Fringe shifts - Michelson Morely Experiment

[mangaroosh]

I would love to get my head around this; I did a quick search for this, but only came across one thread with fringe shift in the title (in the first 5 pages of the search results).


The OP will probably sound like me "thinking out loud", because it largely will be. I'm not entirely sure of what the right question to ask is, but I'm hoping that a point of departure might materialise. I suppose, in short, I'm trying to understand the conclusions drawn from the experiment, and the justification for them.


The speed of light, in the MMX, is not measured in the sense of distance/time; is it the fringe shifts that are used to determine if there is a change in speed along either of the arms?


Some of the questions I have:
- what is a fringe shift?
- what causes a fringe shift?
- does frequency play a part?
- can anything about wavelength be inferred?

 

[Mentz114]

Look up 'interference fringes' in Google and get a basic grasp of interferometry. This is a good place to start
http://en.wikipedia.org/wiki/Interference_(wave_propagation)

 

[Matterwave]

Like the name suggests, the Michelson Morely interferometer experiment is an interference experiment. You have 2 arms of the same length, and part of the light goes along one arm while the other part goes along the other arm (the light beam is split with a half-silvered mirror or some other beam splitter). They then come back to the observer and is recombined. If the light moving along one arm moves at a different speed as the other arm, then you would not expect the light to come back in synch (crest to crest, trough to trough), and so you would expect to see interference effects. The fact that you don't see interference effects suggests the light traveled with the same speed along both arms.

 

[mangaroosh]

thanks guys.

I've done some googling before, but find it more helpful to engage with people on such issues, to try and pose a few questions that might demonstrate where my lack of understanding lies.


my basic understanding is that a single "beam" of light is split, it travels along two different arms of an interferometer and reconvenes, at slightly different angles to each other; the interference patterns (together with the angles) result in a fringe; if the speed of light along either arm changed, it would result in a fringe shift.

Is that in the ball park at all?

 

[ghwellsjr]

Is that in the ball park at all?

That's exactly right.

Also, if a length of a path changes, it will result in a fringe shift

 

[mangaroosh]

That's exactly right.

Also, if a length of a path changes, it will result in a fringe shift

Am I correct in thinking that, because the actual speed, in terms of distance/time, is not measured, that it is the wavelength that gives us the information about speed? That is, the wavelength of the reconvening beams is the same?

 

[NotAName]

When you google "fringe shift"

This article comes up. It has a top-down view of waves that makes sense to me.

http://en.wikipedia.org/wiki/Fringe_shift

 

[NotAName]

Am I correct in thinking that, because the actual speed, in terms of distance/time, is not measured, that it is the wavelength that gives us the information about speed? That is, the wavelength of the reconvening beams is the same?

Wavelength is the same but the phase (how the peaks and troughs line up) is not.

 

[mangaroosh]

Wavelength is the same but the phase (how the peaks and troughs line up) is not.

Thanks NotAName.

I'm trying to get from there to how the speed of light is invariant for all observers, irrespective of their motion relative to the source.

My inital thoughts are that this demonstrates that the wavelength of light, in a vacuum, is constant, but not necessarily the measured speed. I can imagine that light in a vacuum would have an absolute wavelength i.e. the distance between peaks is always the same, but that if I was traveling relative to the light, and measured its speed in terms of distance/speed, that I would get a value less than c.


There's probably something I'm missing though.

 

[mangaroosh]

That's exactly right.

Also, if a length of a path changes, it will result in a fringe shift

cheers gh!

 

[mangaroosh]

btw, thanks guys!

 

[Mentz114]

Thanks NotAName.

I'm trying to get from there to how the speed of light is invariant for all observers, irrespective of their motion relative to the source.

My inital thoughts are that this demonstrates that the wavelength of light, in a vacuum, is constant, but not necessarily the measured speed. I can imagine that light in a vacuum would have an absolute wavelength i.e. the distance between peaks is always the same, but that if I was traveling relative to the light, and measured its speed in terms of distance/speed, that I would get a value less than c.There's probably something I'm missing though.

Remember that frequency x wavelength = speed, and when the light source is moving wrt to you, you'll measure a different wavelength and frequency ( because of the relative velocity) but the same speed.

 

[NotAName]

Just remember that it's utterly different than you would expect from a wave traveling in a medium so your intuitive ideas will not apply to light -as explained by relativity- as they would for, say, sound waves.

