What is Superluminal: Definition and 81 Discussions

Faster-than-light (also superluminal, FTL or supercausal) communications and travel are the conjectural propagation of information or matter faster than the speed of light.
The special theory of relativity implies that only particles with zero rest mass may travel at the speed of light. Tachyons, particles whose speed exceeds that of light, have been hypothesized, but their existence would violate causality, and the consensus of physicists is that they do not exist. On the other hand, what some physicists refer to as "apparent" or "effective" FTL depends on the hypothesis that unusually distorted regions of spacetime might permit matter to reach distant locations in less time than light could in normal or undistorted spacetime.
According to the current scientific theories, matter is required to travel at slower-than-light (also subluminal or STL) speed with respect to the locally distorted spacetime region. Apparent FTL is not excluded by general relativity; however, any apparent FTL physical plausibility is currently speculative. Examples of apparent FTL proposals are the Alcubierre drive, Krasnikov tubes, the traversable wormholes, and quantum tunneling.

View More On Wikipedia.org
  1. Halc

    I Cat Chasing Red Dot Faster Than c: Results & Analysis

    This topic assumes special relativity only, so no gravity is involved. I wanted to know how a superluminal thing would appear to an observer, and two of the 'things' I thought of were a moiré pattern and the red dot projected by a laser pointer. The former can be discussed, but this post will...
  2. A

    I Superluminal Quantum tunneling without FTL messaging

    I recently viewed a PBS "Space time" video The video references two articles of which one is paywalled while the other is not. https://iopscience.iop.org/article/10.1088/1367-2630/abb515/pdf https://www.nature.com/articles/s41586-020-2490-7?proof=tIn the discussion section of the free article...
  3. R

    I A test for a superluminal neutrino?

    The neutrino has been puzzling since its proposal and its experimental confirmation. There have been experimental anomalies with it for every experiment designed to test it. One fact that might be quite key is that measurements of the mass-square of the neutrino has consistently shown it to...
  4. P

    I Choice of signature important for superluminal 4-velocity? (Minkowski)

    I guess my question boils down to "Is choice of signature important when dealing with superluminal 4-velocities"? I wanted to show for superluminal velocities that ##\tilde{U} = \left( \frac{c}{\sqrt{\frac{u^2}{c^2} - 1}}, \frac{\dot{x}}{\sqrt{\frac{u^2}{c^2} - 1}}...
  5. P

    Superluminal moving points 4-velocity

    The first thing I want to do is reconcile the two definitions of 4-velocities ##\tilde{U} = \frac{d \tilde{R}}{d \tau} = c \frac{ d \tilde{R}}{ds}##Start off with the usual definition of 4-velocity##\tilde{U} = \frac{d \tilde{R}}{d \tau}##we know that d \tau^2 = \frac{ds^2}{c^2} \Rightarrow d...
  6. A

    A Superluminal origin of Quantum Mechanics

    A relativistic origin of QM is proposed in https://iopscience.iop.org/article/10.1088/1367-2630/ab76f7 It is proposed that lorentz transformation that include superluminal observers (whether those observers exist or not) explain the indeterministic behavior of QM. Not only that, it also would...
  7. MichPod

    I Epistemic view of the wave function leads to superluminal signal

    Hope, I do not violate any forum rules here, this is not a discussion topic, mostly. I am just asking for help looking for a specific article/work. I just remember reading somewhere that there is a QM theorem or article saying smth. in a sort that if the same physical "ontic" state would be...
  8. P

    A Superluminal Signalling proposal in peer-reviewed journal

    Hi to all, A proposal of mine, that purports to enable faster-than-light communication, has been accepted (a few months ago) by an esteemed peer-reviewed physics journal: Pramana, Indian Academy of Sciences. The article will appear in January. The PDF of the accepted version is attached...
  9. G

    Superluminal speeds -- Princeton study year 2000

    I read an article with this with great interest but also puzzlement. It talked about sending a pulse of light through a section of laser excited Caesium atoms (6cm I think) if traveling at vacuum speeds the pulse should have taken 0.2ns but actually arrived 62ns before it would have done...
  10. ThunderLight

    Faster than Light... Superluminal Group Velocity

    If general relativity in the formal sense constrains all velocities to the speed of light as a maximum, how would superluminal group velocities exceeding speeds of light (at their superpositions) be evaluated in mainstream physics? Would this be a case of General Relativity and Physics...
  11. substitute materials

    I Path integral implies superluminal motion?

