# Analysis of the “Sagnac experiment”

Abstract

This chapter analyses the “Sagnac experiment”, carried out by the French physicist Georges Sagnac in 1912 year. The analysis presented is based on the classical mechanics and relativity of Galileo, which are indisputably valid in our local time-spatial region “on the surface of the Earth”. The experiment demonstrates that in relation to a moving system in the stationary space, the speed of the light differs depending on the speed and the direction of movement of the system in the stationary space. However, the Sagnac experiment is considered as a paradox, because it demonstrates that the speed of light is not the same for all frames of reference – what is not convenient for modern physics, because the special theory of relativity is created on the base of the claim “the speed of light is the same for all frames of reference”. As further evidence of the authenticity of the presented analysis, the derivation of the equation, which is often used in the rotation analyzes is shown.

Georges Sagnac, a French physicist, constructed a device “ring interferometer” (rotating interferometer with two light beams on a closed-loop), also called “Sagnac interferometer”.  The interferometer consists of a light source, a collimator (transforming light or other radiation from a point source into a parallel beam), a beam-splitter (splitting the beam in two directions), a photographic plate, and 4 mirrors of the interferometer, which are all mounted on a spinning disc (0.5m in diameter). In this way, they are all stationary with respect to the disc, but they are actually spinning in the stationary empty space – in the reference system related to the space itself. (Fig. 5.1).

Description of the experiment: A monochrome light beam is split and the resulting two beams follow (in the reference system related to the spinning disk), exactly the same path reflected by the four mirrors. The trajectories of the two beams, however, are in opposite directions, which is actually the brilliant idea of the experiment of Georges Sagnac. The two recombined light beams (unified again after one full cycle), are then focused on a photographic plate, creating a fringe pattern (a series of bright and dark bands, caused by beams of light that are in phase or out of phase with one another), permitting measurement of the interference fringe displacement with high accuracy, as Georges Sagnac described in his article “On the proof of the reality of the luminiferous aether by the experiment with a rotating interferometer” (Sagnac, 1913).

The idea is to demonstrate the different speeds of the two light beams in the frame of reference related to the spinning disk. In this frame of reference, the speed of the beam, moving in the direction of rotation of the disk decreases, and the speed of the other beam, moving in the opposite direction of rotation of the disk increases when the speed of rotation of the disk increases. The experiment demonstrates that the picture of the interference fringes changes when the speed of rotation of the disk changes.

The results of the experiment are precisely fixed. The observed effect is that the displacement of the interference fringes (the bright and dark bands), is changing with the change in the velocity of the disk rotation.

The reported result by George Sagnac is:

“The result of these measurements shows that, in ambient space, light propagates with a velocity V0, independent of the collective motion of the source of light O and the optical system. This property of space experimentally characterizes the luminiferous aether. The interferometer measures, according to the expression (according to the presented equation), the relative circulation of the luminiferous aether in the closed circuit.” (Sagnac, 1913).

It is understandable that the result of the experiment has been explained a century ago with a relative circulation of the luminiferous aether in a closed circuit. It is according to the supposition of Christiaan Huygens (Dutch physicist), that the light travels in a hypothetical medium called “luminiferous aether” – a space-filling substance, thought to be necessary as a transmission medium for spreading of the electromagnetic radiation.

In fact, the conclusion is not that the space has a property that characterizes the “luminiferous aether”, but:

“the “ether” turns out to be the “warped space-time of the Universe” itself.” (Sharlanov, 2011).

5.1. Explanation of the experiment in accordance with the
classical mechanics and the Galilean relativity

The Earth rotates in the surrounding stationary space with a constant angular velocity. The linear velocity of the Earth’s surface, at the latitude where the experiment is carrying out, is constant. The plate (the table on which the rotating disk is mounted), is fixed stationary on the Earth’s surface. Therefore, the influence of Earth’s rotation on the speeds of the two light beams (the displacement of the interference fringes due to the Earth’s rotation), is constant.

Note: The displacement of interference fringes due to the Earth’s rotation is discussed in the next chapter, where the “Michelson-Gail-Pearson experiment” is analyzed.

