Sagnac Effect

Sagnac effect

Multiple examples exist to show that the “MLET template” is actually what is used in most large-scale experiments. By MLET template we mean that moving observers or sensors do not see an isotropic light speed. Because it is virtually impossible to treat each sensor as existing in a separate frame, the SRT magic does not generally function well in any system with multiple sensors. Unfortunately, even though the MLET template is generally used, the SRT is generally called upon for the theoretical explanation of that MLET template. The result is a confusion of theory that provides explanations which are not consistent with either MLET or SRT. The Sagnac effect is a prime example.

Several types of modern gyroscopes function by using the Sagnac effect to measure rotation. Georges Sagnac performed the original experiment in 1913. He split a light beam into two parts, which traveled around the circumference of an area in opposite directions. He then measured the interference fringe effect when the two light beams were brought back together. He found that the fringe shift was a function of the rotational velocity. In other words, the speed of light relative to the rotating sensor was a function of whether the light beam traveled with or against the rotational velocity of the platform. The MLET explanation of the Sagnac effect is again obvious. Simply stated, the motion of the detector (observer) has no effect on the speed of light; and therefore a non-isotropic light speed relative to the moving detector can be expected—which leads to the observed phenomenon.

Explanations for the Sagnac effect within SRT (and GRT) are numerous. However, virtually all of the explanations claim that the speed of light need not be isotropic when rotational phenomena are involved, since rotational phenomena are absolute. But, similar to the situation with the twin paradox, the majority of the explanations are mutually contradictory; and picking out an “official” explanation is a daunting task. However, the most common explanation which I have found in the literature [1], and apparently the one to which Einstein himself subscribed, is that the path around the circumference should be unwrapped into a straight-line path and the Lorentz transformation from the stationary to moving frame applied to this unwrapped moving circumference. This gives the correct fringe shift but directly contradicts the prescription for handling accelerations within SRT, which was cited above for the Thomas precession effect.

The situation has become even more controversial with the advent of precise clocks (and/or transponders) placed upon both interplanetary space probes and upon GPS satellites in orbit around the earth. The 1971 JPL document [2], giving the equations used to model round-trip and one-way signals between a space probe and the earth, prescribed the use of a sun-centered isotropic-light-speed frame. Clearly, both the probe and the detector (or observer) on the earth are moving in this frame. The equations clearly show that the speed of light was not assumed to be isotropic with respect to the observer. Instead, when a signal was in transit from the probe to the earth, the motion of the earth-observer during the transit time was clearly accounted for. This motion included the earth’s spin, the earth’s orbital velocity, and even the motion of the earth caused by its orbit around the earth-moon center of gravity. This accounting is precisely that prescribed by the MLET template rather than the SRT template (isotropic light speed relative to the observer or sensor).

In the GPS system a non-rotating earth-centered isotropic-light-speed frame is assumed. Again, the motion of the receiver during the time the signal transits from the satellite to the receiver must be accounted for to obtain precise navigation results. In the GPS context, this effect is referred to as the one-way Sagnac effect and is blamed upon the rotation of the earth. But the receiver must account for its motion during the transit time no matter the source of the motion. It does not matter whether or not it follows a circular trajectory. The critical range which must be determined is the position of the satellite at the time the signal was transmitted and the position of the receiver at the time of its receipt. The path the receiver followed during the time of flight of the signal is completely irrelevant. This is consistent with the argument of Ives [3] that even the original Sagnac experimental results were not specifically due to rotation. Ives suggested an experimental proof designed to show the effect did not require rotation. In a beautiful modification of Ives suggestion, Ruyong Wang [4] has constructed what he calls a Fiber Optic Conveyer (FOC) which directly verifies that linear motion does not affect the speed of light.

In the examples above the SRT theoreticians attempt to explain the results by claiming that rotation is involved and that because of the rotation non-isotropic light speed can be explained. The MLET explanation is that any inertial frame can be chosen as the isotropic-light-speed frame. But with that assumption clocks moving within that chosen frame must run slower and receivers or observers moving in that frame will not see an isotropic light speed. This is the MLET template, and its use is widespread though largely unrecognized.



  1. Parkinson, et al. (1996) Global Positioning System: Theory and applications Volume I, American Institute of Aeronautics and Astronautics, Washington.
  2. Moyer, T.J. (1971) JPL Technical Report 32-1527, May.
  3. Ives, Herbert E. (1938) “Light Signals Sent Around a Closed Path,” Journal of the Optical Society of America, Vol. 28, August, pp 296-299.
  4. Wang, Ruyong (2002) “Crucial First-Ordwr Fiber Interferometric Experiments to Examine the Constancy of the Speed of Light,” to be published in Galilean Electrodynamics.