Faster-than-lightFrom Wikipedia, the free encyclopedia

Faster-than-light (also superluminal or FTL) communications and travel refer to the propagation of informationor matter faster than the speed of light. Under the special theory of relativity, a particle (that has rest mass) withsubluminal velocity needs infinite energy to accelerate to the speed of light, although special relativity does not forbidthe existence of particles that travel faster than light at all times (tachyons).

On the other hand, what some physicists refer to as "apparent" or "effective" FTL[1][2][3][4] depends on thehypothesis that unusually distorted regions of spacetime might permit matter to reach distant locations in less timethan light could in normal or undistorted spacetime. Although according to current theories matter is still required totravel subluminally with respect to the locally distorted spacetime region, apparent FTL is not excluded by generalrelativity.

Examples of FTL proposals are the Alcubierre drive, and the traversable wormhole, although the physicalplausibility of some of these solutions is uncertain.

Contents1 FTL travel of non-information

1.1 Daily sky motion1.2 Light spots and shadows1.3 Apparent FTL propagation of static field effects1.4 Closing speeds1.5 Proper speeds1.6 How far can one travel from the Earth?1.7 Phase velocities above c1.8 Group velocities above c1.9 Universal expansion1.10 Astronomical observations1.11 Quantum mechanics

1.11.1 Hartman effect1.11.2 Casimir effect1.11.3 EPR Paradox1.11.4 Delayed choice quantum eraser

2 FTL communication possibility3 Justifications

3.1 Faster light (Casimir vacuum and quantum tunnelling)3.2 Give up causality3.3 Give up (absolute) relativity3.4 Space-time distortion3.5 Heim theory3.6 MiHsC/Quantised inertia3.7 Lorentz symmetry violation

3.8 Superfluid theories of physical vacuum4 Time of flight of neutrinos

4.1 MINOS experiment4.2 OPERA neutrino anomaly

5 Tachyons6 General relativity7 Variable speed of light8 See also9 Notes10 References11 External links

11.1 Scientific links11.2 Proposed FTL Methods links

FTL travel of non-informationIn the context of this article, FTL is the transmission of information or matter faster than c, a constant equal to thespeed of light in a vacuum, which is 299,792,458 meters per second (by definition) or about 186,282.4 miles persecond. This is not quite the same as traveling faster than light, since:

Some processes propagate faster than c, but cannot carry information (see examples in the sectionsimmediately following).Light travels at speed c/n when not in a vacuum but travelling through a medium with refractive index = n(causing refraction), and in some materials other particles can travel faster than c/n (but still slower than c),leading to Cherenkov radiation (see phase velocity below).

Neither of these phenomena violates special relativity or creates problems with causality, and thus neither qualifiesas FTL as described here.

In the following examples, certain influences may appear to travel faster than light, but they do not convey energy orinformation faster than light, so they do not violate special relativity.

Daily sky motion

For an earthbound observer, objects in the sky complete one revolution around the Earth in 1 day. ProximaCentauri, which is the nearest star outside the solar system, is about 4 light-years away.[5] On a geostationary viewProxima Centauri has a speed many times greater than c as the rim speed of an object moving in a circle is aproduct of the radius and angular speed.[5] It is also possible on a geostatic view for objects such as comets to varytheir speed from subluminal to superluminal and vice versa simply because the distance from the Earth varies.Comets may have orbits which take them out to more than 1000 AU.[6] The circumference of a circle with a radiusof 1000 AU is greater than one light day. In other words, a comet at such a distance is superluminal in a geostatic—and therefore non-inertial— frame.

Light spots and shadows

If a laser is swept across a distant object, the spot of laser light can easily be made to move across the object at aspeed greater than c.[7] Similarly, a shadow projected onto a distant object can be made to move across the objectfaster than c.[7] In neither case does the light travel from the source to the object faster than c, nor does anyinformation travel faster than light.[7][8][9]

Apparent FTL propagation of static field effects

Main article: Static field

Since there is no "retardation" (or aberration) of the apparent position of the source of a gravitational or electricstatic field when the source moves with constant velocity, the static field "effect" may seem at first glance to be"transmitted" faster than the speed of light. However, uniform motion of the static source may be removed with achange in reference frame, causing the direction of the static field to change immediately, at all distances. This is nota change of position which "propagates", and thus this change cannot be used to transmit information from thesource. No information or matter can be FTL-transmitted or propagated from source to receiver/observer by anelectromagnetic field.

Closing speeds

The rate at which two objects in motion in a single frame of reference get closer together is called the mutual orclosing speed. This may approach twice the speed of light, as in the case of two particles travelling at close to thespeed of light in opposite directions with respect to the reference frame.

Imagine two fast-moving particles approaching each other from opposite sides of a particle accelerator of thecollider type. The closing speed would be the rate at which the distance between the two particles is decreasing.From the point of view of an observer standing at rest relative to the accelerator, this rate will be slightly less thantwice the speed of light.

