From the triangle Sagnac experiment to a practical, crucial experiment of the constancy of the speed of light using atomic clocks on moving objects
Ruyong Wang
Europhysics Letters 43 (1998) 611. According to the triangle Sagnac experiment, between point A and point B that are moving in a circular motion, the travel times for light or radio signals from A to B and from B to A are different. The difference ?t, i.e. the Sagnac effect, equals 2VDL/c2, where D is the foot of the altitude to AB, VD is the speed of point D and L is the distance from A to B. The Sagnac effect exists whether the radius of the circle is as small as only few centimeters, e.g., in fiber-optic gyroscopes, or as big as twenty thousand kilometers, e.g., in GPS. Therefore, if we mount an atomic clock and signal transmitter and receiver on each of two objects moving at the same speed in a circular motion (it is not necessary to synchronize the two clocks beforehand), we will find such a time difference. Practically, using sufficiently large L and VD, this time difference can reach around 1 ns, which is relatively easy to detect with current technology. This experiment would yield both practical applications and theoretical implications. First, it can be used as a verification of Sagnac corrections in GPS. Second, a theoretical problem arises when these two objects change their paths to a straight line. Would the time difference still exist (then it contradicts the principle of the constancy of the speed of light) or does the time difference "jump" to zero? The result of the experiment will be of great interest.
