marcus said:
As an interested observer, I can hardly claim complete knowledge of the QG field!
Personally I can't think of any QG theory that explains the interaction of matter with geometry.
I've often wondered about that. There might be some underlying description of the world in which matter and geometry are fundamentally the same thing. Different aspects of the same mathematical model. Then their interaction might be understandable simply as how that single unified math entity behaves.
You've probably heard about some of the stalled attempts.
Bilson-Thompson et al (matter as braids in spin nets) was one. I haven't seen much in the way of papers for several years now, in braid matter. I'll try to think of others.
Searcing this archive. I found the Bilson-Thompson et al paper. But it doesn't mention about LQG yet you seem to say LQG can encompass it. Why is the paper a stalled attempt? Please elaborate. Thanks.
https://www.physicsforums.com/showthread.php?t=225876
"http://arxiv.org/abs/0804.0037
Particle Identifications from Symmetries of Braided Ribbon Network Invariants
Sundance Bilson-Thompson, Jonathan Hackett, Lou Kauffman, Lee Smolin
9 pages, 7 figures
(Submitted on 1 Apr 2008)
"We develop the idea that the particles of the standard model may arise from excitations of quantum geometry. A previously proposed topological model of preons is developed so that it incorporates an unbounded number of generations. A condition is also found on quantum gravity dynamics necessary for the interactions of the standard model to emerge."
Then you commented in message #2 there:
"So in a sense LQG contains the Standard Model particles as crisscross complications in the network that describes the quantum state of geometry.
There has been some progress: braid-matter contains all three observed generations of particles. Indeed, the LQG particle model actually predicts an infinite series of generations (by increasingly complicated twisting and braiding.) Thus it would be favorable to LQG if the Large Hadron Collider were to detect evidence that the number of generations is not limited to three.
the basic idea is that (the quantum state of) space is like a WEB----a fine network of geometric relationships. And in the fabric of that web there can be little snaggies. And these little snaggly tanglets can propagate and interact with each other and make other snaggles. The way two knots can interweave with each other and make a third knot.
The whole web evolves by local MOVES which reconnect neighbor nodes in different ways. The passage of time is realized by the constant progress of these local moves going on everywhere in the network.
These local reconnection-moves take care of the motion and interaction of particles, as well as the evolution of geometry as required by General Relativity.
That's the idea. It is an ongoing program. You can read the paper and see how far they have come and what the next issues are that they want to investigate.
__________________"
Rovelli not working on it? Know any latest about it? Theory or model that would explain the coupling between matter and geometry is the real quantum gravity, the rest are imitations.
Fascinating question. Several people here may know of ideas along these lines. Mitchell might.
One approach would be to explore the possibility in 3D gravity (spatial 2D plus time) where things are simpler and try to get a toy model. Then the hope would be to jack that up to 4D. I think back in 2005 and 2006 there was some work by Laurent Freidel in 3D gravity where one got a glimpse of matter occurring as topological defects in geometry. If I get some time later today I will try to find a reference. Or somebody else may already know one. Have to do some other stuff now.
