The gravition and Einstein's equivalence principal

[StatusX]

the graviton and einstein's equivalence principle

Einstein's theory of general relativity was motivated by his equivalence principle, that no experiment can distinguish between an accelerated frame and a still frame under the influence of gravity. But if the gravitational force is mediated by a particle, the graviton, then couldn't they be distinguished? (if there are no gravitons, you are accelerating) So has this equivalence principle been abandoned, or is it still somehow true?

 

[marlon]

I wrote a text on this in my journal. Just look for string theory part 1...


regards
marlon

just click on "read my journal" under my name...

 

[marlon]

https://www.physicsforums.com/journal.php?s=&action=view&journalid=13790&perpage=10&page=2

here is the link

just look into string theory part one and go to the subsection General Relativity

regards
marlon

 

[rcgldr]

If the experiment is done within a very tall box, then gravity will diminish with altitude, and acceleration will not.

 

[StatusX]

I'm still confused. Marlon, your journal talked about GR, but I didn't see anything specifically mentioning this conflict. This may sound like a triviality, since principles change all the time (eg, conservation of energy became conservation of mass-energy when we saw they could be interchanged). But Einstein's whole theory arose because of this principle, and if it isn't ture, his theory seems to just become a huge coincidence.

 

[Tzar]

I think the point you raise is one of the main arguments against the existence of the graviton in the first place. It still hasn't been discovered, and I doubt it will be

 

[hellfire]

This is an interesting question. I know about the graviton as a hypothetic particle which results from quantizing a weak perturbation of the flat spacetime metric. The perturbation is quantized in the same way than other quantum fields are quantized on flat spacetime. This procedure is called covariant quantization and I think it is not regarded as very successful, because it considers the gravitational interaction only perturbatively. However, as far as I know string theory claims to fully contain classical gravitation and it does also predict a graviton. And this is somehow puzzling, because in case of an uniform gravitational field a coordinate transformation should remove every graviton. But I am not sure this is really as weird as it seams, since the gravitons which need to be removed by a coordinate transformation (the gravitons responsible for the gravitational interaction) would be virtual gravitons and not real ones. Well, I do not know the answer to your question and whether there is one.

 

[turbo]

Another argument against the existence of the graviton is that the gravitational field (boson=graviton) and the Higgs field (with its theoretical Higgs bosons) are required to be excruciatingly concordant everywhere and everywhen all over the visible Universe. If they were not so concordant, we would see objects in different parts of the universe acting in odd ways as "local" gravitational effects caused objects to more or less gravitationally attractive. Gravity seems to follow the same rules everywhere and everywhen we look, so this is a strong indicator that mass, gravitation, and inertia all arise from matter's interaction with a single field, not from two separate fields, one of which conveys mass and the other of which mediates gravitational attraction.

 

[JesseM]

But I am not sure this is really as weird as it seams, since the gravitons which need to be removed by a coordinate transformation (the gravitons responsible for the gravitational interaction) would be virtual gravitons and not real ones.

That would be my guess too...virtual particles are just things that show up in the equations you use to predict how "real" particles behave, they aren't detectable by experiment (see section s3 of http://arnold-neumaier.at/physics-faq.txt, especially question s3c). I would think that gravitons could only be detected directly in a situation where gravitational waves are passing by you (gravitons being the quanta of gravitational waves just like photons are the quanta of electromagnetic waves--see section 5 of this FAQ on virtual particles), but as I understand it the equivalence principle only deals with arbitrarily small regions of spacetime where things like tidal forces and gravitational waves can be ignored.

 

[hellfire]

Well, it was just a thought but I guess this cannot be the solution. There is a definition of particle which is valid for real particles as well as for virtual particles. I think the question is what happens with gravitons in accelerated reference frames and how to model the fictious force in an uniform gravitational field. I would say it makes no sense to say that the contributions of virtual particles can be transformed away by the coordinate transformation whereas any real particle will remain invariant. It seams to be a very tricky issue, at least I am completely lost...

