A little applet that does 6j and 3j

[marcus]

I just happened onto an online calculator that does 6j symbols

http://www.svengato.com/sixj.html

If you try it, you may find trouble reading the square root
sign. It looked like a vertical bar to my browser.
But you can tell it is the sq.rt. because of what the numbers are.

I like the commonality of things that are online
You don't have to have Mathematica on your computer
for this, anyone in the world can calculate a 6j symbol.

Baez and friends have been calculating spinfoam amplitudes
getting from one geometry to another just as
one calculates feynman diagram amplitudes of some
process. The calculations are reminiscent and depend on
some computer whiz discovering an efficient algorithm
to calculate "10j" symbols. Aiiieeee!

A recent paper based on calculating foams using 10j symbols
is
Baez et al "Spin foam models of Riemannian Quantum Gravity"
arxiv:gr-qc/0202017

have to go

 

[marcus]

Daniele Oriti's paper

http://www.damtp.cam.ac.uk/user/do001/ has written an informative introduction to spin foams

http://lanl.arxiv.org/gr-qc/0106091

6j symbols come in at page 20, in case you were wondering

Oriti is a grad student at Cambridge. I think he writes well. Here's the title and abstract:

Spacetime geometry from algebra: spin foam models for non-perturbative quantum gravity
Daniele Oriti
Department of Applied Mathematics and Theoretical Physics,
Centre for Mathematical Sciences, University of Cambridge,
October 15, 2001

Abstract
This is an introduction to spin foam models for non-perturbative quantum gravity, an approach that lies at the point of convergence of many different research areas, including loop quantum gravity, topological quantum field theories, path integral quantum gravity, lattice gauge theory, matrix models, category theory, statistical mechanics.

We describe the general formalism and ideas of spin foam models, the picture of quantum geometry emerging from them, and give a review of the results obtained so far, in both the Euclidean and Lorentzian case. We focus in particular on the Barrett-Crane model for 4-dimensional quantum gravity.