The Sagnac Experiment was first conducted by French physicist Georges Sagnac in 1913. The experiment is essentially the same experiment as the Michelson-Morley Experiment, except that it is on a horizontal rotating turn-table. The results of the Sagnac Experiment show that when the table is rotated around on a horizontal axis during the experiment, the velocity of light indeed changes. This contradicts the Michelson-Morley Experiment that was designed to use the supposed movement of the earth to change position, and which showed no change in light velocity.
From a paper on the Sagnac Effect one writer states:
“ Since its discovery at the beginning of the XX century the Sagnac effect  has play an important role in the understanding and development of fundamental physics (for a review see ). The Sagnac effect is the dependence of the interference pattern of the rotating interferometer on the direction and speed of rotation. This phenomenon is universal and manifested for any kind of waves, including the matter waves and has found a variety of applications for the practical purposes and in the fundamental physics .
...In the context of the Sagnac effects the null result of the Michelson-Morley experiment is also not clear. Applying the same logic to Sun centered rotating frame in which Earth is fixed, one would expect different light speeds as seen from Earth. ”
A Critical Analysis
“ Einstein's theory of relativity is shown to be a physical theory of limited experimental validity. Twelve different experiments seem to disprove its two postulates. ”
On the subject of the Sagnac Effect, Dr. Monti states:
“ The effect was tested in 1913 by Sagnac(46), and Sagnac’s experimental results disproved the second postulate of special relativity. ”
The geocentrist Malcom Bowden describes the Sagnac Experiment in the following video:
“ An animated explanation of how Sagnac's experiment proved the existence of the aether, thus demolishing Einstein's Theory of Relativity. Complaints by two experienced physicists that when they were at university, they were never informed about this important experiment. When other experiments - Airy's Failure, Michelson-Gale's experiment (also not taught at universities!) and of course the famous Michelson-Morley experiment- a completely different picture of the cosmos emerges - with the Earth at the centre of the universe. ”
University of Geneva
From the Abstract:
“ The Sagnac effect is analysed in both Galilean and Special relativity within a space-time geometrical model previously developed by Langevin and Post. The effect arises because of the different velocities of different light signals relative to the interferometer. The appropriate relativistic relative velocity transformation formulas obtained differ from the velocity transformation formulas of conventional Special relativity, the latter actually predicting that the Sagnac effect vanishes. The Michelson-Morley experiment is analysed using the same model and a nonvanishing fringe shift, albeit below the sensitivity of all such experiments performed to date, is predicted. The Sagnac effect for neutrinos of the CERN CNGS beam is also discussed. The Sagnac effect indicates that the ECI (Earth Centered Inertial) frame is a preferred one in which light signals have a speed close to c, in the vicinity of the Earth, as predicted by General relativity. ”
From the Conclusion:
“ The final conclusions are that the ECI frame constitutes a physically-preferred reference system for light signals or neutrinos in the vicinity of the Earth and that the Sagnac effect is not correctly described by the velocity transformation formulas of conventional special relativity. ”
General Relativity Framework
On the assertion that the Sagnac Effect is explained under the framework of General Relativity theory, Professor Ramzi Suleiman of the University of Haifa writes the following:
“ It is believed that the Sagnac effect does not contradict Special Relativity theory because it is manifest in non-inertial rotational motion; therefore, it should be treated in the framework of General Relativity theory. However, several well-designed studies have convincingly shown that a Sagnac Effect identical to the one manifest in rotational uniform motion is also manifest in transverse uniform motion. This result should have been sufficient to falsify Special Relativity theory. In the present article, we offer theoretical support to the experimental results by elucidating the notion that the dynamics of transverse and rotational types of motion are completely equivalent. Since the transverse Sagnac effect contradicts Special Relativity theory, it follows that the rotational Sagnac effect contradicts Special Relativity theory as well. ”
It has been claimed that Special Relativity does not apply to the Sagnac Effect because the Sagnac Experiment involves rotational acceleration. In 2004, Dr. Ruyong Wang demonstrated that a non-accelerating linearly moving observer can also measure a difference in light speed, falsifying Special Relativity's explanation of the Michelson-Morley Experiment.
Robert Bennett, Ph.D. writes:
“ Belief that the Sagnac test of 1913 only applied to rotational motion was discounted when Ruyong Wang found the same results for linear motion in 2004. The Sagnac result has never been credibly explained, despite its wide application in modern technology. ”
From Challenging Modern Physics: Questioning Einstein's Relativity Theories by Al Kelly (bio) we find:
“ A recent ingenious test by Wang et al. (2003) shows that the Sagnac result is also achieved by sending out and back again light in a straight-line portion of the light path. This is what this author claimed above, but it is so much more convincing when an actual experiment has shown the same thing. I wonder what excuse will be trotted out now! Wang et al. achieved the seemingly impossible by reversing a light beam sent out on a straight line on a moving platform and measuring the difference in time for it to return. This author had the pleasure of meeting Wang in 1997 and corresponded with him during the rests he performed and since then.
