  Elastic scattering of two real photons is the most elusive of the
fundamentally new processes predicted by quantum electrodynamics. This explains
why, although first predicted more than eighty years ago, it has so far
remained undetected. Here we show that in present day facilities elastic
scattering of two real photons can become detectable far off axis in an
asymmetric photon-photon collider setup. This may be obtained within one day
operation time by colliding one millijoule XUV pulses with the broadband
gamma-ray radiation generated in nonlinear Compton scattering of
ultrarelativistic electron beams with terawatt-class optical laser pulses
operating at 10 Hz repetition rate. In addition to the investigation of elastic
photon-photon scattering, this technique allows to unveil or constrain new
physics that could arise from the coupling of photons to yet undetected
particles, therefore opening new avenues for searches of physics beyond the
standard model.