According to the experiment, however, the light source, the collimator (transforming the light beam from a point source into a parallel beam), the beam-splitter (splitting the beam in two opposite directions), the photographic plate, and the four mirrors mounted on the disk, are rotating all together in the stationary space at the speed of the disk. As a result, the different rotational speeds of the disc create different displacements of the interference fringes.

The two frames of reference, which we are considering in the theoretical explanation of the experiment, are:

1) The first one is related to the rotating disk, where the light source, the collimator, the beam-splitter, the photographic plate, and the four mirrors are mounted.

When the observer is on the disk, all devices (the collimator, the beam splitter, the photographic plate, and the four mirrors) mounted on the disk are stationary for the observer (regardless of whether the disc is spinning or not).

2) The second one is related to the stationary space itself.

Appropriate for the explanation of the experiment is, to consider it in a “Disk-Centered Inertial coordinate system” (DCI frame).

The description of this frame of reference is:

• The origin of the “DCI coordinate system” is the center of the disk. If we ignore the displacement of the interference fringes due to the Earth’s rotation (which is constant, regardless of the disk rotation), we actually accept that the origin of the “DCI coordinate system” (the center of the disk, which is a fixed point on the Earth’s surface), is stationary in relation to the surrounding space. Similarly, the North and South poles are stationary in the stationary space when the Earth rotates around its axis.
• The plane of the disk represents the (x,y) plane and the axes of theDCI coordinate system” are stationary in relation to the surrounding stationary space.

It means that the “Disk-Centered Inertial coordinate system” (DCI frame), can be considered as a stationary frame of reference in relation to the surrounding stationary space. In other words, the observer situated in the DCI frame will see how the light source, the collimator, the beam splitter, the photographic plate, and the four mirrors of the interferometer are rotating together with the disc.

Before the examination of the experiment, we can recall that every mechanical or optical experiment actually takes place in the common space of the considered frames of reference.

5.1.1. Examination of the Sagnac experiment in the reference system related to the surrounding stationary space – in the “Disk-Centered Inertial coordinate system”.

In our time-spatial region“in the vicinity of the Earth’s surface”, the intensity of the gravitational field is uniform (the same). According to the abovementioned initial conditions of the experiments (which conditions do not, in fact, contradict the standpoint of contemporary physics): electromagnetic radiation propagates in vacuum (in the stationary space), with a constant speed equal to c. This is actually the speed of light in the stationary in relation to the space DCI frame of reference.

However, everything mounted on the spinning disc is rotating (moving) in the stationary space (what means: in relation to the stationary in the space DCI frame of reference). Therefore, in this frame of reference, the length of the path that the two light beams actually travel in space is different.

This is due to the movement (at the rotation of the disk) of each mirror in the stationary space during the travel of the light beams towards the mirrors.

The two light beams travel in opposite directions. Thus, the path length of one of the light beams (which travels in the opposite direction of the disk rotation) is shortened, and the path length of the other light beam (which travels in the direction of the disk rotation) is extended. As a result of the change of the path lengths of the two light beams (due to different velocities of the disk rotation) – different displacement of the interference fringes is created.

Therefore, the conclusion of the observer, located in the stationary in relation to the space “DCI coordinate system” (where the speed of light is constant and equal to c), is that the displacement of the interference fringes is due to the change of the path lengths travelled by the two light beams, which in turn depends on the velocity of the disk rotation.

5.1.2. Examination of the Sagnac experiment in the frame of reference related to the spinning disk.

Positioned on the spinning disk, the observer will see that all devices (the collimator, the beam splitter, the photographic plate, and the four mirrors) mounted on the disk do not move – that they are stationary. Therefore, the path lengths of the two beams (the distances among the mirrors) are not changing when the disk is spinning, either. As a result, the speeds of the two light beams (measured by the observer), in the reference system related to the spinning disk, will be different. This difference depends on the velocity of the disk rotation: the speed of the beam which travels in the direction of the disk rotation decreases to (c-V), where V is the linear velocity of the mirrors, while the speed of the other light beam, which travels opposite to the direction of the disk rotation – increases to (c+V). In fact, the “light velocity anisotropy” observed in the Sagnac’s experiment is similar to “light velocity anisotropy” in the experiments “One-way determination of the speed light” (see the described cases “Eastward Transmission” and the “Westward Transmission” in chapter 4 of the book).

