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Faster-than-light (also superluminal or FTL) communications and travel refer to the propagation of information or matter faster than the speed of light. This concept is a staple of the science fiction genre, and is also the subject of ongoing scientific study. Faster than light travel In the context of this article, FTL refers to transmitting information or matter faster than c, a constant equal to the speed of light in a vacuum, 299,792,458 meters per second, or roughly 186,000 miles per second. This is not quite the same as travelling faster than light, since: Possibility of FTL Faster-Than-Light travel or communication is problematic in a universe that is consistent with Einstein's theory of relativity. In a hypothetical universe where Newton's laws of motion and the Galilean transformations are exact, rather than approximate, the following would be true: However, according to Einstein's theory of special relativity, what we measure as the speed of light in a vacuum is actually the fundamental physical constant c. This means that all observers, regardless of their acceleration or relative velocity, will always measure zero-mass particles (e.g., gravitons as well as photons) naturally traveling at c. This result means that measurements of time and velocity in different frames are no longer related simply by constant shifts, but are instead related by Poincaré transformations. These transformations have important implications: Because of this, there appear to be only a limited number of ways to justify Faster-Than-Light behavior: Option A: Ignore special relativity This is the simplest solution, and is particularly popular in science fiction. Empirical evidence unanimously affirms that the universe is most-correctly described by Einstein's laws, rather than Newton's, where they disagree. However general relativity is only an approximation due to its incompatibility with quantum mechanics. Special relativity is easily incorporated into nongravitational quantum field theories, however it only applies to a flat Minkowski universe. In particular our expanding universe contains stress-energy which curves the ambient space time and perhaps even has a cosmological constant and so is not Minkowski and in particular is not invariant under Poincaré transformations. However even in the broader context of general relativity, acceleration from subluminal to superluminal speeds does not appear to be possible. Option B: Get light to go faster (Casimir vacuum) Einstein's equations of special relativity posit that the speed of light is invariant in inertial frames. That is, it will be the same from any frame of reference moving at a constant speed. The equations do not specify any particular value for the speed of the light itself. That is an experimentally determined quantity, though it has an exact value because the units of length are defined using the speed of light. The experimental determination has been made in vacuum. However the vacuum we know is not the only possible vacuum which can exist. The vacuum has energy associated with it, called the vacuum energy. This vacuum energy can be changed in certain cases. When vacuum energy is lowered, light itself can go faster than the standard value 'c'. Such a vacuum can be produced by bringing two perfectly smooth metal plates together at near atomic diameter spacing. It is called a Casimir vacuum. Calculations imply light will go faster in such a vacuum. However, there has been no experimental verification, since the technology to detect the change isn't yet available. Einstein's equations of special relativity have an implicit assumption of homogeneity. Space is assumed to be the same everywhere. In the case of the Casimir vacuum, this assumption is clearly violated. Inside the Casimir vacuum, we have homogeneous space, and outside it, we have homogeneous space as well. Inside the Casimir vacuum, the equations of special relativity will apply with the increased value of the speed of light. Outside it, the equations of special relativity will apply with the normal 'c'. However, when considering two frames of reference, one inside the vacuum, and one outside, the equations of special relativity can no longer be applied, since the assumption of homogeneity has been broken. In other words, the Casimir effect breaks up space into distinct homogeneous regions, each of which obey the special relativity laws separately. While this may technically qualify as 'faster-than-light', that is only true relative to two disconnected regions of space. It is unclear whether (and unlikely that) a Casimir vacuum is stable under quantum mechanics, and whether non-trivial communication is possible between two such regions. Option C: Give up causality Another approach is to accept special relativity, but to posit that mechanisms allowed by general relativity (e.g., wormholes) will allow traveling between two points without going through the intervening space. While this gets around the infinite acceleration problem, it still would lead to closed timelike curves (i.e., time travel) and causality violations. Causality is not required by special or general relativity, but is nonetheless considered a basic property of the universe that should not be abandoned. Because of this, most physicists expect (or perhaps hope) that quantum gravity effects will preclude this option. An alternative is to conjecture that, while time travel is possible, it never leads to paradoxes; this is the Novikov self-consistency principle. Option D: Give up (absolute) relativity Due to the strong empirical support for special relativity, any modifications to it must necessarily be quite subtle and difficult to measure. The most well-known attempt is doubly-special relativity, which posits that the Planck length is also the same in all reference frames, and is associated with the work of Giovanni Amelino-Camelia and João Magueijo. One consequence of this theory is a