Faster-than-light
Faster-than-light (superluminal or supercausal)
Particles whose speed exceeds that of light (tachyons) have been hypothesized, but their existence would violate causality and would imply time travel. The scientific consensus is that they do not exist.
According to all observations and current scientific theories, matter travels at slower-than-light (subluminal) speed with respect to the locally distorted spacetime region. Speculative fast-than-light concepts include the Alcubierre drive, Krasnikov tubes, traversable wormholes, and quantum tunnelling.[1][2] Some of these proposals find loopholes around general relativity, such as by expanding or contracting space to make the object appear to be travelling greater than c. Such proposals are still widely believed to be impossible as they still violate current understandings of causality, and they all require fanciful mechanisms to work (such as requiring exotic matter). However, given how little is known about the limits of causality and other speculative concepts related to FTL proposals, physicists continue to research and consider these proposals.
Superluminal travel of non-information
In the context of this article, "faster-than-light" means the transmission of information or matter faster than c, a constant equal to the speed of light in vacuum, which is 299,792,458 m/s (by definition of the metre)[3] or about 186,282.397 miles per second. 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 sections immediately following).
- In some materials where light travels at speed c/n (where n is the refractive index) 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 qualifies as faster-than-light as described here.
In the following examples, certain influences may appear to travel faster than light, but they do not convey energy or information faster than light, so they do not violate special relativity.
Daily sky motion
For an earth-bound observer, objects in the sky complete one revolution around the Earth in one day.
Light spots and shadows
If a laser beam is swept across a distant object, the spot of laser light can easily be made to move across the object at a speed greater than c.[6] Similarly, a shadow projected onto a distant object can be made to move across the object faster than c.[6] In neither case does the light travel from the source to the object faster than c, nor does any information travel faster than light.[6][7][8]
Closing speeds
The rate at which two objects in motion in a single frame of reference get closer together is called the mutual or closing speed. This may approach twice the speed of light, as in the case of two particles travelling at close to the speed 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 the collider 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 than twice the speed of light.
It is instructive to compute the relative velocity of particles moving at v and −v in accelerator frame, which corresponds 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 high speed, the time taken to reach that planet could be less than one year as measured by the traveller's clock (although it will always be more than one year as measured by a clock on Earth). The value obtained by dividing the distance traveled, as determined in the Earth's frame, by the time taken, measured by the traveller's clock, is known as a proper speed or a proper velocity. There is no limit on the value of a proper speed as a proper speed does not represent a speed measured in a single inertial frame. A light signal that left the Earth at the same time as the traveller would always get to the destination before the traveller would.
Phase velocities above c
The
Group velocities above c
The
Cosmic expansion
According to
There are many galaxies visible in telescopes with redshift numbers of 1.4 or higher. All of these have cosmological recession speeds greater than the speed of light. Because the Hubble parameter is decreasing with time, there can actually be cases where a galaxy that is receding from us faster than light does manage to emit a signal which reaches us eventually.[18][19][20]
However, because the expansion of the universe is accelerating, it is projected that most galaxies will eventually cross a type of cosmological event horizon where any light they emit past that point will never be able to reach us at any time in the infinite future,[21] because the light never reaches a point where its "peculiar velocity" towards us exceeds the expansion velocity away from us (these two notions of velocity are also discussed in Comoving and proper distances#Uses of the proper distance). The current distance to this cosmological event horizon is about 16 billion light-years, meaning that a signal from an event happening at present would eventually be able to reach us in the future if the event was less than 16 billion light-years away, but the signal would never reach us if the event was more than 16 billion light-years away.[19]
Astronomical observations
Apparent
Quantum mechanics
Certain phenomena in
The uncertainty principle implies that individual photons may travel for short distances at speeds somewhat faster (or slower) than c, even in vacuum; this possibility must be taken into account when enumerating Feynman diagrams for a particle interaction.[24] However, it was shown in 2011 that a single photon may not travel faster than c.[25] In quantum mechanics, virtual particles may travel faster than light, and this phenomenon is related to the fact that static field effects (which are mediated by virtual particles in quantum terms) may travel faster than light (see section on static fields above). 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 tunnelling phenomenon. Usually, such reports deal with a phase velocity or group velocity faster than the vacuum velocity of light.[26][27] However, as stated above, a superluminal phase velocity cannot be used for faster-than-light transmission of information.[28][29]
Hartman effect
The Hartman effect is the tunneling effect through a barrier where the tunneling time tends to a constant for large barriers.[30][31] 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 non-zero probability that the photon will tunnel across the gap rather than follow the refracted path.
