Apparent Faster-Than-Light Pulse Propagation in Interstellar Space: A
  new probe of the Interstellar Medium
  Radio pulsars emit regular bursts of radio radiation that propagate through
the interstellar medium (ISM), the tenuous gas and plasma between the stars.
Previously known dispersive properties of the ISM cause low frequency pulses to
be delayed in time with respect to high frequency ones. This effect can be
explained by the presence of free electrons in the medium. The ISM also
contains neutral hydrogen which has a well known resonance at 1420.4 MHz.
Electro-magnetic theory predicts that at such a resonance, the induced
dispersive effects will be drastically different from those of the free
electrons. Pulses traveling through a cloud of neutral hydrogen should undergo
"anomalous dispersion", which causes the group velocity of the medium to be
larger than the speed of light in vacuum. This superluminal group velocity
causes pulses containing frequencies near the resonance to arrive earlier in
time with respect to other pulses. Hence, these pulses appear to travel faster
than light. This phenomenon is caused by an interplay between the time scales
present in the pulse and the time scales present in the medium. Although
counter-intuitive, it does not violate the laws of special relativity. Here, we
present Arecibo observations of the radio pulsar PSR B1937+21 that show clear
evidence of anomalous dispersion. Though this effect is known in laboratory
physics, this is the first time it has been directly observed in an
astrophysical context, and it has the potential to be a useful tool for
studying the properties of neutral hydrogen in the Galaxy.
