#### marcus

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Now might be a good time to get some perspective on spin foam, if there are knowledgeable people around willing to help. Baez in some TWF mentioned a paper by Freidel/Louapre with "asymptotic 10j" in the title. It suggests a way to dispell the surprise over unexpected 10j numbers discovered by Baez/Christianson/Egan in mid-2002 IIRC. Rovelli is giving a symposium survey of spin foam in a week, 31 October yes I realize that is halloween, and he might talk about what significance this 10j business has.

But I stand no chance of understanding any of that without some basic perspective, so I will try to sketch out what could be basic perspective on spin foam and hope other people will correct or fill in parts I miss.

It seems that a spin foam is just a path getting you from one spin-net or spin-knot state to another. the original deeply confusing idea is by Feynmann: in a quantum picture trajectories dont exist and a system gets from A to B by following all possible paths---a spinfoam is just one of millions of possible paths for getting from spin-net quantum state of geometry A to spin-net quantum state of geometry B. As insane laughter rises, you AVERAGE all the possible paths with a whole lot of phasecancelation, you ADD UP all these millions of possible paths, and you get the amplitude of evolving from state A to state B. This actually seems rather nice.

I notice that there is a 2003 paper by Livine and Oriti called "Causality in spin foam models for quantum gravity" and I wonder if Rovelli will say anything related to it----there is something attractive about it: a Green function or a propagator of some kind that seems to be comprised of a going forwards piece and a going backwards piece, as if one of the problems that is always coming up is how do you select the right piece. I have a vague suspicion that the problems with spin foam and the problems with hamiltonian are neither of them *prohibitive* problems but are clues to a connection between the two. That is, the spin foam approach is in a fundamental way not all that different from a hamiltonian approach.

In some other thread I mentioned this strangely easy-to-read article "A simple background-independent hamiltonian quantum model" by Colosi and Rovelli. It is a simple toy model of a pendulum or something. I dont have the ability to judge if that article is in any way significant---it seems suggestive to me but I dont know enough to judge. there is a propagator in the toy model that gets you from one situation to another. Is this paper simply a "hamiltonian" toy model or is it a sort of hybrid toy model.

Does this paper, simple as it is, have any bearing on spin foams. Sorry about all the dumb questions. In case anyone wants to take a look the Colosi/Rovelli "simple background-independent quantum model" paper is

http://arxiv.org/gr-qc/0306059 [Broken]

I'll try to steer this back more to the main topic of spin foams proper if I post a follow-up

But I stand no chance of understanding any of that without some basic perspective, so I will try to sketch out what could be basic perspective on spin foam and hope other people will correct or fill in parts I miss.

It seems that a spin foam is just a path getting you from one spin-net or spin-knot state to another. the original deeply confusing idea is by Feynmann: in a quantum picture trajectories dont exist and a system gets from A to B by following all possible paths---a spinfoam is just one of millions of possible paths for getting from spin-net quantum state of geometry A to spin-net quantum state of geometry B. As insane laughter rises, you AVERAGE all the possible paths with a whole lot of phasecancelation, you ADD UP all these millions of possible paths, and you get the amplitude of evolving from state A to state B. This actually seems rather nice.

I notice that there is a 2003 paper by Livine and Oriti called "Causality in spin foam models for quantum gravity" and I wonder if Rovelli will say anything related to it----there is something attractive about it: a Green function or a propagator of some kind that seems to be comprised of a going forwards piece and a going backwards piece, as if one of the problems that is always coming up is how do you select the right piece. I have a vague suspicion that the problems with spin foam and the problems with hamiltonian are neither of them *prohibitive* problems but are clues to a connection between the two. That is, the spin foam approach is in a fundamental way not all that different from a hamiltonian approach.

In some other thread I mentioned this strangely easy-to-read article "A simple background-independent hamiltonian quantum model" by Colosi and Rovelli. It is a simple toy model of a pendulum or something. I dont have the ability to judge if that article is in any way significant---it seems suggestive to me but I dont know enough to judge. there is a propagator in the toy model that gets you from one situation to another. Is this paper simply a "hamiltonian" toy model or is it a sort of hybrid toy model.

Does this paper, simple as it is, have any bearing on spin foams. Sorry about all the dumb questions. In case anyone wants to take a look the Colosi/Rovelli "simple background-independent quantum model" paper is

http://arxiv.org/gr-qc/0306059 [Broken]

I'll try to steer this back more to the main topic of spin foams proper if I post a follow-up

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