Does anyone have any more information on the following (I haven't looked yet)?
http://www.physicstoday.org/pt/vol-54/iss-3/p9.html [Broken]Subluminal Cherenkov radiation. In vacuum, nothing travels faster than light. In transparent substances like water, however, it is possible for high-energy charged particles to exceed the speed of a light beam in that substance. When this happens, the particle will radiate a cone of light called Cherenkov radiation. A team of researchers (University of Michigan and Max Planck Institute for Condensed Matter Research in Stuttgart) has now taken a closer look at the theory, and found that conical Cherenkov emission also occurs at subluminal speeds. The researchers verified the finding experimentally using subpicosecond laser pulses to generate--through a nonlinear optical process--relativistic dipoles that emitted infrared Cherenkov radiation in a zinc selenide crystal. (T. E. Stevens et al., Science 291, 627, 2001.) --pfs
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