Special relativity does not prohibit this. It tells us that it is wrong to use Galilean relativity to compute the velocity ofone of the particles, as would be measured by an observer traveling alongside the other particle. That is, specialrelativity gives the right formula for computing such relative velocity.

It is instructive to compute the relative velocity of particles moving at v and -v in accelerator frame, whichcorresponds to the closing speed of 2v > c. Expressing the speeds in units of c, β = v/c:

Proper speeds

If a spaceship travels to a planet one light-year (as measured in the Earth's rest frame) away from Earth at highspeed, the time taken to reach that planet could be less than one year as measured by the traveller's clock (althoughit will always be more than one year as measured by a clock on Earth). The value obtained by dividing the distancetraveled, as determined in the Earth's frame, by the time taken, measured by the traveller's clock, is known as aproper speed or a proper velocity. There is no limit on the value of a proper speed as a proper speed does notrepresent a speed measured in a single inertial frame. A light signal that left the Earth at the same time as thetraveller would always get to the destination before the traveller.

How far can one travel from the Earth?

Since one might not travel faster than light, one might conclude that a human can never travel further from the earththan 40 light-years if the traveler is active between the age of 20 and 60. A traveler would then never be able toreach more than the very few star systems which exist within the limit of 20-40 light-years from the Earth. This is amistaken conclusion: because of time dilation, the traveler can travel thousands of light-years during their 40 activeyears. If the spaceship accelerates at a constant 1 g (in its own changing frame of reference), it will, after 354 days,reach speeds a little under the speed of light (for an observer on Earth), and time dilation will increase their lifespanto thousands of Earth years, seen from the reference system of the Solar System, but the traveler's subjectivelifespan will not thereby change. If the traveler returns to the Earth, they will land thousands of years into the future.Their speed will not be seen as higher than the speed of light by observers on Earth, and the traveler will notmeasure their speed as being higher than the speed of light, but will see a length contraction of the universe in theirdirection of travel. And as the traveler turns around to return, the Earth will seem to experience much more timethan the traveler does. So, although their (ordinary) speed cannot exceed c, the four-velocity (distance as seen byEarth divided by their proper, i.e. subjective, time) can be much greater than c. This is seen in statistical studies ofmuons traveling much further than c times their half-life (at rest), if traveling close to c.[10]

Phase velocities above c

The phase velocity of an electromagnetic wave, when traveling through a medium, can routinely exceed c, thevacuum velocity of light. For example, this occurs in most glasses at X-ray frequencies.[11] However, the phasevelocity of a wave corresponds to the propagation speed of a theoretical single-frequency (purely monochromatic)component of the wave at that frequency. Such a wave component must be infinite in extent and of constantamplitude (otherwise it is not truly monochromatic), and so cannot convey any information.[12] Thus a phasevelocity above c does not imply the propagation of signals with a velocity above c.[13]

Group velocities above c

The group velocity of a wave (e.g., a light beam) may also exceed c in some circumstances. In such cases, whichtypically at the same time involve rapid attenuation of the intensity, the maximum of the envelope of a pulse maytravel with a velocity above c. However, even this situation does not imply the propagation of signals with a velocityabove c,[14] even though one may be tempted to associate pulse maxima with signals. The latter association hasbeen shown to be misleading, basically because the information on the arrival of a pulse can be obtained before thepulse maximum arrives. For example, if some mechanism allows the full transmission of the leading part of a pulsewhile strongly attenuating the pulse maximum and everything behind (distortion), the pulse maximum is effectivelyshifted forward in time, while the information on the pulse does not come faster than c without this effect.[15]

Universal expansion

The expansion of the universe causes distant galaxies to recede from us faster than the speed of light, if comovingdistance 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 tovelocity calculated locally[16] (see comoving distance for a discussion of different notions of 'velocity' incosmology). Rules that apply to relative velocities in special relativity, such as the rule that relative velocities cannotincrease past the speed of light, do not apply to relative velocities in comoving coordinates, which are oftendescribed 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 to 1030.[17]

There are many galaxies visible in telescopes with red shift numbers of 1.4 or higher. All of these are currentlytraveling away from us at speeds greater than the speed of light. Because the Hubble parameter is decreasing withtime, there can actually be cases where a galaxy that is receding from us faster than light does manage to emit asignal which reaches us eventually.[18][19] However, because the expansion of the universe is accelerating, it isprojected that most galaxies will eventually cross a type of cosmological event horizon where any light they emitpast that point will never be able to reach us at any time in the infinite future,[20] because the light never reaches apoint where its "peculiar velocity" towards us exceeds the expansion velocity away from us (these two notions ofvelocity are also discussed in Comoving distance#Uses of the proper distance). The current distance to thiscosmological event horizon is about 16 billion light-years, meaning that a signal from an event happening at presentwould eventually be able to reach us in the future if the event was less than 16 billion light-years away, but the signalwould never reach us if the event was more than 16 billion light-years away.[19]

