# Approaching the Schwinger Limit

Schwinger (1951) showed that at electric fields corresponding to E_{C} ~ mc^{2} where _{C} is the Compton wavelength, the vacuum will spontaneously create electron- positron pairs as the virtual pairs from vacuum polarisation can acquire enough energy from the field to move on to the mass shell and become real. In an oscillating field the Schwinger mechanism requires that **E.B **and E_{2}-B_{2} should both be non-zero and so it does not apply to a single plane wave. Strongly converging or colliding beam geometry is required and at the intensities available to HiPER the e^{+} e^{-} production rate should be many tens of pairs per pulse. Deviations from the expected rates would again be a likely indicator of new physics since the laser excitation of the vacuum is essentially adiabatic and the QED processes can be calculated very precisely.