 

[pmb_phy]

Einstein's theory of general relativity was motivated by his equivalence principle, that no experiment can distinguish between an accelerated frame and a still frame under the influence of gravity. But if the gravitational force is mediated by a particle, the graviton, then couldn't they be distinguished? (if there are no gravitons, you are accelerating) So has this equivalence principle been abandoned, or is it still somehow true?

No. The ability to detect the graviton is a frame dependant phenomena. Also you stated the EP wrong. It states

Einstein's Equivalence Principle (weak form) - It is impossible to determine whether you're in a static uniform gravitational field or a unifornly accelerating frame of reference.

Einstein's Equivalence Principle (strong form) - The laws of physics are the same in all coordinate systems. Mathematically this means that one must always be able to express the equations of physics in tensor form. This holds whether the gravitational field is uniform or notl

Pete

 

[JesseM]

Well, it was just a thought but I guess this cannot be the solution. There is a definition of particle which is valid for real particles as well as for virtual particles.

What definition are you talking about? My point is just that you cannot detect virtual particles experimentally--do you disagree? Did you read the section on virtual particles from the http://arnold-neumaier.at/physics-faq.txt I mentioned?

 

[hellfire]

What definition are you talking about?

Particles are usually defined according to their properties preserved during isometries in flat spacetime.

My point is just that you cannot detect virtual particles experimentally--do you disagree?

I do not disagree with you, I was just arguing against me (the part of my post you cited above). Sorry for the confusion.

The ability to detect the graviton is a frame dependant phenomena.

But the graviton is usually defined to be a particle described by a second rank tensor. How can this be frame dependent?

 

[JesseM]

I do not disagree with you, I was just arguing against me (the part of my post you cited above). Sorry for the confusion.

But what did you mean when you said this cannot be the solution? If no gravitons will be detectable in an elevator falling through a gravitational field, isn't that a solution of sorts, since the experimenter won't see anything different than if the elevator was moving inertially in deep space?

 

[hellfire]

But what did you mean when you said this cannot be the solution? If no gravitons will be detectable in an elevator falling through a gravitational field, isn't that a solution of sorts, since the experimenter won't see anything different than if the elevator was moving inertially in deep space?

You may consider that gravitons can be transformed away by a coordinate transformation to a free falling frame. Or you may not. Both possibilities must treat real and virtual gravitons in the same manner. If gravitons cannot be transformed away in a free falling frame, then virtual gravitons will contribute to the transition amplitudes in the gravitational interaction. It does not help that they are not measurable.

 

[cosmik debris]

Einstein's theory of general relativity was motivated by his equivalence principle, that no experiment can distinguish between an accelerated frame and a still frame under the influence of gravity.

This principle has many formulations but don't forget it is only valid locally. Locally means in a region of spacetime that is very small and therefore has a flat metric where the laws of SR apply.

But if the gravitational force is mediated by a particle, the graviton, then couldn't they be distinguished? (if there are no gravitons, you are accelerating) So has this equivalence principle been abandoned, or is it still somehow true?

If you are talking about gravitons and forces of gravitation then you are not talking about GR but some alternate theory. Gravitons require a quantum theory of gravitation which has not yet been formulated in general.

 

[JesseM]

You may consider that gravitons can be transformed away by a coordinate transformation to a free falling frame. Or you may not. Both possibilities must treat real and virtual gravitons in the same manner. If gravitons cannot be transformed away in a free falling frame, then virtual gravitons will contribute to the transition amplitudes in the gravitational interaction. It does not help that they are not measurable.

OK, are you saying that even if individual gravitons are not detectable, they could "contribute to the transition amplitudes in the gravitational interaction" in such a way that a person inside a small windowless elevator could do some experiment involving gravity that would tell him he was falling through a gravitational field rather than moving inertially in free space? If so, what type of experiment might yield different results in these two cases? (assume the experimenter has the technology to do any possible experiment allowed by the laws of physics, however unfeasible it may be by present-day standards)