Why are we looking for graviton particles

[RobertsMrtn]

General relativity shows that gravity is a culture of space time.
Suppose that we are in a gravitational field. This can be caused by the presence of matter or being in an accelerating frame of reference. In either case, it is possible to select another accelerating frame of reference for which the gravitational field disappears.
This shows that gravity is not a phenomenon caused by a particle, so why are physicists looking for graviton particles? None have been found and it seems to me that none will be found for the simple reason that they do not exist. Particles do not come in and out of existence depending on the reference frame.

 

[Bill_K]

General relativity shows that gravity is a culture of space time.

Curvature?

Suppose that we are in a gravitational field. This can be caused by the presence of matter or being in an accelerating frame of reference. In either case, it is possible to select another accelerating frame of reference for which the gravitational field disappears.

You can make the local gravitational force equal to zero by going to a different reference frame, but that's not the gravitational field. The gravitational field causes tidal gravitational forces, it's represented by the Riemann tensor which can't be set to zero in this way. A change of coordinates can't make a gravitational wave disappear.

This shows that gravity is not a phenomenon caused by a particle, so why are physicists looking for graviton particles?

Actually they're not. The effects of quantum gravity are far and away too small for us to detect. Even the (semiclassical) Hawking radiation that we discuss so much is too weak to be observed.

Particles do not come in and out of existence depending on the reference frame.

Actually they do! See the Unruh effect.

 

[tom.stoer]

The reason why gravitons may be a useful concept is rather simple: in the weak gravity regime with gravitational waves propagating on a background metric the theory looks similar to a field theory which would result in a quantum theory of a spin-2 particle.

However the are mathematical reasons why this picture does not apply in general: the graviton as fundamental d.o.f. derived for the weak gravity regime fails to provide a consistent quantization of gravity in the non-perturbative regime where quantum effects will become relevant. Therefore all theories of quantum gravity are based on diffent quantization schemes (e.g. LQG) where gravitons are no longer fundamental d.o.f. but derived concepts applicable in a certain approximation, or they are based on a different classical setup (supergravity, strings) where there is hope that the above mentioned difficulties do not apply and where gravitons could play a fundamental role (in string theory the graviton is just one special oscillation of the string)

But in practice the regimes where quantum gravity becomes important are not accessable experimentally! All current searches for gravitational waves are entirely classical and do not deal with gravitons.

 

[RobertsMrtn]

Gravity can be considered a form of radiation which travels through space at the seed of light. If the sun suddenly disappeared, we would continue in orbit around it for approximately 8 minutes before we continued through space in a straight line. However, to call it radiation seems to me to be misleading because it is not electromagnetic radiation which is of course caused by photons which can be considered particles.
If I am in a spaceship with its engines firing accelerating at 1G, the gravitational field created would be identical to the field experienced standing on the Earth (ignoring the non-uniformity of the Earth's gravitational field).
So, if the Earth's gravitational field is caused by this mysterious graviton particle, the same particle must be created inside my spaceship accelerating at 1G. This seems to me to be absurd.

 

[fzero]

Gravity can be considered a form of radiation which travels through space at the seed of light. If the sun suddenly disappeared, we would continue in orbit around it for approximately 8 minutes before we continued through space in a straight line. However, to call it radiation seems to me to be misleading because it is not electromagnetic radiation which is of course caused by photons which can be considered particles.
If I am in a spaceship with its engines firing accelerating at 1G, the gravitational field created would be identical to the field experienced standing on the Earth (ignoring the non-uniformity of the Earth's gravitational field).
So, if the Earth's gravitational field is caused by this mysterious graviton particle, the same particle must be created inside my spaceship accelerating at 1G. This seems to me to be absurd.

Gravitational radiation is a real thing that has been indirectly observed in binary pulsars. See http://en.wikipedia.org/wiki/Hulse-Taylor_binary and links therein. There is very little question that an accelerating object (if it has at least a quadrupole moment) loses energy due to gravitational radiation. The graviton, which has not been observed, would be the quantum excitation from which gravitational radiation is built, in analogy with the EM field and the photon.

