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"Faster than the speed of light" redirects here. For other uses, see Faster than the speed of light (disambiguation).

Faster-than-light (also superluminal or FTL) communications and travel refer to the propagation of information or matter faster than the speed of light. Superluminal communication is the term used to describe the hypothetical process by which one might send Information at Faster-than-light (FTL speeds Interstellar space travel is unmanned or manned Travel between Stars The concept of interstellar travel in Starships is a staple in Science fiction Information as a concept has a diversity of meanings from everyday usage to technical settings Matter is commonly defined as being anything that has mass and that takes up space. Science fiction style space travel, dubbed "True" FTL, in which matter exceeds the speed of light in its own frame of reference, defies known physics. See also Inertial frame A frame of reference in Physics, may refer to a Coordinate system or set of axes within which to

Under the special theory of relativity, a particle (that has mass) with subluminal velocity needs infinite energy to accelerate to the speed of light, although special relativity does not forbid the existence of particles that travel faster than light at all times (see tachyon). Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial A tachyon (from the Greek, takhyónion, from, takhýs, ie swift fast is any hypothetical particle that travels at Faster-than-light

On the other hand, what some physicists refer to as "apparent" or "effective" FTL[1][2][3][4] is the hypothesis that unusually distorted regions of spacetime might permit matter to reach distant locations faster than what it would take light in the "normal" route (though still moving subluminally through the distorted region). SpaceTime is a patent-pending three dimensional graphical user interface that allows end users to search their content such as Google Google Images Yahoo! YouTube eBay Amazon and RSS

Apparent FTL is not excluded by general relativity. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 Examples of apparent FTL proposals are the Alcubierre drive and the traversable wormhole, although the physical plausibility of these solutions is uncertain. The Alcubierre metric, also known as the Alcubierre drive or Warp Drive, is a speculative mathematical model of a spacetime exhibiting features reminiscent of In Physics, a wormhole is a hypothetical topological feature of Spacetime that is fundamentally a 'shortcut' through Space and Time

Outside of mainstream physics, others (often without traditional physics training) have speculated on mechanisms that might allow FTL travel to be achieved, often relying on new conjectures of physics of their own invention, but their ideas have not gained significant acceptance in the physics research community. Fictional depictions of superluminal travel and the mechanisms of achieving it are also a staple of the science fiction genre.

Contents

Faster than light travel

In the context of this article, FTL is 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 about 186,282 miles per second. This is not quite the same as travelling faster than light, since:

Neither of these phenomena violates special relativity or creates problems with causality, and thus neither qualifies as FTL as described here. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial Causality (but not causation) denotes a necessary relationship between one event (called cause and another event (called effect) which is the direct consequence

Possibility of FTL

Faster-than-light communication is, by Einstein's theory of relativity, equivalent to time travel. Albert Einstein ( German: ˈalbɐt ˈaɪ̯nʃtaɪ̯n; English: ˈælbɝt ˈaɪnstaɪn (14 March 1879 – 18 April 1955 was a German -born theoretical This page is about the scientific concept of relativity for philosophical or sociological theories about relativity see Relativism. This article details time travel itself For other uses see Time Traveler. 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. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial This means that all observers, regardless of their relative velocity, will always measure zero-mass particles such as photons traveling at c in a vacuum. In Physics, velocity is defined as the rate of change of Position. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena 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. In Physics and Mathematics, the Poincaré group, named after Henri Poincaré, is the group of isometries of Minkowski spacetime These transformations have important implications:

Albert Einstein elaborated that faster-than-light travel is impossible.

Justifications for FTL travel

Despite the established conclusion that relativity precludes FTL travel, some have proposed ways to justify FTL behavior:

Ignore special relativity

This option is popular particularly in science fiction. However, empirical and theoretical evidence strongly supports Einstein's theory of special relativity as the correct description of high-speed motion,[7] which generalizes the more familiar Galilean relativity, which is actually an approximation at conventional (much less than c) speeds. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial Galilean invariance or Galilean relativity is a Principle of relativity which states that the fundamental laws of physics are the same in all Inertial Similarly, general relativity is an overwhelmingly supported and experimentally verified theory of gravitation, except in the regime of very high energy densities over very short distances, where an as-yet-undeveloped theory of quantum gravity is necessary. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 Particle physics is a branch of Physics that studies the elementary constituents of Matter and Radiation, and the interactions between them In Particle physics and Physical cosmology, the Planck scale is an Energy scale around 1 Quantum gravity is the field of Theoretical physics attempting to unify Quantum mechanics, which describes three of the fundamental forces of nature Special relativity, however, is incorporated easily into quantum field theories. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial In quantum field theory (QFT the forces between particles are mediated by other particles Therefore, even in the broader contexts of general relativity and quantum mechanics, conventional acceleration from subluminal to superluminal speeds is not possible.

