Talk:Sagnac effect

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Contents 1 RING LASERS AND RING INTERFEROMETERS 2 Spacetime geometry is not a prefered frame 3 Edits ready 4 History of the Sagnac Effect 5 Does the Sagnac effect necessarily involve rotation? 6 Erroneous perception of Sagnac effect 7 The proposal of the loop-closing definition. 8 The diagram for the ring laser setup 9 Animation to illustrate the basics of ring lasers

[edit] RING LASERS AND RING INTERFEROMETERS

The second figure has several problems:

1. It is called "Laser Ring Interferometer". There is no such thing. It should be called "Laser Ring" or "Ring Laser"

2. It is not representative at all relative to what is going on in a laser ring. Ati3414 15:35, 4 March 2006 (UTC)

---

There is no such thing as "ring laser interferometers", "dr. Cleonis". For all your superior attitude you make quite a few mistakes. There is "ring lasers" and "ring interferometers". For clarification check the mathpages on the Sagnac link you're quoting, so I fixed it for you. You need to get your terminology straight.

I am making two edits to this page. This first is to remove most of the third paragraph, which is quote improper. There is no prefered frame of reference indicated by the Sagnac effect. It is only due to rotation deviations from inertial motion that this effect occurs, as documented by the remaining portion.

I am also correcting a misspelling of Minknowski ("Minkowsky").

Your statement is incorrect. A sagnac interferometer measures its angular velocity with respect to space-time geometry. This can for example be seen from the Michelson-Gale Sagnac interferometry experiment. The 1925 Michelson-Gale setup had a perimeter of 1.9 kilometer. So how was it calibrated. Michelson started with images that went one way, so there was no interference pattern. The position of each image was recorded. If the central line of the interference pattern would be exactly between the two recorded positions then that would constitute a measurement of zero rotation. If the interference fringes would be shifted from the in-between position then that would provide a quantitative measurement of rotation.

From an article by G E Stedman
Ring-laser tests of fundamental physics and geophysics
External link: StedmanReview1997 1.5 MB PDF document

In a 1953 letter to Shankland (Shankland 1974), Einstein said: ‘my admiration for Michelson’s experiment is for the ingenious method to compare the location of the interference pattern with the location of the image of the light source. In this way he overcomes the difficulty that we are not able to change the direction of the earth's rotation.
--Cleon Teunissen | Talk 22:36, 24 Mar 2005 (UTC)

[edit] Spacetime geometry is not a prefered frame

I think that some history may be quite relevant here:

Before SR, it was believed that there was an special frame of reference in which the rules of electromagnetism (EM) applied exactly: the frame of reference at rest with respect to a "luminiferous aether". This was needed because Maxwell's Equations of EM are not Gallilean Invariant.

SR solved that problem. In SR, Maxwell's Equations are found to be Lorentz Invariant, and no special inertial frame is needed to explain the observed behavior of light in that case. However, SR drove many scientists of the time up a wall. One of them was Sagnac. He figured that if transitional motion was not immediately detectable, perhaps rotational motion should be tried instead. After all, be figured, the speed of light should be constant as viewed in a rotating ring too.

So he did his experiment, and got a non-null result. Immediately he declared this as evidence for the existance of a special frame of reference. However, it was quickly realized that a rotatating ring is not an inertial frame of reference, but instead an accelerated one. In that case, the speed of light elsewhere on the ring as viewed by an observer on and at rest with respect to the ring is not expected to be c with respect to that observer at other positions on the ring. Indeed, the inertial view shows the flaw in Sagnac's argument, as you so ably document.

I will take this page and place it in my sandbox at User:ems57fcva/sandbox/Sagnac%20Effect. I will work on a revision there and hopefully we can come to some agreement on how the article should look soon. (I do not want to play a game of dueling aritcles if I can avoid it. I also am remembering more things about this experiment that should be mentioned anyway.)

