Is the Graviton Fundamental in Quantum Gravity Theories?

[Coin]

Hi jimgraber, that's an extremely interesting overview. To get offtopic for a moment though I'm very curious about this option:

1. Do not quantize gravity. Keep it continuous.

What would be an example of this, besides the "QFT in curved spacetime" people you cite? I am not aware of any work that reconciles gravity with quantum theory without quantizing gravity (and if I understand your quote it sounds like the curved QFT people only have half the equation so to speak, knowing how to perform QFT on surfaces where gravity exists but not knowing how quantum "things" can impact the gravitational field).

 

[Haelfix]

I think Jim means that one proposal is that you have general relativity and field theory, and the two don't mix at all, and they are taken as separate formalisms. Eg three forces are described by field theory, and gravity is done via general relativity and stays classical through all energy scales. Some people still believe this (a minority).

Its important to note that field theory in curved space vs classical general relavitiy have different physical predictions. If you do field theory in curved space, gravitational interactions will receive quantum corrections not unlike how say a photon receives radiative corrections and leads to the Lamb shift. We cannot measure such things yet in the lab or from the cosmos, so no one knows what the answer is, but for instance things like the black hole area-entropy law makes sense if you work with field theory, but doesn't at all with just classical GR.

Now, there is yet another point of view that says.. Ok, well we will take field theory in curved space as fundamental. Who cares if its nonrenormalizable and ceases to be predictive around the Planck scale, that's just a technical problem due to human incompetence. Boom, done, QG is solved! Some people also believe in this (also a minority)

String theory basically takes the last point of view, except that it UV completes the field theory so that it becomes predictive. It 'completes' and finishes the last few orders of magnitude not well described by qft in curved space and limits it to within a larger theory.

 

[Coin]

Haelfix, thanks, that's really interesting. By the way, did you notice that was your 666th post?

Kind of a tangential question:

Now, there is yet another point of view that says.. Ok, well we will take field theory in curved space as fundamental. Who cares if its nonrenormalizable and ceases to be predictive around the Planck scale, that's just a technical problem due to human incompetence. Boom, done, QG is solved!

So if we do this... exactly what kind of predictions are we "losing"? That is to say, is there anything happening around the Planck scale which, if we lost predictivity around the Planck scale, it would impact our ability to solve non-planck-scale problems? I am suddenly, to my slight embarrassment, realizing I cannot think of any real-world problems that we need quantum gravity to solve that do not involve black holes. Are there any good examples of what kind of non-black-hole questions we are limiting ourselves from by taking this "whatever" approach to curved qft?

 

[Haelfix]

Its an open question. To my knowledge the only glimmer of a chance is big bang physics. For instance inflation.

Inflation is an incomplete physical model. What you do is sort of guess the form of some quantum field (something that would naively be involved with QG), and then evolve it (by that point the universe can be described classically via GR). Now we really are interested in the exact nature and shape of that guess, and it would be nice if we could derive it from first principles (no one has yet).

The problem here is that by the very nature of inflation and efolding, it washes away most of the information contained in that initial guess. For instance if you take the slow roll approximation, and inflate the universe through ~60 efolds its really the only last 2 or 3 efolds that have any observational consequence, everything else gets swamped.

So yea, even that might be lost to us.

 

[friend]

Its an open question. To my knowledge the only glimmer of a chance is big bang physics. For instance inflation.

Inflation is an incomplete physical model. What you do is sort of guess the form of some quantum field (something that would naively be involved with QG)...

So if we at present don't have any experimental evidence to support it, why the interest in quantizing gravity? I mean do we suppose GR should be quantized simply because it is just another field? Or are there more substantial reasons that necessitate quantizing gravity? Thanks

 

[Haelfix]

Well, there are plenty of purely theoretical and aesthetic reasons. But for instance the success of inflation begs the question already at an observational level.

It requires some sort of quantum fluctuation in the early universe to plant the seeds necessary for the dynamics to take over. Now these fluctuations occur close enough to the Planck scale that they should in principle have some elements of QG dynamics in them, and its not clear how you would do away with them.

Without inflation you run into the usual problems faced by cosmology in the 80s like
1) Monopole abundance
2) flatness problems
3) Horizon problems

And people view these as serious enough to accept the paradigm (or some slight modification thereof).