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Lightspeed Surpassed

Faster Than Light Communication

John W. Campbell used to use his magazine, Amazing Stories, as a sort of forum to speculate on just such possibilities, before the Second World War.

Two crossed carrier waves, carrying the information as a pulse, faster than light. Like two crossed sticks, opening and closing carrying a ball between them.
 
Hmm. Forgive my ignorance. But shouldn't, according to Einstein, those signals then also travel backwards in time ? Did they recieve the signals before they sent them ? And if no, why not ?
 
My reading of this is that "pulse" may be travelling faster than the speed of light, but that there are no real issues with causality because the time required to set up the "pulse" in the line is greater than (or equal to) the time required for light to travel the distance.

For example, if I wanted to send a signal (using this method) to someone one light minute away, then it will take approximately a minute (or thereabouts ;) ) before the "pulse" leaves me.

Another way of thinking about it is that the pulse is made up of (for example) a superposition of sinusoidal waves. The speed of these waves is a function of wavelength (or frequency). If I select a material in which the speed of a wave is a carefully controlled function of its frequency, then I can set it up so that the pulse moves faster than any of the waves that make it up. (This is the difference between "group velocity" and "phase velocity".) The key thing to remember is that for this to work, at least one of the sinusoidal waves must have had sufficient time to reach the end, otherwise the pulse will "run off the end." In the example in the report, these constituent waves still travel at less than the speed of light, and hence causality is saved. (Phew! ;) )
 
So we can all rest assured that the Law is not (yet) broken :)
Thank you. Now I can sleep better an night.

BTW, didn't someone once claim that they had 'sent' music (Mozart, I believe) faster than the speed of light ? A similar procedure, I suppose.
 
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