Re: [Phys-l] light clock relativity experiment

[John Denker <jsd@av8n.com>]

On 05/01/2008 09:29 AM, Fink Trevor M wrote:
> Thank you for that site, it has been the most helpful of all the
> sites I have reviewed.  

:-)

> I now have a much better understanding of
> time dilation.  

I wish folks wouldn't call it that.  Time dilation (and length
contraction) are pre-1908 concepts and belong in the same bin as 
phlogiston ... i.e. not terrible ideas, but long since supplanted 
by better ideas.

All of SR is most simply understood as the geometry and trigonometry
of spacetime.

> My real question though deals more with the behavior
> of light than with time dilation.  If I could draw your attention to
> Figure 6, we notice that the light pulse is emitted at the same
> position (x) in both the green and blue cars.  My questions is, "If
> light behaves is such a manner that the velocity of the object
> emitting it has no effect on its velocity, why does the light not
> remain at position x and simply change its y,t values?" 

Answer:  Because we engineered it to follow the vertical axis of the
car, in its own proper frame.

There has to be some apparatus in each car to aim the light, perhaps:
 -- a laser cavity
 -- an omnidirectional source plus a lens
 -- an omnidirectional source plus a pinhole
 -- et cetera.
If the apparatus is moving, its aim-point will be moving.

Draw the spacetime diagram for an omnidirectional source plus a
pinhole.  You will see that it _selects_ whatever fraction of the
light is moving in the correct direction, and discards the rest.


There's a nice one-line proof of the proposition "if the apparatus 
is moving, its aim point must be moving".  Otherwise it would break
the fundamental relativity principle.  OK?