  The speed-stress relation for gliding edge dislocations was experimentally
measured for the first time. The experimental system used, a two-dimensional
plasma crystal, allowed observation of individual dislocations at the
"atomistic" level and in real time. At low applied stress dislocations moved
subsonically, at higher stress their speed abruptly increased to 1.9 times the
speed of shear waves, then slowly grew with stress. There is evidence that
immediately after nucleation dislocations can move faster than pressure waves.