Get light to go faster (Casimir vacuum and quantum tunnelling)

Einstein's equations of special relativity postulate that the speed of light is invariant in inertial frames. In Physics, the Casimir effect and the Casimir-Polder force are physical forces arising from a quantized field. In Quantum mechanics, quantum tunnelling is a nanoscopic phenomenon in which a particle violates the principles of Classical mechanics by penetrating a Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial In Physics, an inertial frame of reference is a Frame of reference which belongs to a set of frames in which Physical laws hold in the same and simplest 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. Vacuum energy is an underlying background Energy that exists in Space even when devoid of Matter (known as Free space) This vacuum energy can perhaps be changed in certain cases. When vacuum energy is lowered, light itself has been predicted to go faster than the standard value 'c'. This is known as the Scharnhorst effect. The Scharnhorst effect is a hypothetical phenomenon in which the speed of light is slightly greater between two closely-spaced conducting plates than it is in a normal vacuum 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. In Physics, the Casimir effect and the Casimir-Polder force are physical forces arising from a quantized field. Calculations imply that light will go faster in such a vacuum by a minuscule amount: a photon travelling between two plates that are 1 micrometer apart would increase the photon's speed by only about one part in 1036. [8] Accordingly there has as yet been no experimental verification of the prediction. A recent analysis[9] argued out that the Scharnhorst effect cannot be used to send information backwards in time with a single set of plates since the plates' rest frame would define a "preferred frame" for FTL signalling. However, with multiple pairs of plates in motion relative to one another the authors noted that they had no arguments that could "guarantee the total absence of causality violations", and invoked Hawking's speculative chronology protection conjecture which suggests that feedback loops of virtual particles would create "uncontrollable singularities in the renormalized quantum stress-energy" on the boundary of any potential time machine, and thus would require a theory of quantum gravity to fully analyze (with the hope that such a theory would guarantee the impossibility of a true time machine ever forming). The chronology protection conjecture is a Conjecture by the Physicist Professor Stephen Hawking that the Laws of physics are such as to prevent Other authors argue that Scharnhorst's original analysis which seemed to show the possibility of faster-than-c signals involved approximations which may be incorrect, so that it is not clear whether this effect could actually increase signal speed at all. [10]

Getting light to go faster still doesn't mean one can travel faster than it, it just increases the speed limit from the standard one of 299,792,458 m/s. A comical variant of this explanation is used to justify the superluminal travel of starships on the animated sci-fi series, Futurama (The writers of the show justified ships travelling faster than the speed of light by explaining that the speed of light had been increased). The bouncing ball animation (below consists of these 6 frames Futurama is an Emmy Award -winning animated American sitcom created by Matt Groening, and developed by Groening and

The physicists Günter Nimtz and Alfons Stahlhofen, of the University of Koblenz, claim to have violated relativity experimentally by transmitting photons faster than the speed of light. Günter Nimtz (1936 is a German physicist Working at the 2nd Physics Institute at the University of Cologne ( Universität zu Köln) in Germany he has been conducting [11] They say they have conducted an experiment in which microwave photons - relatively low energy packets of light - travelled "instantaneously" between a pair of prisms that had been moved up to 3ft apart, using a phenomenon known as quantum tunnelling. In Quantum mechanics, quantum tunnelling is a nanoscopic phenomenon in which a particle violates the principles of Classical mechanics by penetrating a Nimtz told New Scientist magazine: "For the time being, this is the only violation of special relativity that I know of. New Scientist is a weekly International science magazine and website covering recent developments in science and technology for a general English -speaking " However, other physicists say that this phenomenon does not allow information to be transmitted faster than light. Aephraim Steinberg, a quantum optics expert at the University of Toronto, Canada, uses the analogy of a train traveling from Chicago to New York, but dropping off train cars at each station along the way, so that the center of the train moves forward at each stop; in this way, the center of the train exceeds the speed of any of the individual cars. [12]

Give up causality

Another approach is to accept special relativity, but to posit that mechanisms allowed by general relativity (e. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 g. , wormholes) will allow traveling between two points without going through the intervening space. In Physics, a wormhole is a hypothetical topological feature of Spacetime that is fundamentally a 'shortcut' through Space and Time While this gets around the infinite acceleration problem, it still would lead to closed timelike curves (i. In a Lorentzian manifold, a closed timelike curve (CTC is a Worldline of a material particle in Spacetime that is "closed" returning to its e. , time travel) and causality violations, as can be seen in this graphical description of a tachyon's pistol duel. Causality describes the relationship between Causes and Effects is fundamental to all natural Science, especially Physics, and has a basis in Causality is not required by special or general relativity, but is nonetheless generally 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. Quantum gravity is the field of Theoretical physics attempting to unify Quantum mechanics, which describes three of the fundamental forces of nature An alternative is to conjecture that, while time travel is possible, it never leads to paradoxes; this is the Novikov self-consistency principle. The Novikov self-consistency principle, also known as the Novikov self-consistency conjecture, is a Principle developed by Dr

