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1696"FTL" experiments for dummies

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  • Mark Gubrud
    Feb 2, 2001
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      No signals and no energy and no matter can propagate faster than c, the
      speed of light in vacuum. This fact is so fundamental that in theoretical
      work one usually assigns the value c=1, i.e. c represents the conversion
      constant between familiar units of time and space.

      Claims of "faster than light" signaling or motion are always based on
      smoke-and-mirrors physics. If you go out to the edges of what is known,
      you can make speculations and hypotheses and no one can contradict you.
      Alternatively, you can stick to what is known, but use verbal sleight of
      hand to suggest what you do not claim outright.

      If you want to claim that _something_ can move faster than light, it's
      easy. There is no reason why you could not set the timers in a string
      of blinking lights so that the blinking would appear to propagate down the
      string at faster than c. This is not FTL signaling because the timers
      were pre-set; everything was determined in advance rather than an actual
      signal propagating down the string. Similarly, if the blades of a very
      long scissors are closing, the point of intersection can move arbitrarily
      fast. This is not FTL signaling again because everything was determined
      in advance. If you tried to signal by squeezing and releasing the handle
      of the scissors, the blades would bend and the motion would propagate down
      the blades at the a velocity (much) less than c.

      The two kinds of FTL claims that have come up recently both involve games
      played with pulses of light. Basically, if you have a long pulse, and in
      passing through a short apparatus it emerges with it's center "advanced"
      relative to where it would be if it had traversed the same length of
      vacuum, you can then claim that its velocity in the apparatus was greater,
      perhaps much greater, than c. However, this is just a verbal game.

      All pulse generators have limited bandwidth. This means that the pulse
      will be made up only of frequencies in a limited band. A band-limited
      pulse never has a definite beginning or end; rather, it typically has a
      leading edge that begins with an exponential buildup, followed by a region
      of roughly linear slope, followed by a flattening. There is a peak or a
      broad plateau around the center of the pulse. The the trailing edge,
      typically a mirror image of the leading edge, ending in an exponential
      decay to zero. Like this:

      ***** *****
      ** **
      * *
      * *
      ** *
      ***** *****
      ********* ****************

      Shifting such a pulse forward is a simple matter of amplifying the leading
      edge and attenuating the trailing edge. There are a lot of ways this can
      be done without seeming to force it too much. You can make an amplifier
      powered by a battery that runs down as the pulse runs through it. That is
      essentially what was done in the experiment with the cesium cell which had
      been pumped to a high-energy state, so that the weak signal at the leading
      edge of the pulse caused the cesium to emit light like a laser, and thus
      fall to its low-energy state, so that it then absorbed light from the
      trailing edge.

      In the tunneling experiments, the entire pulse is attenuated, but the
      leading parts are attenuated less than the remainder, so that again the
      center of the pulse is shifted forward.

      Why isn't this forward-shifting really faster than light propagation?
      Because the actual signal does not arrive any sooner than it would have
      anyway. But we have to say what this means. If the pulse does not have
      any definite beginning, when exactly does it arrive? If we are interested
      in detecting the signal as soon as possible, the answer is not necessarily
      the center of the pulse. We could set some threshold, say, half way up
      the slope, and have our detector tell us the signal has arrived when that
      threshold is reached. If we set the threshold lower, we'll get the
      signal sooner. But we also have to consider the presence of noise. We
      can always reach our threshold of detection sooner if we just amplify the
      signal, but we'll be amplifying noise along with it. That's the problem
      with these schemes. You can amplify the leading edge, attenuate the
      trailing edge, or do both, but you never reach a given signal-to-noise
      ratio at any velocity greater than c. That's what the people who try to
      pass this off as "FTL" never bother to tell you.
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