Talk:Sagnac effect

‹See TfM›

SOrry (then you'll clean this) ... Generated a bit of panic referring to "In 1926 a very ambitious ring interferometry experiment was set up by Albert Michelson and Henry Gale." which differs from the usual "Michelson-Morley experiment that was performed in 1887 by Albert Michelson and Edward Morley at what is now ...." AS TOLD HERE -> [1] ... JUST THERE IS NO MENTION TO THIS EXPERIMENT in that page or I couldn't read it. I apologize in advance for this OffTopic ehm .... The SuperNatural Protocols Anhaestetist.

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)

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)

Edits ready

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

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

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)

Does the Sagnac effect necessarily involve rotation?

No it does not. Lots of *experiments* here showing this http://web.stcloudstate.edu/ruwang/

So, the very first part of the article is totally wrong.

Kommierat (talk) 06:12, 8 January 2008 (UTC)

The Sagnac effect seems as though it must involve rotation, even in the experiments mentioned above. More generally, what is required is that a component of the force be orthogonal to the velocity for at least some part of the motion, as is the case in all the experiments Wang has thus far performed. Stephen Weinberg has shown that this is the effect, in Chapter 2 of his book on General Relativity and Cosmology, namely that the (special) relativistic force, f, (the force obtained by performing a Lorentz boost at each point along the accelerated motion) can be related to the Newtonian force, F, by the equation f = F + v (v . F)/v^2. For the component of the force that is perpendicular to the velocity, f = F, and this is why the Sagnac effect can be solved using either Special Relativity or Newtonian Mechanics. I propose adding this explanation (implicit in Weinberg's text, and explicit in Landau and Lifshitz, Volume 2). If there are no objections by the beginning of next week, I will add the text to the Wikipedia article. P Brandon Rimmer (talk) 21:19, 5 September 2012 (UTC)

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)

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)

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)

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)

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)

The comment by Kommierat

I copy and paste from above:

Does the Sagnac effect effect necessarily involve rotation?

No it does not. Lots of *experiments* here showing this

http://web.stcloudstate.edu/ruwang/

So, the very first part of the article is totally wrong. Kommierat (talk) 06:12, 8 January 2008 (UTC)

I've had a look at the the webpage: http://web.stcloudstate.edu/ruwang/ I quote:

Successful GPS Operations Contradict the Two Principles of Special Relativity and Imply a New Way for Inertial Navigation.

In fact there is no logical contradiction between the Sagnac effect and special relativity. On the contrary, the Sagnac effect is a logical implication of special relativity. It is also a logical implication of classical wave mechanics. Interestingly, the Sagnac effect is so fundamental that it is a theorem of both classical wave mechanics and relativistic physics. It seems that the Sagnac effect transcends the differences between classical wave mechanics and relativistic physics, which is most remarkable.

It happens quite often that people are wrongfooting themselves by assuming that since the Sagnac effect is a theorem of classical wave mechanics it cannot be a theorem of relativistic physic.

Judging from the titles of the articles, the author of http://web.stcloudstate.edu/ruwang/ is unfamiliar with relativistic physics. ( I've struck out my remark about the author of'; no need to become personal) --Cleonis | Talk 22:42, 8 January 2008 (UTC)

The word 'Sagnac e..'

The word don' t point out exactly the relations with the absolute rotation and motion : the Sagnac 'experiment came after the Michelson and Morley ' esperiment (1887) and the Relativity Theory(1905) ; it is connected to that challenge untill now not well resolved. The Sagnac ' effect ( around 1910 ) is a circular interference ' path pointed out over a turning platform ( at least 2 turns for second ) registered by a camera jointed to the platform ; the movement ' effect over the fringes is proportional to the rotational ' speed (and surface) and it contains like residuals the earth rotation around the sun , the sun around the galaxy ..... . In 1925 Michelson and Gale , addressed by Sagnac , repeated the same experiment over a big fixed interferometer ( 650 m. for 400 m. ) : the fringe ' movement was like the absolute spatial rotational speed of the site ( like that-one as reported also by the rotation of Faucould ' pendule ) ; the rigth time to consider was the sideral time ( 86.164 seconds for a day ). In the years 1990 ( experiment Allen-around-the-world ; see H C Hayden , Physics Essays , 8, 366 (1995)) it was calculed the same shift ' quality and quantity in the signals returned between earth and around-the-world satellites with final conclusion that the ligth goes at 300.000 km.s. with respect to own gravity ' center ( and with respect to that center have meaning the accelerations and the movement ' formulas , probabely ). ( Einstein never commented the experiment of Sagnac (today misteriously called 'effect') or Michelson and Gale , although the invariance ' postulate of ligth ' speed was compromised without exact references to the gravitational center! ) The Michelson and Morley ' experiment (1885) remains with negative resultat , but also today its execution ' sensibility is under reliefment ( at our latitudes the Sagnac exp teaches that it should be necessary to relief 0.3 km.s. ; the 30 km.s. ( tried in M&M exp) are 10.000 times larger to releif because 30:0,3 is 100 and 100*100 is 10.000 ( using ligth ' travels go and back) .

