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The electroweak and strong forces seem to be completely different types of forces to gravity. The latter is geometric while the former are not (as far as I'm aware!). So why should they all be described in the same way? - Why are gravitons needed?

Alternatively, I suppose my question is why a theory of everything has to be some form of quantum gravity. Is there a reason why a theory of everything requires gravity to be quantised? Is it possible for the quantum world and gravity to be described by some overarching theory which does not require gravity to be quantised? Or is there some reason why this is not the case?

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Here is a related question: physics.stackexchange.com/q/6980 –  MBN May 19 '11 at 15:15
    
@MBN Thanks, I hadn't seen that one! I've edited my question slightly because of this. –  Calvin May 19 '11 at 15:23
    
holographic principle suggests that those forces are not so disconnected from gravity, they just seem to operate in different domains, one being the boundary of the other –  lurscher Dec 13 '12 at 5:09
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Dear Calvin, if any portion of the world is described by probabilistic wave functions, then the whole world has to be. It's easy to show it. Take a decaying nucleus, connect it to a hammer that kills a cat a that also makes the Sun explode into 2 pieces.

The nucleus is evolving into a linear superposition of "decayed" and "not yet decayed" states. Correspondingly, because of the mechanism, the Sun has to evolve into a linear superposition of "exploded" and "not yet exploded". These two states have different gravitational fields. It proves that in general, the evolution produces linear superpositions of states with different gravitational fields, so the gravitational fields - and any other physical properties of the world - have to be described by linear operators just like any other physical property.

If some building blocks can only be predicted probabilistically, it's clear that everything else that may be affected by these building blocks can only be predicted probabilistically, too. The whole world, including gravitational fields, interacts with itself, so clearly the gravitational waves have to be ultimately described by quantum physics, too.

Gravitons are physical particles that are quanta of gravitational waves. They have to exist because

  • gravitational waves exist
  • the energy stored in a frequency $f$ classical state is always a multiple of $E=hf$

The first point was indirectly, but pretty much conclusively, proved by the observation of the binary pulsar that changes its frequency in the right way, as it emits the gravitational waves and loses energy. Everything agrees with general relativity beautifully. The 1993 physics Nobel prize was given for this confirmation of the gravitational waves.

The second point is a trivial consequence of Schrödinger's equation. Take a classical gravitational wave of frequency $f$ - e.g. similar to what is emitted by the binary pulsar. Divide the corresponding quantum state to energy eigenstate components. They go like $$c_n\exp(E_n t/i\hbar)$$ If all classical observables evaluated in this state are periodic with periodicity $1/f$, it's trivial to see that all the energy differences $E_m-E_n$ must be multiples of $E_0=hf$, so the energy can only be added to the gravitational waves by quanta, the gravitons.

One may also derive the gravitons and their polarizations from the linear approximation of the quantized general relativity. While general relativity has problems at higher-loop level - strongly quantum effects that affect spacetime - this approximation has to work very well, is consistent, and implies the existence of gravitons, spin 2 massless particles, too.

In quantum gravity, virtual gravitons are the messengers of the gravitational force in the same way as photons are messengers of the electromagnetic force. Gravitons and photons have a different spin but otherwise the analogy between them is - and must be - much tighter than the wording of your question is willing to admit. They're described as quantum fields and the effective quantum field theory with Feynman diagrams etc. has to work, with the same interpretation, at least in some approximation.

Also, it's not true that the non-gravitational forces can't be geometric. The Kaluza-Klein theory explains the electromagnetic field as "twists" that include an extra dimension of space. Whether or not the extra dimensions of space have this simple form, string theory generalizes the Kaluza-Klein lesson and every field and particle species may be viewed as a component of a generalized geometry field - because the geometry is generalized in such a way that it includes all other fields as well. Some of the string scenarios are very close to the Kaluza-Klein theory, some of them are far from it, but all of them confirm that gravity ultimately comes from the same underlying physics as everything else.

The term "string theory" may sound too narrow-minded in this context. There are very many different ways how to describe its physics, including gauge theory (just an old quantum field theory!) that lives on the holographic boundary of the AdS space. All of them agree that gravitons have to exist and they're just one particle species among many that share a common origin. So the term "string theory" in all these discussions really means "everything we have ever learned about quantum gravity that has worked".

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Thanks for your answer, but just because matter is quantum, why would space-time (and the effect of matter on space-time) have to be quantum? –  Calvin May 19 '11 at 15:27
    
@Calvin, i think that was exactly what Lubos explained in paragraph 2 –  lurscher May 19 '11 at 16:27
    
Good point @lursher - thanks. –  Calvin May 19 '11 at 16:52
    
I live roughly 45 minutes away from the LIGO facility in Hanford. They're currently in the process of increasing their sensitivity, and after they've calibrated everything they are predicting a tenfold increase, which corresponds to a thousandfold increase in event detection. Exciting! –  voithos May 19 '11 at 17:31
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Lubos, the first argument is not very convincing. The theory, according to which the system will evolve to a superposition of the two states, does not take into account gravity. May be when gravity (classical not quantized) is considered none of that will happen. –  MBN May 19 '11 at 18:26
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