

Offline jeffreyH

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Re: How would the uncertainty principle apply to gravity?
« Reply #12 on: 10/06/2014 22:01:57 »
This question was in relation to a hypothetical graviton or gravitons plural.

We don't know, since there isn't an accepted theory of gravitons.  Presumably since they're quantum objects the uncertainty principle applies to them.  Of course, some theories of quantum gravity are quite exotic, so in those cases maybe not.
Question: Since gravitons are the mediators of the gravitational field doesn't that make them virtual particles? Does the uncertainty principle hold for virtual particles? After all in order to apply the uncertainty principle you have to be able to detect the gravitons and if they're virtual particles they can't be detected.

Short answer: Yes, the uncertainty principle holds for virtual particles. 

Long answer:  The uncertainty principle holds for all quantum particles, period.  If they're particles that have wavelike properties, they necessarily obey uncertainty principles.  The distinction between real and virtual particles is fuzzy because real means that a particle exists for all time.  A virtual particle has a finite existence because it is emitted/absorbed at points in time.  But really nothing exists for all time, so all particles are in a sense virtual. 

The reason we distinguish between them is that typically there are interactions that involve very short-lived particles and interactions that involve very long-lived particles without a lot in between, so it makes sense to categorize them as such.  Because of the energy/time uncertainty principle a short-lived particle has a wide spectrum of allowed energies, whereas a long-lived particle has a narrow spectrum of allowed energies.  This is why we say virtual particles can be off-shell (i.e. they can have energies not allowed for free particles):
http://en.wikipedia.org/wiki/On_shell_and_off_shell
http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

Then again.... That's a good answer.
« Last Edit: 10/06/2014 22:26:23 by jeffreyH »
 

Offline yor_on

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Re: How would the uncertainty principle apply to gravity?
« Reply #13 on: 20/06/2014 23:41:57 »
So, assuming such a thing, as a 'probability space', then there it all must be in some static configuration. With our arrow sorting out what will exist to us. Think I like the idea of 'spaces' better than 'dimensions', as they do not demand me to guarantee their objective existence.
 

Offline jeffreyH

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Re: How would the uncertainty principle apply to gravity?
« Reply #14 on: 22/06/2014 23:46:02 »
We assume the centre of gravity of a mass as reference point for gravitational calculations. This assume a well defined directional component for gravitation. Could the uncertainty principle apply to gravity and if not why not?
I spoke to a friend about this who's a textbook author on the subject. In case you were asking about using the HUP with respect to the field and not a particle moving in the field he wrote
Quote
Yes, the uncertainty principle applies to gravitational fields.
Roughly, the uncertainty principle for the metric tensor says that delta (g_munu) = L*/L, where L* is the Planck length, and L is the size of the region in which we measure the metric tensor. If L is of the same order of magnitude as L*, then the metric tensor becomes totally uncertain.
Does that answer your question?

Thanks Pete. The uncertainty principle applying to the field is something I hadn't considered. I have just ordered "Numerical Relativity: Solving Einstein's Equations on the Computer" by Thomas W. Baumgarte. I am hoping that I can adapt some of this for my own use. May be a hard task.
 

Offline PmbPhy

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Re: How would the uncertainty principle apply to gravity?
« Reply #15 on: 23/06/2014 06:08:41 »
Quote from: jeffreyH
I have just ordered "Numerical Relativity: Solving Einstein's Equations on the Computer" by Thomas W. Baumgarte. I am hoping that I can adapt some of this for my own use. May be a hard task.
Thanks, Jeff. It sounds like an interesting text so I decided to download it from the internet. I can down most textbooks off the internet. I found a place where I can download them for free! :)
 

Offline jeffreyH

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Re: How would the uncertainty principle apply to gravity?
« Reply #16 on: 23/06/2014 22:44:24 »
Quote from: jeffreyH
I have just ordered "Numerical Relativity: Solving Einstein's Equations on the Computer" by Thomas W. Baumgarte. I am hoping that I can adapt some of this for my own use. May be a hard task.
Thanks, Jeff. It sounds like an interesting text so I decided to download it from the internet. I can down most textbooks off the internet. I found a place where I can download them for free! :)

It would be interesting to compare notes.
 

Offline PmbPhy

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Re: How would the uncertainty principle apply to gravity?
« Reply #17 on: 25/06/2014 05:46:56 »
Quote from: jeffreyH
It would be interesting to compare notes.
I printed it out yesterday and then glued the ends of the pages together to make sort of a binding. Tomorrow I'm going to put a makeshift cover on it. Then it will probably sit on the shelf for a long time. I'm currently engulfed in reviewing quantum mechanics. I do this every once in a while to stay fresh on the subject. I'm then going to study EM and then onto GR, both in much more depth and at higher levels. Then I plan on learning Java. Then I'll come back to this. Let me know what you're doing in the mean time. I want to watch what you produce. How will you display the results of the computations you get?
 

