Ansible
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The name and basic function of the device have since been borrowed by authors such as Frank Herbert, Orson Scott Card, Elizabeth Moon, Vernor Vinge, L.A. Graf, Dan Simmons, and Philip Pullman (though his was called a "lodestone resonator"). Other science fiction stories have devices with similar effects that are not called ansibles; perhaps the best known is Star Trek's "subspace radio". One ansible-like device which predates Le Guin's usage is the "Dirac communicator" in James Blish's 1954 short story "Beep". Isaac Asimov's Foundation stories also featured such a device, which he called a "hyper-wave relay".
Le Guin's ansible communicated instantaneously, and so do most other authors'. A notable exception is the ansible in the Vinge short story "The Blabber", which merely communicates faster than light — in a universe where that is believed impossible.
In The Word for World is Forest, Le Guin explains that in order for communication to work with any pair of ansibles at least one "must be on a large-mass body, the other can be anywhere in the cosmos." In The Left Hand of Darkness, the ansible "doesn't involve radio waves, or any form of energy. The principle it works, on the constant of simultaneity, is analogous in some ways to gravity... One point has to be fixed, on a planet of certain mass, but the other end is portable." Le Guin's ansibles are not mated pairs as it is possible for an ansible's coordinates to be set to any known location of a receiving ansible. Moreover, the ansibles Le Guin uses in her stories apparently have a very limited bandwidth which only allows for at most a few hundred characters of text to be communicated in any transaction of a dialog session. Instead of a microphone and speaker, Le Guin's ansibles are attached to a keyboard and small display to perform text messaging.
Orson Scott Card's Ender's Game series is probably the most widely read work to use an ansible ("The official name is Philotic Parallax Instantaneous Communicator," explains Col. Graff in Ender's Game, "but somebody dredged the name ansible out of an old book somewhere"). His description of ansible functions in Xenocide involve a fictional subatomic particle, the philote, and contradicts not only standard physical theory but the results of empirical particle accelerator experiments. In the "Enderverse", the two quarks inside a pi meson can be separated by an arbitrary distance while remaining connected by "philotic rays". In the real world, quark confinement prevents one from separating quarks by more than microscopic distances. Most writers deliberately avoid explaining how their ansibles work; Card elaborated only because philotics became important to later volumes of the series.
There is no known way to build an ansible. While current theories of physics do not absolutely rule out the possibility, the theory of special relativity predicts that any such device would allow communication from the future to the past, which raises problems of causality. For this reason, most physicists believe that they will eventually be proven impossible. Quantum entanglement is often proposed as a mechanism for superluminal communication, but our current understanding of that phenomenon is that it cannot be used for any sort of communication, superluminal or otherwise. See time travel and faster-than-light for more discussion of these issues.
Ansible is also a science fiction fanzine published by Dave Langford, named after the faster-than-light communicator.
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