In another paper (2005), Wang gives further details of the experiment; appended to this paper are comments on the experiment by Hatch and Van Flandern, confirming that Wang had succeeded in proving that the Sagnac effect applies to straight-line motion.
...Any claims that the Sagnac experiment upsets [Special Relativity] were heretofore brushed aside by a statement that Sagnac is a rotational experiment and that SR does not apply to rotational experiments. That defense is now shown to be groundless. ”
“ Since the realization of this [Sagnac] experiment, which has been done with photons , electrons  and neutrons , many trials have been made to interpret the observed results seen, for instance, Selleri . Indeed, Sagnac utilized the habitual linear additive rule and with that he was able to correctly predict the observed results. Still, since his prediction lead to velocities greater than c and consequently are against relativity which claims that the maximal possible velocity is c this raised a large amount of arguing. In fact, many authors tried to explain the results of the experiment in the framework of relativity which assumed that the maximal possible velocity is c. As can be seen in the literature, there are almost as many explanations as the authors that have tried to explain the results in the framework of relativity. In some cases the same author  presents even more than one possible explanation. The complexity of the problem stems mainly from the fact that the experiment is done in a rotating platform. In such case, there may occur a possible accelerating effect leading the explanation of the experiment to fall in the framework of general relativity.
This controversy, whether Sagnac experiment is against or in accordance with relativity, was settled recently by R. Wang et al.  with a very interesting experimental setup they called linear Sagac interferometer. In this case the platform is still, what moves is a single mode optical fiber coil, Fig. 12.
They did the experiment with a 50 meter length linear interferometer with wheels of 30 cm. The observed relative phase shift difference for the two beams of light following in opposite directions along the optical fiber was indeed dependent only on the length of the interferometer and consequently independent of the angular velocity of the wheels. From the experimental results obtained with the linear Sagnac interferometer one is lead to conclude that in this particular case the linear additive rule applies. Consequently we may have velocities greater than c, which clearly shows that relativity is not adequate to describe this specific physical process. ”
In parting, Dr. Croca states:
“ As a final note, I would like to stress that these observed facts [do not] in any way deny the usefulness of relativity. Relativity is a good approach to describe reality at its proper scale of applicability. What is quite wrong is to claim that relativity is the last, the complete and final theory ever devised by mankind. ”
Defeated, and with the ever need to have an 'explanation' to keep the model alive, relativists seem to hint that relativity might operate differently at different scales. Dr. Croca suggests that Special Relativity is useful for explaining why light is consistent in experiments of the Earth going around the Sun (Michelson-Morley Experiment) and with the direction of Starlight (Airy's Failure), but that this theory is not applicable in other experiments such as Sagnac and Wang devices—laboratory devices which do not rely on the motion of the earth. Sagnac and Wang experiments have observed an inconsistency of light speed for different speeds of a detector in motion, including light traveling faster than c, which is directly contradictory to Special Relativity's postulate of a speed consistency of light to all observers and the luminal speed limit of c.
One must remark that it is quite odd and remarkable that Special Relativity remains true despite a direct violation of its postulates with multiple experiments; apparently only applicable to certain situations involving the motion of the Earth as a heliocentric explanation for why the Earth has been tested to be horizontally motionless.
Daniel Y. Gezari
Dr. Gezari (bio) in his Experimental Basis for Special Relativity in the Photon Sector (Archive) shows us that Special Relativity is a theory of limited experimental validity and casts doubt on the invariance of c postulate used to justify the Michelson-Morley and Airy's Failure experiments. We are informed that the invariance of c has never been demonstrated with a moving detector.
A search of the literature reveals that none of the five new optical effects predicted by the special theory of relativity have ever been observed to occur in nature. In particular, the speed of light (0) has never been measured directly with a moving detector to validate the invariance of c to motion of the observer, a necessary condition for the Lorentz invariance of c. The invariance of c can now only be inferred from indirect experimental evidence. It is also not widely recognized that essentially all of the experimental support for special relativity in the photon sector consists of null results. The experimental basis for special relativity in the photon sector is summarized, and concerns about the completeness, integrity and interpretation of the present body of experimental evidence are discussed.
One of the most reassuring things we know about modern physics is that the special theory of relativity has faced a century of experimental challenges, and passed every test. This is generally understood to mean that every aspect of special relativity has been tested and validated, beyond any doubt. But all it really means is that every aspect of special relativity that has been tested has passed the test. This prompts the question, what has been tested and what has not?
Contrary to the popular view, a search of the literature reveals that the experimental basis for the special theory of relativity in the photon sector is not robust.
2.1. Invariance of c.
There are two necessary conditions for the local Lorentz invariance of c: invariance to motion of the source and invariance to motion of the observer. Satisfaction of these two conditions is both necessary and sufficient to validate the invariance of c. Invariance to motion of the emitting source — Einstein’s second postulate has been convincingly validated experimentally (Section 3.1). But conspicuously absent from the experimental record is any published attempt to directly measure the speed of light with a moving detector to test the invariance of c to motion of the observer.
