ltn (01/14/83)
Under certain propagation conditions (not necessarily in a conductor; the same thing can happen to a wave propagating in a plasma), an electrical signal *can* travel at a velocity greater than light. *But* that is only true for the phase velocity. The phase velocity is the velocity of a wave of a single frequency, which means that signal has a constant amplitude for all time. The key point is that the group velocity *cannot* exceed the speed of light. The group velocity is the velocity that a modulation envelope (whether amplitude modulation, on-off cw pulses, or whatever) will travel at. The difference arrises because modulation involves propagating signals of many different frequencies (remember Fourier) which travel with different velocities (the faster-than-light phenomenon only occurs in dispersive media). Thus *information* still cannot travel faster than light, and information propagation is what relativity is concerned with. Les Niles, Bell Labs Murray Hill (aluxz!ltn)
jis (01/14/83)
Although the phase velocity of an elctromagnetic disturbance can be greater
than the speed of light in dispersive media, it strikes me as improper or
misleading to claim that an "electric signal" can travel faster than light.
The concept of a signal is intimately related to communication, and a
"electric signal" travelling at the phase velocity (whatever that might mean)
cannot be used to relly send a "signal" (as is normally connoted by the word)
to anyone at a velocity greater than that of light.
Jishnu Mukerji
ABI Holmdel 1B-425
{vax135, harpo, allegra}!hocsd!jisguy@rlgvax.UUCP (06/03/83)
No, if I remember correctly, Bell's Theorem merely states that no theory which
is:
1) local - i.e., if any thing happening at point A is to affect something at
point B, any "signal" must pass through all the intermediate points at a
finite speed.
2) realistic - i.e., says that the underlying variables of the theory (position,
momentum, field strengths, etc.) must have values independent of the observer.
can NOT exactly reproduce the predictions of quantum mechanics. Therefore,
if somebody concocts a local, realistic theory, there must be an experimental
test to tell whether it or quantum mechanics is correct. In fact, there have
been several such tests; all but one validated quantum mechanics, and the
other one is in doubt.
This does NOT mean that "nature is not local", i.e. you can send "signals"
at infinite speed or via "action at a distance". It merely says "nature is
either local or realistic, not both". I'm not familiar with the mechanics
of the theorem, so I'm sure I'm leaving out other conditions (i.e., "nature
is not local, realistic, and ... all together").
Guy Harris
RLG Corporation
{seismo,mcnc,we13,brl-bmd,allegra}!rlgvax!guymarkb@sdcrdcf.UUCP (06/04/83)
Bell's theorm syas that due to weird quantum effects an event can cause changes in another event via what appears to be faster-then-light messages. But, due to the random nature of the changes the only way to determine the what actually happened to to compare the sequence of events at one place with the sequence of events at the other which need non-random messages which must travel slower then light. This is the same problem observed in the famous twin paradox where as long a constant velocities are maintained the you can never get the twins together to compare there aging and determine which is really older. Mark Biggar