An ansible is a hypothetical machine capable of instantaneous or superluminal communication. Ansibles occur as plot devices in science fiction literature.
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The word, ansible was coined by Ursula K. Le Guin in her 1966 novel, Rocannon's World.[1] Le Guin states that she derived the name from "answerable," as the device would allow its users to receive answers to their messages in a reasonable amount of time, even over interstellar distances.[2] Her award-winning 1974 novel The Dispossessed[3], a book in the Hainish Cycle, tells of the invention of the ansible.
The name of the device has since been borrowed by authors such as Orson Scott Card,[4] Vernor Vinge,[5] Elizabeth Moon,[6] Jason Jones,[7] L.A. Graf,[8] and Dan Simmons.[9] Similar devices are present in the works of numerous others, such as Frank Herbert[10] and Philip Pullman, who called his a lodestone resonator.[11]
Anne McCaffery's Crystal Singer series posited an instantaneous communication device powered by rare, 'Black Crystal' from the planet Ballybran. Black Crystals cut from the same mineral deposit could be "tuned" to sympathetically vibrate with each other instantly, even when separated by interstellar distances, allowing instantaneous telephone-like voice and data communication. Similarly, in Gregory Keyes' series The Age of Unreason, "aetherschriebers" use two halves of a single "chime" to communicate, aided by scientific alchemy. The series is set on an alternate 18th-century Earth. While the speed of communication is important, so is the fact that the messages cannot be overheard except by listeners with a piece of the original crystal.
Stephen R. Donaldson, in his Gap Series, proposed a similar system, Symbiotic Crystalline Resonance Transmission, clearly ansible-type technology, but was very difficult to produce and limited to text messages.
Some hard science fiction stories use small (possibly nano-sized) paired wormholes dedicated to communication by means of a laser which traverses the wormhole.
Charles Stross's books Singularity Sky and Iron Sunrise make use of "causal channels" which use entangled particles for instantaneous two-way communication. The technique has drawbacks in that the entangled particles are expendable and the use of faster-than-light travel destroys the entanglement, so that one end of the channel must be transported below light speed. This makes them expensive and limits their usefulness somewhat.
In Richard K. Morgan's Takeshi Kovacs novels human colonies on distant planets maintain contact with earth and each other via hyperspatial needlecast, a technology which moves information "...so close to instantaneously that scientists are still arguing about the terminology...".
One ansible-like device which predates Le Guin's is the Dirac communicator in James Blish's 1954 short story "Beep". As the title implies, any active device received the sum of all transmitted messages in universal space-time, in a single pulse, so that demultiplexing yielded information about the past, present, and future.
Isaac Asimov solved the same communication problem with the hyper-wave relay in the Foundation series.
Le Guin's ansible was said to communicate "instantaneously",[3] but other authors have adopted the name for devices only capable of finite-speed communication, although still faster than light.
The subspace radio, best known today from Star Trek and named for the method used in the series for achieving faster-than-light travel, was the most commonly used name for such a faster-than-light communicator in the science fiction of the 1930s to the 1950s.
In all the Stargate television series, characters are able to communicate instantaneously over long distances by transferring their consciousness into another person or being anywhere in the universe using "Ancient communication stones". It is not known how these stones operate, but the technology explained in the show usually revolves around wormholes for instant teleportation, faster-than-light, space-warping travel, and sometimes around quantum multiverses.
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."
Unlike McCaffrey's black crystal transceivers, Le Guin's ansibles are not mated pairs: 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 uses the ansible as a plot device. "The official name is Philotic Parallax Instantaneous Communicator," explains Colonel Graff in Ender's Game, "but somebody dredged the name ansible out of an old book somewhere."[4]
Card's description of the ansible's functions in Xenocide involve a fictional subatomic particle, the philote. In the "Enderverse", the two quarks inside a pi meson can be separated by an arbitrary distance while remaining connected by "philotic rays". This concept is similar to quantum teleportation due to entanglement, although even that does not imply a possibility of faster-than-light communication. Also, in the real world, quark confinement prevents quarks from being separated by more than microscopic distances.
The novels' treatment of time dilation is also inconsistent with standard theory. In the novels, the passenger of a starship experiences time compression, resulting in slowed speech when communicating over the ansible. This contradicts the core concept of the theory of relativity by giving the planet a privileged frame of reference. It is no more correct to say that the planet is at rest and the ship moving than to say that the ship is at rest and the planet moving backwards. As such, we find a paradox in which the person at each end of an ansible expects the other person to be time dilated and speaking slowly. In light speed or slower communications, this paradox is avoided due to the expected message delays, as in the Twin paradox. Instantaneous communications, however, does result in a paradox, as each communicator expected the other to be slowed.
The ansible is also feature of the video game Advent Rising, for which Card helped write the story.
There is no currently known way to build an ansible. The theory of special relativity predicts that any such device would allow communication from the future to the past, which raises problems of causality, unless the device used general relativistic curved spacetimes as an integral part.
Quantum nonlocality is often proposed as a mechanism for superluminal communication[11]. A 2008 quantum physics experiment performed in Geneva, Switzerland has determined that in any hypothetical nonlocal hidden-variables theory the speed of the quantum non-local connection would have to be at least 10,000 times the speed of light.[12] Practical applications are made impossible due to the no-cloning theorem, and the fact that quantum field theories preserve causality, so that quantum correlations cannot be used to transfer information.
Nevertheless, consider the EPR thought experiment, and suppose quantum states could be cloned. Assume parts of a maximally entangled Bell state are distributed to Alice and Bob. Alice could send bits to Bob in the following way: If Alice wishes to transmit a "0", she measures the spin of her electron in the z direction, collapsing Bob's state to either |z+> or |z->. To transmit "1", Alice does nothing to her electron. Bob creates many copies of his electron's state, and measures the spin of each copy in the z direction. Bob will know that Alice has transmitted a "0" if all his measurements produce the same result; otherwise, his measurements will have outcomes +1/2 and −1/2 with equal probability. This would allow Alice and Bob to communicate near instantly across space-like separations.
See time travel and faster-than-light for more discussion of these issues.