  It is a striking result the one found by Wang, Kuzmich & Dogariu 1 (see also
2) experimentally last year, that a pulse of light, entered into an atomic
caesium (Cs) vapour cell, appeared at the exit before its entrance (!) by 62
nanoseconds. It has to be mentioned that the shape of the pulse was maintained
(cf. fig. 4 of 1), so that its peak was not shifted forward inside the pulse.
Thus the explanation that the peak of the pulse was advanced at the exit, with
the result of emerging there before the entrance of the peak of the incident
pulse, does not apply.

General Physics