Therefore, the conclusion made by the observer positioned in the frame of reference related to the spinning disk is that the displacement of the interference fringes is due to the difference between the speeds of the two light beams. In turn, that difference (respectively the displacement) changes with the change of speed of the disk rotation.

Finally, we can underline that as early as 1913, Georges Sagnac’s experiment (Sagnac, 1913) actually proved that “the speed of light is not the same in relation to all frames of reference. This was even before the publishing of the general theory of relativity. Is it not surprising that Einstein never commented on this experiment, although certainly knew about its existence…

Georges Sagnac’s experiment is unofficially considered mystical, because so far, none of its explanations have been officially accepted. Of course, there are many “modern scientific explanations” which, however, are based on unscientifically proven hypotheses – or on “scientific” references to false theories (see: What is the Truth and the Proof in the Science?”. Although Sagnac experiment proves that the speed of light is not the same in all inertial reference frames, many modern physics journals publish “scientific” explanations based on the special theory of relativity… which is based on the false claim that “the speed of light is the same in all inertial frames” … In other words, this is a classical “circular reference”! Such an example of a published “scientific” comparison of different explanations is that of Malykin, G.B. “The Sagnac effect: correct and incorrect explanations” (Malykin, 2000). There are other such examples in the scientific literature.

Despite all this mystification, although there was no valid scientific explanation to this day, nowadays, the result of this experiment has many significant applications in the practice. A wide-ranging application is found in space navigation, aviation (optical gyroscope), as well as in everyday Earth positioning needs, where no one has observed any “anisotropy” of the meter as a unit of measurement (which is a claim of the special theory of relativity).

An additional proof of the credibility of the above-mentioned explanation of the Sagnac experiment is given in the next subsection. This theoretical explanation demonstrates the derivation and origin of the most commonly used equation in rotational analyses.

5.2. Derivation of the equation, which is often used in the rotation analyzes

The Sagnac effect is manifested itself in a setup called a ring interferometer. It is the basis of the widely used high-sensitivity fiber-optic gyroscope that fixes the changes in the spatial orientation of the object (airplane, satellite, …).

In general, the fiber-optic gyroscope consists of a rotating coil with a number of optical fiber turns. The optical fibre is a flexible, transparent fiber, made of glass (silica) or plastic. It consists of two separate parts. The middle part of the fiber is called core and that is the fiber optic medium the light travels through. Wrapped around the outside of the core is another layer of glass called the cladding. The cladding’s task is to keep the light beams inside the core. It can do this because it is made of a different type of glass to the core – the cladding has a lower refractive index and acts as countless small mirrors. Each tiny particle of light (photon) propagates down the optical fibre by bouncing repeatedly off the cladding, as though the cladding is really a mirror – it reflects back in again and again. This phenomenon is called total internal reflection, which causes the fiber to act as a waveguide.

We will examine a simple ring interferometer (a coil with only one fiber-optic turn) mounted on a rotating disk with an angular velocity ω radian/sec (see Fig. 5.2).

Two laser beams propagate in the rotating coil: one of them in the direction of the coil rotation, and the other – in the opposite direction of the coil rotation. When the angular velocity of the rotating coil is changing at the turning of the object where it is mounted – and the displacement of the interference fringes changes.

The strength of the Sagnac effect is dependent on the effective area of the closed optical path. However, this is not simply the geometric area of the loop but is enhanced by the number of turns in the coil. The equation that we will derive on the basis of the aforementioned theoretical explanation of the Sagnac experiment is often used in the analyses of rotation:

, where A is the area of the circle bounded by the fiber-optic coil. The optical circuit (the “fiber-optic medium”), mounted on the rotating disc rotates along with the rotation of the disc at a linear speed equal to (Rω), where R is the radius of the optical circuit, ω is the angular velocity of the rotating disk. The speed of light inside the “fiber-optic medium” (where the speed of light is constant for the homogeneous optical medium) is c0.

