An ansible, in its most basic definition, is a faster than light communications system, used mostly in the same way radio is used today. It is a central feature in the works of science fiction authors such as Ursula K. LeGuin, but versions of it are also seen in on-screen science fiction such as Star Trek , Star Wars, and others. It usually allows instant or near-instant communication across many light years of distance.
There have been many different suggestions for how ansibles could work. Some are straight forward, usually as an outgrowth of a particular fiction universe’s means of FTL travel. For example, in the Stargate universe, Stargates allow two-way radio communication when open, even though matter can only be sent one-way through a wormhole.
FTL communications, like FTL travel, is impossible according to our current understanding of the way the universe works. In all likelihood it will never be a reality unless the laws of physics turn out to be much different from what we currently perceive. And if an ansible does someday become a reality, it raises the unpleasant specter of both time travel and violating causality. However, for this article, such implications will be largely ignored, much as they are in science fiction. Time travel and all its implications will be addressed in a future section dedicated to it.
Because ansibles are mostly pure conjecture with only a tangential connection to real world science, their placement on the Tech Level scale is somewhat arbitrary. Like most FTL tech, the physics behind them is assumed to be discovered at Tech Level 16, with actual applications in the Tech Levels after that in order to coincide with their use by space opera civilizations, where they are most prevalently depicted.
In some science fiction sources, only ansibles will exist and no means of FTL travel will be possible, such as in the first two novels of the Ender series. In others, FTL travel may be possible but ansibles cannot be made to work, such as in the Traveller tabletop RPG. Both can lead to very interesting permutations of stereotypical interstellar civilizations.
Quantum entanglement is the phenomenon of particles influencing one another’s quantum states even over great distances, seemingly at FTL speeds. The cause is largely unknown, and is one of the admittedly stranger aspects of quantum mechanics. Even Einstein had trouble accepting the reality of it at first, calling it "spooky action at a distance." Much more detailed explanations of it are available in the links at the end of this article.
Current theory as well as experimental data shows that quantum entanglement cannot be used to pass information. However, an FTL exchange of some sort is clearly implied between the entangled particles. It is possible that future generations, with a far more complete knowledge of quantum mechanics than we may have, may discover an unforeseen loophole that will allow them to take advantage of entanglement for communication.
If quantum entanglement communicators do become possible, they would have no range limits. One could use such a communication system to talk to someone in another galaxy as easily as someone the next street over.
However, the communication would only be possible between devices that had their particles entangled beforehand. In other words, one would have to start out with a base population of particles in one place, and farm them out to the communicators that will be in the network. Communication can only work between these ‘mated’ devices and no others. For better or ill, no other devices can ever ‘listen in’ on the communication network, even other quantum entanglement sets, unless they have some of the previously-entangled particles.
The communicators will also only have one open "channel" between all the sets in a network. There’s no such thing as switching frequencies with this type of communicator.
Quantum entanglement communicators have appeared in several science fiction stories by Stephen Baxter.
The ansible in LeGuin’s works seem thematically similar quantum entanglement communicators, but instead depended on the principle of "simultaneity." Simultaneity in classic physics refers to different events happening at the same time. However, The theory of relativity shows that in the real world simultaneity is also relative; events that seem simultaneous in one frame can seem to occur at different times in another.
LeGuin’s ansible seems to contradict this and instead relies on some unknown universal reference frame where absolute simultaneity can be upheld. Sending and receiving units were ‘entangled’ somehow so that when something happened to one, it simultaneously happened to the other When one communicator is manipulated, the same message typed into it appears on its mate many light years away. This could be quantum entanglement at work, or perhaps its made possible through the use of a higher physical dimension like hyperspace as a universal reference frame.
LeGuin ansibles needed at least one communicating device located within a gravity well of planet, and the other away from one, located almost anywhere out in space. These ansibles didn’t have exclusive ‘mates’ as with a quantum entanglement communicator. One can be tuned to any other ansible, as long as its coordinates were known.
Hyperwaves are also sometimes called Ultrawaves, and mostly come from stories and novels by Isaac Asimov, such as the Foundation Trilogy. The exact inner workings of these devices was usually left conveniently obscure, but the implication was clear that they were related to the same operating principles of the hyperdrive. Other sources that have both hyperdrives and FTL communications (such as Star Wars) probably have similarly related technologies.
In practice, hyperwaves behave, and are mostly used, exactly like mundane radio and television communication sets. They seem to work instantaneously, work in multiple channels and frequencies, can be widely broadcast and picked up by anyone else with a hyperwave device.
The principle that hyperdrives work on is that the ship bypasses the distances of our three-dimensional space by launching itself into a higher physical dimension, or "hyperspace." This hyperspace is not another universe as is often thought, but rather simply another aspect of our own universe that we 3D creatures don’t often have truck with. If its relationship with the ‘lower’ dimensions of our universe align in the right ways, travel through this fourth physical dimension can be much shorter than traversing 3D space exclusively.
A good analogy is surface travel on a globe. Rather than travel over the 2D surface to get to the other side of the globe, a traveler instead goes right through center of the sphere, taking advantage of a higher physical dimension--in this case, the third--in order to shorten his trip. Travel through hyperspace is conceptually similar, only taken up a dimensional step.
If physical objects can pass through hyperspace, there’s little reason to assume radio waves couldn’t do so as well. Hyperspace is just another aspect of our own space/time, and all the physical laws that apply to our familiar 3D surroundings would apply equally in the 4D realm as well.