The Doppler effect in sound waves is easily understood to have the explanation that, when you run into waves more frequently, the "frequency" goes up. This is because you can change the relationship between your speed and the speed of the wave in the medium. This is not true in relativity.

Secondarily, a moving sound source writes waves into the medium at higher and lower frequencies because the waves escape from the source faster or slower thus creating shorter or greater distances between the peaks. They stack up or stretch out. This is also not true with light since it's speed is constant with respect to the emitter.

These intuitive mechanical effects for sound simply cannot exist for light, therefore the Doppler effect in light is currently regarded as a basic truth of reality that has no deeper explanation other than the fact that time and light is different between frames.

Any attempt to explain doppler mechanically requires the total pulse length of a beam of light to be changed improperly and would violate light constancy.

 

[mangaroosh]

Remember that frequency x wavelength = speed, and when the light source is moving wrt to you, you'll measure a different wavelength and frequency ( because of the relative velocity) but the same speed.

How is the wavelength of light measured in an interferometer for example?

Also, would you actually measure a different wavelenght; that is a part I struggle with. I can see how the frequency would change, but not necessarily the wavelength.

 

[mangaroosh]

Just remember that it's utterly different than you would expect from a wave traveling in a medium so your intuitive ideas will not apply to light -as explained by relativity- as they would for, say, sound waves.

The Doppler effect in sound waves is easily understood to have the explanation that, when you run into waves more frequently, the "frequency" goes up. This is because you can change the relationship between your speed and the speed of the wave in the medium. This is not true in relativity.

Secondarily, a moving sound source writes waves into the medium at higher and lower frequencies because the waves escape from the source faster or slower thus creating shorter or greater distances between the peaks. They stack up or stretch out. This is also not true with light since it's speed is constant with respect to the emitter.

These intuitive mechanical effects for sound simply cannot exist for light, therefore the Doppler effect in light is currently regarded as a basic truth of reality that has no deeper explanation other than the fact that time and light is different between frames.

Any attempt to explain doppler mechanically requires the total pulse length of a beam of light to be changed improperly and would violate light constancy.

I find it more intuitive to think about a light wave traveling than a sound wave, because I don't need to take the medium into account with light.

I'm picturing a light wave traveling with a constant speed, and with an absolute wavelenght i.e. the same distance between peaks. If an observer is at rest relative to the source I imagine that they would detect a certain frequency of the light wave.

If an observer moves towards the light source, I imagine that the frequency would increase, but the wavelength wouldn't necessarily change.

I try to apply this to an interferometer and I don't expect that there would be any fringe shift in such an experiment. I wouldn't conclude that the speed of light is constant in all directions though.

Where am I going wrong?

 

[Austin0]

Just remember that it's utterly different than you would expect from a wave traveling in a medium so your intuitive ideas will not apply to light -as explained by relativity- as they would for, say, sound waves.

The Doppler effect in sound waves is easily understood to have the explanation that, when you run into waves more frequently, the "frequency" goes up

.1) This is because you can change the relationship between your speed and the speed of the wave in the medium. This is not true in relativity.

Secondarily, a moving sound source writes waves into the medium at higher and lower frequencies because the waves escape from the source faster or slower thus creating shorter or greater distances between the peaks. They stack up or stretch out.2) This is also not true with light since it's speed is constant with respect to the emitter.

These intuitive mechanical effects for sound simply cannot exist for light, therefore the Doppler effect in light is currently regarded as a basic truth of reality that has no deeper explanation other than the fact that time and light is different between frames.

Any attempt to explain doppler mechanically requires the total pulse length of a beam of light to be changed improperly and would violate light constancy.

The speed of light is independent of the velocity of the emitter yes . And the speed is invariant as measured in any inertial system . But the same mechanical explanation that applies to sound also applies to light. I.e. the motion of the receptor in between wave peaks results in either higher or lower wave lengths /frequencies
The difference is the classic wave equations need a relativistic adjustment , transformation.

 

[Austin0]

I find it more intuitive to think about a light wave traveling than a sound wave, because I don't need to take the medium into account with light.

I'm picturing a light wave traveling with a constant speed, and with an absolute wavelenght i.e. the same distance between peaks. If an observer is at rest relative to the source I imagine that they would detect a certain frequency of the light wave.

If an observer moves towards the light source, I imagine that the frequency would increase, but the wavelength wouldn't necessarily change.

I try to apply this to an interferometer and I don't expect that there would be any fringe shift in such an experiment. I wouldn't conclude that the speed of light is constant in all directions though.

