Faster Than Light

There are phenomena which travel faster than light, but they cannot carry information. To understand this concept, consider this:

Imagine a long conga line of people. Every beat in the music, someone raises their hand in sequence; first the first guy, then the second on the second beat, then the third on the third beat...
Every four beats everyone also takes a step forward.

The "raised hand" will move faster than the individual people will.

Or better, think of a football game when people do "the wave". "The wave" moves around the stadium very quickly but each person stays in their own spot.

If the people are photons, then the "raised hand" or "the wave" equivalent can move faster than c even as the photons move at c.

In the analogy, "the wave" is not the light itself, which has wave characteristics. 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" and THAT is what moves faster than c.

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.

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.

EDIT - I think this describes it:

http://en.wikipedia.org/wiki/Shadow#Shadow_propagation_speed







In a wave, the peaks will seem to "increase" and "decrease" as the wave moves. Like the wiki illustration here:
http://upload.wikimedia.org/wikipedia/commons/b/bd/Wave_group.gif

You can then see that the wave seems to move faster than its shape, because the highest peak eventually starts "decreasing" and the next peak becomes the largest one. So eventually you'll reach the same shape as before, but with the previous peak being the highest one! The speed of that shape is the group velocity.

Signal velocity is the speed at which a wave carries information. Imagine a disturbance in the wave, that disturbance would move at a certain speed which is the signal velocity. The easiest way to think about it is how fast it gets from point A to B from the moment when you originally send it from A (sometimes also called the front velocity, but it's the same AFAIK).

Palynka



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!

Meneer Dries


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".

Palynka


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.

coquette


http://www.playtheimmortalgame.com/board/showthread.php?threadid=124269