In the mid-1980s Langlands turned his attention to physics, particularly the problems of percolation and conformal invariance. In 1995, Langlands started a collaboration with Bill Casselman at the University of British Columbia with the aim of posting nearly all of his writings—including publications, preprints, as well as selected correspondence—on the Internet. The correspondence includes a copy of the original letter to Weil that introduced the -group. In recent years he has turned his attention back to automorphic forms, working in particular on a theme he calls `beyond endoscopy'.