Where am I going wrong?

The wave length would also change. The motion between the peaks would necessarily mean a shorter wave length just as the motion would result in a shorter time interval i.e. higher frequency.
I am afraid it would be exceedingly difficult to get from the interferometer to an understanding of the measured invariance of light. I think it really requires an understanding of length contraction,time dilation and most importantly the relativity of simultaneity.

 

[NotAName]

The speed of light is independent of the velocity of the emitter yes . And the speed is invariant as measured in any inertial system . But the same mechanical explanation that applies to sound also applies to light. I.e. the motion of the receptor in between wave peaks results in either higher or lower wave lengths /frequencies
The difference is the classic wave equations need a relativistic adjustment , transformation.

I'm sorry, I have to disagree. A mechanical explanation makes no sense in this context. If you can "run into" peaks more frequently, then you have changed the relationship between your speed and the speed of light, thus breaking light constancy.

No typical physical or intuitive ideas can be used with light according to modern theory.

 

[Dale]

I'm sorry, I have to disagree.

Austin0 was correct.

If you can "run into" peaks more frequently, then you have changed the relationship between your speed and the speed of light, thus breaking light constancy.

This is not correct. I believe that you are mistakenly assuming that the frequency changes but the wavelength does not.

http://en.wikipedia.org/wiki/Relativistic_Doppler_effect

 

[harrylin]

Just remember that it's utterly different than you would expect from a wave traveling in a medium so your intuitive ideas will not apply to light -as explained by relativity- as they would for, say, sound waves. [..]

That's an often seen misunderstanding which could derail Mangaroosh. As most people here know, an MMX discussion is 100% compatible with a wave traveling in a medium (such as expressed by Maxwell's equations); special relativity emphasizes that the intuitive ideas that do not apply are some specific concepts of Newtonian mechanics.

it's speed is constant with respect to the emitter

Instead, according to SR (as well as Maxwell of course, on which SR was based), the speed of light is independent of that of the emitter. That is the essential characteristic of wave models as opposed to ballistic emission models.

 

[yuiop]

My inital thoughts are that this demonstrates that the wavelength of light, in a vacuum, is constant, but not necessarily the measured speed. I can imagine that light in a vacuum would have an absolute wavelength i.e. the distance between peaks is always the same, but that if I was traveling relative to the light, and measured its speed in terms of distance/speed, that I would get a value less than c.

This is not true. Light in a vacuum does not have an absolute wavelength. Observers with different velocities relative to the source would measure different wavelengths.

Am I correct in thinking that, because the actual speed, in terms of distance/time, is not measured, that it is the wavelength that gives us the information about speed? That is, the wavelength of the reconvening beams is the same?

It is possible for the wavelengths to be the same and still have a fringe change and an implied difference in speeds. A fringe shift occurs if the wavelengths are same but out of phase, or if the wavelengths are different to each other. No fringe shift was detected.

Secondarily, a moving sound source writes waves into the medium at higher and lower frequencies because the waves escape from the source faster or slower thus creating shorter or greater distances between the peaks. They stack up or stretch out. This is also not true with light since it's speed is constant with respect to the emitter.

This is wrong. Light wave peaks still stack up stretch out in the relativistic Doppler effect just like the classic Doppler effect. The only difference is that the frequency of the emitted or received waves is affected by time dilation as well. Light waves still behave like sound waves. The relativistic Doppler effect applies equally to light to sound. The main difference is that the speed of light is different to the speed of sound. The speed of light and sound are both independent of the speed of the source.

If an observer moves towards the light source, I imagine that the frequency would increase, but the wavelength wouldn't necessarily change.

If the frequency changes (f), then the wavelength (w) also changes, because they are the reciprocal of each. w = c/f and since c is a constant w has to change with f.

 

[NotAName]

This is wrong. Light wave peaks still stack up stretch out in the relativistic Doppler effect just like the classic Doppler effect. The only difference is that the frequency of the emitted or received waves is affected by time dilation as well. Light waves still behave like sound waves. The relativistic Doppler effect applies equally to light to sound. The main difference is that the speed of light is different to the speed of sound. The speed of light and sound are both independent of the speed of the source.

Is this seriously the consensus? It is utterly nonsensical to say that the doppler effect in light is the same as in sound. The doppler effect in sound is dependant completely upon the difference in speed between the listener and the speed of sound. The difference

There cannot be a difference if light is constant.

