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7116 clicks; posted to Geek » on 08 Nov 2012 at 6:40 AM (5 years ago)   |   Favorite    |   share:    more»

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It`s like water. If you have a tank of water (laser) and you open one end (fire laser) then the flow goes at the speed water flows on land (speed of light in a vacuum) and you can move a water wheel with this (transmit information) and the max speed the water flows is at the speed of a very fast stream (30mph?). If you already have water there then a compression wave can travel fairly fast under certain circumstances but is not capable of doing work (500mph tsunami wave for example) yet the water itself can not travel so fast. This is how you have two speed limits.
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BarkingUnicorn: Light has two speeds, one of which can be infinite.

Wiki's primer on phase velocity

I'll see if I can do a mathless explanation. Basically, when you have a complex wave system (think in sound, like an instrument playing an A major note), the wave is not a pure sine wave, it's a jumbled mess, just look at any piece of music in some editing software. This jumbled mess of sound travels in a wave-like way at the speed of sound. The speed of sound in this example is the "group velocity" the speed at which the whole system travels in space.

There's a nifty fact about complex wave equations, though, one which was worked out by a guy named Fourier, is that no matter how messed up a waveform looks, you can express it as the simple sum of pure sine waves, given that you intelligently pick what the frequencies and the amplitudes of those sine waves are.

These pure sine waves, which, when added, give you the wave that you hear, also have a speed at which they travel through space, this speed is called the "phase velocity" and does not have to be the same speed as the group velocity.

What this article is saying is that the only part of light waves that have to obey the speed of light is the group velocity, because that's what actually carries the information.

Hope that doesn't make your head worse. I'm going to bed now, but I can try to do better tomorrow.

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