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Frustrated total internal reflection superluminal

  1. Apr 19, 2015 #1
    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 occurs instantly; the light tunnels at infinite speed.
    Couldn't information be sent faster than light using this process, violating the theory of relativity?

    The amplitude of the signal decreases exponentially thru the air gap, so the distance must be small (only a few wavelengths). Microwaves can be used so that the gap could be at least a few centimeters. I know light slows down in glass, but it could be thin glass and the gap made large enough to more than compensate.
    A signal sent thru this glass/air gap/glass combo would beat a signal sent directly thru a vacuum. Is there something I have overlooked? If a superluminal signal can be sent, even over a short distance, could this lead to causally violations and paradoxes?
  2. jcsd
  3. Apr 19, 2015 #2

    Simon Bridge

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    Naturally you are not the first to think of this - see, for eg:
    Carey J, Zawadzka J et al (2000) Noncausal Time Response in Frustrated Total Internal Reflection? Phys Rev Ltt 84(7)
    Tunneling of photons in frustrated total internal reflection has been studied in the time domain with single-cycle femtosecond pulses. It is seen that both the phase and energy of the pulse travel faster than the speed of light in vacuum. Theoretical analysis of the experiments shows that the time-response function for electromagnetic waves propagating in the air gap is noncausal. However, it is found that superluminal signal propagation is not possible in this case because of the inevitable diffractive spreading of the signal beam.

    The standard wisdom is that FTL travel cannot be used for communication... that "inevitable diffractive spreading" mentioned at the end means that only part of the "signal" is FTL: (put very simply) not the part that can be used for sending a message. Thus building a time machine from FTL carriers cannot happen.

    The time that article got published (research done late 90's) there were a few experiments looking at FTL quantum effects especially focussing on tunnelling. The upshot over the last decade or so has been to support the standard wisdom.

    Overview of the earlier Lit.

    Special interest:
    Nimtz G. (2011) Tunnelling confronts Special Relativity Found. Phys. 41(7) 1193-1199
    Superluminal signal velocity is widely assumed to confront Einstein causality and thus the Special Theory of Relativity (STR). Actually, there is only the tunneling mechanism known to permit superluminal signal velocities at present. The demonstrated negative group velocity experiments don't represent superluminal signal velocities. In those experiments only the signal's peak traveled faster than light and the signal was reshaped. In this Note we shall discuss why tunneling allows superluminal signal velocities. However, opposed to text books on STR we have shown that a superluminal signal beam does not allow the design of a time machine.
    ... wish I could find full-text, but you get the idea.
    Last edited: Apr 19, 2015
  4. Apr 20, 2015 #3


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    Please read my reply on this here and references therein:


  5. Apr 21, 2015 #4
    Thank you Simon and ZZ for your references. I will continue to read them.
    But from what I read, stuff like dispersion, pulse width, pulse peak shifting, and so on are subtle optics problems in this difficult experiment. Lets avoid all this with a thought experiment:
    Very intense laser pulses that are big sharp spikes (delta functions) undergo FTIR and tunnel thru a huge air gap. Many tiny sharp spikes are received on the other side of the gap many seconds before a direct pulse of light in vacuum could get there. A complete binary message containing information could be read long before relativity says it should be possible. The width or shifting of the spikes can be negligible if the air gap is huge enough.

    Is there some fundamental law of physics that will prevent this superluminal info transfer from happening? If so, what law? Can anyone summarize what some of the reference publications say about this in simple plain English? Many publications are way too technical for my brain.
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