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?

According to the theory of relativity, one can define simultaneity no 
better than as "nearer in time than in space". For instance, an event 
that happened a year ago (in /our/ frame of reference) two light-years 
away is still happening "simultaneously enough" with whatever you're 
doing right now.

If you were travelling at near speed of light, the spacetime region of 
"simultaneity" does not change, even though you will percieve them as 
happening at other spacetime coordinates - because you're using a 
different spacetime coordinate system.

It's a bit like rotating objects in POV-Ray. Imagine POV's 3D space 
representing a world of 2D space and 1D time. Pick any point and call it 
"here and now". Let's say time is Y, and space is XZ. You can represent 
"here" with a thin vertical cylinder, and "now" with a horizontal plane.

Now assume some object zipping through space at constant speed: It could 
be represented by a slanted cylinder, right? So from that object's 
perspective, "here" would be slanted.

But think about this: Your "now" is a plane perpendicular to your 
"here". Why should that be different for the moving object?

Thus, accelerating an object does not mean to apply a /shearing/ 
transformation that would leave "now" untouched, but instead a 
/rotation/ that redefines both "here" /and/ "now".

(With real spacetime, there is an additional quirk to it: For some 
strange reason the "rotational" transformation associated with 
accelerating an object appears to tilts its "now" plane in the 
/opposite/ direction. Still, the above analogy may illustrate the 
concept of a non-universal "now".)

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