Story: Steorn's free energy seems curiously expensive

Posted by: ralphclark (Saturday 2 June 2007, 4:06 AM)

superluminal joke

What you are talking about with the external field moving superluminally - this has been known for a good long time.

The usual example given is a flashlight. If you point it at a wall and wave it about the spot of light on the wall moves from side to side. Now suppose you are holding an extremely powerful flashlight, so powerful that it can illuminate dust particles millions of miles out in space. If you point the flashlight at the horizon to your left then sweep it across the sky over your head and down to the horizon on your right, the point in the sky which you are illuminating with the beam would cross half the sky in about a second. If we consider the beam as exposed by reflections from particles even as near as 100,000km away, the illuminated spot is crossing the sky at slightly more than the speed of light. If you can illuminate particles twice as far away then the tip of the beam appears to move at twice that speed, etc etc.

Note that it's you holding the flashlight and that Alice (floating in space one light-second past your Eastern horizon) illuminated at the start of the arc, cannot use your superluminal spot of light to transmit information to Bob (floating one light-second past your Western horizon) illuminated at the end of the arc, because here is no causal connection between them - only between you and Alice, and between you and Bob.

Or you could consider a giant pair of scissors of length L, a mere two light seconds. If we call the angle between the blades 2A, then while passing through some particular angle of separation the tips of the blades are closing at an instantaneous speed of L*d(sin(A))/dt [which will of course be less than c], the velocity of the cutting point moves at L*d(cos(A))/dt which may well be more than c. Example: if we consider the final part of snip starting from a tip separation of two arc minutes, and ending with the blades completely closed, and if this is completed within one second, we can calculate the average velocities over that second:

L = c * 2s
t = 1s
sine(0.01666 degrees) = 0.000290771849 =~ 0.0003
cosine(0.01666 degrees) = 0.999999958 =~ 1

tip velocity =~ 0.0006c (well within the laws of physics)
cutting point velocity =~ 2c (wow)

However bear in mind that no physical part of the scissors is actually moving at that speed, or ever at any speed in excess of c. It's only a notional abstract point that is moving, no more physical than the focal point of a lens or a county boundary. As with the flashlight beam you can't use it to transmit information superluminally.

The group velocity of interfering waves is a kind of combination of these two ideas. But although the argument is a bit more complicated in that case, unfortunately the laws of physics have still arranged things so that you cannot use *any* of these methods to transmit information superluminally from A to B.

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