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.

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  1. M

    Calculating Particle Kinetic Energy and Velocities: Where Did I Go Wrong?

    Superluminal Group Velocity? An electron has a De Broglie wavelength L of 2*10^(-12) m. Calculate its kinetic energy, and phase and group velocities. So I used E = p^2 / 2m, p = h / L. Phase velocity is w / k and group velocity is dw/dk. This gave me expressions E/p and p/m...
  2. S

    Superluminal instantaneous influence

    hi, it is well known that after measurement, there is a superluminal instantaneous influence (but not really communication) in entanglement. I wonder if such influence also happens everytime before measurement as well? i would be very grateful to read your reply!
  3. X

    Superluminal motion. How's that possible?

    Homework Statement A friend of mine taking physics course told me about this but I didn't understand it because she wasn't able to explain the ideas easy enough for me to understand. All I know is that nothing can travel faster than the speed of light and how's it even possible? The theory of...
  4. B

    Wormholes expeling particles at superluminal speeds

    Probably a crazy idea but perhaps possible, so let me ask... GR doesn't prevent particles, even those with mass, to travel at speeds higher than the maximal speed of light (C), if that was their initial speed. Now, the only thing traveling faster than C is expansion of our Universe...
  5. J

    Apparent superluminal velocity in M82 source

    An April 18 newscientist.com article, http://www.newscientist.com/article/dn18775-mysterious-radio-waves-emitted-from-nearby-galaxy.html" reports recent observations of a radio source in M82 with an apparent horizontal motion of 4x speed of light. Could someone comment on what particular...
  6. M

    Problem on superluminal motion

    Hi, I have a question about a problem on superluminal motion. Using the following figure (see attachment) I have to derive that: a) \beta = \frac{v}{c} = \frac{1}{cos(\theta)+sin(\theta)} b) Plot \beta as a function of \theta and show where the superluminal motion takes place Thanks...
  7. G

    Superluminal Frame Adjustments?

    In theory the set of Inertial Reference Frames could consist of an infinite set of rectangular grids and distributed clocks, overlaid and sliding through one another without interference. Let us say we are initially at rest at the Origin of IRF K. Relative to K, the grid of moving frame K’...
  8. N

    Exploring Realistic Possibilities of Superluminal Travel

    Hello, I am attempting to write a script (for fun) which outlines a realistic scenario in which humanity travels to the stars within, say, 30 years. Given the short time frame, I'm thinking about using various forms of FTL travel. By my definition, FTL includes any means of getting from A...
  9. G

    Exactly how is causality violated by superluminal travel?

    From my amateur readings in relativity, one of the arguments against tachyons is that causality would be violated locally. But how? Let's say we have observer A and B with synchronized clocks that are separated by a reasonable distance d known to them. A sends B a photon at A's clock t0...
  10. E

    Challenge: superluminal average speed

    או עם 'גי או עם 'גי
  11. C

    Superluminal transfer of information via gravity

    it takes light about 8 minutes to reach Earth from the sun, and I've heard it said information cannot be transmitted faster than the speed of light. if the sun blinked out of existence, steady stream of light indicating to you the the sun still exists would last for 8 minutes, but could you not...
  12. N

    Superluminal velocities & redshift

    We all know that nothing can travel faster than the speed of light in a local frame. We also know that distant galaxies in our observable universe have recession 'velocities' relative to us which are far in excess of the speed of light, c, despite the fact that the velocity in their local...
  13. P

    Understanding Superluminal Jets and Accretion Emissions from Black Holes

    So I've skimmed certain papers several years ago indicating that radio jets and other accretion-related emissions from black holes can be superluminal. Is this true, or is this just the relative motion of particles in some medium other than vacuum that exceeds the speed of light? I'm...
  14. D

    Generalizing entanglement: Aren't all quantum events superluminal?

    Generalizing entanglement: Aren't all quantum "events" superluminal? If as it seems, the speed of the collapsing wave front of entangled particles occurs at a superluminal velocity, what is special about entangled particles? It follows that all quantum changes occur at superluminal rates, e.g...
  15. Z

    Superluminal Jets: 6c Speeds Detected!

    Superluminal jets have been detected with speeds of up to 6c. Does anyone know where these high speed jets are detected? Are they near the centre of galaxies or near the edge of galaxies or somewhere else?
  16. O

    Superluminal Speeds and All That Jazz

    Superluminal Speeds and All That Jazz A popular prejudice is sometimes expressed as “nothing can travel faster than light”. But Special Relativity (SR), from which this prejudice is derived, in fact teaches only that “nothing can be observed to travel faster than light”. The meaning of...
  17. J

    Can Quantum Entanglement Enable Superluminal Communication?

