Stephen A. Lawrence wrote:
[ snip stuff on Sagnac effect]
As I mentioned previously, this can be demonstrated without the use of
any clocks, and in fact it is demonstrated all the time. Current
generation inertial navigation systems use ring-laser gyroscopes which
only work as a result of this effect. In a ring-laser gyro the signal
is a a light pulse carried in a fiber optic cable, and it travels at
roughly 3/4 C relative to the rim of the disk. The signal speed is the
same in both directions, relative to the disk (signal speed on a moving
body is trivial to measure, and if it weren't invariant with respect to
the motion, moving computers would not work). The arrival time
difference is measured by looking at interference fringe shifts between
the counter-traversing pulses, and it's used to determine the rate at
which the disk is turning, which datum is used by the navigation system.
Well, well.
I did a little more reading on this, and the thing I'm describing here
is not a "ring-laser gyroscope" at all.
A ring-laser gyro is a very weird beast which apparently uses a laser
tube in the shape of a ring, and uses some very strange properties of
standing waves in order to operate. Quite different from this.
The thing I was describing is apparently called an "IFOG", which stands
for "Interferometric Fiber-Optic Gyro".
Real devices use very long fiber optic coils, with perhaps 1000 turns in
the coil; 1000 turns results in 1000x the phase change which would
result from a single loop.