variable speed of light, where photon speed would vary with energy, and some zero-mass particles might possibly travel faster than c. While recent evidence casts doubt on this theory, some physicists still consider it viable. However, even if this theory is true, it is still very unclear that it would allow information to be communicated, and appears not in any case to allow massive particles to exceed c. There are speculative theories that claim inertia is produced by the combined mass of the universe (e.g., Mach's principle), which implies that the rest frame of the universe might be preferred by conventional measurements of natural law. If confirmed, this would imply special relativity is an approximation to a more general theory, but since the relevant comparison would (by definition) be outside the observable universe, it is difficult to imagine (much less construct) experiments to test this hypothesis. Option E: Go somewhere where special relativity does not apply A very popular option taken in science fiction novels, movies, television programs, and computer games is to assume the existence of some other realm (typically called hyperspace or subspace) which is accessible from this universe, in which the laws of relativity are usually distorted, bent, or nonexistent, facilitating rapid transport between distant points in this universe, sometimes with acceleration differences - that is, not requiring as much energy or thrust to go faster. To accomplish rapid transport between points in hyperspace/subspace, special relativity is often assumed not to apply in this other realm. An alternative solution sometimes used is to posit that distant points in the mundane universe correspond to points that are close together in hyperspace. This method of faster-than-light travel does not correspond to anything seriously proposed by mainstream science, although there are also no arguments precluding its existence. Option F: Become faster without acceleration An often-implicit assumption about getting something past light speed is that one must get it to light speed as an intermediate step, thus encountering the infinite energy problem. Similar to the idea of using wormholes to instantly change location, there might be a method to instantly change velocity, rather than having to accelerate through all intermediate velocities. The energy required for acceleration hits an asymptote as one approaches light speed. Thus, an object going much faster than light speed might only need energy comparable to an object going much slower than light; the difficulty lies in figuring out how to "convince" particles to move faster than light without resorting to acceleration. (This also gets around the problem of including a human being; inertia is related to acceleration, not velocity, so it would not occur.) As of yet, no method is known of instantly changing the velocity of matter. Option G: SpaceTime Fabric Contrary to popular belief, Einstein never claimed that it was impossible to go faster than light, it was assumed from his equations. He however has no objections to accepting that spacetime fabric can travel faster than light. It is hypothesized that at the beginning of the universe, spacetime fabric travelled faster than light. Therefore, if we could bend spacetime, we could travel faster than light. Miguel Alcubierre theorized that it would be possible to "warp" spacetime by shrinking spacetime in front of you and expanding it behind you. Such warping would require a source of negative energy which would act as anti-gravity. A possible candidate is the dark energy of cosmology. However it is not clear if dark energy even exists and if it does what its properties are. Tachyons In special relativity, while it is impossible to accelerate an object to the speed of light, or for a massive object to move at the speed of light, it is not impossible for an object to exist which always moves faster than light. The hypothetical elementary particles that have this property are called tachyons. Their existence has neither been proven nor disproven. While tachyons have never been observed, they are present in many physical theories. For example tachyons appear in the standard model of interactions in particle physics, in bosonic string theory and even in superstring theory in the presence of an unstable D-brane or in certain compactifications such as an eight-manifold times a discrete quotient of the complex plane. In each of these examples one sees that the tachyon is best thought of not as a particle, but rather as an instability in the theory. In some cases one can demonstrate that this instability leads to a decay to a stable state, for example in the standard model at temperatures below the electroweak scale the instability implies that a condensate forms which is responsible for the mass of all matter. When one expands the theory about the condensed vacuum one finds no tachyons. The open string tachyons attached to unstable D-branes also condense, after which the unstable D-branes disappear. Allan Adams, Eva Silverstein and Joseph Polchinski have shown that the closed string tachyon in the forementioned unstable compactification also condenses, and as it condenses the space-time itself decays and it never reaches a stable state. The fate of the bosonic string after closed string tachyon condensation is still unknown. In each of these cases it has not been useful to think of the propagation of a single tachyonic particle, as a flood of such particles is dynamically formed from the vacuum until a steady state is reached. (See also tachyon condensation.) The equations of relativity do allow faster-than-light travel, however, any particle which is moving faster and faster, at velocities less than c, ends up with more and more kinetic energy. This is true even in the classical model, but with special relativity, as the velocity approaches c, the energy increases without limit. (Sometimes incorrectly stated as 'the energy goes to