An ansible is a category of fictional devices or technology capable of near-instantaneous or faster-than-light communication. It can send and receive messages to and from a corresponding device over any distance or obstacle whatsoever with no delay, even between star systems. As a name for such a device, the word "ansible" first appeared in a 1966 novel by Ursula K. Le Guin. Since that time, the term has been broadly used in the works of numerous science fiction authors, across a variety of settings and continuities. A related term is ultrawave.
Ursula K. Le Guin coined the word "ansible" in her 1966 novel Rocannon's World. The word was a contraction of "answerable", as the device would allow its users to receive answers to their messages in a reasonable amount of time, even over interstellar distances.
The ansible was the basis for creating a specific kind of interstellar civilization – one where communications between far-flung stars are instantaneous, but humans can only travel at relativistic speeds. Under these conditions, a full-fledged galactic empire is not possible, but there is a looser interstellar organization, in which several of Le Guin's protagonists are involved.
Although Le Guin invented the name "ansible" for this type of device, fleshed out with specific details in her fictional works, the broader concept of instantaneous or faster-than-light communication had previously existed in science fiction. For example, similar communication functions were included in a device called an interocitor in the 1952 novel This Island Earth by Raymond F. Jones, and the 1955 film based on that novel, and in the "Dirac Communicator", which first appeared in James Blish's short story "Beep" (1954), which was later expanded into the novel The Quincunx of Time (1973). Robert A. Heinlein in Time for the Stars (1958) employed instantaneous telepathic communication between identical twin pairs over interstellar distances, and like Le Guin, provided a technical explanation based on a non-Einsteinian principle of simultaneity.
In her subsequent works, Le Guin continued to develop the concept of the ansible:
Any ansible may be used to communicate through any other, by setting its coordinates to those of the receiving ansible. They have a limited bandwidth, which only allows for at most a few hundred characters of text to be communicated in any transaction of a dialog session, and are attached to a keyboard and small display to perform text messaging.
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They print Reumere's plans for the ansible. 'What is the ansible?' 'It's what he's calling an instantaneous communication device.'
What matters is we built the ansible. The official name is Philotic Parallax Instantaneous Communicator, but somebody dredged the name ansible out of an old book somewhere and it caught on.
...the technology of the Deep Link, which gives us instant communications access across the deeps.
'It's an ansible.' 'Surely they don't call it that!' 'No. But that's what it is.'
...when I was commissioned, we didn't have FTL communications except from planetary platforms. I was on Boarhound when they mounted the first shipboard ansible, and at first it was only one-way, from the planet to us.
A connection [?ansible] was left; awaiting the next quiet [?peace]; and though destroyed by the threes, it will scream over the void one time.
The two Dax symbionts can communicate with each other across space, instantaneously, because they're composed of identical quantum particles. I've become a living ansible, Benjamin.
I can see Nightenhelser madly taking notes on his recorder ansible.
We are born as memories and meat. The meat was spontaneously created in the ansible's quantum re-creation mechanism, built up from water vapor, hydrogen, carbon dioxide, and various other gases out of storage. The memory is what we carry across from one side of the ansible to the other, into the new flesh.