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There is an only half-facetious precept in Physics stating that "anything which is not prohibited is compulsory". Olexa-Myron Bilaniuk and E C George Sudarshan suggested in 1962, and Gerald Feinberg in 1967, that the idea of a particle that can only travel Faster Than Light does not violate any of the basic maxims of Relativistic physics. Such a hypothetical particle (a tachyon, as opposed to the more familiar tardyon, or slower-than-light particle) might emit Cerenkov radiation analogous to the bow wave of a ship, and thus might perhaps be detected. The mass (or metamass) of a tachyon must be imaginary, in the same sense that the square root of minus one is imaginary.

If tachyons were shown to exist we might have to rethink the idea of causality, since they would appear in some circumstances to go backwards in time, so that to a hypothetical observer the emission of a tachyon would appear to be its absorption. However, a negative-energy tachyon propagating backward in time could be reinterpreted as a positive-energy tachyon propagating forward in time; some physicists think that such a reinterpretation would be the loophole through which the principle of causality might be preserved. J Richard Gott proposed in 1973 that, after the Big Bang, a tripartite Universe may have been formed, consisting of universes of matter, Antimatter and tachyons.

The tachyon became an item of sf Terminology in the 1970s (though never to any great extent), because it suggests a more rational basis on which Time-Travel stories – or (slightly more plausibly, since we cannot, even theoretically, convert tardyonic into tachyonic matter) stories of Communication through time via Time Radio – can be written. The physicist-writer Gregory Benford was the first to do this with some care, in his major novel Timescape (1980), which describes an attempt to change future history by transmitting a tachyonic message from that future to our present. [PN]

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