 

[Bill_K]

RobertsMrtn, You'll find the best approach when you don't know anything about a subject is to ask questions. This works much better than calling the things you don't understand "absurd."

Gravity can be considered a form of radiation which travels through space at the seed of light.

Radiation is one aspect of gravity. Other aspects include the attraction that holds the solar system together, and the dynamics that governs the cosmological expansion. These are not radiation in any sense.

If the sun suddenly disappeared, we would continue in orbit around it for approximately 8 minutes before we continued through space in a straight line.

We often hear this - what if the sun suddenly disappeared?? The nature of gravity makes this impossible. Not just highly unlikely, completely forbidden. Just as electric change can't appear or disappear, because it's conserved. It can at best only flow from one place to another. Mass (actually energy) is the source of the gravitational field, and it can be rearranged but cannot appear/disappear. It's conserved. The sun could, in principle, suddenly explode (e.g. change shape), and in that case indeed a gravitational wave would be produced that would travel outwards at velocity c, and we would feel it 8 minutes later at the same time the light from it reached us.

However, to call it radiation seems to me to be misleading because it is not electromagnetic radiation which is of course caused by photons which can be considered particles.

There's more than one kind of radiation. Puzzles me that you seem to accept electromagnetic radiation and photons but not gravitational radiation and gravitons. They are different, but closely analogous.

If I am in a spaceship with its engines firing accelerating at 1G, the gravitational field created would be identical to the field experienced standing on the earth

As we already said above, the equivalence we talk about between gravity and acceleration is NOT the same thing as the gravitational field that's described in terms of gravitons. Acceleration is unrelated to gravitons.

 

[cmcraes]

I used to ask myself this question periodically until i figured it out. The answer is in QFT. Because according to QFT Something needs to convey the information to the space time that there is mass and that the space needs to curve. In turn the spacetime changes the path of any mass that enters the space. Theoretically The thing that tells space to bend is the graviton. Personally I believe if the graviton did exist we would have found it by now, considering anything with mass and/or momentum should be radiating it at alltimes. But that's just my opinion :)

 

[mfb]

Personally I believe if the graviton did exist we would have found it by now, considering anything with mass and/or momentum should be radiating it at alltimes.

Anything accelerating should be radiating - just the existence is not enough, electrons do not emit electromagnetic radiation all the time even if they have an electric charge and a momentum. But how do you propose to measure the extremely tiny gravitational radiation from accelerated matter here on earth? The earth/sun system emits something like 200W of gravitational radiation. Think about the giant masses which are accelerated there! If you try to measure some rotating rod on earth, the power is probably somewhere in the fW-range.
This is not a matter of belief, by the way. You can calculate that current experiments cannot measure gravitational radiation from stuff on earth.

 

[cmcraes]

Yes i am well aware that belief has nothing to do with it. its more of a personal thing for me, considering I am still in high school, i want there to be something left for me to work on and discover once i pass university. (I know that's absurd because if/when we find the graviton it will only lead to more work needing to be done to unify all the Fundamental forces, but i just feel like were runnning out of gaps to fill haha)

 

[cmcraes]

Also: Side question; Would the discovery of gravitons and possible use of them in future labs help us to understand Dark matter?

 

[mfb]

Would the discovery of gravitons and possible use of them in future labs help us to understand Dark matter?

Maybe. A discovery of gravitons in current (or planned) experiments would require physics beyond the Standard Model, this usually comes together with other new particles.

 

[kurros]

Yes i am well aware that belief has nothing to do with it. its more of a personal thing for me, considering I am still in high school, i want there to be something left for me to work on and discover once i pass university. (I know that's absurd because if/when we find the graviton it will only lead to more work needing to be done to unify all the Fundamental forces, but i just feel like were runnning out of gaps to fill haha)

Yes, fear not, I expect there is a shocking amount left to discover in physics yet. Cooking up new experiments to do the discovering, on the other hand, is getting pretty tough.