As is shown in Fig.5.2, the two light beams (beam 1 and beam 2) are traveling in opposite directions in the same fiber optic circle. Let us analyze one cycle of each of the two beams (from the moment of splitting – up to the moment of directing them to the screen-detector). Fig. 5.2. Schematic presentation of a circular interferometer with one optical coil

Here it must take into consideration two things:

•  the first is that the space inside the optical fibre (the optical medium) is stationary, although each atom of the optical fiber moves at the rotation.

Since the space has no mass, no force can give it acceleration (set it in motion). This is a consequence of Newton’s second law of motion (F = ma). Neither the strength of the chemical bonds between atoms (in the micro-world) nor the gravitational forces (according to Newton’s law of universal gravitation in the macro-world), can force the space to move, because the space has no mass.

•  the second is that at the microscopic level, the cladding of the optical fiber can be seen as a continuous series of millions of mirrors in which electromagnetic waves are reflected in their propagation (in the case of the Sagnac’s experiment, the mirrors are only four).

Similarly, to the Sagnac’s interferometer, each of these “elementary mirrors” shifts at a definite angle from the previous photon reflection when the optical coil is rotated – (the mirrors are moved at a certain distance during the propagation time of the electromagnetic wave in the stationary “micro-space” of the optical medium). Thus, in the stationary space, the path of the photons (of the light beam), moving in the direction of rotation of the optical coil is extended, and the path of the light beam, moving opposite to the rotation of the optical coil, is shortened.

5.2.1. Analysis of one rotation cycle of the light beam “1” that travels in the direction of the disc rotation.

In the stationary in relation to the surrounding space Disk-Centered Inertial (DCI) coordinate frame.

After splitting, the light beam “1” makes one full cycle in the direction of the disk rotation, and reaches again the beam-splitter after time interval t1, to redirect to the display-detector. For the stationary in the space observer (located in the DCI-coordinate system), the distance traveled by the beam ”1” in the stationary space inside the optical medium is longer than the fiber optic coil circumference (2πR) with (Δ = Rωt1). This is because, during the beam travel, the point of redirection to the detector display (as well as the entire optical loop) is moved, due to the disk rotation, at a distance Δ. Therefore, the distance traveled by the light beam “1” in the stationary surrounding space, is (2πR + Rωt1), so for the time interval t1, (the time for one turn of the light beam “1”), the observer in the “DCI frame of reference”) will record:

, where c0 is the speed of light inside the “fiber-optic medium” (where the speed of light is a constant for the homogeneous optical medium).

•  In the frame of reference related to the rotating disk, where the fiber-optic coil is mounted.

For the observer, positioned in this frame of reference (on the rotating disk), the distance traveled by the light beam “1” is 2πR, because the fiber-optic coil does not move in this frame of reference (in relation to the rotating disc). He/she will measure that for the same time interval t1, the speed of light beam “1” will be equal to (c0 – Rω), and for the time interval t1 , (the time for one turn of the light beam “1”), the observer (in the frame of reference related to the rotating disk) will register:

, which is actually equal to t1 from the expression (10) after its transformation.

5.2.2. Analysis of one rotation cycle of the light beam “2”, which travels in the opposite direction to the disk rotation.

•  In the stationary in relation to the surrounding space Disk-Centered Inertial (DCI) coordinate frame.

After splitting, the light beam “2” makes one full cycle in opposite direction to the disk rotation and reaches again the beam splitter after time interval t2 , to be redirected to the display-detector. Actually, the distance, traveled by the beam ”2” in the stationary space inside the optical fibre, is shorter than the fiber optic coil circumference (2πR) with (Δ = Rωt2). This is because, for the time of the beam travel, the point of redirection to the detector (as well as the entire fiber-optic coil) has come closer, due to the disk rotation against the direction of movement of the beam. Therefore, the distance traveled by the light beam “2” in the stationary space (in the “DCI coordinate frame”), is (2πR – Rωt2), and for the time interval t2 ( the travel time for one turn of the light beam “2”), the Observer in the stationary in relation to the surrounding stationary space Disk-Centered Inertial (DCI) coordinate frame, will register:

where c0 is the speed of light in the “fiber optic medium” (where the speed of light for the homogeneous optical medium is constant).