    In the path integral formalism, where we treat a photon as if it takes every possible path, aren't the possible paths limited by the speed of light? If we were to perform the double slit experiment, and shield the detector after a specified time frame to limit the time for a photon to make the...
  12. A

    I Superluminal Messaging and Causality Paradox: Myth or Reality?

    If I could route a signal from here-and-now to an event in my past light cone, then clearly I could make an irresolvable causal paradox by having the arriving signal disable the button that sends it, so I'll choose to believe that I can't send messages back in time TO HERE. Now I've heard it...
  13. Tony Weston

    A Exploring Bell's Theorem with Relativity (No Superluminal Comm.)

    Hi... New to this forum. Be kind! I did not study physics at university, and consider myself an armchair physicist. I am a computer programmer by trade. I first came across Bells inequalities a few years ago, while working with a fello programmer who did have a PHD in physics. Its pretty...
  14. K

    I Fixed Stars Moving Faster Than Light? Problem?

    If I spin around in an open field at night and look up to the stars they appear to be moving relative to me. Additionally, they are very far away and trace out a giant arc length in a very short time (S=rθ). With respect to me, these stars are moving faster than light. Is this a problem? Has...
  15. P

    A Superluminal Energy Transport & Causality: Does It Violate Relativity?

    In superluminal light pulse experiments, as shown in Fig. 4 of the Letter [Nature 406 (2000) 277], the whole pulse intensity profile observed is advanced by 62 ns nearly without any distortions, and the light pulse energy must be transported faster than the speed of light in vacuum...
  16. G

    Imaginary Transverse Space of Superluminal Lorentz Transform

    I was reading this paper: http://dinamico2.unibg.it/recami/erasmo%20docs/SomeOld/RevisitingSLTsLNC1982.pdf It is on superluminal Lorentz transformations and is too advanced for me. :confused: But anyway, take a look at equation(s) (11). For the y' and z' transformations, there is an imaginary...
  17. M

    A What was the explanation of OPERA superluminal neutrinos?

    When I first heard about the OPERA apparent superluminal results on the measurement of neutrinos speed, I tried to locate an explanation using all the possible relativistic effects. All were too small to account for a difference of 18 meters of virtual distance among photons and neutrinos. Then...
  18. quantum.cmptr

    Quantum Entanglement for Superluminal Communications: How Does It Work?

    How is quantum entanglement used in superluminal communications? I'm not sure whether I have the right idea of quantum entanglement (and if I do not, please correct me), but I wonder how quantum entanglement is used if the particles in entanglement are autonomous (that is to say, they can't be...
  19. Z

    Problem of superluminal communication in Non-local HVTs

    Hi there. I have been studying on Bell Inequalities and hidden variables problem for quite some time now however my general knowledge on the problem of superluminal communication is superficial at best. I know that non of the standing interpretations (Everett, Copenhagen, Bohmian, QBist etc.)...
  20. H

    Light Speed Paradox: Xavier's Torch on X

    Suppose planet X is so far away from Earth that due to the expansion of the universe, it is moving at a velocity ##v > c## away from us. Xavier stands on planet X and points a torch away from us. To Xavier, the light emitted from the torch is in front of him. But to us, the light emitted from...
  21. K

    Frustrated total internal reflection superluminal

    When light encounters a boundary between a dense medium (glass) and a less dense one (air), it may undergo total internal reflection. But if there is another glass a short distance away, some light will jump the forbidden region by a tunneling process and enter the second glass medium. This jump...
  22. E

    Superluminal and temporal effect

    In special relativity, anything superluminal can make things go backward in time in some frame.. is there some kind of exception or would there be away to make it not happen? for instance.. if the signal travels via wormhole which appears instantaneous in different points in spacetime, would...
  23. Jesus