infinity'. Incorrect as the energy remains finite at all times, although it becomes arbitrarily large as the velocity approaches 'c'.) If the velocity is greater than c, the energy has no place to go but down. In other words, a particle with mass moving at any speed above c will lose energy when its velocity goes up even further. Put another way, such a particle will speed up when it loses energy. Everything that moves causes a change in the structure of the fabric of space. This change in the structure of the fabric of space causes the formation of gravitational waves that carry away energy. In most cases the change is negligible. However, for a particle with mass moving above c, even a tiny loss of energy is troublesome. As mentioned above, it actually increases the velocity, causing more energy loss, which increases the velocity further. This positive feedback loop causes the particle to soon reach infinite velocity. General relativity General relativity was developed after special relativity to include concepts like gravity. It maintains the principle that no object can accelerate to the speed of light in the reference frame of any coincident observer. However, it permits distortions in spacetime that allow an object to move faster than light from the point of view of a distant observer. One such distortion is the Alcubierre drive, which can be thought of as producing a ripple in spacetime that carries an object along with it. Another possible system is the wormhole, which connects two distant locations as though by a shortcut. Both distortions would need to create a very strong curvature in a highly localized region of space-time and their gravity fields would be immense. To counteract the unstable nature, and prevent the distortions from collapsing under their own 'weight', one would need to introduce hypothetical exotic matter or negative energy. General relativity also agrees that any technique for faster-than-light travel could also be used for time travel. This raises problems with causality. Many physicists believe that the above phenomena are in fact impossible, and that future theories of gravity will prohibit them. One theory states that stable wormholes are possible, but that any attempt to use a network of wormholes to violate causality would result in their decay. In string theory Eric Gimon and Petr Hořava have argued• that in a supersymmetric five-dimensional Gödel universe quantum corrections to general relativity effectively cut off regions of spacetimes with causality-violating closed timelike curves. In particular, in the quantum theory a smeared supertube is present that cuts the spacetime in such a way that, although in the full spacetime a closed timelike curve passed through every point, no complete curves exist on the interior region bounded by the tube. Relative motion It is also possible for two objects to move faster than light relative to each other, but only from the point of view of an observer in a third frame of reference, who naively adds velocities according to Galilean relativity. An observer on either object will see the other object moving slower than light. For example, fast-moving particles on opposite sides of a circular particle accelerator will appear to be moving at slightly less than twice the speed of light, relative to each other, from the point of view of an observer standing at rest relative to the accelerator, and who naively adds velocities according to Galilean relativity. However, if the observer has a good intuition of special relativity, and makes a correct calculation, and the two particles are moving, for example, at velocities and and , then from the observer's point of view, the relative velocity Δβ (again in units of the speed of light c) is , which is less than the speed of light. Phase velocities above c The phase velocity of a wave can easily exceed c, the vacuum velocity of light. In principle, this can occur even for simple mechanical waves, even without any object moving with velocities close to or above c. However, this does not imply the propagation of signals with a velocity above c.• Group velocities above c Under certain circumstances, even the group velocity of a wave (e.g. a light beam) can exceed c. In such cases, which typically at the same time involve rapid attenuation of the intensity, the maximum of a pulse may travel with a velocity above c. However, even this situation does not imply the propagation of signals with a velocity above c, even though one may be tempted to associate pulse maxima with signals. The latter association has been shown to be misleading, basically because the information on the arrival of a pulse can be obtained before the pulse maximum arrives. For example, if some mechanism allows the full transmission of the leading part of a pulse while strongly attenuating the pulse maximum and everything behind, the pulse maximum is effectively shifted forward in time, while the information on the pulse does not come faster than without this effect. Universal expansion The expansion of the universe causes distant galaxies to recede from us faster than the speed of light, if comoving distance and cosmological time are used to calculate the speeds of these galaxies. However, in general relativity, velocity is a local notion, so velocity calculated using comoving coordinates does not have any simple relation to velocity calculated locally. In other words, the galaxies (or more accurately, galaxy clusters) do not physically move away from each other. Rather, the fabric of spacetime between them expands. This also explains the inflationary epoch during the Big Bang, when a volume of the universe a hundred billion times smaller than a proton expanded to approximately one hundred million light years in diameter in just 10-32 seconds. Astronomical observations Apparent superluminal motion is observed in many radio galaxies, blazars, quasars and recently also in microquasars. The effect was predicted before it was observed, and