- #1

TensorAndTensor

What I do find fascinating with these lectures is a chapter in which he starts with the idea of a spin 2 boson - the graviton - and ends up deriving from that a gravitational potential, gauge transformations analogous to those applied in QED, and comes up with the equations relevant to gravity in the weak-field approximation in which the gravitational effects are applied to a flat-space background.

To my knowledge, this language is applicable to the study of gravitational waves as we know them here in 2017, and with the LIGO experiments of the last two years, we have gotten confirmation of the existence of gravitational waves consistent with the weak-field approximation, which Feynman had shown can be derived from the assumption that a spin 2 graviton exists.

So, some questions:

1) Does all of this reasonably settle the question of the existence of gravitons? (independent of the fact that gravity's extreme weakness relevant to other forces makes it unlikely to actually detect a graviton). In other words, here in our relatively low energy world (compared to the big bang or black hole event horizons), the concept of a graviton seems to be a valid one , despite that fact that it is said (at some higher energies), that " gravity is not really a force mediated by a boson in the way the electromagnetic force is. Instead, it's a warping of spacetime masquerading as a force such as electromagnetism".

2) What of non-renormalizability? It is said that this is a sign of a higher theory of undetermined from lurking behind the curtain of high energies.

3) extending 2). What of renormalizability? Does it imply that a theory such as QED is valid, as is, at very high energies? Or do we likely have something still unknown behind that high energy curtain? If the latter, then it would seem that QED is a low-energy approximation to something more involved. If so, are photons really just cousins of the graviton, force-mediating particles that emerge in our low energy world?

--Tensor

[mentors' note: This post has been lightly edited as part of moving it into a thread of its own]