Einstein theory on bending of space

[Wes Tausend]

Einstein's attempt at Unified Field Theory only covered electromagnetism; the strong and weak forces were not well understood at that time and he didn't include them.

Yes. The theory that unifies the strong, weak, and EM interactions is called the Standard Model of particle physics. It does not include gravity (see further comments below on why).

Not really; it affects the difficulty of incorporating gravity into a quantum field theory, but GR is a classical theory, and at the classical level there is no problem with gravity being spin-2; as I think I said before, it's that which makes the GR model of gravity as spacetime curvature possible.

Not really; Einstein sort of intended it to be one, but, as above, he didn't include the strong and weak interactions, and he also didn't take quantum effects into account--during the latter part of his career he became very dissatisfied with quantum theory, even though he had helped to create the field.

"Entirely reconciled" is too strong, and I didn't mean to give that impression. There are several issues:

(1) Gravity, as an interaction, is so weak compared to the others that we have no way of experimentally detecting any quantum aspects that it might have. So we have no way of experimentally confirming that gravitons exist or investigating their properties. We basically believe they exist because all the other interactions have quantum aspects. But there are a number of physicists (Freeman Dyson, who was instrumental in the development of quantum field theory, is one of them) who question whether gravity really has to have quantum aspects the way the other interactions do. Without experimental input there is really no way of resolving this question.

(2) Constructing a quantum field theory of a spin-2 particle has issues that constructing a quantum field theory of particles with spin-1 or lower (which all of the particles in the Standard Model are) does not. The main one is that the QFT of a spin-2 particle is not renormalizable. For a detailed discussion of this we would really need to start a separate thread in the Quantum Physics forum, but the key point is that the Standard Model is entirely renormalizable, and that is an important feature. So there isn't a simple, obvious way to add gravitons to the Standard Model; doing that would break an important feature of the theory.

(3) Even if we assume that finding a quantum theory that includes gravity is the right way to go, it's not entirely clear that doing it by constructing a QFT including a spin-2 graviton along the same lines as the Standard Model is the way to do it. There are a number of different candidates for a quantum gravity theory being investigated, and we don't know at this point how that will play out. (The lack of experimental input is a big issue here.)

When Hawking talks about a Theory of Everything, he's talking about getting all these issues resolved. He's gone back and forth over the years about how close he thinks we are to actually doing it. I personally think we still have quite a way to go.

I really do appreciate the thorough explanation you have presented, Peter. It is exceptionally clear and informative.

The link to the Standard Model is a pleasant surprise. I've seen casual reference to the term before, but took it as a generic phrase for any early science. I had no idea it was a modern specific theory. I am incredibly naive sometimes.

Wes
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[PeterDonis]

I really do appreciate the thorough explanation you have presented, Peter. It is exceptionally clear and informative.

Thanks! Glad it was helpful.