    Superluminal propagation of fields

    I am reading about the Velo-Zwanziger problem in which particles with spin higher than 1 propagates faster than light when the particles are coupled with an electromagnetic field. In the original paper: G. Velo and D. Zwanziger, “Propagation And Quantization Of Rarita-Schwinger Waves In An...
  24. bcrowell

    Gorini vs. Jin: Superluminal Frames in 1+1 Dimensions

    Gorini 1971 (reference below) proves a no-go theorem for attempts to extend the Lorentz group to superluminal velocities in n+1 dimensions, for n\ge3. His assumptions are stated in abstruse notation, but he gives the following translation into plainer language: > Axiom: [...] time has a...
  25. F

    Does Length Contraction Imply Superluminal Speeds?

    Assume a rocket sets off from Earth accelerating to 0.8c within 1 second. The distance to a star originally 1000 light years away contracts thus to 600 lightyears within 1 second. This means the star moves 400 lightyears in one second in the travelers frame. I make this an average speed of more...
  26. K

    Proof of impossibility of superluminal signals?

    Does the following argument prove that any method of faster than light communication is impossible? 1. A transmitter sends a signal at 12:00 from point A to B faster than light by some hypothetical method. 2. A transmitter at point B is moving at a high speed (but less than speed of light)...
  27. MattRob

    Superluminal (Tachyon) Transformations

    Hey all, I've been reading "Time Travel and Warp Drives" by Allen Everett and Thomas Roman, and the book had an interesting section on Tachyons. At one point it presented a system Leonard Parker (of the University of Wisconsin-Milwaukee) created whereby coordinate transformations for Tachyons...
  28. P

    Inflation and superluminal expansion

    Inflation is often referred to as a period of 'superluminal' or 'faster-than-light' expansion (e.g. see article on Wikipedia and hundreds of research papers on the subject). This has always bugged me. What exactly is superluminal about an inflating universe that does not apply to a non-inflating...
  29. T

    Superluminal Gravitational Acceleration

    Can gravitational attraction (generated by a super-massive black hole, for instance) accelerate you faster than c?
  30. B

    Entanglement Swapping Superluminal Communication

    I'm wondering whether or why quantum entanglement swapping or steering could or couldn't be used for superluminal communication. Below is a thought experiment. Setting aside the technological barriers do the laws of physics prevent this? On Earth Base Alpha: EPR Sources Entangle Numbered...
  31. Superposed_Cat

    Could element 137 electrons become superluminal in a retrocausal atom?

    I found the equation for the speed of an electron today. atomic number*fine-structure constant*speed of light. rearranging we get z= v/σc. if you set v to c then you find that at element 137 the electrons become superluminal. This can be possible right? a retrocausal atom! Could someone please...
  32. mrspeedybob

    Adding subluminal and superluminal velocities.

    Suppose we have a long row of rods, each of which is equipped with a clock and a solenoid which can raise and lower the rod. All the clocks are synchronized. Each rod is programed to raise and lower at a slightly different time so that in the rest frame of the apparatus the rods rise and fall in...
  33. E

    What is the current explanation for apparent superluminal jets?

    Im not going to pretend to be more educated than I am. Now I know the usual explanation is that the jet is at a pretty significant angle in relation to the observer. But, http://en.wikipedia.org/wiki/Superluminal_motion makes it seem the jet truly is traveling FTL. I was just...
  34. P

    Superluminal neutrinos now 'pseudoscience'?

    are the supposed superluminal neutrinos at gran sasso now considered 'pseudoscience'?
  35. B

    Why wouldn't this experiment allow superluminal communication?

    I know there's a flaw here as superluminal communicaton is impossible. I was considering this scenario: As with the initial stage of the delayed time quantum eraser experiment, put two light sources through a beta barium borate chrystal creating two pairs of entanged photons. Send one pair...
  36. K

    Possibility of superluminal signals using quantum entanglement

    Superluminal signals using quantum entanglement Please consider this thought experiment. Would this provide a means for communicating faster than light or is there an error somewhere in it? SUMMARY: Person B can instantly know whether or not Person A did measurements on his particles by...
  37. O

    Is this description of superluminal speeds correct?

