Apparent Faster-Than-Light Pulse Propagation in Interstellar Space: A new
probe of the Interstellar Medium
by F. A. Jenet1, D. Fleckenstein1, A. Ford 1, A. Garcia1,R. Miller1, J. Rivera1, K. Stovall1
http://arxiv.org/PS_cache/arxiv/pdf/0909/0909.2445v2.pdf
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.