The speed of light and sound are independent of the source but sound depends upon a medium and is governed by that medium. It only travels at "the speed of sound" with respect to that one single frame. The way you've blandly stated the above seems to glaze over this gigantic critical difference.

You understand that if you are traveling through that medium, according to the frame you are in, sound will be a different speed right? And the doppler effect is proportional to that difference because the difference is the cause. The only other cause of doppler is an emitter moving with respect to the medium in which case the sound traveling in front of it is higher frequency and behind it lower. But that is once again a difference between the speed of the emitter and the speed of sound because sound moving in the ame direction as a moving emitter is "slower" than the speed of sound according to the moving observer and sound moving in the opposite direction is "faster" when considering it from his moving frame.

This is not correct. I believe that you are mistakenly assuming that the frequency changes but the wavelength does not.

Frequency and wavelength are interchangeable for the most part, especially with light.

Now I could believe that (lets say 600nm laser source) emitted in a frame whose time is passing differently from another may perhaps appear to create light of a different wavelength than a similar source in the reference frame. This actually makes sense.

Bu to say that a single peak or trough cannot escape quickly enough before the next peak or trough is emitted would break light constancy. To say you could run into a peak or trough and then hit the next one more quickly than normal would also break constancy.

To say the actual wavelenth of the light is changed because of the time differences makes sense... all other explanations break constancy. Is this mechanical view you guys speak of seriously academic consensus?

 

[Dale]

Is this seriously the consensus? It is utterly nonsensical to say that the doppler effect in light is the same as in sound. The doppler effect in sound is dependant completely upon the difference in speed between the listener and the speed of sound.

This last statement here is incorrect. The listener may be at rest wrt the air and there will still be a Doppler shift if the source is moving. So classically the Doppler formula for sound is not only dependent on the speed of the listener wrt the air, but also the speed of the source wrt the air.

Here is a good reference that derives the general Doppler shift formula for both sound and light and shows how they are the same.

http://mathpages.com/rr/s2-04/2-04.htm

Frequency and wavelength are interchangeable for the most part, especially with light.

Now I could believe that (lets say 600nm laser source) emitted in a frame whose time is passing differently from another may perhaps appear to create light of a different wavelength than a similar source in the reference frame. This actually makes sense.

Bu to say that a single peak or trough cannot escape quickly enough before the next peak or trough is emitted would break light constancy. To say you could run into a peak or trough and then hit the next one more quickly than normal would also break constancy.

To say the actual wavelenth of the light is changed because of the time differences makes sense... all other explanations break constancy. Is this mechanical view you guys speak of seriously academic consensus?

You seem to agree that frequency and wavelength both change in different frames. As long as they change such that λf=c then the speed of light is invariant. All of the rest of your discussion about peaks and troughs escaping (whatever that means) is not important as long as λf=c.

 

[harrylin]

Is this seriously the consensus? It is utterly nonsensical to say that the doppler effect in light is the same as in sound. The doppler effect in sound is dependant completely upon the difference in speed between the listener and the speed of sound. The difference
There cannot be a difference if light is constant. [..]

I cannot parse those sentences; anyway, it's not just "consensus" but a statement of fact that relativistic Doppler (according to special relativity) is obtained by combining the classical Doppler effect and the time dilation effect. And surely you are aware that classical Doppler is the same for light as for sound. If you claim something different (and incompatible), then it's not SR.

BTW, as you appear to think that people here don't understand the basics: most likely everyone who reacted to your comments is experienced with relativistic Doppler calculations.

 

[NotAName]

This last statement here is incorrect. The listener may be at rest wrt the air and there will still be a Doppler shift if the source is moving. So classically the Doppler formula for sound is not only dependent on the speed of the listener wrt the air, but also the speed of the source wrt the air.

Yes, because the speed of sound is different for the moving emitter. Perhaps you didn't understand an emitter can be an observer and lost track because of terminology? As I explained: In one direction, according to the moving emitter's frame, sound travels faster in one direction than in the other.

You seem to agree that frequency and wavelength both change in different frames. As long as they change such that λf=c then the speed of light is invariant. All of the rest of your discussion about peaks and troughs escaping (whatever that means) is not important as long as λf=c.

Actually it is since the discussion is about mechanism and not faith-based reasoning.

 

[harrylin]

Yes, because the speed of sound is different for the moving emitter. Perhaps you didn't understand an emitter can be an observer and lost track because of terminology? [..]