Astronomical observations

Apparent superluminal motion is observed in many radio galaxies, blazars, quasars and recently also inmicroquasars. The effect was predicted before it was observed by Martin Rees and can be explained as an opticalillusion caused by the object partly moving in the direction of the observer,[21] when the speed calculations assumeit does not. The phenomenon does not contradict the theory of special relativity. Interestingly, corrected calculationsshow these objects have velocities close to the speed of light (relative to our reference frame). They are the firstexamples of large amounts of mass moving at close to the speed of light.[22] Earth-bound laboratories have onlybeen 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 fasterthan light. According to the no-communication theorem these phenomena do not allow true communication; theyonly let two observers in different locations see the same event simultaneously, without any way of controlling whateither sees. Wavefunction collapse can be viewed as an epiphenomenon of quantum decoherence, which in turn isnothing more than an effect of the underlying local time evolution of the wavefunction of a system and all of itsenvironment. Since the underlying behaviour doesn't violate local causality or allow FTL it follows that neither doesthe 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 Feynmandiagrams for a particle interaction.[23] It has since been proven that a single photon may not travel faster than c.[24]

In quantum mechanics, virtual particles may travel faster than light, and this phenomenon is related to the fact thatstatic field effects (which are mediated by virtual particles in quantum terms) may travel faster than light (see sectionon static fields above). However, macroscopically these fluctuations average out, so that photons do travel instraight 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 tunnelling phenomenon. Usually, such reports deal with a phasevelocity or group velocity faster than the vacuum velocity of light.[citation needed] However, as stated above, asuperluminal phase velocity cannot be used for faster-than-light transmission of information. There has sometimesbeen confusion concerning the latter point. Additionally a channel that permits such propagation cannot be laid outfaster than the speed of light.[citation needed]

Quantum teleportation transmits quantum information at whatever speed is used to transmit the same amount ofclassical information, likely the speed of light. This quantum information may theoretically be used in ways thatclassical information can not, such as in quantum computations involving quantum information only available to therecipient.

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 largebarriers.[25] This was first described by Thomas Hartman in 1962.[26] This could, for instance, be the gap betweentwo prisms. When the prisms are in contact, the light passes straight through, but when there is a gap, the light isrefracted. There is a nonzero probability that the photon will tunnel across the gap rather than follow the refractedpath. For large gaps between the prisms the tunnelling time approaches a constant and thus the photons appear tohave crossed with a superluminal speed.[27]

However, an analysis by Herbert G. Winful from the University of Michigan suggests that the Hartman effect cannotactually be used to violate relativity by transmitting signals faster than c, because the tunnelling time "should not belinked to a velocity since evanescent waves do not propagate".[28] The evanescent waves in the Hartman effect aredue to virtual particles and a non-propagating static field, as mentioned in the sections above for gravity andelectromagnetism.

Casimir effect

In physics, the Casimir effect or Casimir-Polder force is a physical force exerted between separate objects due toresonance of vacuum energy in the intervening space between the objects. This is sometimes described in terms ofvirtual particles interacting with the objects, owing to the mathematical form of one possible way of calculating thestrength of the effect. Because the strength of the force falls off rapidly with distance, it is only measurable when thedistance between the objects is extremely small. Because the effect is due to virtual particles mediating a static fieldeffect, it is subject to the comments about static fields discussed above.

EPR Paradox

The EPR paradox refers to a famous thought experiment of Einstein, Podolski and Rosen that was realizedexperimentally for the first time by Alain Aspect in 1981 and 1982 in the Aspect experiment. In this experiment, themeasurement of the state of one of the quantum systems of an entangled pair apparently instantaneously forces theother system (which may be distant) to be measured in the complementary state. However, no information can betransmitted this way; the answer to whether or not the measurement actually affects the other quantum systemcomes down to which interpretation of quantum mechanics one subscribes to.

An experiment performed in 1997 by Nicolas Gisin at the University of Geneva has demonstrated non-localquantum correlations between particles separated by over 10 kilometers.[29] But as noted earlier, the non-localcorrelations seen in entanglement cannot actually be used to transmit classical information faster than light, so thatrelativistic causality is preserved; see no-communication theorem for further information. A 2008 quantum physicsexperiment also performed by Nicolas Gisin and his colleagues in Geneva, Switzerland has determined that in anyhypothetical non-local hidden-variables theory the speed of the quantum non-local connection (what Einsteincalled "spooky action at a distance") is at least 10,000 times the speed of light.[30]

Delayed choice quantum eraser

Main article: Delayed choice quantum eraser

Delayed choice quantum eraser (an experiment of Marlan Scully) is a version of the EPR paradox in which theobservation or not of interference after the passage of a photon through a double slit experiment depends on theconditions of observation of a second photon entangled with the first. The characteristic of this experiment is thatthe observation of the second photon can take place at a later time than the observation of the first photon,[31]

which may give the impression that the measurement of the later photons "retroactively" determines whether theearlier photons show interference or not, although the interference pattern can only be seen by correlating themeasurements 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 theother photons in an FTL or backwards-in-time manner.[32][33]