An important point to note is that in general relativity it is possible for objects to be moving apart faster than light because of the expansion of the universe, in some reasonable choice of cosmological coordinates. This is understood to be due to the expansion of the space between the objects, and general relativity still reduces to special relativity in a "local" sense, meaning that two objects passing each other in a small local region of spacetime cannot have a relative velocity greater than c, and will move more slowly than a light beam passing through the region. (See Option F below)

Give up (absolute) relativity

Because of the strong empirical support for special relativity, any modifications to it must necessarily be quite subtle and difficult to measure. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial The best-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. Doubly-special relativity (DSR— also called deformed special relativity or by some extra-special relativity — is a modified theory of Special relativity The Planck length, denoted by \scriptstyle\ell_P \, is the unit of Length approximately 1 Giovanni Amelino-Camelia (born 14 December, 1965) is an Italian physicist of the University of Rome La Sapienza who works on Quantum gravity João Magueijo (born Évora, 1967 is a Portuguese cosmologist and professor in Theoretical Physics at Imperial College London 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. The variable speed of light (VSL concept states that the Speed of light in a vacuum usually denoted by c, may not be Constant in some cases While recent evidence casts doubt on this theory, some physicists still consider it viable. However, even if this theory is accurate, it is still very unclear whether 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. In Theoretical physics, particularly in discussions of gravitation theories, Mach's principle (or Mach's conjecture is the name given by Einstein to a 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. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial

Go somewhere where the speed of light is not the limit

A very popular option in science fiction is to assume the existence of some other realm (typically called hyperspace, subspace, or slipspace) 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. Hyperspace is a fictional Plot device sometimes used in Science fiction. In the Star Trek fictional universe subspace is a feature of space-time which facilitates Faster-than-light transit in the form of interstellar travel " Slipstream " is a Science fiction term for a fictional method of Faster-than-light space travel similar to hyperspace travel Warp Thrust is a reaction force described quantitatively by Newton 's Second and Third Laws. To accomplish rapid transport between points in hyperspace/subspace, special relativity is often assumed not to apply in this other realm, or that the speed of light is higher. Another solution 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.

Distort the space-time fabric

Although the theory of special relativity forbids objects to have a relative velocity greater than light speed, and general relativity reduces to special relativity in a local sense (in small regions of spacetime where curvature is negligible), general relativity does allow the space between distant objects to expand in such a way that they have a "recession velocity" which exceeds the speed of light, and it is thought that galaxies which are at a distance of more than about 14 billion light years from us today have a recession velocity which is faster than light. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 [13] Miguel Alcubierre theorized that it would be possible to create what is called an Alcubierre drive, in which a ship would be enclosed in a "warp bubble" where the space at the front of the bubble is rapidly contracting and the space at the back is rapidly expanding, with the result that the bubble can reach a distant destination much faster than a light beam moving outside the bubble, but without objects inside the bubble locally travelling faster than light. Miguel Alcubierre (Born 1964 is a Mexican Theoretical physicist. The Alcubierre metric, also known as the Alcubierre drive or Warp Drive, is a speculative mathematical model of a spacetime exhibiting features reminiscent of However, several objections raised against the Alcubierre drive appear to rule out the possibility of actually using it in any practical fashion. The Alcubierre metric, also known as the Alcubierre drive or Warp Drive, is a speculative mathematical model of a spacetime exhibiting features reminiscent of Another possibility predicted by general relativity is the traversable wormhole, which could create a shortcut between arbitrarily distant points in space. In Physics, a wormhole is a hypothetical topological feature of Spacetime that is fundamentally a 'shortcut' through Space and Time As with the Alcubierre drive, travelers moving through the wormhole would not locally move faster than light which travels through the wormhole alongside them, but they would be able to reach their destination (and return to their starting location) faster than light traveling outside the wormhole.

Make use of the fact that space and time are quantized

As given by the planck length, there is a minimum amount of 'space' that can exist in this universe (1. The Planck length, denoted by \scriptstyle\ell_P \, is the unit of Length approximately 1 616x10-35 meters). This limit can be used to determine a minimum time quantisation of 1. A chronon is a proposed Quantum of time that is a discrete and indivisible "unit" of time as part of a theory that proposes that time is not continuous 078x10-43 seconds, which corresponds to a beam of light with a wavelength approaching the planck length. The Planck length, denoted by \scriptstyle\ell_P \, is the unit of Length approximately 1 This means that there is a physical limit to how much blue shift a beam of light can endure. Blue shift is the shortening of a transmitted signal's Wavelength, and/or an increase in its Frequency, due to the Doppler Effect, which indicates that According to general relativity there is no limit to this shift, and an infinitesimally small space can exist, but according to well accepted quantum theory these limits do exist.

