The Wang Experiment: Light Traveling Faster Than Light?

A new laboratory experiment at the NEC Research Institute in Princeton claims to have achieved propagation speeds of 310 c (c = speed of light). [i] This made headline news around the world, despite disclaimers by the authors about any violations of special relativity. This type of experiment supplements and extends earlier "quantum tunneling" experiments. [ii] It is still debated whether these experiment types using electromagnetic radiation (e.g., light) can truly send information faster than light. [iii]

Whatever the resolution of that matter, the leading edge of the transmission is an electromagnetic wave, and therefore always travels at lightspeed. See

Figure 1 above and caption below for an overview of the Wang et al. experiment. If the trailing edge of the light pulse went continuously from A to C, it would indeed exceed the speed of light. However, reality is probably that a smaller light pulse shape is created at the exit to the chamber when the leading edge of the pulse arrives, because the trailing edge of the exiting pulse leaves the chamber even before the trailing edge of the original pulse has entered the chamber. That makes the appearance that the trailing edge traveled faster than light an illusion.

However, such experiments have served to raise public consciousness about the faster-than-light-propagation concept, for which good evidence involving electrodynamic and gravitational forces (rather than light) exists.

2000/08/24 -- tvf

[i] L.J. Wang, A. Kuzmich, A. Dogariu, Nature 406 (2000).277-279.

[ii] W. Heitmann, G. Nimtz, Phys. Lett. A 196 (1994) p. 154.

[iii] P. Weiss, Sci. News 157 (2000) p. 375.

Figure 1. The Wang experiment: A light pulse A-B is timed when its leading edge B and its trailing edge A enter the opening of a chamber (near B in the figure), moving in the direction of the arrow. The leading edge D and trailing edge C of the same pulse, now weaker, are also timed when they exit the chamber through another opening on its far wall (near D in the figure). The leading edge BgD travels through the chamber at the speed of light, c. Because the pulse shape is retained but its amplitude is weaker, the trailing edge AgC appears to travel faster than c. In fact, C exits the chamber before A enters it.