Observation of negative impulse velocity in free space

TL;DR

This paper demonstrates both theoretically and experimentally that in free space near a source, individual electromagnetic impulses can appear to slow down or reverse, challenging the assumption that the entire wave-packet always travels at light speed.

Contribution

It provides the first experimental evidence of negative impulse velocity in free space, revealing a near-field effect that affects wave-packet propagation.

Findings

Impulses can slow down or reverse in free space near the source.

Wave-front travels at light speed, but impulses can behave differently.

This effect may explain some superluminal measurements.

Abstract

Since the 1983 definition of the speed of light in vacuum as a fundamental constant with the exact value of 299792458 m/s the question remains as to what apart from the wavefront travels at that speed. It is commonly assumed that the entire wave-packet or an impulse of the electromagnetic radiation in free space does. Here it is shown, both theoretically and experimentally, that there exists a region close to the source, where, while the wave-front travels at the speed of light, the individual impulses comprising the body of the wave-packet appear to slow down and even go backwards in time. This three-dimensional near-field late-time effect may also explain some of the free-space superluminal measurements.