Term used by this encyclopedia for specialized Time Machines which allow information to be sent, without gross physical interaction, from the future to the past. (The reverse may also be possible but generally lacks sf interest: unless worldwide Holocaust or a sizeable Time Abyss has intervened, physical records – from paper to fossils – will normally suffice to transmit information from past to future.) The instantaneous Dirac Communicator introduced by James Blish in "Beep" (February 1954 Galaxy; exp as The Quincunx of Time 1973) is a well-known example: all Dirac messages are potentially retrievable by all operating communicators, whether in the past, present or future.
The time radio in James P Hogan's Thrice Upon a Time (1980) depends on an Imaginary-Science "tau radiation" which propagates backwards through time. Gregory Benford's more impressive novel Timescape (1980) – published just a few months later – invokes the existing concept of Tachyons as carriers of a message to the past which only reasonably well-equipped Scientists are able to read; to make the story work, it was necessary to ignore the Physics which indicates that tachyons, though not necessarily impossible, cannot carry actual information backwards through time. Time: Manifold 1 (1999) by Stephen Baxter likewise avoids this issue in its deployment of a "Feynman Radio" transmission from future to past; the name of course derives from the celebrated physicist Richard Feynman (1918-1968). Alastair Reynolds's Absolution Gap (2002) describes the "Exordium" project which acquires information from a future, though not (neatly eliminating the possibility of Time Paradoxes) the precise future of the recipients.
A kind of natural time radio features in Colin Kapp's The Patterns of Chaos (February-May/June 1972 If; 1972), whose Imaginary Science of Chaos posits that violent events generate shockwaves propagating backward through time, allowing the time and place though not the specific nature of a future Disaster to be inferred via appropriate detectors. [DRL]
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