An experimental test of relativistic wave-packet collapse is presented. The
tested model assumes that the collapse takes place in the reference frame
determined by the massive measuring detectors. Entangled photons are measur
ed at 10 km distance within a time interval of less than 5 ps. The two appa
ratuses are in relative motion so that both detectors, each in its own iner
tial reference frame, are first to perform the measurement. The data always
reproduces the quantum correlations and thus rule out a class of collapse
models. The results also set a lower bound on the 'speed of quantum informa
tion' to 2/3 10(7) and 3/2 10(4) times the speed of light in the Geneva and
the background radiation, reference frames, respectively. The very difficu
lt and deep question of where the collapse takes place-if it takes place at
all-is considered in a concrete experimental context.