 Forbidden and hidden asks : does exist the absolute movement ?  can we have a normal solution (see:  www.bbamateur.blogspot.com ) observing that the angular movement can be always detected (for Sagnac) and the distance from the gravitational center too ...are existing only sensibility ' problems ?

Critics to Wang exp.

 The Wang experiment :1) Wang  generalizes Sagnac and Fizeau , but he uses max. speeds 70 times inferior to F. and 30 times less than the minimum-one of S.;

2) the F. ' generarization (proposed by W.) seems indipendent from the refraction index , showed always to modifie the answers of moving matter  ;

3) F. had already showed that the ligth ' speed could not surpass 300.000 km.s.; W uses an optic fiber ( semplifing : some moving mirrors ) and , if the ligth ' speed cannot change , it was clear that the phase should change exactly in proportion to the effective used velocity , like showed by W ;

4) in S all was moving , in W source and detector are fixed ;if its were moving too , what happens to anwers datas ; 5) in F the quantity of matter crossed by ligth (at the detector) was changing with the speed , in W is fixed (but moving ) ; 6) W could should arrive to same numbers , measuring the external movements ; 7) W speaks to measure the nanometers , but he says nothing about the lock-in ;

8) in W it is hard to see relations with the absolute motion and rotation ' challenge  (  S had to use the sideral time and W ?) Often Physical Revew edits what 'Nature' and 'Science' don't edit .188.153.162.128 (talk) 10:12, 13 February 2011 (UTC)oldogf@yahoo.it

Powered by Blogger

For a nice discussion of Wang's experiments, see A. Tartaglia, M.L. Ruggiero arXiv:gr-qc/0401005. The experiment is consistent with special relativity, since it is really only about the independence of the speed of light from the speed of the source. Thus the length of the light paths is not the same for the counter-propagating rays - in every inertial frame of reference. --D.H (talk) 11:40, 16 February 2011 (UTC)

Thanks for D.H... i must apologise with Wang , because i liked too much Fizeau and Sagnac..but , but ... beyond the savant formulations of Wang , it ' s a words ' play to say : W generalizes S or to say W is a minimalization of S ...: if the W ' table is stopped , the W ' detector should measure the movement of Michelson & Gale (or S, but not detectable because too little for W!) ; when the W ' wheels are going (the movement!) , the interferences ' bands are going proportionally to the wheels (of course!)and that are jointing to the movement of M & G (or S).... substantially : W measures the relative movement of the mirrors (optic fiber) against the detector, which is stopped ... S measures the absolute movement of the laboratory containing W ..: doing the necessary proportions , S should feel ashamed to measure what W measures very well (opinion of 188.153.183.102 (talk) 09:28, 19 February 2011 (UTC) oldogf@yahoo.it )

Again apologies for Wang..The complete sistem (for the absolute movement'measure) joints a tridimentional gyroscope -three Sagnac ' gyroscopes with different axial positions- and a gravity ' indicator -with measure of gravitational center ' distance-....: for example , an inertial movement shows a regular variation in gravity and in gyroscopes.. 188.153.176.45 (talk) 09:42, 21 February 2011 (UTC)oldogf@yahoo.it

It also works with linear motion, a Sagnac accelerometer?

Does it only measure motion or would it also show change in the interference pattern when aligned with gravity? And does it measure acceleration relative to what? --TiagoTiago (talk) 07:18, 6 November 2011 (UTC)

It works linearly in regard to producing fringe shifts when accelerated. This can easily be understood by considering the Doppler effect and the retardation that the light does along the linear path. The Doppler effects do not cancel out since there is a delay in mixing source and destination signals. Redo the experiments with light frequency changing over time, for example as a ramp function, to get an effect on speed and not only acceleration. If there is a linear Sagnac effect even in this case the beat frequency would differ at different speeds. Do this experiment on a rotating frame as well. The common understanding is that the rotating frame would be affected by speed and the linear interferometer would not. David Jonsson 16:04, 20 August 2012 (UTC)

Source of Images.

