Understanding the Nature of Gravity: Debunking the Need for Gravitons

[WrenGiles]

OK, So as Einstein showed us that the effect that we call gravity is actually caused by the warping of spacetime.
I.e. The Sun, for example, does not "pull" on the Earth directly.
The Sun warps spacetime, thus altering the route which the Earth takes as it travels through spacetime - the perceived affect that we call gravity.
To reiterate - there is no direct force between the Sun & the Earth.
Thus - gravity is NOT a force - so gravitons (the force carrier of gravity) don't need to exist.
So why do physicists tell us they probably do exist & are actively searching for them?
Unless this "force" of gravity is referring to the action that energy in the universe has on the fabric of spacetime (i.e. warping it).
So gravity (and gravitons) is the force that acts between energy & spacetime. Hmm.??
Is this right? or perhaps I should ask, why is this wrong?

 

[HallsofIvy]

Because physicists do research in both Quantum Physics and Relativity. "Gravitons" (particles mediating forces in general) are a feature of Quantum Physics rather than Relativity.

 

[WrenGiles]

I understand that - each force requires a mediator, eg. photons for Electromagnetism, gluons for the strong force and W+, W- & Z bosons for the weak force.
But my points is that no gravitons would be needed (even in Quantum theory) if gravity was not a force - which it appears not to be, simply an effect of warped spacetime.

 

[Dickfore]

Einstein's field equations are non-linear, unlike Maxwell's equations. In weak fields (meaning the metric tensor differs from the Minkowski tensor by terms that are very small), we may linearize these equations - linearized gravitation. Among other things, these equations predict that a small perturbation in the metric may propagate through space by the speed of light - gravitation. In fact, as any linear system of equations, the weak field may be expanded in normal modes, which are the gravitons.

If we write the action of a free particle in a gravitational field and expand it up to terms linear in the small corrections to the metric tensor, this will describe an effective interaction between any particle and the graviton.

In this way we have an effective field theory for gravitation that should be correct as long as the fields are sufficiently weak.

NOTE: We may consider small fluctuations of the field around a curved background space-time metric - gravitational waves in curved space-time.

 

[PeterDonis]

I understand that - each force requires a mediator, eg. photons for Electromagnetism, gluons for the strong force and W+, W- & Z bosons for the weak force.
But my points is that no gravitons would be needed (even in Quantum theory) if gravity was not a force - which it appears not to be, simply an effect of warped spacetime.

From the standpoint of quantum field theory, the other "forces" aren't really forces either; all you have are excitations of quantum fields. What we see as "forces" are really just particular patterns of those excitations. In this viewpoint, spacetime itself should be one of the quantum fields, and so it should have excitations, called "gravitons", just as the other quantum fields do. The main issue with this viewpoint currently is that we don't fully know how to make a consistent theory of quantum fields that includes spacetime as one of the fields, since all the ways we know of to make a consistent quantum field theory require a background spacetime that doesn't have any dynamics--it stays the same regardless of what the quantum fields do. String theory is one of the current candidates for fixing this issue. In the meantime, however, we can do physics at "low" energies--which means all of the energy regimes we can currently test with experiments and observations--by using our current theories, including General Relativity, as "effective" field theories that correctly capture the low-energy physics but are not expected to be correct in energy regimes far above those we can currently observe. So since General Relativity predicts gravitons (as Dickfore's post described), we expect to observe gravitons if we can make detectors sufficiently sensitive (experiments such as LIGO are attempts to do this).

