Superluminal communication is a hypothetical process in which information is sent at faster-than-light (FTL) speeds. The current scientific consensus is that faster-than-light communication is not possible, and to date it has not been achieved in any experiment.
Superluminal communication is believed to be impossible because, in a Lorentz-invariant theory, it could be used to transmit information into the past. This contradicts causality and leads to logical paradoxes.
Tachyonic particles are hypothetical particles that travel faster than light. These would allow superluminal communication, and for this reason are widely believed not to exist. By contrast, tachyonic fields - quantum fields with imaginary mass - certainly do exist, and exhibit superluminal group velocity under some circumstances. However, such fields have luminal signal velocity and do not allow superluminal communication.
Quantum mechanics is non-local in the sense that distant systems can be entangled. Entangled states lead to correlations in the results of otherwise random measurements, even when the measurements are made nearly simultaneously and at far distant points. The impossibility of superluminal communication lead Einstein, Podolsky, and Rosen to propose that quantum mechanics must be incomplete (see EPR paradox).
However, it is now well understood that quantum entanglement does not allow any influence or information to propagate superluminally. Technically, the microscopic causality postulate of axiomatic quantum field theory implies the impossibility of superluminal communication using any phenomena whose behavior can be described by orthodox quantum field theory. A special case of this is the no-communication theorem, which prevents communication using the quantum entanglement of a composite system shared between two spacelike-separated observers. Some authors have argued that using the no-communication theorem to deduce the impossibility of superluminal communication is circular, since the no-communication theorem assumes that the system is composite.
If wormholes are possible, then ordinary subluminal methods of communication could be sent through them to achieve superluminal transmission speeds. Considering the immense energy that current theories suggest would be required to open a wormhole large enough to pass spacecraft through, it may be that only atomic-scale wormholes would be practical to build, limiting their use solely to information transmission. Some hypotheses of wormhole formation would prevent them from ever becoming "timeholes", allowing superluminal communication without the additional complication of allowing communication with the past.
Ultrawave and hyperwave
The terms "ultrawave" and "hyperwave" have been used by several authors, often interchangeably, to denote faster-than-light communications. Examples include:
- E. E. Smith used the term "ultrawave" in his Lensman series, for waves which propagated through a sub-ether and could be used for weapons, communications, and other applications.
- In Isaac Asimov's Foundation series, "ultrawave" and "hyperwave" are used interchangeably to represent a superluminal communications medium. The hyperwave relay also features.
- In the Star Trek universe, subspace carries faster-than-light communication (subspace radio) and travel (warp drive).
- The Cities in Flight series by James Blish featured ultrawave communications which used the known phenomenon of phase velocity to carry information, a property which in fact is impossible. The limitations of phase velocity beyond the speed of light later led him to develop his Dirac communicator.
- Larry Niven used hyperwave in his Known Space series as the term for an FTL method of communication. Unlike the hyperdrive that moved ships at a finite FTL speed, hyperwave was essentially instantaneous.
- 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".
The Dirac communicator features in several of the works of James Blish, notably his 1954 short story "Beep". As alluded to in the title, 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.
The ansible is a major plot element, nearly a MacGuffin, in Elizabeth Moon's Vatta's War series. Much of the story line revolves around various parties attacking or repairing ansibles, and around the internal politics of ISC (InterStellar Communications), which holds a monopoly on the ansible technology.
- In Ernest Cline's novel Armada, alien invaders possess technology for instant "quantum communication" with unlimited range. Humans reverse engineer the device from captured alien technology.
- In the Mass Effect series of video games, instantaneous communication is possible using quantum-entanglement communicators placed in the communications rooms of starships.
- In the Avatar continuity, faster-than-light communication via a subtle control over the state of entangled particles is possible, but for practical purposes extremely slow and expensive: at a transmission rate of three bits of information per hour and a cost of $7,500 per bit, it is used for only the highest priority messages.
- 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.
Psychic links, generally considered to belong to pseudoscience, have been described as explainable by physical principles or unexplained, but they are claimed to operate instantaneously over large distances.
In 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.
Anne McCaffrey'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, "aetherschreibers" use two halves of a single "chime" to communicate, aided by scientific alchemy. 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 same original crystal.
Stephen R. Donaldson, in his Gap cycle, proposed a similar system, Symbiotic Crystalline Resonance Transmission, clearly ansible-type technology but very difficult to produce and limited to text messages.
