[TheForge] YAK relativity- WAY OT, or not...

[Mike Linn]

> > If I'm understanding you, then you don't understand the
> > observation * it's not an assumption * that the velocity of light
> > in a vacuum is a constant.
> >
> No (and again I hope I understand your viewpoint), what I'm saying is that
> the speed of light is NOT a constant. It's only constant in a theoretical
> vacuum in a theoretical void (no gravity) in a theoretically
> static frame of reference.

In special relativity, the speed of light is constant when measured in ANY
inertial frame.  In general relativity, the appropriate generalization is
that the speed of light is constant in any reference frame at rest relative
to a source of gravity.  In such a frame, the speed of light can differ from
c, basically because of the effect of gravity. But observers will ALWAYS
measure that speed the same regardless of their individual frames.


Assume Gloria is sitting still and shines her flashlight at George who is
moving toward her at 0.8c.
Gloria measures the speed as 1c  George measures the light and also measures
it as 1c. How can this be??
George should measure it at 1.8c right?

No, because relativity shows us that space-time shrinks in the direction of
motion. Since speed is distance/time, George measures it as 1c, because his
ruler is shorter and his stopwatch is slower.

The speed of light in a vacuum is defined as exactly 299,792,458 m/s. Not
because its a true physical constant as such, but because in 1983 the BIPM
defined the meter as length of the path traveled by light in vacuum during a
time interval of 1/299,792,458 of a second. So the speed of light c is set
because of a desire for uniform and equivalent measurements throughout the
world. Its better than the platinum-indium rod that was used prior to 1983
and was defined using the best measurements of the time, but it may change
at a later date...  sorta like the volt did it 1990.


mike