"Constantine" wrote in message ...

Hold on. The speed of light depends on the frame of reference?

Correct. But it's more accurate to say that the speed of light depends
on the system of coordinates. An inertial frame of reference is just a
platform that moves at a fixed velocity with an observer attached.
This observer is free to synchronize his clocks in any way he likes.
In other words, an inertial frame of reference can have more than one
system of coordinates. It turns out that the speed of light does
depend on the definition he chooses.

Does it depend on the direction it is travelling?

Thank you for that delightfully perceptive question. You made me see
that I wasn't speaking as clearly as I thought. My aim is to answer as
clearly as possible. Thanks for keeping me on the straight and narrow.

Permit me to try again.

I will give you a picture that represents my equations. You may forget
about the visualization just a soon as you understand it. The ancients
called this model that I'm about to recite "the luminiferous ether."
They really believed in it. I do not. To us it will serve as a naïve
illustration of the physics and math, useful only for teaching
purposes.

Imagine all of space being filled with an invisible fluid. If you're
at rest with respect to this fluid, then the speed of light is c in
all directions. Now imagine that you're moving through this
"luminiferous ether" in a specific direction. For this illustration,
let's say that you send out a pulse of light in both the fore and aft
directions. I'm calling the direction that you're traveling toward,
the positive direction. The direction you're leaving (moving away
from) I call the negative direction. The equations say that the
measured speed of light, in my moving coordinate system, is

C(v) for the light pulse moving directly ahead and in front of you and
-

C(v) for the light pulse directed toward where you come from.
-

So the system is symmetric.

Eugene Shubert
http://www.everythingimportant.org