•  In the frame of reference related to the rotating disk.

For the observer, positioned in this frame of reference (on the rotating disk), the distance traveled by the light beam “2” is exactly 2πR, because the fiber-optic coil does not move in relation to the rotating disc (in the observer’s frame of reference). He/she will measure that for the same time interval t2 , the speed of light beam “2” will be equal to (c0 + Rω), and for the travel time for one turn of the light beam “2”, the observer in the frame of reference related to the rotating disk will register:

, which is actually equal to t2 from the expression (12) after its transformation.

5.2.3. The results.

On the basis of the analysis, it was found that:

• the time t2 for one complete tour of the light beam “2” is the same for both frames of reference;
• the time t1 for one complete tour of the light beam “1” is the same for both frames of reference.
• However, the time for one complete tour of the light beam “1” (which moves in the direction of the rotation of the optical coil) is more than the time for one complete tour of the light beam “2” (which moves in the opposite direction of the rotation of the optical coil).

In the frame of reference related to the rotating disk, for the difference between the time for one tour of the light beam “1” and the time for one tour of the light beam “2”, we get (after subtracting (13) from (11):

, because

Equation (14) is actually the equation (9) we had to derive. The result will be the same for the Disk-Centered Inertial (DCI) coordinate frame, if we subtract the equation (12) from (10). So, there is no “relativistic difference in time” …

Therefore, the demonstrated derivation of the equation, which is often used in rotation analyzes, proves the veracity of the theoretical explanation of the Sagnac’s experiment (in accordance with classical mechanics and Galilean relativity).

5.3. Conclusion

The moving reference system in the stationary space at the Sagnac experiment is the “spinning disc”.  The moving reference system in the stationary space at the experiments “One-way measurement of the speed of light” is the “moving/rotating Earth’s surface”.

The observed effects of displacement of the interference fringes in the case of “Sagnac’s ring interferometer”, as well as “light speed anisotropy” (the difference in the speed depending on the direction of the light beam) in the case of “one-way determination of the speed of light”, clearly demonstrate that:

The speed of light in relation to the stationary space (in vacuum) is a constant in our local time-spatial region with the uniform intensity of the gravitational field (or in relation to the stationary space inside a homogeneous optical environment).

However, it appears that in relation to a moving system in the stationary space, the speed of the light differs depending on the speed and the direction of movement of the system in the stationary space.

Before revealing the defect of the fatal Michelson-Morley experiment, performed in 1887, we will analyze the Michelson-Gale-Pearson experiment (carried out in 1925), as it also proves that the speed of light is not the same for all frames of reference.

Before examining the inappropriate conceptual design, embedded in the interferometer construction, used in the experiment “Michelson-Morley” (held in 1887), we will analyze the experiment “Michelson-Gale-Pearson” (held in 1925). The experiment “Michelson-Gale-Pearson” proves again that, in the reference system related to the moving Earth’s surface, the measured speed of light is influenced by the rotation of the Earth (by the movement of the Earth’s surface) – that the speed of light is not the same for all frames of reference.

=> to the parent webpage

If you haven’t read the analysis of “One-way measurement of the speed of light” yet, it is worth to read it here!

If you haven’t read the analyses the “Michelson-Gale-Pearson” experiment yet, it is worth to read it here!

The revealing fact that the inappropriate conceptual design, embedded in the construction of Michelson’s interferometer, however indisputably shows that the claim “the speed of light is the same in all inertial frames of reference” is a great delusion and the “Michelson-Morley experiment” is actually the primary root cause for the biggest blunder in physics of the 20th century – the special theory of relativity.

Furthermore, the analysis of the article “On the Electrodynamics of Moving Bodies”, where Einstein has published the special theory of relativity shows exactly where and how the claim “the speed of light is the same in all inertial frames of reference” was applied …