It seems that not all image files of this article are in commons, I request them in order to translate properly this page to Spanish language (with the help of Wikibhasha), I mean, I don't want to download those pictures since here, and then upload to commons, with the goal to call them since the new page ¿May you tell me at what web page are the images located?.--Paritto 16:27, 29 November 2012 (UTC) — Preceding unsigned comment added by Paritto (talk • contribs)

Addition of LIGO section, reply to Stigmatella aurantiaca

This is in response to the two posts left on my talk page by Stigmatella aurantiaca

Thanks for your feedback. I think you were a little hasty in reverting (of course I could do the same, but let's talk about it). Let me briefly address your concerns:

You did a rather crude cut-and-paste of material from the Common path interferometer article to Sagnac effect. The material that you cut out relied on the illustration in the Common path interferometer article, and made references to other material in the Common path interferometer article that do not exist in the Sagnac effect article.

Well the "cut" may have been crude, but I don't think the paste was. I rewrote parts of that text in order to make it fit into the Sagnac article (such as removing reference to the figure that no longer was present). Could you kindly go back to my version of the Sagnac page and identify exactly what if anything is wrong? I would be glad to further edit it....

I can also draw a custom illustration for you to illustrate how the zero area is achieved.

Well yes, that would be great, and then the text could be attached to a figure.

Also, the whole point of the zero area Sagnac interferometer is that, while being a common path interferometer that exhibits the high stability inherent to the Sagnac configuration, it does not exhibit the Sagnac effect because of the net zero enclosed area, so its presence in an article about the Sagnac effect is confusing without some degree of rewrite.

Well right, it doesn't exhibit a NET Sagnac effect for that very reason, which also makes it instructive for someone trying to learn about the Sagnac effect (the point of that article) as they will appreciate that the rotation is relative to an axis and that reverse rotations can accumulate Sagnac effects of opposite sign. I thought it belonged more in the Sagnac article since it is an interesting application of the Sagnac effect (measuring a differential effect due to graviational waves, I gather). Of course it more especially belongs in an article about gravitational wave detection, but that is beyond my competance so I didn't want to touch it.

I thought it did NOT belong in the common-path interferometer article, because it was distracting to the point (two light waves travelling along the same path) and in relation to the point (common path) it added nothing to what is unique about the Sagnac configuration (two waves taking a common path but in OPPOSITE directions, unlike the other common-path schemes) but in which the NET area happens to be zero, which has nothing to do with the point of that article. It was so far off the point of the article that I considered it out of place. For instance, in an article about an electronic component, you don't mention the use of that component in every electronic device that uses it. So I would have thought that mentioning a zero-area Sagnac interferometer was already rather redundant, but going into gravitational wave detection was really out of place!

Also, I'll quickly mention that having this text in the Sagnac interferometer article is seen by 8x as many readers as in the common-path interferometer article. My inclination is to restore it on this page at least, along with an additional illustration if you can supply one (ideally not the one inside figure 1, but rather one that more corresponds to the LIGO proposal, if available). Taking it out of the other page is less important, but addresses one of my pet peaves about material that is so unrelated to a subject or so obscure that it turns off a casual reader who then won't see the subject in perspective.

I look forward to your response (or anyone one else), but I hope for this text to find a home in a relevant article, probably this (Sagnac) page.

Interferometrist (talk) 16:15, 10 February 2013 (UTC)

Look, I went ahead and restored that section to this page, with some further editing that I hope answers your concerns. But if you still have concerns we can discuss them. However this is certainly one "practical use" albeit obscure, so belongs on this page (if not on the gravitational wave detection page). And yes, if you could supply a figure of the proposed interferometer in space (or otherwise of a generic zero-area Sagnac) then please do that! Interferometrist (talk) 17:10, 10 February 2013 (UTC)

I have removed it per wp:NOR. The analysis is yours, not the source's. - DVdm (talk) 17:22, 10 February 2013 (UTC)

Uh, I'm sorry, which analysis? What I placed there was just the text that is on the Common path interferometer page (inappropriately, I believe), and which I further edited in response to Stigmatella aurantiaca's remarks on my talk page, saying that the material wasn't well explained. I don't believe I added any factual material (and certainly not original research!!) that wasn't already covered in the article. But if you think I did, then why not edit out those particular parts (or call them to my attention) and go from there. I am sure that there are reliable sources for any of the (meager) principles I invoked or implied, such as areas around a loop being measured as polar vectors. Or do you think this section is so obscure that it doesn't belong anywhere. I might have agreed, but it certainly is one of the few possible applications of Sagnac interferometers.

