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From MathPages.com:
Unfortunately we frequently read in the newspapers about how someone has succeeded in transmitting a wave with a group velocity exceeding c, and we are asked to regard this as an astounding discovery, overturning the principles of relativity, etc. The problem with these stories is that the group velocity corresponds to the actual signal velocity only under conditions of normal dispersion, or, more generally, under conditions when the group velocity is less than the phase velocity. In other circumstances, the group velocity does not necessarily represent the actual propagation speed of any information or energy. For example, in a regime of anomalous dispersion, which means the refractive index decreases with increasing wave number, the preceding formula shows that what we called the group velocity exceeds what we called the phase velocity. In such circumstances the group velocity no longer represents the speed at which information or energy propagates.
To see why the group velocity need not correspond to the speed of information in a wave, notice that in general, by superimposing simple waves with different frequencies and wavelengths, we can easily produce a waveform with a group velocity that is arbitrarily great, even though the propagation speeds of the constituent waves are all low. A snapshot of such a case is shown below. In this figure the sinusoidal wave denoted as "A" has a wave number of kA = 2 rad/meter and an angular frequency of wA = 2 rad/sec, so it's individual phase velocity is vA = 1 meter/sec. The sinusoidal wave denoted as "B" has a wave number of kB = 2.2 rad/meter and an angular frequency of wB = 8 rad/sec, so it's individual phase velocity is vB = 3.63 meters/sec.
The sum of these two signals is denoted as "A+B" and, according to the formulas given above, it follows that this sum can be expressed in the form 2cos(kx-wt)cos(Dkx-Dwt) where k = 5, w = 2.1, Dk = 0.1, and Dw = 3. Consequently, the "envelope wave" represented by the second factor has a phase velocity of 30 meters/sec. Nevertheless, it's clear that no information can be propagating faster than the phase speeds of the constituent waves A and B. Indeed if we follow the midpoint of a "group" of A+B as it proceeds from left to right, we find that when it reaches the right hand side it consists of the sum of peaks of A and B that entered at the left long before the current "group" had even "appeared". This is just one illustration of how simple interfering phase effects can be mis-construed as ultra-high-speed signals. In fact, by simply setting kA to 2.2 and kB to 2.0, we can cause the "groups" of A+B to precess from right to left, which might mistakenly be construed as a signal propagating backwards in time!
Needless to say, we have not succeeded in arranging for a message to be received before it was sent, nor even in transmitting a message superluminally. Examples of this kind merely illustrate that the "group velocity" does not always represent the speed at which real information (or energy) is moving.
Originally posted by Figher Master FIN
And the speed is not "changeble" it's 300 000 km/second... It's now and it will always be, I believe that it's prooven with math...
[edit on 20-8-2005 by Figher Master FIN]
Originally posted by spacedoubt
The article doesn't say that.
Simcity was merely posing the question.
Interesting idea..If light is faster, then are all the other relationships to that new speed also adjusted?
There is a slightly longer article here, as well..
www.livescience.com...
Originally posted by Amorymeltzer
grad_student: The permittivity of free space is 8.85x10^-12 F/m. The impendance, however, is 377 Ohms, or exactly 120 times pi (3.14159 and so on and so forth) and is, iirc, the square root of the permeability divided by the permittivity. The way those two numbers relate is relaly cool.
Originally posted by FLYIN HIGH
Interesting idea..If light is faster, then are all the other relationships to that new speed also adjusted?
Originally posted by d1k
Where does it say they made it go 300x faster? I don't see that anywhere in the article or any of the releated articles.