 

[PAllen]

From the standpoint of quantum field theory, the other "forces" aren't really forces either; all you have are excitations of quantum fields. What we see as "forces" are really just particular patterns of those excitations. In this viewpoint, spacetime itself should be one of the quantum fields, and so it should have excitations, called "gravitons", just as the other quantum fields do. The main issue with this viewpoint currently is that we don't fully know how to make a consistent theory of quantum fields that includes spacetime as one of the fields, since all the ways we know of to make a consistent quantum field theory require a background spacetime that doesn't have any dynamics--it stays the same regardless of what the quantum fields do. String theory is one of the current candidates for fixing this issue. In the meantime, however, we can do physics at "low" energies--which means all of the energy regimes we can currently test with experiments and observations--by using our current theories, including General Relativity, as "effective" field theories that correctly capture the low-energy physics but are not expected to be correct in energy regimes far above those we can currently observe. So since General Relativity predicts gravitons (as Dickfore's post described), we expect to observe gravitons if we can make detectors sufficiently sensitive (experiments such as LIGO are attempts to do this).

Gravity waves are detectible, but strong arguments can be made that gravitons will never be detected (e.g. for photons, observing its particle behaviors rather than its wave behaviors).

http://arxiv.org/abs/gr-qc/0601043

 

[PeterDonis]

Gravity waves are detectible, but strong arguments can be made that gravitons will never be detected (e.g. for photons, observing its particle behaviors rather than its wave behaviors).

Good point, I should have said that LIGO and other such experiments are attempting to detect gravitational waves, but they will certainly not be anywhere near sensitive enough to detect quantum phenomena associated with those waves, which is what "detecting gravitons" would mean.

 

[stevenb]

So since General Relativity predicts gravitons (as Dickfore's post described), we expect to observe gravitons.

I disagree that GR predicts gravitons. This is like saying that Maxwell's equations predict photons. Both of these classical theories predict waves, but quantization is a quantum effect not predicted by Maxwell's EM or Einstein's GR equations. While I agree that there are good reasons to expect gravitons (unrelated to GR theory itself), unless we do observe them, we just don't know if they are real. For that matter, even gravity waves are unverified, even though we expect they should exist.

 

[bcrowell]

It's true that GR doesn't say anything about gravitons. However, there are generic reasons why you can't make a workable theory where one field interacts with another and one field is treated classically while the other is quantized. There's a good discussion of this in The quantum challenge: modern research on the foundations of quantum mechanics, by Greenstein and Zajonc. Since gravity is coupled to fields like electromagnetism, and electromagnetism is quantized, it's not possible for gravity to be purely classical.

[EDIT] This paper is probably useful: Carlip, "Is Quantum Gravity Necessary?," http://arxiv.org/abs/0803.3456

For that matter, even gravity waves are unverified, even though we expect they should exist.

They haven't been sensed directly with a detector, but there is very clear empirical verification from the Hulse-Taylor binary pulsar that gravitational waves do exist.

 

[bcrowell]

The Rothman paper that PAllen linked to in #6 ("Can Gravitons Be Detected?") is interesting, but I don't think it should be interpreted to mean more than the title literally says.

For comparison, spectroscopists in the 19th century found a bunch of cases where atomic lines showed an mysterious, exact relationship $1/\lambda_1=1/\lambda_2+1/\lambda_3$. These guys went to their graves without ever detecting a photon, but if they'd known about photons they could have figured out the reason for the mysterious relationship. When Hertz observed the photoelectric effect, he didn't detect individual photons, but Einstein was able to explain the effect successfully in terms of photons. Ditto for blackbody radiation.

I think it's probably hopeless to get any empirical evidence that would guide us in constructing a theory of quantum gravity, but I would characterize that level of hopelessness as much less severe than the level of hopelessness of detecting individual gravitons. It's conceivable that we'll get a working, empirically testable theory of quantum gravity that has gravitons in it, but we'll just never detect a graviton.

 

[John232]

I was reading an abstract in english of Einsteins General Theory from an online book dealer and I noticed something that looked to me to be a proof that gravity couldn't have a carrier since if you where to take into account the number of force lines that would be acting on an object there would have to be an infinite amount of the carriers on the surface.

Say you where to try and account for all the gravitons affecting the Earth from every other body acting on it gravitationaly in the universe. The Earth is really small in comparison and the gravitons would overwelm the surface of the planet, the surface wouldn't be large enough to contain them all. I think this is why Einstein went with his approuch to gravity, and why he didn't describe it as haveing a carrier of the force.

