A New Bound on Light-by-Light Scattering in the Low-Energy Regime

The superposition principle at the heart of optics states that electromagnetic fields do not interact in vacuum. This principle is not universal, instead electromagnetic fields couple weakly via quantum vacuum fluctuations. Striking consequences are light-by-light scattering and vacuum birefringence. Experimental results from colliding X-ray free-electron and optical high-intensity lasers combined with dark-field detection and polarimetry for background suppression are reported. This approach yields the most stringent bound for light-by-light scattering in the low-energy regime achieved to date -- improving upon previous experimental limits by ten orders of magnitude -- and paves the way for precision measurements.