In Ivan Yefremov's 1957 novel Andromeda Nebula, a device for instant transfer of information and matter is made real by using "bipolar mathematics" to explore use of anti-gravitational shadow vectors through a zero field and the antispace, which enables them to make contact with the planet of Epsilon Tucanae.
In Cordwainer Smith's Instrumentality novels and stories, interplanetary and interstellar communication is normally relayed from planet to planet, presumably at superluminal speed for each stage (at least between solar systems) but with a cumulative delay. For urgent communication there is the "instant message", which is effectively instantaneous but very expensive.
In Liu Cixin's novel The Three-Body Problem, the alien Trisolarans, while preparing to invade the Solar System, use a device with Ansible characteristics to communicate with their collaborators on Earth in real time. Additionally, they use spying/sabotaging devices called 'Sophons' on Earth which by penetration can access any kind of electronically saved and visual information, interact with electronics, and communicate results back to Trisolaris in real-time. The technology used is "single protons that have been unfolded from eleven space dimensions to two dimensions, programmed, and then refolded" and thus Sophons remain undetectable for humans.
In Howard Taylor's web comic series Schlock Mercenary, superluminal communication is performed via the hypernet, a galaxy-spanning analogue to the internet. Through the hypernet, communications and data are routed through nanoscopic wormholes, using conventional electromagnetic signals.
- Bell test experiments
- Delayed choice quantum eraser
- Near and far field
- Quantum teleportation
- SETI Institute
- Wheeler–Feynman absorber theory
- Tipler, Paul A.; Llewellyn, Ralph A. (2008). Modern Physics (5th ed.). New York: W.H. Freeman & Co. p. 54. ISBN 978-0-7167-7550-8.
... so existence of particles v > c ... Called tachyons ... would present relativity with serious ... problems of infinite creation energies and causality paradoxes.
- Lisa Randall, Warped Passages: Unraveling the Mysteries of the Universe's Hidden Dimensions, p.286: "People initially thought of tachyons as particles travelling faster than the speed of light...But we now know that a tachyon indicates an instability in a theory that contains it. Regrettably for science fiction fans, tachyons are not real physical particles that appear in nature."
- Eberhard, Phillippe H.; Ross, Ronald R. (1989), "Quantum field theory cannot provide faster than light communication", Foundations of Physics Letters, 2 (2): 127–149, Bibcode:1989FoPhL...2..127E, doi:10.1007/BF00696109
- Peacock, K.A.; Hepburn, B. (1999). "Begging the Signaling Question: Quantum Signaling and the Dynamics of Multiparticle Systems". Proceedings of the Meeting of the Society of Exact Philosophy. Bibcode:1999quant.ph..6036P. arXiv: .
- Moon, Elizabeth (September 2004). Trading in Danger (mass ppb. ed.). Del Rey. p. 111. ISBN 0-345-44761-1.
Attack on instersystem ansibles is just...just unthinkable
- James Cameron's Avatar: An Activist Survival Guide – pg 156-157
- Hines, Terence. (2003). Pseudoscience and the Paranormal. Prometheus Books. pp. 117–145. ISBN 1-57392-979-4
- Diaconis, Persi. (1978). Statistical Problems in ESP Research. Science New Series, Vol. 201, No. 4351. pp. 131–136.
- Herbert, Frank (April 1970) . The Whipping Star. Worlds of If magazine.
- Pullman, Philip (2001-10-02) . The Amber Spyglass (His Dark Materials, 3. mass pbk. ed.). New York: Del Rey. p. 156. ISBN 0-345-41337-7.
'Well, in our world there is a way of taking a common lodestone and entangling all its particles, and then splitting it in two so that both parts resonate together.'
- Keyes, J. Gregory (March 4, 2009). The Shadows of God. Random House LLC. ISBN 9780307559609. Retrieved 15 March 2014.
My aetherschreiber was lost when the Coweta captured us.
- Smith, Cordwainer. "On the Storm Planet" (February 1965), Chap. XII, pp. 148–149 in: Dozois, Gardner, ed. (October 28, 2014). Modern Classic Short Novels Of Science Fiction. New York: St. Martin's Press. pp. 94–163. Retrieved 14 August 2015.
The communicator was the kind they mount in planoforming ships right beside the pilot. The rental on one of them was enough to make any planetary government reconsider its annual budget. ... She pressed a button. "Instant message." ... Casher, knowing the prices of this kind of communication, almost felt that he could see the arterial spurt of money go out of Henriada's budget as the machines reached across the galaxy, found Mizzer and came back with the answer.