FTL communication possibilityFaster-than-light communication is, by Einstein's theory of relativity, equivalent to time travel. According toEinstein's theory of special relativity, what we measure as the speed of light in a vacuum is actually the fundamentalphysical constant c. This means that all inertial observers, regardless of their relative velocity, will always measurezero-mass particles such as photons traveling at c in a vacuum. This result means that measurements of time andvelocity in different frames are no longer related simply by constant shifts, but are instead related by Poincarétransformations. These transformations have important implications:

The relativistic momentum of a massive particle would increase with speed in such a way that at the speed oflight an object would have infinite momentum.To accelerate an object of non-zero rest mass to c would require infinite time with any finite acceleration, orinfinite acceleration for a finite amount of time.Either way, such acceleration requires infinite energy.Some observers with sub-light relative motion will disagree about which occurs first of any two events thatare separated by a space-like interval.[34] In other words, any travel that is faster-than-light will be seen astraveling backwards in time in some other, equally valid, frames of reference[citation needed], or need toassume the speculative hypothesis of possible Lorentz violations at a presently unobserved scale (for instancethe Planck scale)[citation needed]. Therefore any theory which permits "true" FTL also has to cope with timetravel and all its associated paradoxes,[35] or else to assume the Lorentz invariance to be a symmetry ofthermodynamical statistical nature (hence a symmetry broken at some presently unobserved scale).In special relativity the coordinate speed of light is only guaranteed to be c in an inertial frame, in a non-inertial frame the coordinate speed may be different than c;[36] in general relativity no coordinate system on a

large region of curved spacetime is "inertial", so it's permissible to use a global coordinate system whereobjects travel faster than c, but in the local neighborhood of any point in curved spacetime we can define a"local inertial frame" and the local speed of light will be c in this frame,[37] with massive objects movingthrough this local neighborhood always having a speed less than c in the local inertial frame.

Justifications

Faster light (Casimir vacuum and quantum tunnelling)

Raymond Y. Chiao was first to measure the quantum tunnelling time, which was found to be between 1.5 to 1.7times the speed of light.

Einstein's equations of special relativity postulate that the speed of light in a vacuum 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 specifyany particular value for the speed of the light, which is an experimentally determined quantity for a fixed unit oflength. Since 1983, the SI unit of length (the meter) has been defined using the speed of light.

The experimental determination has been made in vacuum. However, the vacuum we know is not the only possiblevacuum which can exist. The vacuum has energy associated with it, unsurprisingly called the vacuum energy. Thisvacuum energy can perhaps be changed in certain cases.[38] When vacuum energy is lowered, light itself has beenpredicted to go faster than the standard value c. This is known as the Scharnhorst effect. Such a vacuum can beproduced by bringing two perfectly smooth metal plates together at near atomic diameter spacing. It is called aCasimir vacuum. Calculations imply that light will go faster in such a vacuum by a minuscule amount: a photontraveling between two plates that are 1 micrometer apart would increase the photon's speed by only about one partin 1036.[39] Accordingly there has as yet been no experimental verification of the prediction. A recent analysis[40]

argued that the Scharnhorst effect cannot be used to send information backwards in time with a single set of platessince the plates' rest frame would define a "preferred frame" for FTL signalling. However, with multiple pairs ofplates in motion relative to one another the authors noted that they had no arguments that could "guarantee the totalabsence of causality violations", and invoked Hawking's speculative chronology protection conjecture whichsuggests that feedback loops of virtual particles would create "uncontrollable singularities in the renormalizedquantum stress-energy" on the boundary of any potential time machine, and thus would require a theory of quantumgravity to fully analyze. Other authors argue that Scharnhorst's original analysis which seemed to show thepossibility of faster-than-c signals involved approximations which may be incorrect, so that it is not clear whetherthis effect could actually increase signal speed at all.[41]

The physicists Günter Nimtz and Alfons Stahlhofen, of the University of Cologne, claim to have violated relativityexperimentally by transmitting photons faster than the speed of light.[27] They say they have conducted anexperiment in which microwave photons—relatively low energy packets of light—travelled "instantaneously"between a pair of prisms that had been moved up to 3 ft (1 m) apart. Their experiment involved an opticalphenomenon known as "evanescent modes", and they claim that since evanescent modes have an imaginary wavenumber, they represent a "mathematical analogy" to quantum tunnelling.[27] Nimtz has also claimed that "evanescentmodes are not fully describable by the Maxwell equations and quantum mechanics have to be taken intoconsideration."[42] Other scientists such as Herbert G. Winful and Robert Helling have argued that in fact there isnothing quantum-mechanical about Nimtz's experiments, and that the results can be fully predicted by the equationsof classical electromagnetism (Maxwell's equations).[43][44]

