Four-Dimensional Twisted Group Lattices

Four-dimensional twisted group lattices are used as models for space-time structure. Compared to other attempts at space-time deformation, they have two main advantages: They have a physical interpretation and there is no difficulty in putting field theories on these structures. We present and discuss ordinary and gauge theories on twisted group lattices. We solve the free field theory case by finding all the irreducible representations. The non-abelian gauge theory on the two-dimensional twisted group lattice is also solved. On twisted group lattices, continuous space-time translational and rotational symmetries are replaced by discrete counterparts. We discuss these symmetries in detail. Four-dimensional twisted group lattices can also be used as models for non-trivial discrete compactifactions of certain ten-dimensional spaces.Comment: 31 pages (no figures) in Late

On the Speed of Gravity and the v/cv/c Corrections to the Shapiro Time Delay

Using a relatively simple method, I compute the v/c correction to the gravitational time delay for light passing by a massive object moving with speed v. It turns out that the v/c effects are too small to have been measured in the recent experiment involving Jupiter and quasar J0842+1845 that was used to measure the speed of gravity.Comment: 8 pages, LaTeX (or Latex, etc), one figure, which is also available at http://www-theory.lbl.gov/~samuel/sog_figure.pdf; Revised version is the one to appear in Phys. Rev. Lett