colin wrote:
"Chuck Harris" wrote in message
...
colin wrote:

Get an rf signal generator, 100 MHz maybe, near an oscilloscope. Run a
long coax to a laser (pointer type is fine) from the generator. Run
another long coax from a pin photodetector (with maybe a NON AGC
amplifier) back to the scope. Start with the laser close to the pin
and measure phase shift. Move them apart, ditto.

This would work with a pulse generator, measuring arrival time, too.
but the signal traveling down the coax is governed by the speed of light,
by the time the received signal is brought next to the transmited signal
it has undergone a roundtrip,
moving the optical devices apart just alters the total trip, not just the
trip in one direction only.

Colin =^.^=

Well, it depends on what you are trying to do. Are you looking for
currents
in the once presumed ether?

no I just find it fascinating that its so difficult to arrange an experiment
that measures it in one direction only,

I guess it depends on how literally you need it to be in one direction.
The reason it is difficult, is there is no way to convey the information from
the finish line to the start line without doing a round trip...

However, if you consider light traveling through a fiber, or light bouncing
off of mirrors not to be round trip, you could do something with that.

1) Coil up a few kilometers of optical fiber on a spool, and pop your laser
pulse into one end, and measure it with your biased PIN diode on the other,
and let your scope do its thing...

2) Arrange two almost parallel mirrors a few hundred meters apart, and introduce
a beam at a slight off orthogonal angle, and let the beam bounce back and forth
between the mirrors hundreds, of times. You can arrange the mirrors so the beam
enters and leaves on the same vector. (Hint, if the mirrors are slightly
off of parallel, the bounce can be made to travel from one side of the
mirror plane to the other and return back)

Of course, strictly speaking, either method involves the beam making a
return trip.


that I dont think anyone has done it. everything seems to be extrapolated
form round trip experiments.

You could take two highly stable synchronized time standards, A and B, and
move B to the destination end of the experiment, and turn on the
laser when A reaches a predefined second, and record when the light pulse
comes to B after that same predefined second.

its effectivly been done with two fixed hydrogen masers in buildings some
distance apart.
the results seemed to cuase some controversy in some cirlces.

If you are worried about relativistic effects on the time standards, you
can bring A and B back together to recheck their synchronism.


moving the clocks introduces an error too, trying to move them realy slowly
may not be enough.
the two hydrogen masers were also moving at different speeds ever so
slightly.

Terribly sorry about that, but Heisenberg's uncertainty principle doesn't
allow you to know position and velocity at the same time.


-Chuck