Anomalous Dispersion, not Faster than Light

In the 20 July 2000 issue of Nature, Wang, Kuzmich & Dogariu talk about faster than light transmission. However, this is really just anomalous dispersion, as the authors readily admit. A pulse of light can be decomposed into a sum of sine waves with different wavelengths, as shown below.

The black curve shows the sum of the red, green and blue sine waves. When all of the waves are delayed, but the longer wavelengths [red] are delayed more and the shorter wavelengths [blue] are delayed less, then the overall pulse appears to be advanced in time!

This is how the pulse in the Wang et al. (Nature, 20 Jul 2000) experiment was advanced by 62 nanoseconds while passing through a gas cell only 6 cm long. Even if the speed of transmission was infinite, that could only shave 0.2 ns off the time. The actual advance was much larger, representing time travel into the past, not faster than light travel. But it was really only anomalous dispersion. Normal dispersion in glass delays the blue light more than the red light, while in anomalous dispersion the red light is delayed more than the blue. As shown above, this causes the pulse peak to shift earlier in time.