Lawbreakers? The physics of superluminal sources

John Singleton, LANL, MST-NHMFL

The emission of electromagnetic radiation from a superluminal (faster-than-light in vacuo) charged particle was first studied by Sommerfeld in 1904. However, the Special Theory of Relativity was published just a few months later; prevailing scientific opinion then effectively curtailed the research field until Ginzberg and coworkers pointed out in the 1980s that no physical principle forbids emission by extended, massless superluminal sources. A displacement current density (dD/dt; see Maxwell's fourth equation) can provide such a source; the individual charged particles creating the polarization do not move faster than the speed of light, and yet it is relatively trivial to make the envelope of the displacement current density to do so.

Based on this idea, the author and coworkers have constructed and demonstrated a machine (the Polarization Synchrotron) for animating superluminal displacement currents. The emitted electromagnetic radiation has several intriguing features; e.g. there is a component with an intensity that decays as 1/distance, rather than as the inverse square law, and the simultaneous spherically-decaying emission shows a fixed angular width out to hundreds of Fresnel distances.

This talk will give a simplified description of the theory of superluminal sources. A particularly important point is that the radiation detected by a point-like observer at a single time in his/her inertial frame can correspond to emission over an extended period of source time. This becomes especially marked with a rotating superluminal source (like the Polarization Synchrotron), where a volume of the source can approach the observer at the speed of light with zero acceleration; under such conditions, the emission from this volume is effectively coherent (i.e. like a laser, although the mechanism is completely different). The predictions of theory will be shown to account for the recent experimental data obtained from the Polarization Synchrotron.

The "Lawbreakers" part of the title is a quotation from an article in the "Economist" magazine about the Polarization Synchrotron. In spite of the (at first sight) surprising nature of some of the data, no physical laws are broken by the machine; it should merely be thought of as an experimental implementation of a little-considered problem in electrodynamics!

(Two papers on the theory of superluminal sources are in press at J. Optical Society of America A; preprints may be found under the name "Singleton" at the xxx.lanl.gov server. This colloquium will be the first public showing of the experimental data from the Polarization Synchrotron.)

The P/T Colloquium is
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