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The graviton and general relativity


Eldad Eshel
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I have some questions regarding the graviton. Let's say it was discovered, would this be the end of general relativity's idea of gravity ? Or do they somehow work together ? With the discovery of the graviton, what exactly will happen with general relativity ? In graviton theory, how does sending particles back and forth cause objects to be attracted to one another and not the opposite, as in pushing each other away ? Is it something like quantum electro dynamics, where 2 charges can attract each other ? Even there in a well confirmed theory (or is it ?), still they don't have a solid explanation of the attraction. Is this just the weirdness of the universe ? Or is something not right even with QED ?

Edited by Eldad Eshel
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I have some questions regarding the graviton. Let's say it was discovered, would this be the end of general relativity's idea of gravity ? Or do they somehow work together ? With the discovery of the graviton, what exactly will happen with general relativity ?

 

My understanding is that they would be two different but equivalent descriptions of the same thing. In the same way you view light as photons or as classical electromagnetic waves.

 

The challenge is producing a theory that includes both general relativity and gravitons.

 

In graviton theory, how does sending particles back and forth cause objects to be attracted to one another and not the opposite, as in pushing each other away ? Is it something like quantum electro dynamics, where 2 charges can attract each other ?

 

Just the same: gravity would be described as the exchange of virtual gravitons, in the same way that electric and magnetic forces are described in terms of the exchange of virtual photons.

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It is unlikely that the graviton will be "discovered" in the terms of being detected. The graviton would be a quantum of a gravity wave. Gravity waves themselves are so weak and hard to measure, that we would be hard pressed to measure a single quantum of one.

Gravitons would be part of a quantum gravity theory and within it would fulfill the same role that photons do within QED. Gravity would be mediated by virtual gravitons.

As far as GR is concerned, it would still be viable in the domain it is presently being used and any quantum gravity theory would have to make the same predictions it does in that domain at least to an equal degree as we have tested GR (just like SR and Newtonian physics are near indistinguishable at low relative speeds) . What a QGT theory would do is extend the applicable domain into regions where GR presently is inapplicable.

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It is unlikely that the graviton will be "discovered" in the terms of being detected. The graviton would be a quantum of a gravity wave. Gravity waves themselves are so weak and hard to measure, that we would be hard pressed to measure a single quantum of one.

Gravitons would be part of a quantum gravity theory and within it would fulfill the same role that photons do within QED. Gravity would be mediated by virtual gravitons.

As far as GR is concerned, it would still be viable in the domain it is presently being used and any quantum gravity theory would have to make the same predictions it does in that domain at least to an equal degree as we have tested GR (just like SR and Newtonian physics are near indistinguishable at low relative speeds) . What a QGT theory would do is extend the applicable domain into regions where GR presently is inapplicable.

The graviton would be not just the quanta of a gravity wave but of all gravitation. It is actually harder to imagine in the head since it quite different than in the case of light. It is pretty simple to imagine a quanta of something moving like an electromagnetic wave, but in the case of gravity and general relativity there is nothing moving, it is sort of like a side of a slope of a hill, it is a bend, a distortion of space. So it is quite hard to imagine a quanta of this. This is also why these 2 theories, general relativity and graviton theory seem so different to me. But maybe they can co-exist as it is not as simple as a slope of a hill. In the case of the slope there is a force pushing down, In the bending of space, there is no additional force, it seems to be the bending itself that causes the force. Anyway these 2 theories are alot more detached than in the case of light and the photon.

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You can have a static electric field, which does not require any (real) photons. Similarly a static gravitational field would not require (real) gravitons. A changing electromagnetic field is quantised as the photon. A changing gravitational field would (hypothetically) by quantised as the graviton.

 

If there is a static electric field causing a force on a charged particle then that will be mediated by (modelled as) the exchange of virtual photons. If there is a static gravitational field causing a force on a massive particle then that would be mediated by (modelled as) the exchange of virtual gravitons.

Edited by Strange
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Let's say it was discovered, would this be the end of general relativity's idea of gravity ?

It would not be the end of general relativity.

 

 

Or do they somehow work together ?

They work together in exactly the same was as quantum electrodynamics and classical electrodynamics work together.

 

In fact, if you want to discuss gravitons then we should do this in the context of quantum general relativity. Although the theory is not renormalisable, ie. it is fundamentally sick, we can still work with it as an effective theory. One can calculate graviton scattering amplitudes to 2 loops (if I recall right).

 

With the discovery of the graviton, what exactly will happen with general relativity ?

As general relativity can only be treated as a effective theory, it is possible that the discovery of gravitons would show us the way to a more complete theory. This is all very speculative so I am not sure if much more can be said.

 

You should also be aware of the notion of asymptotic safety. Basically, the idea is that quantum general relativity, or something close to it, is perfectly well defined, just not using perturbation theory. This means that one could have a fully working quantum version of general relativity, but without gravitons!

 

 

This is also why these 2 theories, general relativity and graviton theory seem so different to me.

The problem with your thinking is that you are thinking about a 'theory of gravitons'. But what is this?

 

You need to discuss gravitons in the context of a theory, and the best we have so far quantum general relativity (as an effective theory). You should keep in mind that nobody thinks of quantum electrodynamics quite independently of Maxwell's equations

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