Offline jeffreyH

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Re: How would the uncertainty principle apply to gravity?
« Reply #18 on: 25/06/2014 17:43:26 »
Quote from: jeffreyH
It would be interesting to compare notes.
I printed it out yesterday and then glued the ends of the pages together to make sort of a binding. Tomorrow I'm going to put a makeshift cover on it. Then it will probably sit on the shelf for a long time. I'm currently engulfed in reviewing quantum mechanics. I do this every once in a while to stay fresh on the subject. I'm then going to study EM and then onto GR, both in much more depth and at higher levels. Then I plan on learning Java. Then I'll come back to this. Let me know what you're doing in the mean time. I want to watch what you produce. How will you display the results of the computations you get?

I haven't even considered how to display results yet. I have been looking into the open source Einstein Tools but that only runs on unix\linux based systems. I could run it on cygwin but will probably rewrite it or base something else on it. I am currently examining how they handle the Ricci calculations. I'll keep you posted on this as I progress.
 

Offline PmbPhy

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Re: How would the uncertainty principle apply to gravity?
« Reply #19 on: 26/06/2014 02:42:03 »
Quote from: jeffreyH
I haven't even considered how to display results yet. I have been looking into the open source Einstein Tools but that only runs on unix\linux based systems. I could run it on cygwin but will probably rewrite it or base something else on it. I am currently examining how they handle the Ricci calculations. I'll keep you posted on this as I progress.
What are "Ricci calculations" and why are you so interested in them? Are you referring to how to calculate the Rici tensor? If so then why? Don't confuse the Rici tensor with a measure spacetime curvature because it isn't. People often make that mistake.
 

Offline jeffreyH

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Re: How would the uncertainty principle apply to gravity?
« Reply #20 on: 27/06/2014 04:42:45 »
Quote from: jeffreyH
I haven't even considered how to display results yet. I have been looking into the open source Einstein Tools but that only runs on unix\linux based systems. I could run it on cygwin but will probably rewrite it or base something else on it. I am currently examining how they handle the Ricci calculations. I'll keep you posted on this as I progress.
What are "Ricci calculations" and why are you so interested in them? Are you referring to how to calculate the Rici tensor? If so then why? Don't confuse the Rici tensor with a measure spacetime curvature because it isn't. People often make that mistake.

No it was just the first example code I came across to analyse. Yes it was the tensor calculations. I am also looking at flat affine geometry. This also is not related to curvature but there are reasons for studying it. You do of course have affine curves.
« Last Edit: 27/06/2014 05:09:31 by jeffreyH »
 

Offline PmbPhy

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Re: How would the uncertainty principle apply to gravity?
« Reply #21 on: 27/06/2014 13:52:54 »
Quote from: jeffreyH
I am also looking at flat affine geometry. This also is not related to curvature but there are reasons for studying it. You do of course have affine curves.
Yes. I see. And of course Iím intimately aware of these facts since Iíve been studying it for a very long time now.

I recommend that you pick up the following textbook

A General Relativity Workbook by Thomas Moore, Springer Press, (2012)

If you want to really know general relativity as well as I think you do then this is the perfect textbook for you. Iím taking my own advice too since I just bought it. :)
 

Offline jeffreyH

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Re: How would the uncertainty principle apply to gravity?
« Reply #22 on: 01/07/2014 16:53:29 »
Quote from: jeffreyH
I am also looking at flat affine geometry. This also is not related to curvature but there are reasons for studying it. You do of course have affine curves.
Yes. I see. And of course Iím intimately aware of these facts since Iíve been studying it for a very long time now.

I recommend that you pick up the following textbook

A General Relativity Workbook by Thomas Moore, Springer Press, (2012)

If you want to really know general relativity as well as I think you do then this is the perfect textbook for you. Iím taking my own advice too since I just bought it. :)

I will be buying the workbook but first I have gotten a book called General Relativity Without Calculus which uses only high school mathematics. For some of the early preparation this is ideal as the basic equations of the Lorentz transformations, relativistic velocity addition, curvilinear coordinates and the Schwarzschild metric are all treated in the text.
 

Offline jeffreyH

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Re: How would the uncertainty principle apply to gravity?
« Reply #23 on: 01/07/2014 17:00:04 »
Currently I am working on spherical surface area and the particle density distribution with respect to field strength. G can actually be determined via a combination of direct experimental observation and calculation. As long as the properties of the mass under consideration are accurately known.
 

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Re: How would the uncertainty principle apply to gravity?
« Reply #23 on: 01/07/2014 17:00:04 »

 

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