    Most of us know that Einstein came up with the theory that superluminal speed was impossible. And most physicists nowadays believe that and have accepted that as fact. I just want to know if this is a technically correct way of stating this: Light travels at approx. 299,800 km/s in a vacuum...
  38. J

    Is superluminal communication impossible according to STR?

    It is often said that superluminal communication is prohibited by special theory of relativity. Do you think this is true? I have doubts about this claim. Imagine a communication between two astronauts in two distant inertial frames A,B with any mutual velocity. Suppose that both...
  39. Physics Monkey

    Entanglement, classical correlation, and questions about superluminal signalling

    The quantum physics forum is full of questions about why entanglement can't be used to signal superluminally. My question is this: do these questions usually still make sense if we replace entanglement by classical correlation? For example, I can send Alice and Bob each a random bit but with...
  40. A

    Superluminal Neutrino Breaks SR and GR ?

    Superluminal Neutrino Breaks SR and GR...? I have reading a lot of articles and editorials about apparent neutrinos that went faster than the speed of light by 60 nanoseconds, I read some that say yes and no, so I'm a bit confused. So is it true?? Are there neutrinos that went faster than the...
  41. andrewkirk

    What Aspect of GR Forbids Superluminal Motion?

    I am trying to understand the expansion of the universe and the implications of distant galaxies receding from us at superluminal speeds. To properly understand how this is not forbidden by GR, I wanted to focus on exactly what it was about GR, what 'rule', that said an object can't travel...
  42. L

    Stimulated generation of superluminal light pulses via four-wave mixing (NIST)

    Concerning this paper: Stimulated generation of superluminal light pulses via four-wave mixing. I have two questions: 1) How can we check that information cannot be propagated faster than light with this system? 2) What is the meaning of the last sentense of this paper: Thanks
  43. Simon Bridge

    Superluminal Light Propagation in Optical Fibres?

    I'm figuring this belongs in quantum - sounds like a quantum effect. http://www.opticsinfobase.org/abstract.cfm?id=63756 (my emph) There's lots of papers like this ... also been in the blogosphere: Sounds like some people are claiming to have done it for real...
  44. P

    Proof that a negative mass causes superluminal speeds?

    I've heard this theory, based on the assumption that if negative mass objects exist, they would travel faster than light. I planned on mentioning this in a physics paper I have to write for my class, but I just realized that I have no idea what formula shows this correlation. Can somebody help...
  45. B

    Just a Little Mind Game re: Superluminal Neutrino Velocities

    By now, I've listened several times to Brian Greene's talk about the OPERA experiment. And, although I largely agree with him, there's one nagging question I have: Gravitation was, at a time soon following the Big Bang, a repulsive force. The existence of "Dark Energy" suggests the...
  46. M

    Time loops and those superluminal neutrinos

    I believe there needs to be an intelligent discussion somewhere about the possibility that theories with time loops can be rendered consistent by nondeterministic (probabilistic) physics, and specifically about the possibility that genuinely spacelike neutrino effects - which, let us recall...
  47. M

    Old experiments with superluminal speed

    Hey guys, I'm just wondering what the general opinion is of these two articles: http://www.electrogravityphysics.com/html/speed_of_light.html http://www.telegraph.co.uk/science/science-news/3303699/We-have-broken-speed-of-light.html It seems strange to me that if these two...
  48. B

    Is superluminal communication possible?

    I know the answer to this is most likely "No", and that there have been other threads on this topic before. But recently someone explained a technique for superluminal communication using entanglement and watching for wave form collapse to transmit data. I've always know this not to be...
  49. Q

    Superluminal Speed: Exploring Lorentz Transformations

    Superluminal speed... Have any scientist ever tried to put superluminal speed in lorentz transformations...? If they have then, have they found anything significant...??...
  50. K

    Is There a Superluminal Signal When Turning Off a Harmonic Potential?

    Consider we initially have a ground state particle of a harmonic oscillator: \psi = \exp ( - a{x^2}) (neglecting nomalization) And we have a detector far in space monitoring the local probability of finding a particle. Now if we suddenly turn off the harmonic potential, the wavefunction will...