clipka wrote:
> Tim Cook schrieb:
>> I also have trouble with the notion that light emitted at the same 
>> time from point x and point y, point x stationary to and point y 
>> moving rapidly relative to point z, both beams of light will arrive at 
>> z at the same moment, regardless of distance.  (Which was one of the 
>> bits mentioned in a simplified explanation of relativity I read once.)
> 
> The crucial thing here being "at the same time": Are you perfectly sure 
> what exactly constitutes simultaneity?

Just a quick pedantic note to the otherwise good explanation -- since 
the light beams in his example are arriving at the same point, z, he can 
of course be sure what "arrive at the same time" means.


As far as Tim's concern here goes, you can actually resolve it perfectly 
well without need for relativity.  Think of ripples in a pond.  If I 
throw a pebble into the pond, or drag a stick through it, the ripples 
from these two sources will move at the same speed, even though the 
stick is moving with respect to the surface of the water and the pebble 
is not (assume of course that the stick is moving slower than the 
ripples do).  Viewing light in this way as waves propagating through a 
medium (rather than particles as it seems you're thinking) is very close 
to the pre-relativity view of light, and you can see how the speed of 
light in this case would be independent of the speed of the source.

The surprising bit in relativity, of course, is that it's also 
independent of the speed of the observer.  And in fact, as clipka has 
alludes to before, this has actually been verified by experiment: 
http://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment among 
others.

Intuitively, the concept might be made more palatable by an analogy to 
classical mechanics.  Let's say you're locked inside a train moving 
straing at a constant speed with no windows on a perfectly smooth track. 
  What experiment could you do that would tell you how fast you were 
moving?  If you drop a ball you'll see that it always looks to you like 
it falls straight down and gives no hit as yo your speed.  You'll find 
that any mechanical experiment you concoct behaves the same way -- 
exactly as it would if it were at rest.  In some sense this isn't too 
surprising since we hardly notice the the earth is racing through space 
at thousands of miles per hour.

So on some level, it's intuitive that you can't do an experiment to tell 
you what your "absolute" velocity is, you can only determine relative 
velocities, for instance determining the velocity of the train relative 
to the earth by looking out a window (if there were one).

The theory of relativity derives from assuming that this principle 
*also* applies to experiments involving electromagnetic phenomena, such 
as light, and that these too can't be used to determine your absolute 
velocity.  It turns out that this being the case leads to a view of 
space and time which is different from the naive one in a manner such as 
clipka described, but omitting this counter-intuitive result the 
postulates leading to it are actually quite sensible.

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