Interferometrist (talk) 17:39, 10 February 2013 (UTC)

Hi, DVdm! Interferometrist's proposed rewrite looks fairly reasonable to me. It is missing a bit that I would emphasize (the exceptional stability of the Sagnac topology, known at least as far back as Michelson when he re-ran the Fizeau experiment — this was long before the topology was known as the "Sagnac" topology), and I'll try to search out additional references to show that what he has written really isn't original research. Unfortunately, one of the major sources when I originally expanded the Common path interferometer article is no longer available online, and I need to scout out some alternatives. Stigmatella aurantiaca (talk) 21:04, 10 February 2013 (UTC)

Hi. Well, perhaps I didn't search sufficiently careful, but I really didn't find backup for this edit in cited sources for phrases like

"...it does have one interesting proposed application..."

"...scientists will need to contend with thermal distortion..."

"...A variety of competing optical systems are being explored for third generation enhancements beyond Advanced LIGO..."

DVdm (talk) 21:49, 10 February 2013 (UTC)

• "...it does have one interesting proposed application..." — I didn't like that wording either, and would definitely be changing it.
• "...scientists will need to contend with thermal distortion..." — Sun et al. state: "An advanced LIGO interferometer must address the issues of laser noise, interferometer fringe contrast ratio, thermally induced figure changes, birefringence, and depolarization."
• "...A variety of competing optical systems are being explored for third generation enhancements beyond Advanced LIGO..." — This was directly taken from my writing in the Common path interferometer article. I have access to over a dozen journal articles on this subject, only a few of them freely available over the internet, and I didn't want to clutter my writing with references to original research articles that require a trip to a university library to access. After all, the original article was about common path interferometers, not on LIGO. The following is a review article that may be of interest to you: The Einstein Telescope: a third generation gravitational wave observatory
• I'll dig out Interferometrist's edit and will place it in my sandbox. He and I can work on it there until it's ready to place in the Sagnac effect article.

Stigmatella aurantiaca (talk) 22:32, 10 February 2013 (UTC)

Hi Interferometrist!
I placed the text in my sandbox.
We can work on it together until we are both happy. Stigmatella aurantiaca (talk) 22:43, 10 February 2013 (UTC)

DVdm, Interferometrist —
Tell me what you think of the revised section that I inserted. Thanks! Stigmatella aurantiaca (talk) 02:54, 11 February 2013 (UTC)

Looks good! - DVdm (talk) 10:14, 11 February 2013 (UTC)

Hi, yes all of the text is good (and I trust the OR issue with DVdm is resolved), but I would mention two changes. The first paragraph is fine, but does NOT belong in a section about the zero-area Sagnac because it describes a property of the Sagnac interferometer, being a common-path interferometer (remember, that's how this all got started?!). I think it really does belong in the article, though, and I made some modifications to it on your sandbox page you can look at.

BTW, I wasn't expecting to have this issue of OR with DVdm (but I understand why s/he may have felt that the claims were not properly cited, in which case a {CN} would have been appropriate). The ONLY reason I got involved in adding (after editing) that paragraph to this article is because I thought (and still think!) it doesn't belong in the common path interferometer page, but I didn't want to just throw away some interesting material that someone had written if it had a proper place in WP, which would be this page and/or the page on gravitational wave detection. Interferometrist (talk) 23:03, 11 February 2013 (UTC)

Oh, I should have looked first, but I see that you went ahead and uploaded what you had proposed on your sandbox page to the article. Well, read my changes there and (please) consider my points, most of all that the first paragraph (or my rewrite of it?) belongs earlier in the article, certainly not in that section. Interferometrist (talk) 23:26, 11 February 2013 (UTC)

Except for minor wording issues, I'm OK with your revisions. So go ahead and make your changes, and I'll follow up with a word tweak here and a word tweak there. Thanks! Stigmatella aurantiaca (talk) 01:01, 12 February 2013 (UTC)

I added some notes to my sandbox. Stigmatella aurantiaca (talk) 10:19, 12 February 2013 (UTC)

Alright, I will look at that a bit later when I'm done with my "real" work, and then maybe we can converge on a version we all agree with. I trust that changes that someone disagrees with will not be summarily reverted but discussed here so the issues can be addressed. In addition to what I wrote in Stigmatella aurantiaca's sandbox regarding the LIGO section, I noticed some other clear problems on the page which I will edit, and again we can resolve any differences in our understandings. Interferometrist (talk) 18:19, 12 February 2013 (UTC)