can be explained as an optical illusion caused by the object moving in the direction of the observer, when the speed calculations assume it does not. The phenomenon does not contradict the theory of special relativity. Interestingly, corrected calculations show these objects have velocities close to the speed of light (relative to our reference frame). They are the first examples of large amounts of mass moving at close to the speed of light. Earth-bound laboratories have only been able to accelerate small numbers of elementary particles to such speeds. Quantum mechanics Certain phenomena in quantum mechanics, such as quantum entanglement, appear to transmit information faster than light. These phenomena do not allow true communication; they only let two observers in different locations see the same event simultaneously, without any way of controlling what either sees. The fact that the laws of physics seem to conspire to prevent superluminal communications via quantum mechanics is very interesting and somewhat poorly understood. In recent years, however, the position against quantum entanglement allowing FTL has strengthened due to developments in quantum decoherence. Wavefunction collapse can be viewed as an epiphenomen of quantum decoherence, which in turn is nothing more than an effect of the underlying local time evolution of the wavefunction of a system and all of its environment. Since the underlying behaviour doesn't violate local causality or allow FTL it follows that neither does the additional effect of wavefunction collapse, whether real or apparent. The uncertainty principle implies that individual photons may travel for short distances at speeds somewhat faster (or slower) than c, even in a vacuum; this possibility must be taken into account when enumerating Feynman diagrams for a particle interaction. To quote Richard Feynman: "… there is also an amplitude for light to go faster (or slower) than the conventional speed of light. You found out in the last lecture that light doesn't go only in straight lines; now, you find out that it doesn't go only at the speed of light! It may surprise you that there is an amplitude for a photon to go at speeds faster or slower than the conventional speed, c"• However, this does not imply the possibility of superluminal information transmission, as no photon can have an average speed in excess of the speed of light. There have been various reports in the popular press of experimentally based of faster-than-light transmission in optics — most often in the context of a kind of quantum tunneling phenomenon. Usually, such reports deal with a phase velocity or group velocity faster than the vacuum velocity of light. But recall from above, that a superluminal phase velocity cannot be used for faster-than-light transmission of information. There has sometimes been confusion concerning the latter point. Quantum teleportation transmits quantum information at whatever speed is used to transmit the same amount of classical information, likely the speed of light. This quantum information may theoretically be used in ways that classical information can not, such as in quantum computations involving quantum information only available to the recipiant. However, this does not allow additional classical information to be extracted from the quantum information. In science fiction, quantum teleportation is either used as a basis for teleportation of physical objects at the speed of light, presumably preserving some important aspect of the entanglment between the particles of the object, or else is miss-represented as allowing faster-than-light communication. As it is currently understood, quantum mechanics is completely consistent with special relativity, and doesn't allow for faster-than-light communication. Variable speed of light In conventional physics, the speed of light in a vacuum is assumed to be a constant. There exist theories which postulate that the speed of light is not a constant. The interpretation of this statement is as follows. The speed of light is a dimensionful quantity and so, as has been emphasized in this context by João Magueijo, it cannot be measured.• Measurable quantities in physics are, without exception, dimensionless, although they are often constructed as ratios of dimensionful quantities. For example, when you measure the height of a mountain you really measure the ratio of its height to the length of a 50,000Ft. long stick. Our universe has seven basic dimensionful quantities, which can be taken to be distance, mass, time, electric current, temperature, amount of substance, and luminous intensity•. These units are thought to be independent and cannot be described in terms of each other. For example, temperature and electric current are not simply just another form of energy. One can then reduce all measurements to dimensionless quantities by constructing the ratios with respect to the 7 independent reference quantities. For example, in quantum gravity one usually fixes Newton's constant, the speed of light and Planck's constant. By manipulating these basic constants one can construct the Planck time, Planck length and Planck energy which make a good system of units. João's proposal used a different set of units, as one should never do, a choice which he justifies with the claim that some equations will be simpler in these new units. In the new units he fixes the fine structure constant, a quantity which some people, using units in which the speed of light is fixed, have claimed is time dependent. Thus in the system of units in which the fine structure constant is fixed, the observational claim is that the speed of light is time-dependent. While it may be mathematically possible to construct such a system, it is not clear what additional explanatory power or physical insight such a system would provide, assuming that it does indeed accord with existing empirical data. See also | |||||||
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