1696"FTL" experiments for dummies
- Feb 2, 2001No 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
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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