From: Adrian Tymes (wingcat@pacbell.net)
Date: Fri Jun 21 2002 - 19:12:59 MDT
John Leppik wrote:
> If light can only travel at one absolute speed in a vacuum, how does it
> know what that is? It would know if it were a propagation in a
> stationary medium. It would know if it had an instantaneous reference
> to a stationary point. How else could it know?
It doesn't "know" anything. It just is. Though perhaps it would be
better to phrase it, "from the point of view of an external observer,
light travels at one absolute speed in a vacuum" - equivalent in the
original sense, but better addresses the sense you're thinking of.
> If I were traveling at half the speed of light with respect to my
> unknown stationary reference point, what would the absolute speed of
> light be for a light pointed forward? And what for a light pointed
> backward? How would light beam know to behave thus?
Time dialates as one approaches the speed of light. If I observed
you travelling at half the speed of light, and you shot a laser
forwards, both you and I would see the laser (or, more accurately, any
effects of the laser, like photons of the laser scattered off any dust
in the way) going at the speed of light relative to ourselves. This is
only possible because your clock and my clock would have different
concepts of how long a second was. I would observe it going forwards
X distance in what I measured to be a second; you would observe it going
forwards X distance in what you measured to be a second. If we were to
try to synchronize our clocks, we would notice them constantly going out
of sync in the same direction. (This last bit has been measured by
satellites in orbit - at Mach 20-something - vs. stationary clocks on
the ground, though the time dialation there is of course extremely tiny,
such that one needs large amounts of time and clocks as precise as
atomic clocks to detect anything.)
> If light has a reference to absolutely immobility in an apparently
> dynamic universe, where would that reference be? How does light
> reference it? What is our speed here on Earth with respect to immobility?
There is no absolute immobility. Distance and motion can only be
measured with respect to something else. (It's so easy for most uses to
make the "else" be the Earth that it's almost never stated, but it is
there.)
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