"Superluminal" wave propagation
The lecture demonstration below shows the propagation of a
mechanical wave through a barrier formed by 5 bars with red ends. The
barrier is produced by rubber bands (visible in the background) which
give an effective mass to the "field" in that area.
Observe that the peak of the outgoing pulse leaves the barrier before
the peak of the incoming pulse has entered. This corresponds to what
certain authors claim is propagation of a signal backward in time.
The white arrow shows the peak of the ingoing pulse and the red arrow
shows the peak of the outgoing pulse. The vertical line shows the
"front" - the first place where the bars are displaced from rest.
This is the point to which the information that I started to make the
pulse has propagated. It moves forward in time at a steady velocity.
kdo@cosmos.phy.tufts.edu