To remove: Sagnac effect in translational motion

I do not think this section is appropriate because it is not about the Sagnac effect per se. It seemed dubious at first, but having briefly looked at the paper itslef I do believe it is valid (the sensitivity of a fiber path to a linear motion of one part w/r/t another). However it is improper to refer to it as the Sagnac effect, as it has nothing to do with the area inside a loop or rotation, but just the time transit of light in a moving path. I did a literature search, and there is no other paper ever published in the subsequent 8 years using the term "generalized Sagnac effect." In other words, this is definitely NOT recognized as part of the Sagnac effect, though it indeed explains the Sagnac effect just as does the article in an earlier section. I propose to remove it. Interferometrist (talk) 17:47, 10 February 2013 (UTC)

I think it would be better to improve the section rather than remove it. As I stated before, the papers from this group have gotten a lot of attention from crackpots who think that it somehow proves something or other about relativity. Indeed, at least one of the authors appears to be somewhat of a relativity skeptic himself, and has published in the fringe journal Galilean Electrodynamics. The effect is very straightforward, and the proportionality of the conventional Sagnac to the area inside a loop merely has to do with the way that the terms drop out when you rotate. If you wrapped the fibers in the generalized Sagnac around a circular core, you'd see the same proportionality, so I don't think that proportionality to area constitutes any sort of fundamental distinction between conventional Sagnac and this trivial variation. Stigmatella aurantiaca (talk) 22:57, 10 February 2013 (UTC)

I did some more research. R. Wang and associates are definitely anti-relativity. But that is no reason to exclude discussion of a well-conducted set of experiments. After all, one of the cornerstone experiments supporting relativity, the Ives-Stilwell experiment, was conducted by an anti-relativist. Likewise, the Beckmann-Mandics experiment was conducted by a couple of relativity skeptics. Quirino Majorana was a relativity skeptic, but conducted two of the earliest experiments demonstrating the non-viability of emission theory. Even Michelson was a disbeliever. There are some other examples, but I can't bring them immediately to mind. Stigmatella aurantiaca (talk) 11:06, 11 February 2013 (UTC)

Conceptually, a conventional fibre optic gyro (FOG), shown on the left, can be divided into two semicircular sections with extended fibre connecting the end sections as shown on the right, creating a fibre optic conveyor (FOC). Use of different lengths for the straight-fiber segments with the same semicircular end sections demonstrates that the travel time difference between two beams through the loop is a summation of all the travel time differences between two beams in each segment of the loop, regardless whether the fibre is straight or rotating. Other configurations, not shown, show that the travel time difference in FOG and FOC is not dependent on the enclosed area, but rather conforms to the equation Δt = 2vL/c², whose derivation is based on the constant speed of light. Hence, the operation of the FOC is completely consistent with relativity.

Well I don't want to belabour the point, but what you have found concerning the CONTEXT of this paper should tell you everything you need to know about where this research belongs, which is NOT on WP especially if there isn't a single non-fringe scientist who has taken up this line of research (either theoretically or experimentally) or who (NOTE:) has ever used the same -- inappropriate! -- term "generalized Sagnac effect." The whole point, beauty, of the Sagnac interferometer is that it measures absolute rotation when the entire apparatus is rotated as a solid body. What an interferometer does when its shape changes (which is what the experiment described is) is quite another matter.

I had said I did a literature search and found nothing. But I also did a citation search, and found EXACTLY what you just mentioned. The only people who found this paper interesting were writing in fringe journals. I believe the results of the experiment could have been predicted using standard theory of light propagation. The fact that the author could derive the Sagnac effect from a misnamed restatement of that standard theory, is not interesting. And above all, it is NOT the Sagnac effect and wouldn't belong in the article anyway. Please do me a favor and delete it so I don't have to.  :-) Interferometrist (talk) 23:19, 11 February 2013 (UTC)

If you look at the earlier sections of this Talk page, you'll see that, like it or not, there is a lot of interest in the linear Sagnac interferometer by people who are honestly curious about what they have heard about the experiment. We can either run away, or we can hold our ground and explain, very simply and firmly, that the effect is very simply explained as a consequence of the constant speed of light. Stigmatella aurantiaca (talk) 01:13, 12 February 2013 (UTC)

See proposed illustration. Most of the caption should actually be in the text, of course. Stigmatella aurantiaca (talk) 08:21, 12 February 2013 (UTC)

Pro: Those experiments are trivial confirmations of the source independence of light speed and special relativity, and have (at least mathematically) some similarity to the standard Sagnac effect. Contra: the only non-fringe references I found are arXiv:gr-qc/0401005 and arXiv:gr-qc/0403111 - both are not peer reviewed. So this section indeed has a problem with WP:Undue and WP:Secondary. --D.H (talk) 08:54, 12 February 2013 (UTC)

