# What is sagnac: Definition and 18 Discussions

The Sagnac effect, also called Sagnac interference, named after French physicist Georges Sagnac, is a phenomenon encountered in interferometry that is elicited by rotation. The Sagnac effect manifests itself in a setup called a ring interferometer or Sagnac interferometer. A beam of light is split and the two beams are made to follow the same path but in opposite directions. On return to the point of entry the two light beams are allowed to exit the ring and undergo interference. The relative phases of the two exiting beams, and thus the position of the interference fringes, are shifted according to the angular velocity of the apparatus. In other words, when the interferometer is at rest with respect to a nonrotating frame, the light takes the same amount of time to traverse the ring in either direction. However, when the interferometer system is spun, one beam of light has a longer path to travel than the other in order to complete one circuit of the mechanical frame, and so takes longer, resulting in a phase difference between the two beams. Georges Sagnac set up this experiment in an attempt to prove the existence of the aether that Einstein's theory of special relativity had discarded.A gimbal mounted mechanical gyroscope remains pointing in the same direction after spinning up, and thus can be used as a rotational reference for an inertial navigation system. With the development of so-called laser gyroscopes and fiber optic gyroscopes based on the Sagnac effect, bulky mechanical gyroscopes can be replaced by those with no moving parts in many modern inertial navigation systems. A conventional gyroscope relies on the principle of conservation of angular momentum whereas the sensitivity of the ring interferometer to rotation arises from the invariance of the speed of light for all inertial frames of reference.

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1. ### I Sagnac gyroscope affected by linear acceleration?

As some of you know I have been coding a 2+1D relativistic simulator that can handle (constant proper) acceleration in any direction on a plane. So far it is going quite well. I have now decided to simulate a Sagnac gyroscope inside the simulation by sending signals in opposite directions...
2. ### B Sagnac effect for matter beams

Hello, I have recently come across this article by Rizzi and Ruggiero, called "The Relativistic Sagnac Effect: Two Derivations": https://arxiv.org/pdf/gr-qc/0305084.pdf In section 3, the authors derive the Sagnac proper time difference for all beams (light beams and matter beams, including...
3. ### I Explaining Sagnac vs. SRT Violation & Reconciling Lorentz Transform

Can someone please explain how a Sagnac ring laser gyro or fiber-optic gyro seems to violate SRT, since the second postulate of SRT holds that the speed of light does not vary in vacuum regardless of the position or motion of any observer (in this case, receivers)? And how is it even possible to...
4. ### I Two Photon Sagnac Source

In this paper, Cramer references a Sagnac source and mentions it has a half wave plate to vary the entangement vs. coherence by rotating the plate. Look just below figure 1: https://arxiv.org/pdf/1409.5098.pdf What is this setup? My guess is that the half wave plate is right before the...
5. ### I What causes the Sagnac effect?

Wikipedia explains the Sagnac effect as a result of the rotating disk, which moves the target so that one of the light beams has farther to travel and consequently, will arrive later than the other light beam which goes around the disc in the same angular direction as the rotating disc. However...
6. ### Minimizing Specular Reflections in a Sagnac based Fiber Gyro

My project is to build an airspeed flowmeter using the design of a signac fiber optic based gyro. There is a group that has already accomplished this task (http://www.ncbi.nlm.nih.gov/pubmed/18301479), however they used a commercial fiber gyroscope whereas I am trying to build the entire setup...
7. ### The Sagnac Effect: Exploring Length Contraction with a Thought Experiment

So, I've been reading on GR and I've come across this. Awhile ago I read a smaller book on relativity that ended with a neat thought experiment: take a disk, lay out the circumference with many small rods, and start spinning it at relativistic speeds. Because of SR length contraction, each of...
8. ### Sagnac Effect on Equatorial Plane: MG-P Exp & GR Twist

If one were to send two light beams around the equator east and west, would the Sagnac effect occur? I know Michelson-Gale-Pearson measured it, but in their experiment the two beams would each travel both directions in separate rectangles, so I wonder if the effect would be weaker/stronger. How...
9. ### Does the Sagnac Effect Indicate Rotation in Non-Rotating Gyroscope Frames?

As you may already know, given a time-like congruence describing some extended body with world-tube ##\mu## embedded in space-time, there are various different characterizations of what it means for this extended body to be non-rotating. Of particular interest for me is the setting of...
10. ### MMX and the earth's rotational sagnac

MMX proves light is isotropic in all directions. Hand held GPS units apply the Earth rotational sagnac correction which means light is not isotropic in all directions at the unit from the satellites. If a hand held GPS unit is placed at the same location as an MMX experiment, one...
11. ### Sagnac Effect and Special Relativity

http://www.mathpages.com/rr/s2-07/2-07.htm Today I met a man who was claiming that the Sagnac effect, particularly in the case of light moving in a circular pattern with a rotating observing "arm" as in the first figure in the link, is contrary to special relativity. He claimed that since...
12. ### Current Sagnac Effect Theory: PF Viewpoint

In the view of PF posters, what is the "current" [meaning most accepted] theory explaining the Sagnac effect? I have heard of something called Klauber's NTO [Non-Time-Orthogonality] theory and am wondering if it is "accepted"? -Harry Wertmuller
13. ### Understanding the Sagnac Effect in GPS Systems: Explained

I've read that the GPS system has to take into account the Sagnac effect when sending signals from the satellites to the Earth. For those of us who are unfamiliar with the Sagnac effect, it is the delay in receiving(or shorter time in receiving) the signal due to the rotation of the Earth while...
14. ### How Does the Sagnac Effect Cause Interference Patterns in a Rotating System?

I'm having trouble understanding the Sagnac Effect. So far all I know is that a ring cavity is formed where 2 beams travel in opposite directions. My question is: Why do the 2 beams form an interference pattern if the entire cavity is rotating at some angular velocity?
15. ### GR, SR and the Sagnac effect question

I've read in multiple locations that GR is necessary to properly explain a co-rotating perspective/observer of the experiment. Additionally, SR can explain the experience of non-inertial observer. The above two statements seem somewhat conflicting and leads to my question. Why is it that...
16. ### Linear vs Sagnac: Examining Arrival Time Differences

Okay I've got a question about the difference between the sagnac effect in an inertial and non-inertial environment. Let me give you an example to base the info off of. I want to make a really long linear sagnac-like arrangement to factor out any cross propagation. In the following example...
17. ### What Causes the Sagnac Effect and How to Calculate Reduction in Light Intensity?

in this webpage: http://www.mathpages.com/rr/s2-07/2-07.htm we have an explanation to the saganc effect and in page 2 and 3 i got stumbled: i can't get to the quadratic equation in page 3 i.e: [c^2-R^2*w^2*cos(theta)]*T^2+-[2R^2*w*sin(theta)]*T-2R^2[1-cos(thata)]=0 from this...
18. ### Explaining Sagnac Experiment: Einstein & Constant Velocity of Light

Is there anyone here that can explain this phenomena using Einsteins postulat that the velocity of light is constant? I haven't found any explanation to the phenomena on the Internet.