Nimtz told New Scientist magazine: "For the time being, this is the only violation of special relativity that I know of."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 traintraveling from Chicago to New York, but dropping off train cars at each station along the way, so that the center ofthe ever shrinking main train moves forward at each stop; in this way, the speed of the center of the train exceedsthe speed of any of the individual cars.[45]

Herbert G. Winful argues that the train analogy is a variant of the "reshaping argument" for superluminal tunnelingvelocities, but he goes on to say that this argument is not actually supported by experiment or simulations, whichactually show that the transmitted pulse has the same length and shape as the incident pulse.[43] Instead, Winfulargues that the group delay in tunneling is not actually the transit time for the pulse (whose spatial length must begreater than the barrier length in order for its spectrum to be narrow enough to allow tunneling), but is instead thelifetime of the energy stored in a standing wave which forms inside the barrier. Since the stored energy in the barrieris less than the energy stored in a barrier-free region of the same length due to destructive interference, the groupdelay for the energy to escape the barrier region is shorter than it would be in free space, which according to Winfulis the explanation for apparently superluminal tunneling.[46][47]

A number of authors have published papers disputing Nimtz's claim that Einstein causality is violated by hisexperiments, and there are many other papers in the literature discussing why quantum tunneling is not thought toviolate causality.[48]

It was later claimed by the Keller group in Switzerland that particle tunneling does indeed occur in zero real time.Their tests involved tunneling electrons, where the group argued a relativistic prediction for tunneling time should be500-600 attoseconds (an attosecond is one quintillionth (10−18) of a second). All that could be measured was 24attoseconds, which is the limit of the test accuracy.[49] Again, though, other physicists believe that tunnelingexperiments in which particles appear to spend anomalously short times inside the barrier are in fact fully compatiblewith relativity, although there is disagreement about whether the explanation involves reshaping of the wave packetor other effects.[46][47][50]

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 getsaround the infinite acceleration problem, it still would lead to closed timelike curves (i.e., time travel) and causalityviolations. Causality is not required by special or general relativity[citation needed], but is nonetheless generallyconsidered a basic property of the universe that cannot be sensibly dispensed with. Because of this, most physicistsexpect that quantum gravity effects will preclude this option.[citation needed] An alternative is to conjecture that,while time travel is possible, it never leads to paradoxes; this is the Novikov self-consistency principle.

Give up (absolute) relativity

Because of the strong empirical support for special relativity, any modifications to it must necessarily be quite subtleand difficult to measure. The best-known attempt is doubly special relativity, which posits that the Planck length isalso the same in all reference frames, and is associated with the work of Giovanni Amelino-Camelia and JoãoMagueijo. 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.[citation needed] However, even if this theory isaccurate, it is still very unclear whether it would allow information to be communicated, and appears not in any caseto 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'sprinciple), which implies that the rest frame of the universe might be preferred by conventional measurements ofnatural law. If confirmed, this would imply special relativity is an approximation to a more general theory, but sincethe relevant comparison would (by definition) be outside the observable universe, it is difficult to imagine (much lessconstruct) experiments to test this hypothesis.

Space-time distortion

Although the theory of special relativity forbids objects to have a relative velocity greater than light speed, andgeneral relativity reduces to special relativity in a local sense (in small regions of spacetime where curvature isnegligible), 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 morethan about 14 billion light-years from us today have a recession velocity which is faster than light.[51] MiguelAlcubierre theorized (http://www.iop.org/EJ/abstract/0264-9381/11/5/001) that it would be possible to create anAlcubierre drive, in which a ship would be enclosed in a "warp bubble" where the space at the front of the bubble israpidly contracting and the space at the back is rapidly expanding, with the result that the bubble can reach a distantdestination much faster than a light beam moving outside the bubble, but without objects inside the bubble locallytraveling faster than light. However, several objections raised against the Alcubierre drive appear to rule out thepossibility of actually using it in any practical fashion. Another possibility predicted by general relativity is thetraversable wormhole, which could create a shortcut between arbitrarily distant points in space. As with theAlcubierre drive, travelers moving through the wormhole would not locally move faster than light which travelsthrough the wormhole alongside them, but they would be able to reach their destination (and return to their startinglocation) faster than light traveling outside the wormhole.

Dr. Gerald Cleaver, associate professor of physics at Baylor University, and Richard Obousy, a Baylor graduatestudent, theorize that by manipulating the extra spatial dimensions of string theory around a spaceship with anextremely large amount of energy, it would create a "bubble" that could cause the ship to travel faster than thespeed of light. To create this bubble, the physicists believe manipulating the 10th spatial dimension would alter thedark energy in three large spatial dimensions: height, width and length. Cleaver said positive dark energy is currentlyresponsible for speeding up the expansion rate of our universe as time moves on.[52]

Heim theory

In 1977, a paper on Heim theory theorized that it may be possible to travel faster than light by using magnetic fieldsto enter a higher-dimensional space.[53]

MiHsC/Quantised inertia

A new theory has been proposed that Modifies inertia by assuming it is due to Unruh radiation subject to a Hubblescale Casimir effect (MiHsC, or quantised inertia). MiHsC predicts a minimum possible acceleration[54] even atlight speed, implying that this speed can be exceeded.