Interferometrist is strong contra, I'm pro, you're both pro and contra. (I agree with you about the possibility of WP:Undue, by the way, especially if I add an eye-catching illustration.) We need another opinion to establish the consensus. DVdm? Stigmatella aurantiaca (talk) 09:06, 12 February 2013 (UTC)

At least the primary sources were published in PRL and PLA, so I'm slightly on the pro side... BTW: Your illustration is great! --D.H (talk) 10:25, 12 February 2013 (UTC)

The illustration is fine but the second part of it has nothing to do with the Sagnac effect. There is nothing recognized as the "generalized Sagnac effect" outside of one paper which is of particular interest only to those involved in fringe science (questioning special relativity). Once that is recognized, then the analysis of an interferometric configuration in which the shape changes has nothing to do with this article. The unique aspect of a Sagnac interferometer is that it measures absolute rotation with rigid-body motion only, and this is anything but a rigid body.Interferometrist (talk) 18:30, 12 February 2013 (UTC)

The ONLY effect worth noting here, is that since the start/end point is in motion relative to other parts of the closed light path, a light pulse going in the forwards direction has to travel a further distance than a light pulse going in the reverse direction. The math is exactly the same whether the motion of the loop is a fixed rotation, or the motion of the loop is longitudinal around a closed path. The only thing special about rotation, is that it allows you to combine terms so that you can get a formula dependent on area and angular velocity. You are fixated on the absolute rotation aspect of the conventional Sagnac apparatus. The Sagnac apparatus works because light travels at a constant speed independent of the speed of the emitting body. There is nothing magic about rotation. Stigmatella aurantiaca (talk) 02:11, 13 February 2013 (UTC)

I notice that my argument is essentially identical to that of Tartaglia and Ruggiero, the first of the two non-peer-reviewed references that D.H provided. Stigmatella aurantiaca (talk) 08:56, 13 February 2013 (UTC)

"The Sagnac effect for a uniformly moving observer is discussed. Starting from a recent measurement we show that the Sagnac effect is in fact not a consequence of the rotation of the observer, but simply of its even, inertial motion with respect to the device resending light towards the (moving) source." — Tartaglia and Ruggiero (2004)

I fixed the little jitter at the beginning. One of the frames was bad. DVdm voted contra, so now I'm just waiting for Srleffler. He's not on every day, so we may have to wait a bit. Stigmatella aurantiaca (talk) 08:38, 13 February 2013 (UTC)

I've placed requests on DVdm and Srleffler's talk pages to help us reach a consensus. Hopefully I've phrased my posts in such a way that I am innocent of WP:Canvassing. Stigmatella aurantiaca (talk) 22:43, 12 February 2013 (UTC)

I can only go along with D.H.'s above contra per slight WP:Undue, but serious WP:Secondary. A secondary source should do the job. DVdm (talk) 07:59, 13 February 2013 (UTC)

From what has been said here, it seems that references to a "generalized Sagnac effect" and to the "Sagnac effect in translational motion" should be removed, because these are not neutral descriptions, and no reliable secondary sources for these ideas have been presented. There are a few primary sources that use these terms and concepts, but the treatment here probably places undue emphasis on this work. In any event, without reliable secondary sources we can't evaluate the importance of this material within the field.

That said, it seems like there might be room for a section on phenomena related to the Sagnac effect. A neutrally-written discussion of the behavior of light in an interferometer that is moving relative to the light source could perhaps aid readers in understanding how a Sagnac interferometer works. --Srleffler (talk) 02:32, 14 February 2013 (UTC)

• The consensus is that the section should be removed because of a lack of peer-reviewed secondary sources, so I have done so. That being said, the Theory section already mentions, more or less correctly but in a completely unsourced fashion, that "the shift in interference fringes can be viewed simply as a consequence of different distances light travels due to the rotation of the observer [sic]. The simplest derivation is for a circular ring rotating at an angular velocity of ω, but the result is general for loop geometries with other shapes."
• Since the primary source authors are demonstrably non-neutral concerning their own work, the consensus would presumably be that the primary source authors should not be cited as backup for the above statement. But even though Tartaglia and Ruggiero (2004) and Wucknitz (2004) are non-peer-reviewed, the authors of these two works have excellent credentials, such that I believe it is admissible to cite at least the first of these two works (Wucknitz's paper is too technical to be understood by the general public) as a secondary source in support of the aforementioned statements in the Theory section.
• I will also look for other secondary sources to back up this section. One of the best online secondary sources, unfortunately, is by an author who is determined to keep his true identity secret. See The Sagnac Effect, by "Kevin Brown"
• See the faculty description for Angelo Tartaglia and the web pages for ML Ruggiero and Olaf Wucknitz