This is precisely what happens towards the center of a black hole; the incoming light becomes blue shifted past the planck length as it approaches the region of discontinuity within our universe. A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e Blue shift is the shortening of a transmitted signal's Wavelength, and/or an increase in its Frequency, due to the Doppler Effect, which indicates that The Planck length, denoted by \scriptstyle\ell_P \, is the unit of Length approximately 1 The Universe is defined as everything that Physically Exists: the entirety of Space and Time, all forms of Matter, Energy The argument is: if a black hole with finite mass can create such a discontinuity in the fabric of space and time, why would people be unable to do the same thing using a finite amount of energy and acceleration? (According to general relativity, the space-time distortions caused by gravity are fundamentally identical to space-time distortions caused simply by accelerating your reference frame). General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916

Tachyons

Main article: Tachyon

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. A tachyon (from the Greek, takhyónion, from, takhýs, ie swift fast is any hypothetical particle that travels at Faster-than-light The hypothetical elementary particles that have this property are called tachyons. In Particle physics, an elementary particle or fundamental particle is a particle not known to have substructure that is it is not known to be made A tachyon (from the Greek, takhyónion, from, takhýs, ie swift fast is any hypothetical particle that travels at Faster-than-light Their existence has neither been proven nor disproven, but even so attempts to quantise them show that they may not be used for faster-than-light communication. In Digital signal processing, quantization is the process of approximating a continuous range of values (or a very large set of possible discrete values by a relatively-small [14] Physicists sometimes regard the existence of mathematical structures similar to Tachyons arising from theoretical models and theories as signs of an inconsistency or that the theory needs further refining. [15]

General relativity

General relativity was developed after special relativity to include concepts like gravity. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial Gravitation is a natural Phenomenon by which objects with Mass attract one another 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. SpaceTime is a patent-pending three dimensional graphical user interface that allows end users to search their content such as Google Google Images Yahoo! YouTube eBay Amazon and RSS 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. A distortion is the alteration of the original shape (or other characteristic of an object image sound waveform or other form of information or representation The Alcubierre metric, also known as the Alcubierre drive or Warp Drive, is a speculative mathematical model of a spacetime exhibiting features reminiscent of SpaceTime is a patent-pending three dimensional graphical user interface that allows end users to search their content such as Google Google Images Yahoo! YouTube eBay Amazon and RSS Another possible system is the wormhole, which connects two distant locations as though by a shortcut. In Physics, a wormhole is a hypothetical topological feature of Spacetime that is fundamentally a 'shortcut' through Space and Time 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. Exotic matter is a hypothetical concept of Particle physics. It covers any material which violates one or more classical conditions or is not made of known baryonic particles

General relativity also agrees that any technique for faster-than-light travel could also be used for time travel. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 Travel is the change in location of people on a trip through the means of Transport from one location to another This article details time travel itself For other uses see Time Traveler. This raises problems with causality. Causality (but not causation) denotes a necessary relationship between one event (called cause and another event (called effect) which is the direct consequence Many physicists believe that the above phenomena are in fact impossible, and that future theories of gravity will prohibit them. Gravitation is a natural Phenomenon by which objects with Mass attract one another 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[16] 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. String theory is a still-developing scientific approach to Theoretical physics, whose original building blocks are one-dimensional extended objects called strings Petr Hořava is a Czech string theorist. He is well-known for his articles written with Edward Witten about the Horava-Witten domain walls in In Particle physics, supersymmetry (often abbreviated SUSY) is a Symmetry that relates elementary particles of one spin to another particle that The Gödel metric is an exact solution of the Einstein field equations in which the Stress-energy tensor contains two terms the first representing the 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.

Superficially FTL phenomena which do not carry information

Closing speeds

An observer may conclude that two objects are moving faster than the speed of light relative to each other, by adding their velocities according to the principle of Galilean relativity. The velocity-addition formula is one of two physics equations that relates the velocities of a moving object in different reference frames Galilean Addition Of Velocities A principle of relativity is a criterion for judging physical theories, stating that they are inadequate if they do not prescribe the exact same laws of physics in

For example, two fast-moving particles approaching each other from opposite sides of a 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. This correctly reflects the rate at which the distance between the two particles is decreasing, from the observer's point of view and is called the closing speed. However, it is not the same as the velocity of one of the particles as would be measured by a hypothetical fast-moving observer travelling alongside the other particle. To obtain this, the calculation must be done according to the principle of special relativity. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial If the two particles are moving at velocities v and -v, or expressed in units of c, β and − β, where

\beta \equiv v/c \,\!

then this relative velocity (again in units of the speed of light c) is

\beta_{rel} = { \beta - (-\beta) \over 1 + \beta ^2 } = { 2\beta \over 1 + \beta^2 },

which is less than the speed of light.

