Speaker: Wendell T. Hill, III of University of Maryland
Program Description:
Multi-petawatt laser pulses of short duration have placed us at the threshold of a new era where novel experimental investigations of nonlinear aspects of electrodynamics – quantum electrodynamics (QED) – will be possible. Never before feasible tests of QED from the photon side, and the intimate coupling between QED and the quantum vacuum are on the horizon. The very essence of the vacuum is entangled with a fundamental tenet of quantum physics – quantum fluctuation – virtual particles and antiparticles (e.g., electron-positron pairs) fluctuating into and out of existence. Quantitative measurements of virtual particles not only will challenge calculations from the 1930s, they will set strenuous limits for add-ons to the Standard Model of Particle Physics. The Bosonic nature of photons allows arbitrarily large numbers of photons to pile up in one location. Classical electrodynamics would naively suggest than nothing unusual will happen when focusing laser pulses to higher and higher intensity in the vacuum until the so-called Schwinger limit, Icr ≃ 2×1029 W/cm2 is reached, where real matter is coaxed, albeit not so gently, out of negative energy states. Long before Icr, however, QED warns us that the linear response of light propagating in vacuum, as Maxwell equations demand, gives way to a nonlinear behavior – light-light interaction. Post-Maxwellian theories, such as QED and Born-Infeld, allow virtual pairs to mediate this interaction between photons that can be viewed, to some extent, as light propagating through material. Before real pairs emerge, however, QED predicts virtual pairs will cause the vacuum to become birefringent. This effect is expected to become unambiguously discernible in photon-only experiments between 1023 and 1025 W/cm2, arguably just around the corner. In this talk we will discuss one possible way to measure the birefringence via photon-photon scattering, what new physics can be learned in such a measurement, and the tools and conditions required to realize such a measurement. In addition, I will present our recent results toward addressing one of the biggest challenges in this and other strong-field experiments well into the relativistic regime, a direct way to measure the intensity at full power.
