Here we complete the "quantum field theory" of a vibrating string. (Note: My voice is lower and slower than normal - I was coming down with a cold.)
Errors:
At 0:42 I say "minus c-squared times q-k..." I should have said, "minus c-squared times quantity k pi over L squared, times q-k..."
At 6:00 the last two "kets" on the lower right should have n_k in place of n_k+1. The number operator does not change the photon occupation numbers.