I think if they did claim to find a graviton that it would only be a representation of some kind of microscopic curvature of spacetime.

As for the gravity waves, I don't think they have been able to detect them directly because they are using the wrong experiment to try and find them. They are trying to detect a difference in the speed of light hitting between two mirrors to find it. This approuch is wrong because it goes against one of the founding principals of relativity that predicts them in the first place. If they continue with that method they will never find them.

 

[bcrowell]

I was reading an abstract in english of Einsteins General Theory from an online book dealer and I noticed something that looked to me to be a proof that gravity couldn't have a carrier since if you where to take into account the number of force lines that would be acting on an object there would have to be an infinite amount of the carriers on the surface.

Say you where to try and account for all the gravitons affecting the Earth from every other body acting on it gravitationaly in the universe. The Earth is really small in comparison and the gravitons would overwelm the surface of the planet, the surface wouldn't be large enough to contain them all. I think this is why Einstein went with his approuch to gravity, and why he didn't describe it as haveing a carrier of the force.

No, this is completely wrong. Einstein didn't even have the option of describing gravity in terms of an exchange of gravitons, because quantum mechanics didn't exist in 1915.

As for the gravity waves, I don't think they have been able to detect them directly because they are using the wrong experiment to try and find them. They are trying to detect a difference in the speed of light hitting between two mirrors to find it. This approuch is wrong because it goes against one of the founding principals of relativity that predicts them in the first place. If they continue with that method they will never find them.

Aha, obviously the entire international community of physicists working on gravity wave detection lacks a basic understanding of special relativity. I bet they'll be really red-faced when you point out their elementary error to them. I wonder if they'll give back all their grant money.

 

[Dickfore]

...because quantum mechanics didn't exist in 1915.

Yes it did.

 

[bcrowell]

...because quantum mechanics didn't exist in 1915.

Yes it did.

I guess it depends on what you mean by quantum mechanics. There was the very beginning of the "old quantum theory" (the Bohr model dates to 1913). There was no Schrödinger equation, no uncertainty principle. The state of the art was definitely not sufficient to allow anyone even to speculate about gravitons.

 

[John232]

No, this is completely wrong. Einstein didn't even have the option of describing gravity in terms of an exchange of gravitons, because quantum mechanics didn't exist in 1915.


Aha, obviously the entire international community of physicists working on gravity wave detection lacks a basic understanding of special relativity. I bet they'll be really red-faced when you point out their elementary error to them. I wonder if they'll give back all their grant money.

I thought Einsteins biggest blunder was that he didn't try to work with particle physics and tried to go against it. Since, he thought god doesn't play dice with the universe. It may be the main reason why we don't have any unification of relativity and quantum mechanics.

As far as I have seen so far over the internet a majority of people do lack a basic understanding of basic relativity concepts. I hope someone points out the error to them so that the grant money can go to someone more deserving. The gravitational waves should be here and they have not detected them with their current setup. How long will it take before they realize that they are not going to detect gravitational waves doing it? After they retire?

If michelson and morley found that the speed of light varied in their experiments relativity the history of relativity would have went a lot differently. Although he didn't use the experiment to derive the theory, I think it would have been a big stumbling block in developing the theory.

 

[stevenb]

I thought Einsteins biggest blunder was that he didn't try to work with particle physics and tried to go against it. Since, he thought god doesn't play dice with the universe. It may be the main reason why we don't have any unification of relativity and quantum mechanics.

As far as I have seen so far over the internet a majority of people do lack a basic understanding of basic relativity concepts. I hope someone points out the error to them so that the grant money can go to someone more deserving. The gravitational waves should be here and they have not detected them with their current setup. How long will it take before they realize that they are not going to detect gravitational waves doing it? After they retire?

If michelson and morley found that the speed of light varied in their experiments relativity the history of relativity would have went a lot differently. Although he didn't use the experiment to derive the theory, I think it would have been a big stumbling block in developing the theory.

"I award you no points and may god have mercy on your soul."