The experimental validation of the invariance of c is plagued by misconceptions and errors of interpretation. There is a common misconception that Einstein’s second postulate says that c is invariant to "motion of the source and motion of the observer" and it is incorrectly presented this way in most textbooks. But the second postulate says nothing about the observer: “Light is propagated in empty space with a definite velocity 0 which is independent of the state of motion of the emitting body” (Einstein 1905). The second postulate was not a new idea in 1905 and it is not unique to special relativity (recall that the classical wave theory of light also holds that c is invariant to motion of the emitting source). So observations of moving sources cannot discriminate between special relativity and the old ether hypothesis, and do not favor one over the other. Of course, it could be argued that experiments with moving sources and moving observers should be equivalent and indistinguishable, so the second postulate would apply to the observer as well as to the source. But in other phenomena involving propagating light (e.g., the Doppler effect in an optical medium, stellar aberration) motion of the source and motion of the observer have entirely independent consequences. To claim that source and observer motions are equivalent without experimental confirmation would be invoking the theory to validate itself. Observations of moving sources certainly cannot validate the universal Lorentz invariance of 0 without observations with moving detectors, or at least experimental validation of the equivalence of source and observer motions for propagating light, and these things have not yet been accomplished.
It would seem that elements of the classical Sagnac effect conflict directly with special relativity, however, the prevailing view is that the rotating instrument is a non—inertial system to which special relativity does not apply (as first argued by Langevin 1921). The argument goes further to say that an observer viewing the rotating experiment from any inertial frame would be permitted under the rules of special relativity to measure relative speeds that differed from c, so the apparent speeds c + v and c - v of the counter— propagating beams in the instrument frame would still be consistent with special relativity. However, recently Wang et al. (2003, 2004) demonstrated the Sagnac effect in a non- rotating, inertial reference frame using a fiber optic linear motion sensor (FOLMS) interferometer. They showed that the light travel time in a straight optical fiber in inertial motion has a first—order dependence on the fiber speed in the local stationary frame, just as the light travel time in a rotating Sagnac effect fiber optic gyro has a first-order dependence on the tangential rotation speed of the fiber. The effect was obtained using both solid and hollow (air core) fibers. If the Sagnac effect can be produced by inertial motion then the rules of special relativity would have to be applied after all, and the linear Sagnac experiment would violate special relativity.
Considering the weakness of the present experimental support for the invariance of c — the fact that observations of moving sources cannot discriminate between special relativity and the old ether hypothesis, the absence of speed-of-light measurements with moving detectors, the lack of experimental validation of the equivalence of source and observer motions, doubts about the interpretation of the classical ether-drift experiments, concerns about the applicability of the modern isotropy experiments, and the fact that all of the unambiguous tests of special relativity in the photon sector have produced null results — it cannot yet be claimed that the local Lorentz invariance of c has been convincingly validated by observation or experiment. ”
Note: A brief reference is made in the paper at the end of p.6 to the Michelson-Gale Experiment. See the Michelson-Gale-Pearson Experiment
“ The logical existence of the incremental Sagnac effect implies... that there is some compelling physical reason why the effect cannot be observed at the surface of the Earth....We hold that until something new is brought to the table, this question simply cannot be resolved. No currently accepted theory reveals why, like a Cheshire cat, the Sagnac effect shows itself in one kind of experiment but not in another. ”-- Howard C. Hayden and Cynthia K Whitney, "If Sagnac and Michelson-Gale Why Not Michelson-Morley?" Galilean Electrodynamics, vol. 1, no. 6, Tufts University, Nov./Dec. 1990, pp. 73-74.
“ According to the second postulate of Einstein's Special Theory of Relativity the speed of light is independent of the uniform motion of its source. Direct experimental evidence by W. Kantor of the US Navy Electronics Laboratory, San Diego, leads him to the surprising conclusion that it may be untenable (Journal of the Optical Society of America, Vol. 52, No. 8, p. 978)...[...]..If Einstein's postulate is correct there should be no displacement between the two sets of interference fringes on spinning the disc, because the light from the approaching and receding windows, respectively, should all have the same velocity. In fact, an unambiguous, easily noted shift of the fringes was apparent when the mirrors were in motion (maximum linear velocity: 4,690 cm per sec) and Kantor deduces that Einstein's second postulate is incorrect. The fringe shift, moreover, appeared to depend on the speed of the disc. If the present work turns out, on more rigorous research, to be flawless and free from experimental artefacts, and if there is no obvious alternative explanation for the observed effects, there may be a need to reconsider some basic ideas in physics. ”-- 'Light Velocity Dependant on Speed of Source?', New Scientist 1 Nov 1962 p276.
“ The high-velocity experiments on mesons, such as those at CERN, are definite evidence of the meson lifetime's functional relationship to their velocity with respect to the Earth, but have nothing whatsoever to do with the 'time-dilation' of Special Relativity. The experiments also are yet another 'ether-drift' investigation, with the usual answer: the velocity of the Earth with respect to a fundamental frame is zero. ”-- 'The "Time Dilation" of Mesons Re-Examined', Donald T. MacRoberts, Galilean Electrodynamics, Volume 3, No. 5, pp. 83-84.