— Stigmatella aurantiaca (talk) 09:17, 14 February 2013 (UTC)

Interferometrist —

• Please read the references before you go around labeling statements that I've written, "couldn't be true" or "invalid assertion, misleading". Stigmatella aurantiaca (talk) 04:20, 16 February 2013 (UTC)
• Please note that, following the consensus decision, I have assiduously avoided any reference to the problematical primary sources by R. Wang et al. The secondary sources that I have used so far are by authors of excellent reputation. Angelo Tartaglia has 85 papers related to cosmology and/or general relativity, most of them published in mainstream journals, with an average of about two citations each, some of them much more. Kevin Brown is listed on the Numericana Hall of Fame alongside such web luminaries as John Baez, Leonard Susskind and Sten Odenwald. Stigmatella aurantiaca (talk) 04:17, 17 February 2013 (UTC)
• The more you attack this section, the longer the section has to be to respond to your attacks. To me, this is a trivial issue, and I did not wish to use more than a few sentences to discuss it. The current version is much longer than my original version precisely because I have had to respond to your multiple attacks, and it is taking up a degree of importance in the Theory section that I did not intend. Multiple mainstream authors agree that the effect shown by the fibre optic conveyor is a genuine manifestation of the Sagnac effect. For example, see my quote from MathPages below. Stigmatella aurantiaca (talk) 10:28, 16 February 2013 (UTC)

Kevin Brown on MathPages has these amusing things to say about the Sagnac conveyor:

The belt loop arrangement is rather trivial, and might not be worth mentioning, although a couple of papers were published (in Phys Rev Let A) in 2003 and 2004 presenting results of measurements from such a device. Needless to say, the measured results agreed with the above formula, based on special relativity, for all shapes of the loop, and for all indices of refraction, exactly as one would expect. As to why such a trivial phenomenon deserved to be “tested”, the authors of those papers revealed (in another, less reputable, publication) that they believe it “falsifies the principle of the constancy of the velocity of light”, and therefore invalidates special relativity. Unfortunately, they don’t explain how experimental results that agree exactly with the predictions of special relativity can invalidate special relativity.

Stigmatella aurantiaca (talk) 04:42, 16 February 2013 (UTC)

No calibration

The following statement was marked "dubious"

With passive ring interferometry there is no way of establishing which position of the interference fringes corresponds to zero angular velocity of the ring interferometer setup.

As a matter of fact, this statement may be true for the fibre-optic gyro. It was not true for Michelson-Gale-Pearson, since in the absence of rotation, the central fringe of equal path length in the larger loop would be centered between the two images of the slit in the smaller loop. One can envision an analogous short-loop long-loop arrangement for FOG devices, but if such an arrangement were practical, I would have expected a patent on it, and I've just finished a patent search with no results. There are tons of FOG patents, though, so I could easily have missed one. :-(

Overall, discussion of the relative merits of FOGs versus RLGs is not handled at all well in this article. I'll take a stab at providing a decent comparison of the two.

Stigmatella aurantiaca (talk) 10:02, 14 February 2013 (UTC)

Please check the way I handled this issue. Thanks! Stigmatella aurantiaca (talk) 17:17, 15 February 2013 (UTC)

Invariance or Constancy

I'm happy to see the improvements that were added by Stigmatella Aurantiaca last february.

I created the Sagnac effect article back in 2005, my last edit was in March 2009.

Indeed, the principle that underlies Sagnac interferometry is the frame invariance of the speed of light. I think it's right to put that in the introduction paragraph.

I have to say, I prefer the expression 'invariance of the speed of light' to 'constancy of the speed of light.' I know that the word 'constancy' is quite common in this context, presumably intended as interexchangeable with 'invariance'. But the word 'invariance' underlines that it is to be understood in the general context of invariance principles.

I propose: the sensitivity of the ring interferometer to rotation arises from the invariance of the speed of light for all inertial frames of reference.

The underlying point (which need not be stated explicitly in the opening paragraph) is as follows: It's not only the case that for an observer located at the geometric center of a ring the speed of light along the perimeter is the same in both directions. It's also the case that for any sub-section along the perimeter for an observer moving at any velocity along the perimeter the speed of light is the same in both directions.