A hyperwave, therefore, may simply be a small, self-contained hyperdrive unit that opens an aperture into hyperspace just large enough to allow a radio signal to be broadcast through that realm, or to receive such a broadcast in turn. How exactly this is done would depend on how the hyperdrive physics worked in that universe. Like with a hyperdrive, the radio broadcast may need to be locked onto a specific hyperspatial trajectory at the time of transmission in order to be received at the desired destination. In this case, it can only be ‘beamed’ at one destination at a time.
Because it is only sending a radio signal instead of a multi-ton starship into hyperspace, its assumed hyperwave units need much less energy to operate than standard hyperdrives. Hence, why hyperwave units are usually depicted as smaller and more portable than their ship-moving cousins. Also, this implies that hyperdrives could be turned into hyperwave radios with some modification.
Hyperspace may not just be an empty void, however. It may have phenomena that may block, interfere, or distort a hyperwave signal much in the same way that three-dimensional space does.
Subspace radio is the main means of FTL communication in the Star Trek universe.
Subspace is an odd concept, that is used (some would say overused) to explain away a lot of the odd anomalies and FTL physics in the various Star Trek series. It is not a separate plane of existence, nor is it a hyperspace, exactly. Rather, it is part of the make-up of our own universe with an infinite number of integral levels.
One way to conceptualize subspace is to think of our universe as the number 3. Let's pretend 3 is the very critical number that defines existence, because if the universe added up to even a tiny smidgen less than 3, the whole thing would fall apart and the universe would blink into nothingness. If everything didn't add up to 3, nothing would exist at all.
Subspace would therefore be like all the tiny decimals that exist between zero and three. A layer of subspace would be at 0.5, another would be at 1.4, and still another at 2.9998, etc. All of these numbers must exist for 3 to exist, and if you take away even the smallest of them the whole thing collapses. So think of all these small, fractal, semi-dimensions of subspace being layered and woven together intricately to form our universe. Some of these subspace fractal dimensions are intricately tied into our everyday existence (such as the ones that determine known physical laws), other may exist only tangentially to our common experience of reality (such as the subspace layers used in Star Trek to allow FTL travel), still others may have collapsed into the planck-scale level of the universe and may manifest themselves only in certain circumstances (such as some of the "spatial anamolies" found in the various Star Trek series.)
Subspace communications would use designated layers of subspace, such as those that allow FTL travel, in order to propagate radio and other signals at superluminal speeds similarly to a hyperwave. Alternately, it may manipulate these layers directly in some way in order to create a superluminal phenomenon that can convey information. In the latter case, different "channels" may simply mean switching the signal to a different layer of subspace.
The Federation and other civilizations of the Star Trek universe use a network of subspace relays to amplify and rebroadcast subspace communications throughout their territory. Within the network, communication can be accomplished in real-time even across the breadth of hundreds of light-years. Without these relays, however, communications even at FTL speeds could take weeks or even years to reach the intended party, depending on distance and conditions.
Current theory spells out explicitly that nothing material can ever be accelerated to the speed of light, much less beyond it. But what if a particle came into existence that was already moving faster than lightspeed ? Since it was never accelerated from a lower velocity, it does not violate relativity. This type of purely-theoretical particle is known as a tachyon.
Tachyons are a theory that goes back decades, and have never been shown to exist in the real word. But if they were proven a reality, they would have a number of very unusual properties. Tachyons would forever exist on the other side of the lightspeed barrier in a strange mirror to our own universe. At their lowest energy levels they would have literally infinite speed, and as they gain energy, they slow down. The slowest a tachyon can ever go is the speed of light, but for that it would need nigh-infinite energy. Even more oddly, their arrow of time is the opposite of ours. They can only ever travel backward in time. More complete explanations of tachyons and tachyon theory are discussed in the links at the end of this article.
It is thought that very high energy collisions, such as those produced by cosmic rays high in the atmosphere, can produce tachyons. If this bears out, these conditions could eventually be reproduced in particle accelerators to give future physicists a source of the particles to work with. A tachyon burst could then be aimed in the direction of the receiving target and broadcast much like a directed radio beam.
However, unlike the other ansible schemes outlined here, the prospect of time travel is much harder to ignore with tachyons. By their very nature, tachyons must travel backward in time, and this causes many potential paradoxes and causality violations. If you use a tachyon communicator, your message will always be received before you even send it.
How far into the past it goes may be relative, however, and risk factors can be minimized. If low-energy tachyons have nigh-infinite speed, they can reach anywhere in the universe in a fraction of a second. For example, the receiver a galaxy away may get your signal in .0000000004 seconds before you send it, such a tiny span of time that it may pose no significant consequences to the causality in either frame of reference on a macroscopic scale. Its with the higher-energy signals, using more pumped up, slower moving tachyons, that paradoxes and other causality problems may pop up.
Because of the need to stick to low-energy signals for tachyon communications to avoid the worst of the potential paradoxes, much more sophisticated technology is needed to detect the comparatively weak signals. Hence why Tachyon Communications has a much higher tech level than the other ansible schemes outlined here.
On Subspace:http://memory-alpha.org/en/wiki/Subspace http://memory-alpha.org/en/wiki/Subspace_communication
On Quantum Entanglementhttp://plato.stanford.edu/entries/qt-entangle/ http://everything2.com/index.pl?node_id=1507278
On Tachyonshttp://scienceworld.wolfram.com/physics/Tachyon.html http://en.wikipedia.org/wiki/Tachyon