Don't worry, everyone here (or almost everyone, and certainly dalespam) does understand that

Actually it is since the discussion is about mechanism and not faith-based reasoning.

:rofl: This discussion is based on SR (explaining how a theory works has nothing to do with "faith" but everything with correct understanding), but the topic that we should be developing here - "fringe shifts-MMX" - would be derailed by too much of other elaborations. So, please start a new topic about the second postulate and or mechanism if you want to elaborate more about those.

Managaroush, do you need more explanations about fringe shifts and light propagation in an MMX apparatus, as determined with different reference systems?

 

[NotAName]

I cannot parse those sentences; anyway, it's not just "consensus" but a statement of fact that relativistic Doppler (according to special relativity) is obtained by combining the classical Doppler effect and the time dilation effect. And surely you are aware that classical Doppler is the same for light as for sound. If you claim something different (and incompatible), then it's not SR.

BTW, as you appear to think that people here don't understand the basics: most likely everyone who reacted to your comments is experienced with relativistic Doppler calculations.

No, I think you have an acute lack of understanding the mechanisms which cause effects and instead substitute what you have learned for understanding. If I need to explain the difference between understanding and memory then this discussion is moot anyway but I will try. Understanding is derived from the connection between data, not simply the data itself. Understanding is meta-data. You seem to lack certain meta-data.

I have no doubt of your ability to run a formula correctly. I have no doubt of your ability to select the formula you have been trained to select. I just prefer to differentiate between machine intellect and human intellect within myself. I'd ask you to do the same for yourself.

You can "know" many things as "fact" but there is a difference between the information you were simply given and the information you could logically derive yourself.

The post I just answered with DaleSpam seemed to indicate that he did not understand the idea that waves are stacked up in front of a moving emitter because of the fact that each compression of the medium cannot escape as quickly as it normally would before the next compression of the medium occurs. This is the mechanism of a shortened wavelength being written into the medium.

This is the lack of understanding I'm talking about.

When I try to explain how a microwave works you start talking to me about hitting the buttons on the front when I'm trying to tell you how the buttons relay a signal to the processor and the other steps in the chain of events that cause your food to warm.

I am however also aware that a great chef, usually only needs know how the buttons on the stove or microwave work to be able to create a masterpiece meal. If he had spent his time learning all the circuitry that causes those devices to work he may have never become a great chef.

I'm asking you to be aware that you, like every other functioning human, do compartmentalize your understandings and leave certain things up to faith. (A placeholder for knowledge of inner workings) That does not mean that nothing lies beneath but it does mean that sometimes you never realize that something lies beneath.

 

[harrylin]

No, I think you have an acute lack of understanding the mechanisms which cause effects and instead substitute what you have learned for understanding. [..]

You could hardly be more mistaken as all the information that I give here happens to be what I verified for myself; so don't hesitate to start a discussion on your understanding of SR but please, do not try to hijack Mangaroosh's topic of MMX fringe shifts - that's against the rules. In this thread I will focus on answering Mangeroosh's questions.

 

[NotAName]

And just as a side note, I've spend enough time studying optics that when I found that there was no explanation of "Friinge Shift" on the internet, I created the wikipedia page still used today.

I've spend so much time in researching optics that I accidentally got a Honeywell employee almost fired for revealing information that became restricted after 911. Ring laser gyoscopes, iFOGs and rFOG are part of most military guidance systems.

Much more information about them used to be readily available but are now restricted. For instance, did you know that all laser-based interferometer devices require dithering or the introduction of multiple frequencies to eliminate something loosely called "phase-locking"?

Lasers are so powerful that a difference in phase between to beams of similar/same frequency is instantly eliminated. Christiaan Huygens discovered that occilators of similar frequency which can communicate with each other in some way will automatically synchronize their phase.

For Huygens it was just all the pendulums of clocks on a wall that synchronized...

For us it means that all high-school/college interferometer experiments which are used to demonstrate light constancy are based upon a fundamental lack of understanding the components of the device. Ergo, the experiment is a fallacy.

Doesn't mean light constancy isn't true... it just means that we are ignorantly touting something as proof that simply isn't proof of anything. As a matter of fact it is interesting to note that the original Michelson-Morely, while sodium-light was used to calibrate it, used white light. Therefore, because of the numerous frequencies, would have eliminated the phase locking effect. Thus the results of the original experiment are still valid though our modern experiments are worthless.

I guess that's why Michelson won a Nobel and the rest of us haven't ;)

 

[berkeman]

Thread closed for Moderation...