Phase velocities above c

The phase velocity of an electromagnetic wave, when traveling through a medium, can routinely exceed c, the vacuum velocity of light. The phase velocity (or phase speed) of a Wave is the rate at which the phase of the wave propagates in space Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. For example, this occurs in most glasses at X-ray frequencies[17]. X-radiation (composed of X-rays) is a form of Electromagnetic radiation. However, the phase velocity of a wave corresponds to the propagation speed of a theoretical single-frequency (purely monochromatic) component of the wave at that frequency. Monochrome comes from the Greek μονόχρωμος ( monochromos) meaning “of one color” which is a combination Such a wave component must be infinite in extent and of constant amplitude (otherwise it is not truly monochromatic), and so cannot convey any information[18]. Thus a phase velocity above c does not imply the propagation of signals with a velocity above c. In the fields of communications, Signal processing, and in Electrical engineering more generally a signal is any time-varying or spatial-varying quantity

Group velocities above c

The group velocity of a wave (e. The group velocity of a Wave is the Velocity with which the variations in the shape of the wave's amplitude (known as the modulation or envelope g. a light beam) may also exceed c in some circumstances. In such cases, which typically at the same time involve rapid attenuation of the intensity, the maximum of the envelope 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. In the fields of communications, Signal processing, and in Electrical engineering more generally a signal is any time-varying or spatial-varying quantity 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. The Universe is defined as everything that Physically Exists: the entirety of Space and Time, all forms of Matter, Energy In standard cosmology, ' comoving' distance and ' proper distance' are two closely related distance measures used by cosmologists to define distances between 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. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 [19] Rules that apply to relative velocities in special relativity, such as the rule that relative velocities cannot increase past the speed of light, do not apply to relative velocities in comoving coordinates, which are often described in terms of the "expansion of space" between galaxies. This expansion rate is thought to have been at its peak during the inflationary epoch thought to have occurred in a tiny fraction of the second after the Big Bang (models suggest the period would have been from around 10-36 seconds after the Big Bang to around 10-33 seconds), when the universe may have rapidly expanded by a factor of around 1020 - 1030. See also Cosmic inflation In Physical cosmology the inflationary epoch was the period in the evolution of the early universe when according to Inflation The Big Bang is the cosmological model of the Universe that is best supported by all lines of scientific evidence and Observation. [20]

Astronomical observations

Apparent superluminal motion is observed in many radio galaxies, blazars, quasars and recently also in microquasars. In Astronomy, superluminal motion is the apparently Faster-than-light motion seen in some radio galaxies, Quasars and recently also in some galactic An active galactic nucleus ( AGN) is a compact region at the centre of a Galaxy which has a much higher than normal luminosity over some or all of the Electromagnetic A blazar is a very compact and highly variable energy source associated with a presumed Supermassive black hole at the center of a Host galaxy. A quasar (contraction of QUASi-stellAR radio source) is an extremely powerful and distant Active galactic nucleus. The effect was predicted before it was observed by Martin Rees and can be explained as an optical illusion caused by the object partly moving in the direction of the observer,[21] when the speed calculations assume it does not. Martin John Rees Baron Rees of Ludlow, OM, PRS (born 23 June 1942 in York) is an English cosmologist This article is about visual perception See Optical Illusion (Album for information about the Time Requiem album The phenomenon does not contradict the theory of special relativity. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial 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. [22] 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. Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons Quantum entanglement is a quantum mechanical Phenomenon in which the Quantum states of two or more objects are linked together so that one object According to the No-communication theorem 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. In Quantum information theory, a no-communication theorem is a result which gives conditions under which instantaneous transfer of information between two observers is impossible Wavefunction collapse can be viewed as an epiphenomenon 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. In certain interpretations of quantum mechanics, wave function collapse is one of two processes by which Quantum systems apparently evolve according to the laws of 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. In Quantum physics, the Heisenberg uncertainty principle states that locating a particle in a small region of space makes the Momentum of the particle uncertain Motivation and history When calculating Scattering cross sections in Particle physics, the interaction between particles can be described To quote Richard Feynman:

… there is also an amplitude for light to go faster (or slower) than the conventional speed of light. Richard Phillips Feynman (ˈfaɪnmən May 11 1918 – February 15 1988 was an American Physicist known for the Path integral formulation of quantum 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. [23]

– Richard Feynman

However, macroscopically these fluctuations average out, so that photons do travel in straight lines over long (i. e. non-quantum) distances, and they do travel at the speed of light on average. Therefore, this does not imply the possibility of superluminal information transmission.