BTW - Some of your recent changes I like, but the gyroscope paragraph seems to be a reaction to me and unfortunately does not add anything to the article.

--EMS 03:28, 25 Mar 2005 (UTC)

[edit] Edits ready

See User:Ems57fcva/sandbox/Sagnac_Effect. I look forward to your comments.

--EMS 05:36, 25 Mar 2005 (UTC)

[edit] History of the Sagnac Effect

"The first to perform a ring interferometry experiment ... was performed by the Frenchman G. Sagnac in 1913, which is why the effect is named for him. Later an experiment conducted in 1911 by F. Harress, ..." Ungrammatical, and time goes backwards. Probably an error. GangofOne 04:52, 23 September 2005 (UTC)

I now fixed its readability. Harald88 22:55, 11 December 2005 (UTC)

[edit] Does the Sagnac effect necessarily involve rotation?

Herbert Ives claimed that considering rotation is not a necessity.

In GPS technology, all signals, including signals that do not complete a circumnavigating loop, are adjusted for what is referred to as 'the Sagnac effect'. It is referred to as Sagnac effect because if the signals would complete a circumnavigating loop, then the Sagnac effect would be obvious.

What the two situations have in common is that both the emitter and the reciever are accelerating with respect to the inertial frame of refererence. In the case of a Sagnac interferometer, the point of entry of light is the same point as the point of exit, hence both points are accelerating. In the case of a GPS ground station and a GPS satellite: both are accelerating with respect to the inertial frame that is co-moving with the center of mass of the Earth.

So technically Ives had a point in claiming that considering rotation is not a necessity.

I propose to take the loop-closing as the fundamental characteristic. My proposal is: whenever a there is a loop-topology involved, it is a Sagnac scenario. --Cleonis | Talk 10:36, 1 March 2006 (UTC)

I will keep an open mind on this. However, you certainly seem to have a point, and I will acknowledge that any loop-closing scenario as potentially being subject to the Sagnac effect. So I am willing to go along with this and see where it leads. --EMS | Talk 20:04, 1 March 2006 (UTC)

I came across the loop-closing theme in the article by Olaf Wucknitz, titled: Sagnac effect, twin paradox, and space-time topology. gr-qc/0403111. Stedman and Neil Ashby point out that in relativistic context there is a clock version of the sagnac effect.
A sagnac scenario and the twin scenario have in common that they are loop-closing scenarios. In a sense one might say that in the twin scenario each twin travels at most half a loop, while in the case of circumnavigating time-dissemination-relays both counterpropagating signals complete a loop. It is referred to as 'Sagnac effect' if there is a difference in pathlength; difference of spatial pathlength as mapped in an inertial coordinate system. Topologically, a Sagnac scenario and a twin scenario are related.
There is no need to elaborate on such cross-connections in the Sagnac effect article itself, but it can influence ideas on how the Sagnac effect is best presented in the context of Minkowski space-time. (For an overview of how I see Minkowski space-time see this Sandbox article about time dilation --Cleonis | Talk 22:19, 1 March 2006 (UTC)

---

You need to look at the math and at the expected physical effect:

1. Loop is necessary

2. Having the two beams of light travel in opposite directions is necessary

3. Generating an interference pattern that MOVES (VARIES) wrt to some physical entity is necessary

4. You cannot get point 3. unless the loop MOVES making one beam of light "chase" and the other beam "run into" the point where the interference happens. Rotation is the most obvious way. The angular speed ("omega") is the obvious physical entity mentioned at 3.

Conclusion: Ives did not understand the Sagnac experiment. He was mixing it with his own flavor (the Ives Stilwell experiment) which indeed does not need rotation and relies on a mirror in order to make the two beams converge on the same screen. In the Ives-Stilwell experiment the measured physical entity (correspondent to Sagnac's "omega") is the speed "v" of the ions.

