Re: How to measure one-way light speed.


karandash2000@xxxxxxxxx wrote:
shevek wrote:

The first point is that a "measurement of the speed of light" is today
not a valid concept, because our definition of distance relies on the
speed of light being exactly c. If you claim to measure the time taken
by light to travel between point A and point B, you are really just
making a precise measurement of the distance between those two points.
The speed is by definition c.

The experiment was described by Paul Anderson originally as a setup
capable of measuring ANY speed, inclusive of speeds higher than c.

You are mixing experiment with definition. Of course that in SR the
speed of light is by definition 1/sqrt(nu*eps) (consistent with the
speed of the EM waves deduced from Maxwell's equation). By definition
as well, in SR the speed of light is isotropic (this is not the case
in test theories like Mansouri-Sexl).
This is why we we run experiments in order to verify the above.
Now, there are quite a few experiments (see Tom Roberts FAQ) that
VERIFY that light speed is isotropic.
In Paul's setup , the light speed from A to B is equal to the light
speed from B to A. Therefore, instead of using the standard TWLS way of
measuring light speed, we can safely use the measurement from A to B
for example.
Now, we have one more step: we need to match the delays in the two
fibercables that lead from scintillators A and B to the UNIQUE clock in
the system (the oscilloscope). We do that by observing that if we
COMBINE the fact that the TW delays can be made equal with the light
speed isotropy, we get the equality of the OW delays.
We are done:
-the light speed can be measured with the Paul's setup
-a one way measurement (A to B or B to A) is sufficient
-one clock (the oscilloscope) is sufficient

Let's say that A and B are seperated by a distance L and that they are
moving through the aether at a velocity V. It would take light L/(C+V)
to make the trip one way and L/(C-V) the other way. It doesn't matter
which is down wind or up wind, you get the same round trip time
measuring from either point. That proves the two way speed of light is
isotropic using a single clock at either point.

Next we sync the clocks assuming that light is isotropic. The clocks
are out of sync because we allowed for a L/C delay where there was
actually a L/(C+V) delay. If we now use the two clocks to measure the
one way speed of light we will measure the light to travel at C rather
than (C+V) because the clocks are out of sync.

If an electric signal travels through wire at nearly the speed of
light, making the return trip by wire is for all practical purposes the
same as a two way light signal.

Paul thought that all of the above was self evident and left the
thinking as an exrcise for the rest of us. I am quite sure that if he's
watching this exchange, he must be chuckling :-)



The second point is that what we are really doing in such an experiment
is measuring the round trip travel time of a light signal, and assuming
that the one-way travel time is one half that value. A direct one-way
travel time measurment requires two clocks at each endpoint of the
signal path.

No. See above.

The usual methods of synchronization of those clocks
(Einstein synch., or slow transport), or of simplifcation of the setup
to one clock as you suggest, make implicit the result that the one-way
travel time is one half the round trip travel time.


Since there is only one clock, there is no synchronization to speak
about.


To make myself more clear, consider the experiment as drawn by Paul,
with the scintilators and the single clock. The delay from the arrival
of signal at point B to the arrival of the signal through the cable at
the clock is a one-way travel time along that cable. If we assume that
this is 1/2 of the round trip travel time in the cable, as you
suggested, we are wasting our time with this experiment, as the
conclusion is already clear. Assumption of isotropy -> isotropy. We
aren't proving that one-way light speed is isotropic in such an
experiment, we are proving that we can make precise measurements -
perhaps not entirely a useless exercise.


As explained above, we are measuring the light speed with a one clock
setup, we are NOT trying to prove that it is isotropic. That was
already DONE by other experiments.

Then you need to look at those experiments to see exactly what they
proved. As I showed above, proving that the two way speed of light is
isotropic does not prove that the one way speed of light is isotropic.

.

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