Observation of locally negative velocity of the electromagnetic field in free space.

Since the 1983 definition of the speed of light in vacuum as a fundamental constant with the exact value of 299 792 458 m/s the question has remained as to what apart from the wave front travels at that speed. It is commonly assumed that the entire electromagnetic waveform in free space does. Here it is demonstrated, both theoretically and experimentally, that the near- and intermediate-field dynamics of the vectorial electromagnetic field is much more complex than simple outwards propagation. In particular, it is shown that there exists a region close to the source, where, while the wave front travels outwards at the speed of light, the main body of the waveform appears to go inwards or back in time. The same effect may also lead to apparent superluminal results in free space.