Lorentz symmetry violation

Main articles: Modern searches for Lorentz violation and Standard-Model Extension

The possibility that Lorentz symmetry may be violated has been seriously considered in the last two decades,particularly after the development of a realistic effective field theory that describes this possible violation, the so-called Standard-Model Extension.[55][56][57] This general framework has allowed experimental searches by ultra-high energy cosmic-ray experiments[58] and a wide variety of experiments in gravity, electrons, protons, neutrons,neutrinos, mesons, and photons.[59] The breaking of rotation and boost invariance causes direction dependence inthe theory as well as unconventional energy dependence that introduces novel effects, including Lorentz-violatingneutrino oscillations and modifications to the dispersion relations of different particle species, which naturally couldmake particles move faster than light.

In some models of broken Lorentz symmetry, it is postulated that the symmetry is still built into the mostfundamental laws of physics, but that spontaneous symmetry breaking of Lorentz invariance[60] shortly after the BigBang could have left a "relic field" throughout the universe which causes particles to behave differently depending ontheir velocity relative to the field;[61] however, there are also some models where Lorentz symmetry is broken in amore fundamental way. If Lorentz symmetry can cease to be a fundamental symmetry at Planck scale or at someother fundamental scale, it is conceivable that particles with a critical speed different from the speed of light be theultimate constituents of matter.

In current models of Lorentz symmetry violation, the phenomenological parameters are expected to be energy-dependent. Therefore, as widely recognized,[62][63] existing low-energy bounds cannot be applied to high-energyphenomena; however, many searches for Lorentz violation at high energies have been carried out using theStandard-Model Extension.[59] Lorentz symmetry violation is expected to become stronger as one gets closer tothe fundamental scale.

Another recent theory (see EPR paradox above) resulting from the analysis of an EPR communication set up, hasthe simple device based on removing the effective retarded time terms in the Lorentz transform to yield a preferredabsolute reference frame. This frame cannot be used to do physics (i.e., compute the influence of light-speed limitedsignals) but it provides an objective, absolute frame all could agree upon, if superluminal communication is possible.If this sounds indulgent, it allows simultaneity, absolute space and time and a deterministic universe (along withdecoherence theory) whilst the status-quo permits time travel/causality paradoxes, subjectivity in the measurementprocess and multiple universes.

Superfluid theories of physical vacuum

Main article: Superfluid vacuum

In this approach the physical vacuum is viewed as the quantum superfluid which is essentially non-relativisticwhereas the Lorentz symmetry is not an exact symmetry of nature but rather the approximate description valid onlyfor the small fluctuations of the superfluid background.[64] Within the framework of the approach a theory wasproposed in which the physical vacuum is conjectured to be the quantum Bose liquid whose ground-statewavefunction is described by the logarithmic Schrödinger equation. It was shown that the relativistic gravitationalinteraction arises as the small-amplitude collective excitation mode[65] whereas relativistic elementary particles canbe described by the particle-like modes in the limit of low momenta.[66] The important fact is that at very highvelocities the behavior of the particle-like modes becomes distinct from the relativistic one - they can reach thespeed of light limit at finite energy; also the faster-than-light propagation is possible without requiring moving objectsto have imaginary mass.[67][68]

Time of flight of neutrinos

MINOS experiment

Main article: MINOS

In 2007 MINOS collaboration reported results measuring the flight-time of 3 GeV neutrinos yielding a speedexceeding that of light by 1.8-sigma significance.[69] However, those measurements were considered to bestatistically consistent with neutrinos traveling at the speed of light.[70] After the detectors for the project wereupgraded in 2012, MINOS corrected their initial result and found agreement with the speed of light. Furthermeasurements are going to be conducted.[71]

OPERA neutrino anomaly

Main article: Faster-than-light neutrino anomaly

On September 22, 2011, a paper[72] from the OPERA Collaboration indicated detection of 17 and 28 GeV muonneutrinos, sent 730 kilometers (454 miles) from CERN near Geneva, Switzerland to the Gran Sasso NationalLaboratory in Italy, traveling faster than light by a factor of 2.48×10−5 (approximately 1 in 40,000), a statistic with6.0-sigma significance.[73] On 18 November 2011, a second follow-up experiment by OPERA scientists confirmedtheir initial results.[74][75] However, scientists were skeptical about the results of these experiments, the significanceof which was disputed.[76] In March 2012, the ICARUS collaboration failed to reproduce the OPERA results withtheir equipment, detecting neutrino travel time from CERN to the Gran Sasso National Laboratory indistinguishablefrom the speed of light.[77] Later the OPERA team reported two flaws in their equipment set-up that had causederrors far outside of their original confidence interval: a fiber optic cable attached improperly, which caused theapparently faster-than-light measurements, and a clock oscillator ticking too fast.[78]