As an interesting aside, Einstein was the one to suggest both the Ives-Stilwell experiment and the sagnac experiment. The irony is that, these experiments, run by stubborn antirelativists, figure prominently in proving the special relativity (Silwell is one of the pillar tests, along with Michelson-Morley and Kennedy-Thorndike).Ati3414 22:26, 1 March 2006 (UTC)

Strictly speaking, there is no such thing as an experiment proving a theory. The maximum result is that an experiment rules out all proposed theories except one.

Michelson agreed reluctantly to undertake the experiment that is known as the 1925 Michelson-Gale experiment. It is my understanding that Michelson considered the Michelson-Gale experiment to be an empty gesture, as he was aware that the Sagnac effect is part and parcel of all existing theories.

For wave propagation (sound waves and electromagnetic waves), the Sagnac effect is a theorem of classical mechanics, it is a theorem of special relativity, and it is a theorem of general relativity. The Sagnac effect is a theorem of any geometry that is suitable for formulating physics in: euclidean, minkowskian, and riemannnian.

That is why I am drawn to seeing the topological characteristic as key; I am looking for something that is common to euclidean, minkowskian and riemannian geometry. --Cleonis | Talk 23:26, 1 March 2006 (UTC)

I don't understand what all the above philosophy has to do with the question that started it. You asked the question, I answered : Ives was wrong, he did not understand (never managed to) relativity and he didn't understand the Sagnac experiment. Ati3414 00:46, 2 March 2006 (UTC)

And what didn't he understand, do you think? Quite to the contrary, some of his publications have helped me to understand some issues a lot better, although one or two I only understood well in a second reading. I know of no erroneous prediction by him. Moreover, his "linear Sagnac effect" is indeed common use in GPS, as Cleonis mentions above; and note that for such no acceleration needs to be involved. Probably both of you misunderstood the point Ives tried to make, which is that the Sagnac effect is not (can not be!) fundamentally different from relativity of simultaneity: all motion is slightly curved motion, straight line motion is just a special (ideal) case. Harald88 14:18, 2 March 2006 (UTC)

There is an experimental setup that has an interesting similarity to a Sagnac interferometer setup. Normally lasers are designed to facilitate that a single frequency of light is generated. On the other hand, ring laser gyroscopes are designed to facilitate going out of frequency lock, so that there are two frequencies of light in the laser cavity if the ring laser is rotating. Now what about a linear laser, that is accelerating? (with respect to the local inertial frame). Similar to a ring laser, you expect the laser light to split into two frequencies then (if going out of frequency lock is facilitated). One might decide to refer to that as 'the linear Sagnac effect'.

It rather depends on where you put the focus. I have proposed to take the loop-closing as the defining feature, as I prefer that, but it is in itself not wrong to take the involvement of acceleration as the defining feature. I don't think this is a matter of principle. I think this is rather a matter of convention. Ives may well have opted to apply that other convention --Cleonis | Talk 09:12, 2 March 2006 (UTC)

[edit] Erroneous perception of Sagnac effect

I decided to remove the tortuous section about the erroneous perception of the Sagnac effect as disproof of relativity. Especially, there is no need to mention Herbert Ives.

The essence is in noting that occurrence of the Sagnac effect is frame-independent.

As to GPS-technology, it is my understanding that the correction factor that in GPS-technology is referred to as Sagnac effect has been incorporated from the beginning. Neil Ashby describes that at executive level there was doubt whether gravitational time dilation would have to be corrected for or not. Once the gravitational time dilation was corrected for, optimal accuracy could be achieved. --Cleonis | Talk 12:32, 2 March 2006 (UTC)

[edit] The proposal of the loop-closing definition.

Earlier on this Talk page I proposed to take the loop-closing characteristic as the fundamental characteristic. I propose to add to that another demarcation criterium.

A twin scenario can be realized with one dimension of space (plus, of course, the dimension of time). Plotted in a Minkowski space-time diagram it will look like a loop, but the idea is that one dimension of space suffices to enable a twin scenario.

