Sommerfeld Theory Colloquium, 2015 is the International Year of Light, and of its purposes is “to raise

awareness of optical technologies”. One such technology, high-power

lasers of the petawatt class and beyond, provides the most intense light sources created by humankind so far. The intensities and field

strengths in question are in excess of 1022 W/cm2 and 1014 V/m, respectively

magnitudes that correspond to concentrating the total solar

radiation on a pinhead. This talk will present an overview of the uses

and consequences of such extreme environments within the realm of

particle physics. The relevant theory is strong-field QED, with the

laser beams providing a rather peculiar electromagnetic background

field. The magnitudes above are such that a nonperturbative treatment

of the background becomes a necessity. Using appropriate theoretical

tools, a number of phenomena will be addressed, including

radiation reaction, nonlinear Thomson/Compton scattering, laserstimulated

pair production and photon-photon scattering. A particular

incarnation of the latter, polarisation flip forward scattering,

or vacuum birefringence, will be discussed in some detail, with an

outlook on a planned experiment at the European XFEL at DESY.