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marcus

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I notice that Loll calls what she does "nonperturbative quantum theory of gravity" and she calls CDT (which she and several others developed) a METHOD to do nonperturbative QG with.

And the organizers of this years conference Loop 05 say it is the annual conference of researchers in "nonperturbative/background independent" quantum gravity.

http://loops05.aei.mpg.de/

And at the conference they have people basically united by a common goal, all trying to discover or build the same theory, but using various methods:

CDT

spin foams

other kinds of path integrals

canonical LQG

causal sets

...

...

So I am beginning to try to see this as a single emerging theory where there is pretty good agreement about what it should do (classical limit is Gen Rel, quantum theory of spacetime geometry, quantum spacetime dynamics, maybe include topological variation).

Of course I am not an expert and I don't have this as clearly in mind as I would like, but I sense that these people understand each other's concerns and methods and aims, and they are engaged in trying various methods towards a common purpose. And often encountering analogous problems. Getting experience one place that will apply somewhere else. And I see often the same person will try several lines of research. So the Loop 05 conference is not a gathering of several separate competing ball teams, with sharp lines allegiance drawn. It is looking like a single non-partisan workforce all working by various schemes to bring to light the same theory.

Sideline observers (like myself) can probably at this point form some mental pictures of how the nonperturbative QG theory is going to turn out and they may not be too far wrong, if we are careful.

I get my mental imagery of QG largely from Loll's CDT, but I have a strong suspicion that WHATEVER the formalism and methods that might eventually be used and might replace causal dynamical triangulation methods, what comes out will likely be at least ANALOGOUS to what I can see now.

For example it will have to start out with no spacetime shape at all, and introduce a MICROSCOPIC DYNAMICAL PRINCIPLE that operates at small scale and (as its influence expands outwards) generates largescale spacetime geometry behavior matching Einstein Gen Rel. And this microscopic operating principle is going to have to generate a world that appears 4D at large scale (but may have different behavior down small scale) and thus it must EXPLAIN why we have macroscopic 4D spacetime.

And the theory will have to be able to generate the myriad quantum possibilities of geometry----all differerent shapes of the 4D continuum---and provide a place for matter particles to run around in or for matter fields to ripple around in----and provide some mechanism for the matter and the spacetime geometry to interact.

And it will have to get rid of the Big Bang singularity, and see what is beyond the classical theory breakdown. And the Black Hole singularity.

Because the examples of CDT and LQC show us that is the kind of thing that QG theories do: they smooth over classical singularities. And, following the example of LQC it should provide a natural generic mechanism for inflation---so we don't need a gimmick.

So what we are seeing now, and what people will talk about at Loop 05, in October, is teaching us WHAT TO EXPECT of a nonperturbative QG theory, and what things to look for as one gradually rises out of the swamps of our curiosity and confusion.

And we can probably use a thread, like this, to be about the dim emerging shape of nonperturbative QG, from the glimpses we can get in its various current formats.

And the organizers of this years conference Loop 05 say it is the annual conference of researchers in "nonperturbative/background independent" quantum gravity.

http://loops05.aei.mpg.de/

And at the conference they have people basically united by a common goal, all trying to discover or build the same theory, but using various methods:

CDT

spin foams

other kinds of path integrals

canonical LQG

causal sets

...

...

So I am beginning to try to see this as a single emerging theory where there is pretty good agreement about what it should do (classical limit is Gen Rel, quantum theory of spacetime geometry, quantum spacetime dynamics, maybe include topological variation).

Of course I am not an expert and I don't have this as clearly in mind as I would like, but I sense that these people understand each other's concerns and methods and aims, and they are engaged in trying various methods towards a common purpose. And often encountering analogous problems. Getting experience one place that will apply somewhere else. And I see often the same person will try several lines of research. So the Loop 05 conference is not a gathering of several separate competing ball teams, with sharp lines allegiance drawn. It is looking like a single non-partisan workforce all working by various schemes to bring to light the same theory.

Sideline observers (like myself) can probably at this point form some mental pictures of how the nonperturbative QG theory is going to turn out and they may not be too far wrong, if we are careful.

I get my mental imagery of QG largely from Loll's CDT, but I have a strong suspicion that WHATEVER the formalism and methods that might eventually be used and might replace causal dynamical triangulation methods, what comes out will likely be at least ANALOGOUS to what I can see now.

For example it will have to start out with no spacetime shape at all, and introduce a MICROSCOPIC DYNAMICAL PRINCIPLE that operates at small scale and (as its influence expands outwards) generates largescale spacetime geometry behavior matching Einstein Gen Rel. And this microscopic operating principle is going to have to generate a world that appears 4D at large scale (but may have different behavior down small scale) and thus it must EXPLAIN why we have macroscopic 4D spacetime.

And the theory will have to be able to generate the myriad quantum possibilities of geometry----all differerent shapes of the 4D continuum---and provide a place for matter particles to run around in or for matter fields to ripple around in----and provide some mechanism for the matter and the spacetime geometry to interact.

And it will have to get rid of the Big Bang singularity, and see what is beyond the classical theory breakdown. And the Black Hole singularity.

Because the examples of CDT and LQC show us that is the kind of thing that QG theories do: they smooth over classical singularities. And, following the example of LQC it should provide a natural generic mechanism for inflation---so we don't need a gimmick.

So what we are seeing now, and what people will talk about at Loop 05, in October, is teaching us WHAT TO EXPECT of a nonperturbative QG theory, and what things to look for as one gradually rises out of the swamps of our curiosity and confusion.

And we can probably use a thread, like this, to be about the dim emerging shape of nonperturbative QG, from the glimpses we can get in its various current formats.

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