Dept. of Physics and Astronomy Brigham Young University Provo, UT 84602 email@example.com
During the past century, some confusion has arisen surrounding the meaning of group velocity in situations where it exceeds the speed of light in vacuum or where it becomes negative. Such situations arise for narrowband pulses in the neighborhood of absorption of amplifying resonance structures where broadband pulses typically suffer severe distortion. This has led to the common perception that group velocity loses meaning near a resonance (at least for broadband pulses). We have developed a context in which group velocity always retains meaning, regardless of the spectral bandwidth of the pulse. A linear spectral superposition of group delay tracks the arrival of field energy.
So-called superluminal behavior is an
artifact of paying attention only to the field energy while ignoring
energy before being transferred out of or after being transferred
into of the medium. The total energy in a system obeys strict
luminality. A causal linear dielectric exchanges energy with
the front of a pulse differently than with the back, which can
lead to superluminal motion (or in other circumstances to highly
subluminal motion) of the locus of field energy. The medium responds
to the instantaneous spectrum of the field, that is the spectrum
of the electric field pulse truncated at each new instant (as
a given locale in the medium experiences the field).
©Copyright 1999 KSU Department of Physics