However, 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".[32] The evanescent waves in the Hartman effect are due to virtual particles and a non-propagating static field, as mentioned in the sections above for gravity and electromagnetism.
Casimir effect
In physics, the
EPR paradox
The EPR paradox refers to a famous
An experiment performed in 1997 by
Delayed choice quantum eraser
The
Superluminal communication
Faster-than-light communication is, according to relativity, equivalent to
- The relativistic momentum of a massive particle would increase with speed in such a way that at the speed of light 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, or infinite 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 that are separated by a Lorentz invarianceto be a symmetry of thermodynamical 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 from c.[41] In general relativity no coordinate system on a large region of curved spacetime is "inertial", so it is permissible to use a global coordinate system where objects 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,[42] with massive objects moving through this local neighborhood always having a speed less than c in the local inertial frame.
Justifications
Casimir vacuum and quantum tunnelling
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 simply the vacuum energy, which could perhaps be altered in certain cases.[43] 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. Such a vacuum can be produced by bringing two perfectly smooth metal plates together at near atomic diameter spacing. It is called a Casimir vacuum. Calculations imply that light will go faster in such a vacuum by a minuscule amount: a photon traveling between two plates that are 1 micrometer apart would increase the photon's speed by only about one part in 1036.[44] Accordingly, there has as yet been no experimental verification of the prediction. A recent analysis[45] argued 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 signaling. 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. 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.[46]
It was later claimed by Eckle et al. that particle tunneling does indeed occur in zero real time.[47] Their tests involved tunneling electrons, where the group argued a relativistic prediction for tunneling time should be 500–600 attoseconds (an attosecond is one quintillionth (10−18) of a second). All that could be measured was 24 attoseconds, which is the limit of the test accuracy. Again, though, other physicists believe that tunneling experiments in which particles appear to spend anomalously short times inside the barrier are in fact fully compatible with relativity, although there is disagreement about whether the explanation involves reshaping of the wave packet or other effects.[48][49][50]
Give up (absolute) relativity
Because of the strong empirical support for
Spacetime distortion
Although the theory of
Gerald Cleaver and Richard Obousy, a professor and student of Baylor University, theorized that manipulating the extra spatial dimensions of string theory around a spaceship with an extremely large amount of energy would create a "bubble" that could cause the ship to travel faster than the speed of light. To create this bubble, the physicists believe manipulating the 10th spatial dimension would alter the dark energy in three large spatial dimensions: height, width and length. Cleaver said positive dark energy is currently responsible for speeding up the expansion rate of our universe as time moves on.[56]
Lorentz symmetry violation
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.[57][58][59] This general framework has allowed experimental searches by ultra-high energy cosmic-ray experiments[60] and a wide variety of experiments in gravity, electrons, protons, neutrons, neutrinos, mesons, and photons.[61] The breaking of rotation and boost invariance causes direction dependence in the theory as well as unconventional energy dependence that introduces novel effects, including Lorentz-violating neutrino oscillations and modifications to the dispersion relations of different particle species, which naturally could make particles move faster than light.
In some models of broken Lorentz symmetry, it is postulated that the symmetry is still built into the most fundamental laws of physics, but that spontaneous symmetry breaking of Lorentz invariance[62] shortly after the Big Bang could have left a "relic field" throughout the universe which causes particles to behave differently depending on their velocity relative to the field;[63] however, there are also some models where Lorentz symmetry is broken in a more fundamental way. If Lorentz symmetry can cease to be a fundamental symmetry at the Planck scale or at some other fundamental scale, it is conceivable that particles with a critical speed different from the speed of light be the ultimate constituents of matter.