There have been various reports in the popular press of experiments on faster-than-light transmission in optics — most often in the context of a kind of quantum tunneling phenomenon. In Quantum mechanics, quantum tunnelling is a nanoscopic phenomenon in which a particle violates the principles of Classical mechanics by penetrating a Usually, such reports deal with a phase velocity or group velocity faster than the vacuum velocity of light. The phase velocity (or phase speed) of a Wave is the rate at which the phase of the wave propagates in space The group velocity of a Wave is the Velocity with which the variations in the shape of the wave's amplitude (known as the modulation or envelope 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. Quantum teleportation, or entanglement-assisted teleportation, is a technique used to transfer information on a Quantum level usually from one Particle 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 recipient. 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 entanglement between the particles of the object, or else is misrepresented as allowing faster-than-light communication.

Hartman effect

Main article: Hartman effect

The Hartman effect is the tunnelling effect through a barrier where the tunnelling time tends to a constant for large barriers. The delay time for a tunneling Particle is independent of the thickness of the opaque barrier [24] This was first described by Thomas Hartman in 1962. Thomas Hartmann may refer to Thomas W Hartmann, American lawyer and officer in the United States Air Force Reserve Thomas Hartmann (USN, [25] This could, for instance, be the gap between two prisms. When the prisms are in contact, the light passes straight through, but when there is a gap, the light is refracted. There is a finite probability that the photon will tunnel across the gap rather than follow the refracted path. For large gaps between the prisms the tunnelling time approaches a constant and thus the photons appear to have crossed with a superluminal speed. [26]

However, an analysis by Herbert Winful from the University of Michigan suggests that the Hartman effect cannot actually be used to violate relativity by transmitting signals faster than c, because the tunnelling time "should not be linked to a velocity since evanescent waves do not propagate". [27] Winful means by this that the photons crossing the barrier are virtual photons only existing in the interaction and could not be propagated into the outside world.

Casimir effect

In physics, the Casimir effect or Casimir-Polder force is a physical force exerted between separate objects due to resonance of vacuum energy in the intervening space between the objects. In Physics, the Casimir effect and the Casimir-Polder force are physical forces arising from a quantized field. Vacuum energy is an underlying background Energy that exists in Space even when devoid of Matter (known as Free space) This is sometimes described in terms of virtual particles interacting with the objects, due to the mathematical form of one possible way of calculating the strength of the effect. Because the strength of the force falls off rapidly with distance, it is only measurable when the distance between the objects is extremely small. Energy appears suddenly as if it came from the vacuum. See Option B above for a discussion of whether or not this effect could actually be used to send signals faster than c or violate causality.

EPR Paradox

We can also quote the spectacular case of the thought experiment of Einstein, Podolski and Rosen (EPR paradox) which could be realized in experiments for the first time by Alain Aspect in 1981 and 1982. A thought experiment (from the German Gedankenexperiment) is a proposal for an Experiment that would test a Hypothesis or Theory In Quantum mechanics, the EPR paradox is a Thought experiment which challenged long-held ideas about the relation between the observed values of physical quantities In this case, the measurement of the state on one of the quantum systems of an entangled pair forces the other system to be measured in the complementary state. Quantum entanglement is a quantum mechanical Phenomenon in which the Quantum states of two or more objects are linked together so that one object Thus functions quantum teleportation. Quantum teleportation, or entanglement-assisted teleportation, is a technique used to transfer information on a Quantum level usually from one Particle

An experiment performed in 1997 by Nicolas Gisin at the University of Geneva has demonstrated nonlocal quantum correlations between particles separated by over 10 kilometers. [28] But as noted earlier, the nonlocal correlations seen in entanglement cannot actually be used to transmit classical information faster than light, so that relativistic causality is preserved; see no-communication theorem for further information. In Quantum information theory, a no-communication theorem is a result which gives conditions under which instantaneous transfer of information between two observers is impossible

Delayed choice quantum eraser

Delayed choice quantum eraser (The experiment of Marlan Scully) is an alternative of the paradox EPR in which the observation or not of interference after the passage of a photon through a double slit experiment depends on the conditions of observation of a second photon entangled with the first. A delayed choice quantum eraser is a cross between a Quantum eraser experiment and Wheeler's delayed choice experiment. Marlan Orvil Scully is a physicist best known for his work in theoretical quantum optics The characteristic of this experiment is that the observation of the second photon can take place at a later time than the observation of the first photon, [29] which may give the impression that the measurement of the later photons "retroactively" determines whether the earlier photons show interference or not, although the interference pattern can only be seen by correlating the measurements of both members of every pair and so it can't be observed until both photons have been measured, ensuring that an experimenter watching only the photons going through the slit does not obtain information about the other photons in an FTL or backwards-in-time manner (see the delayed choice quantum eraser article for further information). A delayed choice quantum eraser is a cross between a Quantum eraser experiment and Wheeler's delayed choice experiment.

