Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Superluminal Light Propagation in Optical Fibres?

  1. Jan 7, 2012 #1

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    I'm figuring this belongs in quantum - sounds like a quantum effect.
    (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:
    http://scienceblog.com/light.html [Broken]

    These are representative samples from the less crazy end of the spectrum.

    I could imagine the leading-edge of a wave-function faster than c so long as the group velocity was less. So I'm betting there is some crucial info left out which makes the result more sensational sounding than it should be.

    What did I miss?
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jan 7, 2012 #2

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

  4. Jan 8, 2012 #3

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Sure - with classical waves, dispersion can result in the leading edge of a pulse moving faster than the nominal wavespeed in the medium.

    But that animation is very useful - the superluminal pulse is an interference effect.
    The lay reports seem to be missing that part out - (par for the course really). There are people who think someone just sends a flash of light down a fiber and it travels FTL.

    Some reports seem to be saying that this effect can be used to send information faster then light. But now I look back at it, nobody actually says this - only that there are "great (but unspecified) implications" for communication. Then it is left up to the audience to draw their own conclusions.

    <sigh> Don't you love journalists?

    I'm thinking I should collect the different "legal" FTL things together and do a pop-science paper.
    Should we move the thread?
    Last edited: Jan 8, 2012
  5. Jan 8, 2012 #4
    I'm sick of scientific reporters making false claims that are published. For example, one reported that an experiment refuted any hidden-variable QM, but upon emailing the people who conducted the experiment, they said that was not the case.
  6. Jan 8, 2012 #5

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Yeah - always go to the actual paper, or prepublication.
    Shame most people can't read these things.

    I've been having fun coming up with a lay-description of this which does not scream "FTL communication" ... I settled for saying that the brightness of the beam varies, and it's the bright bit that the researchers are calling a pulse. It's this brightness-pulse that travels ftl - not the light itself.

    It's still not great.
  7. Jan 8, 2012 #6
    Yes, I recall reading something similar to that. The title went something like " Could this be the end of quantum mechanics", I think it spoke of how a "quantum state cannot be interpreted statically" ( which turned out to be an unverified publication )
  8. Jan 8, 2012 #7
    Nope, not that article. But I did read the one you mention. Did it turn out be unverified? Wow.
  9. Jan 9, 2012 #8
    I heard that the optical fiber slows the light down, so a sort of "imaginary photon" carries on in front at c. At some point, the real light needs to catch up the imaginary light, and the transition appears as if is FTL, even though it isn't really. Is that roughly right?
  10. Jan 9, 2012 #9

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    You missed the meaning of the very words that you highlighted: "the group velocity of the pulse exceeds c." Had the article said something about the signal velocity of the pulse exceeding c, now that would have been earth shattering. No such experiment has yet been performed, nor is one likely to ever be performed.

    Suppose you aimed an extremely bright laser at the moon, bright enough to see the spot. Suppose the laser is mounted on a pivot so the spot can be maintained on one location or moved from location to location. Swivel the laser smoothly and the spot will appear to move rather than jump. Swivel the laser at even a moderate rotation rate (7.5 rpm) and the spot will move across the face of the Moon at faster than c. That superluminal spot is no more a violation of relativity than is that superluminal group velocity.
  11. Jan 9, 2012 #10

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Oh yeah - I was thinking of the group velocity a little more narrowly.
    Of course, in this case the "group velocity" in question is in the direction the laser is pointing rather than transverse movement of the spot. Not quite the same effect.

    (I was aware of the rotating source btw - there are pulsars which have odd effects by them as their beam sweeps FTL around a gas halo exciting the gas to emmission. I'm putting that badly...)

    Off jetwaterluffy - that does not sound right: if you think how a water-wave behaves - the water itself does not move very fast or far but the wave can be quite a lot faster. A "pulse" on the surface of a lake is seen as a change in the relative height - you see a hump moving but it is different water in the hump at different places.

    Now, if you have a stream of water, you can send a pulse along the stream faster than the water itself is travelling - so it would have a positive group velocity faster than the speed of water. You can also send the pulse back upstream so it has a negative group velocity.

    Its like that only instead of a hump in height, the "hump" is the brightness (shown by the amplitude in the animation) of the beam, and the role of the water is played by the stream of photons.

    Thing is I can just make a bit of the beam brighter by putting more photons into that bit - but if I do that, the bright bit still travels at the speed of the light it is made of. To set up a bright pulse travelling along the beam faster than the photons travel means being tricky with the interference between the different colors in the beam.

    Also, I think, in the stream analogy, the signal velocity of the pulse is the same as the group velocity... but that's not the same as the light. Otherwise the analogy sounds good.
  12. Jan 9, 2012 #11

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    I did find some reputable claims of FTL signal velocity:
    eg http://www.nature.com/nature/journal/v429/n6987/full/nature02586.html ... it's been investigated since of course and that Nature reference is part of an ongoing discussion.

    An accessible description comes from a conference:
    Vanner M. (2009) Superluminal Pulse Propagation by Photonic Tunneling Presented at seminar: Recent progress in nanooptics and photonics (April 2, 2009). (Not letting me copy the link - so google it. Powerpoint and pdf.)

    They are talking about optical tunneling - but this should be nice context for people who are interested in the neutrino thing and the general cut and thrust of scientific research.
    Vanner points out that group velocity should not be confused with signal velocity and that quantum noise can mask part of the pulse making the data appear to say more than it does... he says these are (interpretation of data) mistakes in Nimtz.

    Also found some sort of overview in J.Opt (2010) but they want US$80 for the full article.
    ... looks like what I want to see but too expensive.

    There seem to be quite a few papers claiming FTL signal velocity, also claiming some theoretical framework where this does not violate causality. Not had a chance to sift through them more.
    Last edited by a moderator: May 5, 2017
  13. Jan 9, 2012 #12


    User Avatar
    Science Advisor

    Well, a brief communication is not refereed in the manner an article is and neither are conference proceedings. The explanation Nimtz used is usually regarded as nonsense. All they do is a clever kind of pulse shaping which leads back to the group velocity discussion.

    A more sensible approach to the same experiment carried out with single photons is given in:
    A. M. Steinberg, P. G. Kwiat,R. Y. Chiao Measurement of the Single-Photon Tunneling Time, Physical Review Letters, 71, 1993,p. 708.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook