SR Predictions of Sagnac and Wang

[Quoi]

The Sagnac and Wang (generalized Sagnac) effects have been discussed in several threads, and I thought it would be best to combine it and start afresh here.


While it is quite healthy for the site (and encouraged) to have lively discussion and debate about the interpretations of experimental data, it is not healthy to debate the predictions of a theory. While "ThoughtClearly" may have said it very harshly, it is true that there is an absolute right/wrong when discussing the mathematical predictions of a consistent theory.

For this reason (the good of the site and our understanding), we need to come to an agreement on this. What exactly is preventing an agreement? If we got several university physics professors to work out the calculations, would that be sufficient?

I don't want to argue or fight. I want everyone to agree on this. It will put us all on the same page (on this topic that is) and make the community stronger.

Since people on both sides seem to be repeating themselves, what is the best way to continue from here? How do we resolve this?

[TheAntiRelative]

Okay perhaps there is someone who thinks that I care if a theory is mathematically internally consistent without having meaning in the real world. I don't. I believe that relativity is internally consistent and couldn't care in the slightest. The mathmeatical model that created the movie "Toy Story" is also mathematically consistent and somewhat representative of reality. Is it reality? Could I use that world to solve real world physics problems? No.
Internal consistency in a mathematical model is absolutely meaningless except on the most basic level of intellegence that requires it at least react the same to the same stimuli. Whoopdee-freakin-doo

For instance I'm going to create a mathematical theory I'm going to call countonomics:
When I take a step towards a person I'm going to give myself a value of +.75 and give them a value of +.5. They are going to do the same thing for me but stopping walking causes me to do a calculation of .25 x steps and add this to the value for them. They reciprocate.
When we meet up after all calculations we'll both have the same number.

Does this theory accomplish a thing? If I start talking about the flaws in this theory, what do you think I'm going to be arguing about? The ability for them to come to the same answer that is represntative of nothing? Or do you think that I'm going to argue that this theory is a useless excersize that has no bearing on reality.

I would think it even has less use if when they get together they both have different answers for the other.



The purpose of Wang's experiment is not to try to prove some internal inconsistency with relativity, it is to disprove it's link with reality. Putting all parts of an interferometric experiment in an inertial frame (linear motion) wrt the earth should change nothing in the measurement according to relativity. There should be zero reading.
There is a reading according to Wang's experiment, the MMX, Millers, and the recent cryotest in germany.

I think the funniest thing is looking at a graph of the MMX reveals a positive and negative periodicity that exacltly correlates with one full revolution of the interferometer as would be required by a wind.
A look at the night time results shows the exact opposite direction of the wind just as it should be. (peak and trough of graph in exactly opposite position) The crappy ass 1881 MMX itself was good enough to prove aether.

[Quoi]

I realize that you don't care that SR (and GR) are internally consistent (at least we agree on that though). But the point is, you are trying to show evidence against relativity: you do this well by arguing your interpretation of Miller's data, etc. But you do not do this well by using Sagnac and Wang's data. That is because the results of Sagnac and Wang do agree with the theory of relativity. This is what I am referring to, the misunderstanding of the predictions of the theory.

The purpose of Wang's experiment is not to try to prove some internal inconsistency with relativity, it is to disprove it's link with reality.

But it doesn't disprove the theory of relativity's link with reality, because the experimental data matches the predictions of the theory. That is where we are disagreeing... the predictions of the theory.

Putting all parts of an interferometric experiment in an inertial frame (linear motion) wrt the earth should change nothing in the measurement according to relativity. There should be zero reading.

Not quite.
For there to be zero reading, all the pieces of the interferometer must be in the same inertial frame. This doesn't happen for the Sagnac, nor the Wang experiment. The parts of the aparatus itself are moving relative to each other in any inertial frame you choose.

The Sagnac and Wang (generalized Sagnac) experiments are in agreement with the predictions of relativity (and Aether theory as far as I know).

What is the best way to come into agreement here?

[TheAntiRelative]

Edit: Wow... what a stroke of absolute idiocy this post was!

[TheAntiRelative]

Ohhh... so that's where you are misunderstanding.

In wangs experiments the detector and the emiiter are traveling with the fiber. They are all in the same frame.

I thought I said that already....


Edit: Wait just a tick...

all the pieces of the interferometer must be in the same inertial frame. This doesn't happen for the Sagnac, nor the Wang experiment. The parts of the aparatus itself are moving relative to each other

Yes they are in both. In a sagnac device, none of the components move with respect to eachother in the slightest. All of them spin.

The pro-relative argument is that it doesn't count because they are spinning and that makes it a non-inertial frame because of the intrinsic acceleration at all times.

Wang factors out the spin component and moves the fiber linearly, and detects linear motion. (emitter and detector in the line!)

[Guest]

You are missing the point here. You only need to worry about the acceleration if you are trying to go into a frame where all parts of the device are at rest (because this is a non-inertial frame).

We can analyze it just fine (and much simpler) if we just choose an inertial frame to do the calculations in. So the acceleration isn't the main point here, it is:

The parts of the aparatus itself are moving relative to each other in any inertial frame you choose.

Ohhh... so that's where you are misunderstanding.

In wangs experiments the detector and the emiiter are traveling with the fiber. They are all in the same frame.

I thought I said that already....

I am not misunderstanding. Because no, they are not all in the same inertial frame.

There is a parallelogram that is changing shape. Go into the inertial frame of the emitter/detector, and there are still portions of the fiber that are not at rest.

The point is: there is no inertial frame you can choose where all parts of the device are not moving relative to each other.

all the pieces of the interferometer must be in the same inertial frame. This doesn't happen for the Sagnac, nor the Wang experiment. The parts of the aparatus itself are moving relative to each other

Yes they are in both. In a sagnac device, none of the components move with respect to eachother in the slightest. All of them spin.

Go into an inertial frame. You will see parts of the Sagnac device move down while some move up.

To "spin with it" is to enter a non-inertial frame, so general relativity is needed in that frame. It is much easier to just analyze it in an inertial frame.

I'm sorry if I'm not explaining this well. I'm trying my best.

[Quoi]

Oops... that was me (I forgot to log in). Sorry about that.

[TheAntiRelative]

In the wang experiment there is a constant difference in the two beams that you would say is caused by the rotation. He adds linear motion that causes an effect IN ADDITION to the constant rotation.

I think that's what I'm not getting across...

Edit: and the important part I forgot to reiterate is that the amount of difference measureed is the linear speed.


On the sagnac effect, all the pieces are accelerated the same amount so general relativity actually solves nothing when trying to determine why the path is considered shorter and longer and we're not allowed to say that in a linear motion. It simply falls into the category of an accelerated frame and we start saying that its shorter and longer according to a non co-rotational frame

[Quoi]

Edited by TheAntiRelative: Basically this is me blabbing out some temporary insanity that I now have the priveledge of hiding because I'm the administator. Yay Me! (oh yeah... it was acceleration and sagnac stuff)

I will discuss this later (in another thread) if you want, but we are having enough trouble agreeing on the predictions of SR. So I don't really want to go into discussing the predictions of GR. We can choose an inertial frame and use SR. If you accept that the theory of relativity is self consistent, then that is enough to show that the more complicated GR calculations would produce the same predictions.

So let's just discuss SR here.

In the wang experiment there is a constant difference in the two beams that you would say is caused by the rotation. He adds linear motion that causes an effect IN ADDITION to the constant rotation.

I think that's what I'm not getting across...

Edit: and the important part I forgot to reiterate is that the amount of difference measureed is the linear speed.

I understand the setup. I read the paper.

The point is: there is no inertial frame you can choose where all parts of the device are not moving relative to each other.

Let's keep it simple to get to the heart of the matter. Consider no rotation, and just a parallelogram of fiber with the detector/emitter on one side of the parallelogram. If you change the shape of the parallelogram by moving the sides relative to each other ... even if it is in a nice constant motion ... this effectively shortens the path of the light travelling against the motion of a section of the fiber, and lengthens the path of the light travelling with the motion of a section of the fiber.

This is actually a classical effect, and relies only on the speed of all the fiber sections and the finite velocity of light. That is why both SR and Aether give the same prediction here.

This experiment proves nothing against the predictions of SR.

[TheAntiRelative]

Hmm. We definately either have a communication impasse or a totally huge difference in classification of effects.

How is your above explanation different than saying the following?:

Make an interferometer with the detector and an emitter in the center and two mirrors disposed opposite. Put this interfermoter on a rocket sled lined up with the optical paths. When it reaches an inertial state in linear motion, the optical path length of the forward propagating light beam is greater than the optical path length of the rearward propagating light beam.

Because of the differing path lengths, there will be a different arrival time and the forward light will be red-shifted on it's return trip from the forward mirror (propagating backwards) and the rear light will be blue shifted on its return trip from the rear mirror (propagating forwards)

Edit: Didn't put enough thought into this one and didn't closely examine his argument later. Okay, I'm human. The reversal of the effects by the reflection somehow escaped me so this was a bad example. I've got a better one later in the thread though.


If there is no difference between what I've just said and wht you are describing as a classical effect, I will explain further in my next response. If there is a difference then we can focus on that once you describe it.

[Quoi]

There are major differences between the Wang setup and what you just described. And therefore the comparisons are not correct.

The differences in the device you described (as compared to Wang's):

First, even at rest, the two beams of light are not travelling the same path (unlike the Wang experiment). But since you set it up such that the mirrors are the same distance from the emitter, and both light paths are parallel to the rocket's motion ... it turns out that this first difference does not matter here. (However, this difference is huge when considering say, the MMX type setups.)

The second difference:
In the case you described above, there _is_ an inertial frame where every part of the device is not moving with respect to each other (in fact, this is true in every inertial frame for this device).

Now let's find the predictions of the theory for this setup:

In the rest frame of the device, imagine two pulses of light being released from the center (the emitter). They will travel, hit both mirrors at the same time, and then return to the center (the detector) at the same time.

...

Okay, now let's look at the same device from another inertial frame (where the whole device is moving linearly). Yes, in this frame the path of the forward propagating light is longer, and the path of the reverse propagating light is shorter. So the reverse propagating light will hit the mirror first. The two frames no longer agree on what is simultaneous (if the events do not occur at the same point).

Now, after the light reflects off the mirror, the roles are reversed! The light that was forward propagating is now propagating against the motion of the device. Similarly for the other light pulse. Since the linear speed of the device is constant, the change in path length will exactly cancel the previous change in path length ... that is, both pulses will still arrive in the center (the detector) at the same time. No phase shift will be measured.

Note that a similar explanation to the last paragraph can be used for "moving in the Aether". Aether theory also predicts no phase shift here.

...

As we both know, if the 'legs' of the interferrometer are placed at right angles to themselves (instead of in the same line like your explanation), then Aether and SR no longer give the same prediction when the device is moving. But again, as explained briefly above, this is also different from the Wang experiment. Is this the source of confusion?

Anyway, I hope we're at least getting closer to agreement.

[TheAntiRelative]

There are major differences between the Wang setup and what you just described. And therefore the comparisons are not correct.

The differences in the device you described (as compared to Wang's):

First, even at rest, the two beams of light are not travelling the same path (unlike the Wang experiment). But since you set it up such that the mirrors are the same distance from the emitter, and both light paths are parallel to the rocket's motion ... it turns out that this first difference does not matter here. (However, this difference is huge when considering say, the MMX type setups.)


The second difference:
In the case you described above, there _is_ an inertial frame where every part of the device is not moving with respect to each other (in fact, this is true in every inertial frame for this device).

The only thing that matters is that the emitter and the detector are in the same frame at all times and inside that frame there is no change in the path length.

Now let's find the predictions of the theory for this setup:

In the rest frame of the device, imagine two pulses of light being released from the center (the emitter). They will travel, hit both mirrors at the same time, and then return to the center (the detector) at the same time.

...

Okay, now let's look at the same device from another inertial frame (where the whole device is moving linearly). Yes, in this frame the path of the forward propagating light is longer, and the path of the reverse propagating light is shorter. So the reverse propagating light will hit the mirror first. The two frames no longer agree on what is simultaneous (if the events do not occur at the same point).

Now, after the light reflects off the mirror, the roles are reversed! The light that was forward propagating is now propagating against the motion of the device. Similarly for the other light pulse. Since the linear speed of the device is constant, the change in path length will exactly cancel the previous change in path length ... that is, both pulses will still arrive in the center (the detector) at the same time. No phase shift will be measured.

Note that a similar explanation to the last paragraph can be used for "moving in the Aether". Aether theory also predicts no phase shift here.

Not entirely the point. The sagnac effect is not a simple phase change.
There are two types of detection via interferometry. Phase shift and frequency difference.
Phase shift is used in the MMX.
Frequency difference is what is used in RLGs and FOGs

If there is no frequency differnce between the two beams entering an interferometer there will be a stable pattern somewhat like a bullseye. A change in the phase of the two beams will cause the circles of the bullseye to shift inward or outward by a certain amount and then stop.

Two beams of not exactly the same frequency will cause a constantly moving bullseye pattern. The pattern and speed of shift is representative of a periodic shifting from destructive to constructive "phases". IE if the interfermoter was not contructed to show the bullseye pattern through a slight deviation in angle at the detector site, the interference pattern of the two beams directed at one spot would rise and fall in intensity in a periodicity that is relative to the difference between the two lights. Faster for a larger difference, slower for a smaller difference.

In the example I gave, Aether theory would predict a detection of different frequencies.

As we both know, if the 'legs' of the interferrometer are placed at right angles to themselves (instead of in the same line like your explanation), then Aether and SR no longer give the same prediction when the device is moving. But again, as explained briefly above, this is also different from the Wang experiment. Is this the source of confusion?

Anyway, I hope we're at least getting closer to agreement.

I believe then source of confusion may be in the mechanism of the two types of detection.

[Quoi]

The only thing that matters is that the emitter and the detector are in the same frame at all times and inside that frame there is no change in the path length.

That is true if the frame is an inertial frame.

For the Wang setup, the emitter and detector are in the same frame (and it is indeed an inertial frame). But parts of the fiber are still moving relative to the emitter/detector ... so the second part of that statement "and inside that frame there is no change in the path length" is not true for the Wang setup.

In the example I gave, Aether theory would predict a detection of different frequencies.

I don't agree. Consider releasing a series of pulses (we will consider each corresponding to a "peak" of a continuous wave). As I explained, each "pair" of pulses released will return to the detector at the same time (no phase shift). A train of them released at a certain rate, will return at the same rate (no frequency shift). A detector at rest in the Aether would see a frequency shift (two different ones, corresponding to waves going with the motion of the detector and waves going against the motion of the detector ... notice that the frequency of the wave hitting the mirror changes during reflection), but since the detector is moving with the interferometer, it will see no shift.

So the interferometer experiment you described would show no frequency or phase shift according to Aether theory or the theory of relativity.

If we disagree on the predictions of Aether theory, and you want to discuss it, please continue this discussion in another thread. I'm sorry I brought up Aether (I was just trying to use that to stress that the Sagnac and Wang effects are classical).

I want to stay on topic here, which is the predictions of the theory of relativity in the Sagnac and Wang (generalized Sagnac) experiment. So let's stick to that.

[TheAntiRelative]

I think what we are talking about is closely related to the topic anyway.

The speed of sound is constant in it's medium. In air's frame.
The speed of light is constant in the Aether frame.

A detector moving through a medium will detect counter-propagating waves more frequently and co-propagating waves les frequently. The true wavelength should only be measured in the medium's rest frame.

An emitter travelling through a medium will inscribe co-propagating waves into the meduim closer together so therefore a 10 hrtz emitter will create one wavelength travelling and a different one stationary even though the time between vibrations does not change.

I think you are imagining light propagation speed being increased by the speed of the emitter for Aether. That is only a conceptualization you'll get from relativity when considering another frame's estimation of light speed in comparison to your same estimation of the same light should be if it behaved like a conventional particle of matter. However that is not applicable to Aether which treats light only as a wave and the only particulate property of it is the particles of Aether moved by the waves as they go through it just like particles of air as sound moves through.

In my doppler paradox section there is a sub-page with some drawings that help visualize sound propagation.

Edit: Basically you only detect c+v in the case of a detector moving through aether.

[Quoi]

I think you are imagining light propagation speed being increased by the speed of the emitter for Aether.

I am not thinking that and I believe you are misunderstanding what I was saying about the Aether predictions. But this is not on topic, so I'm sorry I brought up Aether.
If we disagree on the predictions of Aether theory, and you want to discuss it, please continue this discussion in another thread.

I want to stay on topic here, which is the predictions of the theory of relativity in the Sagnac and Wang (generalized Sagnac) experiment. So let's stick to that.

I assume we still disagree on the predictions of the theory of relativity here, yes? If so, what is causing the disagreement?