Variable speed of light

Main article: Variable speed of light

In conventional physics, the speed of light in a vacuum is assumed to be a constant. The variable speed of light (VSL concept states that the Speed of light in a vacuum usually denoted by c, may not be Constant in some cases Physics (Greek Physis - φύσις in everyday terms is the Science of Matter and its motion. This vacuum means "absence of matter" or "an empty area or space" for the cleaning appliance see Vacuum cleaner. There exist theories which postulate that the speed of light is not a constant. The word theory has many distinct meanings in different fields of Knowledge, depending on their methodologies and the context of discussion. 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. João Magueijo (born Évora, 1967 is a Portuguese cosmologist and professor in Theoretical Physics at Imperial College London [30] Measurable quantities in physics are, without exception, dimensionless, although they are often constructed as ratios of dimensional quantities. For example, when you measure the height of a mountain you really measure the ratio of its height to the length of a meterstick. The conventional SI system of units is based on seven basic dimensional quantities, namely distance, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity[31]. Distance is a numerical description of how far apart objects are Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object For other uses see Time (disambiguation Time is a component of a measuring system used to sequence events to compare the durations of Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. Thermodynamic temperature is the absolute measure of Temperature and is one of the principal parameters of Thermodynamics. The amount of substance, n, of a sample or system is a Physical quantity which is proportional to the number of elementary entities present In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a Light source in a particular direction per unit Solid These units are defined to be independent and so cannot be described in terms of each other. UNIT ( U nited N ations I ntelligence T askforce later the UN ified I ntelligence T askforce is a fictional military Dependent variables and independent variables refer to values that change in relationship to each other As an alternative to using a particular system of units, one can reduce all measurements to dimensionless quantities expressed in terms of ratios between the quantities being measured and various fundamental constants such as Newton's constant, the speed of light and Planck's constant; physicists can define at least 26 dimensionless constants which can be expressed in terms of these sorts of ratios and which are currently thought to be independent of one another. The gravitational constant, denoted G, is a Physical constant involved in the calculation of the gravitational attraction between objects with mass The Planck constant (denoted h\ is a Physical constant used to describe the sizes of quanta. [32] By manipulating the basic dimensional constants one can also construct the Planck time, Planck length and Planck energy which make a good system of units for expressing dimensional measurements, known as Planck units. In Physics, the Planck time ( tP) is the unit of Time in the system of Natural units known as Planck units. The Planck length, denoted by \scriptstyle\ell_P \, is the unit of Length approximately 1 In Physics, the unit of Energy in the system of Natural units known as Planck units is called the Planck energy, denoted by E P Planck units are Units of measurement named after the German physicist Max Planck, who first proposed them in 1899

João's proposal used a different set of units, a choice which he justifies with the claim that some equations will be simpler in these new units. UNIT ( U nited N ations I ntelligence T askforce later the UN ified I ntelligence T askforce is a fictional military 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. The fine-structure constant or Sommerfeld fine-structure constant, usually denoted \alpha \ is the Fundamental physical constant characterizing 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. Mathematics is the body of Knowledge and Academic discipline that studies such concepts as Quantity, Structure, Space and