TachyonsMain article: Tachyon

In special relativity, it is impossible to accelerate an object to the speed of light, or for a massive object to move atthe speed of light. However, it might be possible for an object to exist which always moves faster than light. Thehypothetical elementary particles with this property are called tachyonic particles. Attempts to quantize them failedto produce faster-than-light particles, and instead illustrated that their presence leads to an instability.[79][80]

Various theorists have suggested that the neutrino might have a tachyonic nature,[81][82][83][84][85] while others havedisputed the possibility.[86]

General relativityGeneral relativity was developed after special relativity to include concepts like gravity. It maintains the principlethat no object can accelerate to the speed of light in the reference frame of any coincident observer.[citation needed]

However, it permits distortions in spacetime that allow an object to move faster than light from the point of view ofa distant observer.[citation needed] 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 connectstwo distant locations as though by a shortcut. Both distortions would need to create a very strong curvature in ahighly 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 hypotheticalexotic matter or negative energy.

General relativity also agrees that any technique for faster-than-light travel could also be used for time travel. Thisraises problems with causality. Many physicists believe that the above phenomena are in fact impossible, and thatfuture theories of gravity will prohibit them. One theory states that stable wormholes are possible, but that anyattempt to use a network of wormholes to violate causality would result in their decay[citation needed]. In stringtheory, Eric G. Gimon and Petr Hořava have argued[87] that in a supersymmetric five-dimensional Gödel universe,quantum corrections to general relativity effectively cut off regions of spacetime with causality-violating closedtimelike curves. In particular, in the quantum theory a smeared supertube is present that cuts the spacetime in such away that, although in the full spacetime a closed timelike curve passed through every point, no complete curves existon the interior region bounded by the tube.

Variable speed of lightMain article: Variable speed of light

In conventional physics, the speed of light in a vacuum is assumed to be a constant. However, theories exist whichpostulate that the speed of light is not a constant. The interpretation of this statement is as follows.

The speed of light is a dimensional quantity and so, as has been emphasized in this context by João Magueijo, itcannot be measured.[88] Measurable quantities in physics are, without exception, dimensionless, although they areoften constructed as ratios of dimensional quantities. For example, when the height of a mountain is measured, whatis really measured is the ratio of its height to the length of a meter stick. The conventional SI system of units is basedon seven basic dimensional quantities, namely distance, mass, time, electric current, thermodynamic temperature,amount of substance, and luminous intensity.[89] These units are defined to be independent and so cannot bedescribed in terms of each other. As an alternative to using a particular system of units, one can reduce allmeasurements to dimensionless quantities expressed in terms of ratios between the quantities being measured andvarious fundamental constants such as Newton's constant, the speed of light and Planck's constant; physicists candefine at least 26 dimensionless constants which can be expressed in terms of these sorts of ratios and which arecurrently thought to be independent of one another.[90] By manipulating the basic dimensional constants one canalso construct the Planck time, Planck length and Planck energy which make a good system of units for expressingdimensional measurements, known as Planck units.

Magueijo's proposal used a different set of units, a choice which he justifies with the claim that some equations willbe 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 whichthe 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 explanatorypower or physical insight such a system would provide, assuming that it does indeed accord with existing empiricaldata.

See also

Main pages: Category:Faster-than-light travel and Category:Faster-than-light communication

Intergalactic travelKrasnikov tubeAlcubierre drive

Wheeler–Feynman absorbertheoryFaster-than-light neutrinoanomalyTachyonic field

Tachyon

Science Fiction

Animorphs (Zero Space)Battlestar (reimagining)Jump driveJumpgateTARDIS (Doctor Who)Warp drive (Star Trek)HyperdriveStarburst (Farscape)

Slipstream (sciencefiction)Skip driveInfinite ImprobabilityDriveInertialess driveStargate (device)Ansible

Mass Effect RelayMacross Space FoldFTL:2448 by Tri TacGamesInterdimensional Drive(Earth Final Conflict)Kearny-Fuchida jumpdrive (BattleTech)Ultrawave

FTL drive(BattlestarGalactica)FTLengine(Eureka)

Notes

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2. ^ Loup, F.; Waite, D.; Halerewicz, E. Jr. (2001). "Reduced total energy requirements for a modified Alcubierrewarp drive spacetime". arXiv:gr-qc/0107097 (http://arxiv.org/abs/gr-qc/0107097) [gr-qc(http://arxiv.org/archive/gr-qc)].

