If gravity is the shape of spacetime, why is there the need for a graviton messenger particle?

If gravity is the shape of spacetime, why is there the need for a graviton messenger particle?
As the other 3 fundamental forces (electromagnetism, weak nuclear force and strong nuclear force) are mediated by forcecarrier particles (bosons) i.e. electromagnetism is mediated by photons, weak nuclear force by W and Z bosons and strong nuclear force by gluons. So, one can only assume that gravity could be mediated by a forcecarrying boson such as a hypothetical graviton particle. However, no evidence has been seen of such a boson as yet and neither has evidence been found of gravitational waves which would be the waveparticle duality partners of gravitons, I assume.
General Relativity is a geometric deterministic theory. It does not predict gravitons.
What a lot of theoretical physicists are working towards, is a unified theory encompassing all known forces and including gravity. The other three forces have been given a quantum field description with QED and QCD but gravity refuses to cooperate and be descibed by a nice quantum field theory.
Force at adistance has always troubled physicists and GR, being strictly geometric gets around this. Similarily, quantum field theories use force carrier virtual bosons to mediate the forces they carry and so again get around the idea of force at a distance. It is for this reason that a quantum gravity field theory must also have a boson ( graviton ) to carry it.
Quantum gravity theories are many ( string, M, supergravity, etc ) but none have made any sensible predictions yet. They can only be checked at Planck scale energies which would need a collider about 10 lightyrs in diameter.
Are Quantum Mechanical forces such as the other three fundemental forces of interaction observed with 3 dimensions? I wonder if a thought experiement has been devised to see how Quantum Mechanics holds up within black holes. Are gluons still strong enough to keep mediating Quarks to keep atoms together? Or are they completley stripped down? Could this be a dimensional effect as opposed to an observable one I wonder as I don't believe that gravity can be quantised and althought this seems hard to believe, I believe Quantum physics is a dead end in science. I would love to know what Einstein as working on before he died. I think he knew that Quantum physics wouldn't do it. Thoughts?
Hi guys, thanks very much for your feedback.
I don't pretend to understand string theory beyond a very superficial degree, but I understand it predicts gravitons.
Gravity definitely has an influence, and I'm happy to believe there might be a boson to act as a messenger particle for it, but maybe it isn't like the other forces because it is only forcelike, a geometrical feature of spacetime and not a particle with charges and spins.
Anyway, thanks again for the feedback.
As I thought, thanks. If I am not mistaken, efforts are under way to try and detect the gravity waves from the big bang, enabling us to look further back than the CMBR. That would be really exciting!
What would it mean if gravitons are never detected? Will it destroy Mtheory and/or the validity of the field equations?
Einstein@Home is currently running gravity wave detection experiments:
2Physics: Deepest AllSky Surveys for Continuous Gravitational Waves
There have been numerous attempts at a quantum field theory of gravity, which would naturally predict gravitons, however, none of these theories are renormalizable. They seem perfectly fine until you try to make a calculation, then end up with infinities in your answer which cannot be cancelled out like in QED and QCD. This is because elementary particles are considered to be dimensionless points. String and Mtheory instead consider elementary particles to be loops or strings of vibrating space/time of Planck length dimension.
Even if a viable predictive theory is ever put foreward, at the low energies ( compared to Planck energy ) that we can reach in our colliders, gravitons may never be detected because the interaction strength is 10^39 times that of EM interaction.
I've always wondered about how a Black Hole generates gravity.
If gravity from body A requires a graviton to affect motion of body B, then a Black Hole gravitationally effects other bodies by gravitons. Except, nothing can escape the event horizon of a Black Hole. Therefore, all the gravitons generated by the gravity of the Black Hole are trapped in the Black Hole.
If gravitons do not exist and gravity is a geometrical force without particle the problem resolves itself nicely.
Perhaps this is why no one has  yet  formulated a combined theory and equations bringing gravity into the fold of electromagnetic, strong and weak nuclear forces?
When we talk about gravitons being the force carrying particle for gravity, we are really talking about virtual gravitons. Gravitons would play the same role for gravity and photns do fro electromagnetism. Photon are quanta of electromagnetic radiation and the electromagnetic force is mediated by virtual photons. Gravitons would be the quanta of gravitational radiation (gravity waves) and the gravitational force would be mediated by virtual gravitons. Virtual particles are not bound by the same rules as "Normal" particles. Photons and gravitons cannot escape the event horizon, but virtual photons and virtual gravitons can make their respective forces be felt beyond the event horizon.
Wow, let's makes sure we are real here.
Let us introduce confessions.
If gravitons exists, then their force carrying abilities exists over vast distances.
However, that implies they are political in nature. That means, they compromise with other gravitons that are in the region like the .moon or something such that the overall force carrying particle makes the local gravity viable.
Can you explain under the standard model the mathematical equations for this "political" compromise under the logic of the graviton?
JanusSo, what is the difference between a 'virtual' graviton and a 'real' graviton? I have a feeling it's like the difference between a 'virtual' kiss and a 'real' kiss.When we talk about gravitons being the force carrying particle for gravity, we are really talking about virtual gravitons.
Okay, a virtual particle exists only for a brief period of time and in a limited region of space. I find that odd in the sense of a virtual graviton enticing another bit of mass past the event horizon of a Black Hole. It's simpler in my mind for gravity to be a geographical effect of the shape of space. (Here's to William of Ockham.)
We (human science) can identify a photon, for instance. There are various experiments to 'capture'  in reality point and look at  a neutrino; with some success, I understand There have been experiments and observations to 'capture' a graviton and no one has yet done so. If one is discussing a 'virtual' graviton, does one bother trying to 'capture' one?
One way of searching for a black hole, is to look for matter that is orbiting, what seems to be an unseen compact massive object, a number of such systems have been observed. Something must then escape a black hole, whether it is virtual or not remains to be seen. I am going to go and sit in a darkened room.
There is no difference between virtual force carrying particles and real force carrying particles. They act and interact exactly the same. The only difference is that the virtual force carrying particles must annihilate after a period of time determined by Heisenberg's Uncertainty Principle. In effect, they pop in and out of existence due to the energy of the field between interacting particles.
You're assuming the gravitons need to emanate from the possible singularity central to the black hole, and so cannot possibly escape the gravitational well. However, the event horizon preserves three qualities of everything collapsed or swallowed by the black hole, mass, charge and angular momentum. We may then assume that the gravitons that mediate the interaction are virtually formed at the event horizon, not inside it.
If this was not the case how would charge, which is mediated by photons, make itself felt from a black hole ???
OK, I have been reading " The Universe In A Nutshell " by S Hawking. He says, that virtual particles appear and annihilate one another close to the event horizon of a black hole. One of the pair falls into the black hole while its twin is free to escape to infinity. To someone far from the black hole the escaping particle appears to have been radiated from the black hole. I guess this is exactly what you are saying ( fonts messed up ) every time that I think that I understand what a black hole is, I find out that my understanding is limited, just got to keep reading.
You're thinking of Hawking radiation. The means by which small black holes evaporate. This radiation also gives an indication of the BH's temperature and entropy.
What I said was that the preserved properties of a black hole, mass, charge and angular momentum, are preserved in the event horizon, not in the possible singularity. The gravitational and electromagnetic field would then emanate from the horizon and so would the force mediating bosons associated with a quantum description of these fields.
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