(John Baez) wrote in message ...

Given all this, I'm delighted to see some real progress on getting 4d
spacetime to emerge from nonperturbative quantum gravity:

3) Jan Ambjorn, Jerzy Jurkiewicz and Renate Loll, Emergence of a 4d world
from causal quantum gravity, available as hep-th/0404156.

This trio of researchers have revitalized an approach called "dynamical
triangulations" where we calculate path integrals in quantum gravity by
summing over different ways of building spacetime out of little 4-simplices.
They showed that if we restrict this sum to spacetimes with a well-behaved
concept of causality, we get good results. This is a bit startling,
because after decades of work, most researchers had despaired of getting
general relativity to emerge at large distances starting from the dynamical
triangulations approach. But, these people hadn't noticed a certain flaw
in the approach... a flaw which Loll and collaborators noticed and fixed!

This is pretty exciting. It is sort of obvious that you can formally
put gravity on a lattice, but I always thought that there wasn't a
continuum limit. If the numerical evidence in this paper is true, and
it seems quite strong, then we see a new field open up here, perhaps
like when Wilson invented lattice gauge theory in 1974. A lot of
interesting things can be done, e.g. to apply standard techniques in
lattice models, introduce gauge and fermion fields, and try to find
different continuum formulations. I would not be surprised if this is
the next bandwagon and a lot of smart people will jump onto it.