In the section about precision synchronization of clocks that are located far apart (such as needed in neutrino speed measuring setups.):

I don't think I've ever encountered a description of a land-based high precision synchronization procedure (for large distance). In the case of the neutrino-speed-measurement setups I got the impression that both the distance and the synchronization were established by means of GPS signals.

Currently the article seems to suggest that there's actual landbased high-precision long-distance synchronization. The case of stations situated on the equator is hypothetical, obviously; it's a thought experiment. The current wording goes seamlessly from the thought experiment to asserting that 'the sagnac effect is widely recognized and employed in the precision synchronization of clocks'.

I intend to edit that section of the article, for better separation between thought experiment and actual deployment, such as in GPS operation.

--Cleonis | Talk 13:43, 27 April 2013 (UTC)

The understanding of how the Sagnac effect works out in the physics of ring laser gyroscopes

In the current article the section that discusses the physics of ring laser gyroscopes is extensive.

I opted for that extensive discussion because the physics of ring laser gyroscopes offers a special perspective on the general physics of the Sagnac effect. Ring laser gyroscopes underline the two fundamental features of the Sagnac effect: the invariance of the speed of light for all inertial frames of reference, and the fact that there is a loop that spans around an area.

Equally important: we agree on the following:
[...]ring laser interferometers do not require calibration to determine the output that corresponds to zero angular velocity. Ring laser interferometers are self-calibrating. The beat frequency will be zero if and only if the ring laser setup is non-rotating with respect to inertial space.

Calibration is with respect to a reference. The reference of a ring laser gyroscope is the principle that for all inertial frames of reference the speed of light is invariant.

I created the animation with the blue dots and the red dots several years ago to emphasize that the light propagates with the same velocity in both directions. (The upload history of that animation shows that initially I made the biggest mistake possible: confused about the direction I had the dots going the wrong way. To my horrible embarressment it wasn't until 2009 that I realized the mistake, and I uploaded a version with the dots moving in the correct direction.)

I think the animation has its use, but there's a problem. In the case of the small ring laser gyroscopes for commercial use the gas that serves as the lasing medium is obviously not stationary with respect to the cavity. To overcome the lock-in problem the whole gyro device is vibrated (The usual term for it is 'dithering'). The effects of the dithering cancel out; the measurement after removing high frequency components in the signal is the sought after measurement. Given the dithering it makes no sense to think of the lasing medium as stationary with respect to the laser cavity. That pretty much blows away the explanation (that I wrote) that is in the current version of the article.

Now that big improvements have been made to the article I hope more work can be done on the weak parts that remain.
--Cleonis | Talk 18:29, 28 April 2013 (UTC)

Undoubtedly I am misunderstanding your statement "the gas that serves as the lasing medium is obviously not stationary with respect to the cavity", but phrased in the precise way that you have chosen, the statement appears to be wrong. Real ring laser gyroscopes are not continuous loops within which the gas is capable of circulating. For example see this illustration or this photo from Commons. The ends are kept in phase by an electronic feedback loop. So the gas that serves as the lasing medium is necessarily stationary with respect to the cavity. I had meant to address this point of misunderstanding by creating a new image to replace the existing one, but I've been preoccupied with other Wiki projects. Stigmatella aurantiaca (talk) 21:25, 28 April 2013 (UTC)

Ah, yes, I need to rephrase/clarify my above assertions.
I will use the expression 'glass block' to mean the following: the entire conduit of the laser light, plus the beam sampling point with beam splitter and prism and photo sensitive components that is used to obtain a beat frequency reading.

The glass block is a solid whole (it needs to be), and that glass block is doing the dithering. The motion of the gas inside the cavity will lag behind the motion of the glass block (simply because of the inertia of the gas). In actual ring laser gyro operation the effects of that lag cancel out over time, so it's not a problem for the intended gyroscope purpose of the device.

Currently the caption of the counterpropagating light animation describes resonance of molecules in the laser cavity as a factor. But on the time-scale of the actual laser process the gas molecules in the glass block do move in unison relative to the cavity (the gas will lag behind the dithering).

In general, velocity of the gas relative to the cavity will affect the measurement. For instance, there are also continuous loop designs, and an asymmetry in the radio excitation can induce a flow, and that would introduce a bias in the ring laser gyroscope readings.

Summerizing:
Averaged over time the gas can be considered stationary with respect to the cavity, that's one thing.
My concern is that the current form of the explanation requires that the resonators in the cavity are instantaneously co-moving with the glass block, which is not the case. So I think the explanation is flawed. (I wrote it several years ago to the best of my ability, but now I think it's flawed.)
--Cleonis | Talk 06:37, 29 April 2013 (UTC)