It is well-known fact that the one-way speed of light is not a
measurable parameter, because it is actually defined to be the same as
the two-way speed of light in all inertial frames. Put differently, we
define the synchronization of two clocks (that are not co-located, but
stationary relative to each other) in such a way that the one-way speed
of light comes out the same as its two-way speed. Hence to measure the
one-way speed of light using such clocks does not make much sense. The
outcome is a given before the test.
However, people have not stopped trying to see if it could be done
without synchronized clocks. All 'official' efforts have so far been
refuted, proving that they are in fact two-way measurements. Physicist
Don Lincoln of Fermi-lab has told me about a method they use that is
(almost?) an irrefutable one-way test.

Here is a quote from his post on another Blog. The sketch is my own doing, upper is cable calibration and lower is laser pulse speed test:
[quote=Don Lincoln]
Take two photon detectors. These can be arbitrarily thin - less than a
millimeter if necessary. Take the two detectors and place them side by
side. From each detector take a cable of a convenient length. Put both
of those cables into fast electronics (a modern digital oscilloscope
will work just fine).
Fire a light pulse through both detectors.
Since these two detectors are adjacent to one another, the transit time
from one to the other is of order (1 mm)/(speed of light) = (1 x 10-3 m)/(3 x 108 m/s) = 3 x 10-12 seconds. If sub- 3 picosecond speed is needed, there are ways.
Using
your oscilloscope, you can calibrate your cables to establish what
"simultaneous" means. In the abstract, the cables can be of identical
length. This means that the signals from the two detectors will arrive
simultaneously at your oscilloscope.
Now move one detector far
away...maybe 1000 feet. Do not disconnect the cables, so you have
identical conditions. Fire the light pulse (use a laser) through one
detector to hit the other. The signals from the two detectors will
transit the cables and hit your oscilloscope at a single spatial point.
Since you have already established that the transit time in the cables
of both detectors are identical, the only difference between the signal
arrival time at your detector is the transit time of light from one to
the other. If you have measured the distance exactly, you can then
determine the speed of light by distance over time.
If you do not want to measure the distance between the two detectors,
you can verify the isotropy of space (and consequently, the identical
nature of the 1-way speed of light). First do as I said, and fire a
laser that first hits detector 1 and then hits detector 2. Record the
transit time seen in your oscilloscope. Now have a laser pointing in
the opposite direction, hitting detector 2 and then detector 1. Again,
record the transit time.
Since the distances are the same, and
the only difference is the direction in which the light is travelling,
you can establish that light going one way takes the same speed as the
other way. I believe that within the uncertainties of your
equipment, this detector configuration will establish that the speed of
light is the same in either direction."
[/quote]
What do you think? Did Don measure the one-way speed of light, or has he got a 'hidden two-way' assumption in there somewhere?
-J
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