  Einstein's theory of relativity establishes the speed of light in vacuum, c,
as a fundamental constant. However, the speed of light pulses can be altered
significantly in dispersive materials. While significant control can be exerted
over the speed of light in such media, no experimental demonstration of altered
light speeds has hitherto been achieved in vacuum for ``twisted'' optical
beams. We show that ``twisted'' light pulses exhibit subluminal velocities in
vacuum, being slowed by 0.1\% relative to c. This work does not challenge
relativity theory, but experimentally supports a body of theoretical work on
the counterintuitive vacuum group velocities of twisted pulses. These results
are particularly important given recent interest in applications of twisted
light to quantum information, communication and quantum key distribution.