In current models of Lorentz symmetry violation, the phenomenological parameters are expected to be energy-dependent. Therefore, as widely recognized,[64][65] existing low-energy bounds cannot be applied to high-energy phenomena; however, many searches for Lorentz violation at high energies have been carried out using the Standard-Model Extension.[61] Lorentz symmetry violation is expected to become stronger as one gets closer to the fundamental scale.
Superfluid theories of physical vacuum
In this approach, the physical
FTL neutrino flight results
MINOS experiment
In 2007 the
OPERA neutrino anomaly
On September 22, 2011, a preprint
Tachyons
In special relativity, it is impossible to accelerate an object to the speed of light, or for a massive object to move at the speed of light. However, it might be possible for an object to exist which always moves faster than light. The hypothetical elementary particles with this property are called tachyons or tachyonic particles. Attempts to quantize them failed to produce faster-than-light particles, and instead illustrated that their presence leads to an instability.[81][82]
Various theorists have suggested that the neutrino might have a tachyonic nature,[83][84][85][86] while others have disputed the possibility.[87]
General relativity
General relativity was developed after special relativity to include concepts like gravity. It maintains the principle that no object can accelerate to the speed of light in the reference frame of any coincident observer.[citation needed] However, it permits distortions in spacetime that allow an object to move faster than light from the point of view of a 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 connects two distant locations as though by a shortcut. Both distortions would need to create a very strong curvature in a highly localized region of space-time and their gravity fields would be immense. To counteract the unstable nature, and prevent the distortions from collapsing under their own 'weight', one would need to introduce hypothetical exotic matter or negative energy.
General relativity also recognizes that any means of faster-than-light
In fiction and popular culture
FTL travel is a common trope in science fiction.[89]
See also
- Faster-than-light neutrino anomaly
- Intergalactic travel
- Krasnikov tube
- Slow light
- Variable speed of light
- Wheeler–Feynman absorber theory
Notes
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That group found, although with less precision, that the neutrino speeds were consistent with the speed of light.
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- ^ Randall, Lisa; Warped Passages: Unraveling the Mysteries of the Universe's Hidden Dimensions, p. 286: "People initially thought of tachyons as particles travelling faster than the speed of light...But we now know that a tachyon indicates an instability in a theory that contains it. Regrettably for science fiction fans, tachyons are not real physical particles that appear in nature."
- Gates, S. James (2000-09-07). "Superstring Theory: The DNA of Reality".)
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: CS1 maint: numeric names: authors list (link - .
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References
- Falla, D. F.; Floyd, M. J. (2002). "Superluminal motion in astronomy". S2CID 250863474.
- ISBN 978-0-7139-9992-1.
- 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. (eds.). Frontiers of Propulsion Science. ISBN 978-1-56347-956-4.
External links
- Measurement of the neutrino velocity with the OPERA detector in the CNGS beam
- Encyclopedia of laser physics and technology on "superluminal transmission", with more details on phase and group velocity, and on causality
- Markus Pössel: Faster-than-light (FTL) speeds in tunneling experiments: an annotated bibliography Archived 2010-01-23 at the Wayback Machine
- Alcubierre, Miguel; The Warp Drive: Hyper-Fast Travel Within General Relativity, Classical and Quantum Gravity 11 (1994), L73–L77
- A systemized view of superluminal wave propagation
- Relativity and FTL Travel FAQ
- Usenet Physics FAQ: is FTL travel or communication Possible?
- Relativity, FTL and causality
- Yan, Kun (2006). "The tendency analytical equations of stable nuclides and the superluminal velocity motion laws of matter in geospace". Progress in Geophysics. 21: 38. Bibcode:2006PrGeo..21...38Y.
- Glasser, Ryan T. (2012). "Stimulated Generation of Superluminal Light Pulses via Four-Wave Mixing". Physical Review Letters. 108 (17): 173902. S2CID 46458102.
- Conical and paraboloidal superluminal particle accelerators
- Relativity and FTL (=Superluminal motion) Travel Homepage