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4. ^ Visser, M.; Bassett, B.; Liberati, S. (1999). "Perturbative superluminal censorship and the null energy condition".AIP Conference Proceedings 493: 301–305. arXiv:gr-qc/9908023 (http://arxiv.org/abs/gr-qc/9908023).doi:10.1063/1.1301601 (http://dx.doi.org/10.1063%2F1.1301601). ISBN 1-56396-905-X.

5. ̂a b See Salters Horners Advanced Physics A2 Student Book, Oxford etc. (Heinemann) 2001, pp. 302 and 3036. ^ see http://www.oarval.org/furthest.htm7. ̂a b c Gibbs, Philip (1997). Is Faster-Than-Light Travel or Communication Possible?

(http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html). University of California, Riverside.Retrieved 20 August 2008

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(http://books.google.com/books?id=4m14K1PpJwMC). Oxford University Press. p. 180. ISBN 0-19-969461-3.,Extract of page 180 (http://books.google.com/books?id=4m14K1PpJwMC&pg=PA180)

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78. ^ Strassler, M. (2012) "OPERA: What Went Wrong" (http://profmattstrassler.com/articles-and-posts/particle-physics-basics/neutrinos/neutrinos-faster-than-light/opera-what-went-wrong/) profmattstrassler.com

79. ^ Randall, Lisa; Warped Passages: Unraveling the Mysteries of the Universe's Hidden Dimensions, p. 286: "Peopleinitially thought of tachyons as particles travelling faster than the speed of light...But we now know that a tachyonindicates an instability in a theory that contains it. Regrettably for science fiction fans, tachyons are not realphysical particles that appear in nature."

80. ^ Gates, S. James. Superstring Theory: The DNA of Reality.81. ^ Chodos, A.; Hauser, A. I.; and Kostelecký, V. Alan; The Neutrino As A Tachyon, Physics Letters B 150, 431

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ReferencesFalla, D. F.; Floyd, M. J. (2002). "Superluminal motion in astronomy". European Journal of Physics 23:69–81. Bibcode:2002EJPh...23...69F (http://adsabs.harvard.edu/abs/2002EJPh...23...69F).doi:10.1088/0143-0807/23/1/310 (http://dx.doi.org/10.1088%2F0143-0807%2F23%2F1%2F310).Kaku, Michio (2008). "Faster than Light". Physics of the Impossible. Allen Lane. pp. 197–215.ISBN 978-0-7139-9992-1.Nimtz, Günter (2008). Zero Time Space. Wiley-VCH. ISBN 978-3-527-40735-4.Cramer, J. G. (2009). "Faster-than-Light Implications of Quantum Entanglement and Nonlocality". In Millis,M. G.; et al. Frontiers of Propulsion Science. American Institute of Aeronautics and Astronautics.pp. 509–529. ISBN 1-56347-956-7.

External links

Scientific links

Measurement of the neutrino velocity with the OPERA detector in the CNGS beam(http://iysn.org/2011/10/19/measurement-of-the-neutrino-velocity-with-the-opera-detector-in-the-cngs-beam/)Encyclopedia of laser physics and technology on "superluminal transmission" (http://www.rp-photonics.com/superluminal_transmission.html), with more details on phase and group velocity, and oncausalityJuly 22, 1997, The New York Times Company: Signal Travels Farther and Faster Than Light(http://dustbunny.physics.indiana.edu/~dzierba/HonorsF97/Week1/NYTJuly22.html)Markus Pössel: Faster-than-light (FTL) speeds in tunneling experiments: an annotated bibliography(http://www.aei-potsdam.mpg.de/~mpoessel/Physik/FTL/tunnelingftl.html)Alcubierre, Miguel; The Warp Drive: Hyper-Fast Travel Within General Relativity, Classical andQuantum Gravity 11 (1994), L73–L77 (http://www.yellowknife.com/warp/)A systemized view of superluminal wave propagation(http://www.eleceng.adelaide.edu.au/personal/dabbott/publications/PIE_withayachumnankul2010.pdf)Relativity and FTL Travel FAQ (http://www.physicsguy.com/ftl/index.html)Usenet Physics FAQ: is FTL travel or communication Possible?(http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html)Superluminal (http://scienceworld.wolfram.com/physics/Superluminal.html)Relativity, FTL and causality (http://www.theculture.org/rich/sharpblue/archives/000089.html)Superluminal velocity (http://adsabs.harvard.edu/abs/2006PrGeo..21...38Y) fusing with Einstein specialrelativityStimulated Generation of Superluminal Light Pulses via Four-Wave Mixing(http://prl.aps.org/abstract/PRL/v108/i17/e173902)

Proposed FTL Methods links

Conical and paraboloidal superluminal particle accelerators (http://www.petar-bosnic-petrus.com/science-articles/conical-and-paraboloidal-superluminal-particle-accelerators/)Relativity and FTL (=Superluminal motion) Travel Homepage (http://www.physicsguy.com/ftl/)

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