X-Rays May Pluck Pairs from Vacuum

Daniel S. Burgess

According to quantum electrodynamics, a vacuum is never empty but rather is seething with virtual pairs of electrons and positrons. A team from Universität Tübingen in Tübingen, Germany, and Argonne National Laboratory in Argonne, Ill., predicts that the x-ray free-electron lasers under development at Deutsches Elektronen Synchrotron in Hamburg, Germany, and at Stanford Linear Accelerator Center in Menlo Park, Calif., will offer the vacuum sufficient energy to generate real pairs of particles.

Using a formula developed by Julian Schwinger in 1951, the researchers calculate that an external electric field of 1.3 x 10¹⁸V/m would be necessary to generate a significant number of particle pairs. Based on a quantum kinetic equation, however, they estimate that the 1.3 x 10¹⁷-V/m fields produced by the 80-fs, 0.15-nm pulses of the free-electron lasers should produce up to approximately 1000 pairs -- if the radiation can be focused to a diffraction-limited spot. Such electrons and positrons would spring into existence in time with the frequency of the laser.

Sebastian M. Schmidt and Reinhard Alkofer, researchers on the project from the university, said that it should be possible to measure pair production at the facilities within a decade -- again, if suitable x-ray optics can be developed.

Such work would not only test the theory, but also enhance astrophysicists' understanding of neutron stars and black holes, which may feature intense electromagnetic fields.