I've learned about this phenomenon in school.
It's called subluminal (or superluminal, not sure) light and it is indeed the group velocity of waves that can be faster than c.
This is the velocity of peaks you get when adding all the different waves in a medium.
It occurs when waves of different wavelenghts have different wave speeds.
It seems unnatural but you have to remember that nothing is really moving with the velocity, it only seems that way. You can't use it to send information faster than light. How it all works is still a bit alien to me too.
Anyway, you can see an applet here:
http://gregegan.customer.netspace.net.au/APPLETS/20/20.html
Fun fact: it is also possible to produce 'stopped light' with this!
My favourite example of the difference between group and phase velocity is in internal gravity waves in a stratified fluid. In that case, the two are at right angles!
imagine two spiral arcs of two different galaxies that are crossing paths.
The arcs are closing like scissors.
The "phenomenon" of movement .. .the point of the crossing, can move faster than the speed of light, but not particular particle or wave is moving faster than the speed of light.
phenomena easily move faster than the speed of light.
Originally posted by coquette imagine two spiral arcs of two different galaxies that are crossing paths.
The arcs are closing like scissors.
The "phenomenon" of movement .. .the point of the crossing, can move faster than the speed of light, but not particular particle or wave is moving faster than the speed of light.
phenomena easily move faster than the speed of light.
Originally posted by sonhouse Wouldn't it be more accurate to say 'effects' move faster than C but 'influences' can not? To me, 'things' imply matter.
To me 'effect' also implies that something can have an effect on other at faster than the speed of light. I guess it's just semantics, though...but I can't resist! 'Patterns'?
Originally posted by wolfgang59 But isnt that the speed of the wave? (ie c for radio waves)
No, it's the velocity of the "wave" that is formed when you combine light beams of different wavelengths. Different wave.
The waves themselves are doing "the wave".
Another example:
A shadow can move faster than c. If you have a flashlight and an object in front of it, the shadow from this arrangements will move in an arc. Close up moving the flashlight 45 degrees might make the shadow move a meter; if you aim it farther away, in the same time the same movement will make the shadow move two meters.
Extrapolate this pattern and you can see you can make the shadow move faster than light as long as you aim it at something far enough away.
Originally posted by AThousandYoung No, it's the velocity of the "wave" that is formed when you combine light beams of different wavelengths. Different wave.
The waves themselves are doing "the wave".
Another example:
A shadow can move faster than c. If you have a flashlight and an object in front of it, the shadow from this arrangements will move in an arc. Close up moving ...[text shortened]... make the shadow move faster than light as long as you aim it at something far enough away.
Are you sure? Covering something up doesn't destroy the photons that have already passed, so how can it be faster than light?
Originally posted by Palynka Are you sure? Covering something up doesn't destroy the photons that have already passed, so how can it be faster than light?
No I'm not sure. That was the example that introduced me to this concept.
I think the idea is that nothing travels with the shadow. It's simply that the photons have moved apart. If a flashlight is moved from directly out of the north pole to perpendicular to the north pole in one second, after a minute one photon goes one light minute out of the north pole, and another goes one light minute perpendicular to this, nd they are now more than one light minute apart, right? Square root of two light minutes apart, and the "beam of light" and any shadow it caries with it has moved that whole distance in one second.
Originally posted by AThousandYoung No, it's the velocity of the "wave" that is formed when you combine light beams of different wavelengths. Different wave.
The waves themselves are doing "the wave".
Another example:
A shadow can move faster than c. If you have a flashlight and an object in front of it, the shadow from this arrangements will move in an arc. Close up moving ...[text shortened]... make the shadow move faster than light as long as you aim it at something far enough away.
Yes I'm aware of this phenomenon. The shadow does travel faster than light. (The key here is that a shadow doesnt move.)
Originally posted by wolfgang59 Yes I'm aware of this phenomenon. The shadow does travel faster than light. (The key here is that a shadow doesnt move.)
I'm still struggling with the wave though!
Meneer Dries applet is excellent for visualizing it. Did you try it? You can see how group velocity can be much faster than the wavelets that form it. It's the same principle, as group velocity is just a velocity of a "pattern".
faster than c
09 Jan '10 22:59
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