See also

Fiction

References

Footnotes

  1. ^ Gonzalez-Diaz, Pedro F. Günter Nimtz (1936 is a German physicist Working at the 2nd Physics Institute at the University of Cologne ( Universität zu Köln) in Germany he has been conducting Intergalactic travel is travel between galaxies, and is considered much more technologically demanding than even Interstellar travel. Hyperspace is a fictional Plot device sometimes used in Science fiction. A jump drive (or with a capital J is one of the speculative inventions in Science fiction, a method of traveling faster than light (FTL In Science fiction, Jumpgate (or jump gate) refers to a device that creates Wormholes allowing fast travel between two points Warp drive is a form of Faster-than-light propulsion in the Fictional universe of Star Trek, capable of propelling spacecraft or objects to many multiples This article contains information about fictional alien races in the Farscape universe " Slipstream " is a Science fiction term for a fictional method of Faster-than-light space travel similar to hyperspace travel Warp In Quantum mechanics, quantum tunnelling is a nanoscopic phenomenon in which a particle violates the principles of Classical mechanics by penetrating a The inertialess drive is a fictional means of accelerating to close to the speed of light or Faster-than-light travel originally used in Triplanetary and A Stargate is fictional device that allows practical rapid travel between two distant locations An ansible is a hypothetical machine capable of Superluminal communication and used as a Plot device in Science fiction literature Ultrawaves are a concept used in Science fiction to represent transmissions or signals that may propagate Faster than light through either normal space or alternate Mass Effect is an Action role-playing game developed by BioWare for Xbox 360 and Microsoft Windows. is a long-running series of Science fiction Anime, created by Shoji Kawamori of Studio Nue in 1982 (2000). "Warp drive space-time". Physical Review D 62: 044005-1—044005-7.   arXiv:gr-qc/9907026
  2. ^ http://arxiv.org/pdf/gr-qc/0107097
  3. ^ PDF for gr-qc/9810026
  4. ^ PDF for gr-qc/9908023
  5. ^ Einstein, A, Relativity:the special and the general theory, pp25-27, Methuen & Co, 1927. The arXiv ( pronounced " Archive " as if the "X" were the Greek letter Chi, χ is an Archive for electronic
  6. ^ Gott, J. Richard (2002). "Time Travel in Einstein's Universe".   pp82-83
  7. ^ What is the experimental basis of Special Relativity?
  8. ^ http://www.nat.vu.nl/~scharnh/m16scine.htm Science News: Secret of the vacuum: Speedier light
  9. ^ S. Liberati, S. Sonego and M. Visser, Faster-than-c signals, special relativity, and causality, Annals Phys. 298, 167-185 (2002) preprint.
  10. ^ [0706.0553] Can Light Signals Travel Faster than c in Nontrivial Vacuua in Flat space-time? Relativistic Causality II
  11. ^ Macroscopic experiments with virtual photons
  12. ^ Anderson, Mark (August 18-24, 2007), “Light seems to defy its own speed limit”, New Scientist 195 (2617): 10, <http://www.eurekalert.org/pub_releases/2007-08/ns-lst081607.php> 
  13. ^ Charles H. New Scientist is a weekly International science magazine and website covering recent developments in science and technology for a general English -speaking Lineweaver and Tamara M. Davis. "Misconceptions about the Big Bang", Scientific American, March 2005.  
  14. ^ Feinberg, Gerald (1967). Gerald Feinberg ( 27 May 1933 – 21 April 1992) was a Columbia University Physicist and Futurist. "Possibility of Faster-Than-Light Particles". Physical Review 159: 1089-1105.  
  15. ^ Gates, S. James. "Superstring Theory: The DNA of Reality".  
  16. ^ Gimon, Eric G. ; Petr Horava (May 2004). Over-rotating black holes, Gödel holography and the hypertube. Retrieved on 2006-06-05. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 70 - Titus and his Roman Legions breach the middle wall of Jerusalem in the Siege of Jerusalem
  17. ^ Hecht, Eugene (1987). Optics, 2nd ed. , Addison Wesley, p. 62. ISBN 0-201-11609-X.  
  18. ^ MathPages - Phase, Group, and Signal Velocity. Retrieved on 2007-04-30. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 313 - Roman emperor Licinius unifies the entire Eastern Roman Empire under his rule
  19. ^ Cosmology Tutorial - Part 2
  20. ^ Inflationary Period from HyperPhysics
  21. ^ Rees, M. J. , "Appearance of relativistically expanding radio sources", Nature 211, 468, 1966.
  22. ^ R. D. Blandford, C. Roger D Blandford FRS is an Astronomer and Astrophysicist. He is a Fellow of the Royal Society and Fellow of the American Academy F. McKee and M. J. Rees, "Super-luminal expansion in extragalactic radio sources", Nature 267, 211, 1977.
  23. ^ Feynman. "Chapter 3", QED, 89.  
  24. ^ J. C. Martinez, and E. Polatdemir, "Origin of the Hartman effect", Physics Letters A, Vol 351, Iss 1-2, 20 February 2006, pp31-36.
  25. ^ T. E. Hartman, "Tunneling of a wave packet", J. Appl. Phys. 33, 3427 (1962).
  26. ^ G. Nimtz, A. A. Stahlhofen, "Macroscopic violation of special relativity", 5 Aug 2007 available as eprint [1]
  27. ^ Winful, H, "Tunneling time, the Hartman effect, and superluminality: A proposed resolution of an old paradox", Physics Reports, Vol 436, Iss 1-2, December 2006, pp1-69.
  28. ^ http://www.quantumphil.org/history.htm
  29. ^ Delayed Choice Quantum Eraser
  30. ^ Magueijo, João (1999). João Magueijo (born Évora, 1967 is a Portuguese cosmologist and professor in Theoretical Physics at Imperial College London A time varying speed of light as a solution to cosmological puzzles. In Chemistry, a solution is a Homogeneous Mixture composed of two or more substances Cosmology (from Greek grc κοσμολογία - grc κόσμος kosmos, "universe" and grc -λογία -logia) is study Retrieved on 2006-06-05. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 70 - Titus and his Roman Legions breach the middle wall of Jerusalem in the Siege of Jerusalem
  31. ^ SI base units.
  32. ^ constants.

Notations

External links

The implications of changeable fundamental "constants" are so profound that any hint that this might be proved true makes news. But the real news is the dramatic increase in precision of the relevant measurements. That is going on in laboratories around the world right now.

Dictionary

faster-than-light

-adjective

  1. Refers to speeds faster than the speed of light in a vacuum, the theoretical speed limit for travel within the universe.
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