Causality in superluminal pulse propagation

Robert W. Boyd

Daniel J. Gauthier

Paul Narum

pp. 175-204

in: Gonzalo Muga, Andreas Ruschhaupt, Adolfo del Campo (eds), Time in quantum mechanics II, Berlin, Springer, 2009

Abstract

The theory of electromagnetism for wave propagation in vacuum, as embodied by Maxwell's equations, contains physical constants that can be combined to arrive at the speed of light in vacuum c. As shown by Einstein, consideration of the space–time transformation properties of Maxwell's equations leads to the special theory of relativity. One consequence of this theory is that no information can be transmitted between two parties in a time shorter than it would take light, propagating through vacuum, to travel between the parties. That is, the speed of information transfer is less than or equal to the speed of light in vacuum c and information related to an event stays within the so-called light cone associated with the event. Hypothetical faster-than-light (superluminal) communication is very intriguing because relativistic causality would be violated. Relativistic causality is a principle by which an event is linked to a previous cause as viewed from any inertial frame of reference; superluminal communication would allow us to change the outcome of an event after it has happened.