I propose to call it a 'Sagnac scenario' if and only if the spatial paths enclose an area. In the case of a ring interferometer, the size of the enclosed area of the counterpropagating beams is an operative factor for the magnitude of the Sagnac effect.

By adopting a convention that a spatial area must be enclosed, such a thing as a 'linear Sagnac effect' is excluded from the definition. Again, I think these are matters of conventions of how things are named, not of physics principles.

I expect the following: a laser with a linear resonance cavity will not show frequency splitting when it is moving inertially. I expect that a linear cavity laser can have frequency-splitting only when it is being accelerated with respect to the local inertial frame.
I take the Canterbury university ring laser setup as the purest realisation of a ring laser rotation sensor. The laser process generates two frequencies of light if the setup as a whole is rotating around an axis.

Cleon - Do be careful here. Please remember the Wikipedia is an encyclopedia. It is our job to report how terms such as "Sagnac effect" are defined, not to define them ourselves. I am willing to be tolerant of this effort if not be outright supportive of it, but when the chips are down you need to have in this article a definition that is very much supported by external sources. As a fallback, I can accept a statement that "in this article we will define the Sagnac effect as being ..." if you can show that this is an appropriate consolidation of the way that the term is used in external sources. Otherwise, you will be creating a neologism, and that violates the No original research policy. --EMS | Talk 15:48, 2 March 2006 (UTC)

As far as I can tell, different authors use the expression 'Sagnac effect' somewhat differently, depending on what their focus is. Thus, there is room for babylonian confusion. In order to discuss the question 'does the Sagnac effect require rotation' a crisp definition is necessary. I agree with you that a definition proposed within wikipedia is at best only valid within wikipedia. I agree with you that if used in the article then only as a explicitly stated fallback.

My proposal is designed precisely to represent general usage as much as possible while removing ambiguity. The path of the light in a Sagnac interferometer can have any shape, as long as there is a closed loop, and an area is enclosed. --Cleonis | Talk 17:13, 2 March 2006 (UTC)

Then my request is to document what you are doing and why in this article. That said, I agree that you are well within your rights to choose a definition given an ambiguity on that count so that a coherent article can be written. In that case, I can back you up easily under WP:IAR. Even a neologism can be accepted here IMO as long as it genuinely reflects and/or consolidates current useage(s) and is well documented as doing so. --EMS | Talk 17:41, 2 March 2006 (UTC)

[edit] The diagram for the ring laser setup

It has been pointed out that there is a spelling mistake in the diagram: exitation instead of excitation. I will correct that.

That diagram is a remake of the diagram that is presented on this page of the website of the ring laser measuring station in Germany. I always try to keep diagrams as simple and as bare as possible. --Cleonis | Talk 16:35, 4 March 2006 (UTC)

Try adding the arrows showing the two counter-rotating beams.

Add a box for the resonator. This two simple things should fix it somewhat. Ati3414 17:03, 4 March 2006 (UTC)

I don't think arrows are necessary. In the other diagram of the Sagnac interferometer, I added arrows to distinguish between were there are counterpropagating beams present and where not. But in the case of a ring laser the light is counterpropagating everywhere anyway. Maybe a diagram like this one would be best, because it shows that there is an enclosing tube. --Cleonis | Talk 18:48, 4 March 2006 (UTC)

I agree, this it is much better than what it is on the site right now. Ati3414 23:53, 4 March 2006 (UTC)

[edit] Animation to illustrate the basics of ring lasers

I have made an animation to illustrate the basic principle of the Sagnac effect in the case of ring lasers and synchronisation procedures.

I have created a Sandbox to display my proposal for a rewrite of two of the sections of the article: 'ring lasers' and 'synchronisation procedures'. Please have a look.
http://en.wikipedia.org/wiki/User:Cleonis/Sandbox/Sagnac_effect
--Cleonis | Talk 20:41, 27 July 2006 (UTC)