The quantum vacuum is not empty: in fact it is inherently unstable, and the application of an external electric field can lead to the production of electron-positron pairs. This "Heisenberg-Schwinger effect" was one of the first non-trivial predictions of quantum electrodynamics (QED), but the effect is so weak that it has not yet been directly observed. However, new developments in ultra-high intensity lasers come tantalizingly close to opening a new window on this unexplored extreme ultra-relativistic regime. This has prompted a fresh look at both experimental and theoretical aspects of this and other nonlinear QED effects. I review the basic physics of the problem and describe some recent theoretical ideas aimed at making the elusive Heisenberg-Schwinger effect observable, by careful shaping of laser pulses. This is an example of an emerging new field using ultra-intense lasers to probe fundamental problems in particle physics, gravity and quantum field theory.

Coffee: 3:30 pm, 245 Compton