    My speculation relates to “Superluminal Communication” in the context of a paper by Robert Desbrandes and Daniel Van Gent titled “Intercontinental quantum liaisons between entangled electrons in ion traps of thermoluminescent crystals” (see http://arxiv.org/abs/quant-ph/0611109). In their...
  18. J

    Superluminal Recession & Cosmological Redshift

    I'm taking the liberty of revising and restating this topic which started in a separate thread. Comments are welcome. A lively debate is underway today by mainstream cosmologists as to whether the expansion of the universe implies that empty space between galaxies is also expanding. When...
  19. J

    The Mystery of Superluminal Recession Velocities in Cosmology

    Let's conduct a thought experiment involving a galaxy far, far away ("Galaxy FFA") from earth. Galaxy FFA is observed from Earth to be receding at twice the speed of light. Let's hire very fast (.9c) alien spaceships to simultaneously release 999 test particles equally spaced across the...
  20. S

    Apparent superluminal velocity of galaxies

    I just posted about what is termed "apparent superluminal velocity of galaxies" and gave a website of an article written by Prof-Dr. L. Schatzer, which was said to be not "main stream". My posting was moved because it contained this website that was not considered " main stream" and no...
  21. marcus

    Superluminal recession speed is common

    A lot of objects are observed to have redshift z in range 2 - 6. These would have been receding from us faster than c at the time the light which we are now receiving from them was emitted by them and began its journey to us. Do you have any questions about this? There is a popular...
  22. B

    How Would Headlights Behave If Your Car Traveled Faster Than Light?

    Example: If you had a car that could travel quicker than the speed of light (Theoretically speaking), and there was another car infront of you traveling at the same speed, how would the light from your headlights travel ? From what i understand light always travels at the same speed...
  23. A

    Superluminal Velocity: Reports & Implications

    there are some reports on superluminal velocity www.space.com/scienceastronomy/generalscience/faster_than_c_000719.html news.bbc.co.uk/1/hi/sci/tech/841690.stm I just want to know what you might think of this. I personally think that this might lead us to something practical.
  24. berkeman

    Superluminal Matter at Two Stages of Big Bang ?

    Sorry if this is a repost, but I saw something on a science documentary show last night that completely surprised me. The show was on the Big Bang and evolution of the Universe, start to finish. It was on a show called Naked Science on one of the science channels on DTV, and it seems to do a...
  25. Robert100

    Common misconceptions of cosmological horizons and superluminal expansion

    I recently came across this paper on arXiv.org http://arxiv.org/abs/astro-ph/0310808 Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe Authors: Tamara M. Davis, Charles H. Lineweaver Comments: To appear in Publications of...
  26. S

    Exploring Superluminal Speed with Airships A & B

    If we have two airships going at speed 0.6c in the opposite directions. then they will be separated at a speed higher than light. let's name the airships , A and B My question is , Let's say we have the two airships pull an elastic material. take a point of the material on Airship A name it a...
  27. M

    Exploring the Horizon of Superluminal Expansion

    I have read that the expansion of the universe is not a relative velocity and therefore not restricted to the limitation of the speed of light. I read that this creates a horizon which we cannot see beyond due the fact that the portions of the universe are moving away from us at a speed greater...
  28. T

    Superluminal events which do not transmit information or energy

    What are the examples of superluminal events which do not transmit information or energy, and hence not violating causality but travels faster than light? My book here gives an example of a shadow of a bug flying across a projector. Any others?
  29. Y

    Griffiths Afterword and Superluminal Shadows

    Hi, I'm trying to understand Griffiths' (Introduction to Quantum Mechanics) argument that shadows can't transmit information. In his Afterword, he mentions that there are many things that travel faster than light, to quote, "If a bug flies across the beam of a movie projector, the speed of...
  30. P

    Superluminal spacecraft: impossible?

    Einstein asserted that the mass would have become infinite and/or created a black hole. But then came Alcubierre in 1994 who said it might be possible by contracting space in front of the ship and expanding space behind. They said this would violate laws of energy conservation, but the physicist...
  31. marcus

    Are Superluminal Recession Velocities Consistent with Special Relativity?

    I found an article by Tamara Davis and Charley Lineweaver called "Superluminal Recession Velocities" http://astro-ph/0011070 the date shown is Jan 2001, though the number suggests late 2000. It is 4 pages so easy to download and it is specifically on that topic-----Lineweaver's longer...
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