Best Approach to Quantum Gravity

[rogerl]

Which of the following is the most promising road to quantum gravity?

1. quantising General Relativity
2. quantising a different classical theory, while still having
general relativity emerge as a low-energy (large-distance) limit.
3. having general relativity emerge as a low-energy limit
of a quantum theory that is not a quantization of a classical
theory
4. having both general relativity and quantum theory emerge
from a theory very different from both

By the way. Is the problem of Quantum Gravity the same as the question
of how quantum matter got coupled to Spacetime even on macroscopic
scale or are the 2 concepts distinct such that Quantum Gravity deals
only with the quantization of gravity?

 

[marcus]

Which of the following is the most promising road to quantum gravity?

3. having general relativity emerge as a low-energy limit
of a quantum theory that is not a quantization of a classical
theory

By the way. Is the problem of Quantum Gravity the same as the question
of how quantum matter got coupled to Spacetime even on macroscopic
scale or are the 2 concepts distinct such that Quantum Gravity deals
only with the quantization of gravity?

Your BTW is really interesting. I think that QG is just a step in the direction of understanding the coupling of matter and geometry. QG should simply be a theory able to cope with the very high density at the start of expansion, and reproduce classical at lower density, and be testable (by cosmological observation).
The testing arena is basically cosmology and maybe other astrophysics. It would be great if there were several testable QG.

Understanding the common root of matter and geometry would come later I guess.

About your question what is the BEST way to get a qg theory. Pragmatically whatever works. Here is an example of an approach that so far is working well that is going along the lines of your option 3

http://arxiv.org/abs/1102.3660

Basically he considers your problem---he describes the alternative ways you COULD derive the QG and which converge to a surprising extent but no one completely covers it. Then he chooses not to derive (by some type of "quantization" from a classical start) but to conjecture a theory (I guess some call it "top down") and then check to see what evidence that you get the right largescale limit.

And he lists evidence, partial checks that have been performed, there's quite a lot.

So that is an example of someone using your "option 3" and explaining why he chooses that way and not some other way like 1 or 2.

I think if you want to make up your mind about the "best" approach, you need to study examples of people trying these various ways you list. So there is one to look at if you want.

To my mind the best is "whatever works" and I hope people try various quite different approaches.

 

[Fra]

IMO, to try to "refine" the question is part of the problem. And here it's clear that different people also have different ambitions, as to how much baggage in various forms you can accept. So regardless of what technical progress there might be in any direction, one must ask oneself which questions we really want answered.

As I see it, analysing all these issues suggest to me at least that the problem of combining QM and GR, becomes inseparable from the unification program. I think any separation is artificial. This is why I think that some ideas, even if the "did work" would not answer the right questions anyway due to beeing insufficiently ambitious.

It's also debatable wether it's easier or harder, to solve an artifiially separated sub-problem from a bigger problem. Some people argue that the artificial separation of quantizing pure gravity from seeing the bigger picture where quantized matter is sourcing gravity is creating more problems than it solves.

So not answering which was is the EASIEST or most promising, but rather which is more ambitous I vote for #4.

IMHO: Any idea assuming quantum theory as it stands without explanation is missing a central question. Also any idea starting with a "classical theory" and then quantizing is also missing a central questions. Therefore these aren't interesting options for me personally. That's not to say some partial progress aren't possible, but it is still not answering all questions. Questions which I think are unavoidable.

The problem of quantum gravity is more than just curing diverging expectation calulations, though I think some people see it more as a technical problem, looking for a technical solution.

/Fredrik