Interferometry experiments - the MMX

roosh

I'm still wondering about the possible deductions from interferometry experiments, such as the MMX.

From my understanding, the speed of light is not measured in the sense that the distance the light travels along the arm of an interferometer, isn't divided by the time it took to complete the journey. Instead, what is measured is the interference patterns, or fringe shifts of the re-converging light waves.

My understanding is that the light reflected from the mirrors in the interferometer converge at slightly different angles, which results in a fringe; if the speed of light, along any of the arms of the interferometer were to change, then it would result in a fringe shift.

I'm just wondering, what is it that would cause the fringe shift; I know we say that a changing speed of light would, but because we don't actually measure the speed, in terms of distance/time, what is it that would cause the fringe shift - if one were to occur; would it be a change in wavelength; a change in frequency of the light wave; or some other phenomenon?

krd

They were expecting that light in the direction of the Earth's motion would have a short path than light travelling in the opposite direction. That the fringes would be slightly different depending on which direct the table was turned relative to the motion of the Earth. They were expecting to see the effect of the aether. There were no changes in the fringes, no matter which direction they turned the table - so it meant there probably was no aether.

roosh

krd wrote: »

They were expecting that light in the direction of the Earth's motion would have a short path than light travelling in the opposite direction. That the fringes would be slightly different depending on which direct the table was turned relative to the motion of the Earth. They were expecting to see the effect of the aether. There were no changes in the fringes, no matter which direction they turned the table - so it meant there probably was no aether.

cheers krd. I understand that they were testing for the aether, and that the conclusion is that no such aether exists; I suppose, I'm trying to understand the experiment in the context of no aether.

I'm picturing the experiment, without the concept of an aether, where two light waves converge at angles to each other; the speed of the light, in terms of distance/time isn't measured; how does a lack of a fringe shift tell us that the speed of light was the same; is it that the wavelength of the light doesn't change, or that the frequency of light doesn't change, or something along those lines?

krd

roosh wrote: »

I'm picturing the experiment, without the concept of an aether, where two light waves converge at angles to each other; the speed of the light, in terms of distance/time isn't measured; how does a lack of a fringe shift tell us that the speed of light was the same;

The theory before the experiment was that light propagated itself through the aether. Like ripples in water propagate themselves through the water.

The concept of the aether is wrong. But you have to imagine there's this static invisible substance, that allows solid matter to pass through it. And in theory of light for the time, it was believed light waves moved through this as ripples.

The interference pattern is dependant on the phase of the light waves. If the path of either wave is made longer or shorter, the phase will change - the position of the fringes will change.

So with the experiment, it was believed, that light travelling away from the motion of the earth would have a longer path than light travelling in the same direction. The extra path length - or which ever way you would turn the table - which with the aether theory would mean a change in the path length. Which would cause a phase change in the light, which would be visible in the interference pattern.

There was no change in the path length regardless of which way the table was turned.

is it that the wavelength of the light doesn't change, or that the frequency of light doesn't change, or something along those lines?

Neither the wavelength nor the frequency changes. It's meant to be the path length - the distance the light has to travel. In the experiment there was no change in the path length. They were expecting a change, and didn't get one.

roosh

krd wrote: »

The theory before the experiment was that light propagated itself through the aether. Like ripples in water propagate themselves through the water.

The concept of the aether is wrong. But you have to imagine there's this static invisible substance, that allows solid matter to pass through it. And in theory of light for the time, it was believed light waves moved through this as ripples.

The interference pattern is dependant on the phase of the light waves. If the path of either wave is made longer or shorter, the phase will change - the position of the fringes will change.

So with the experiment, it was believed, that light travelling away from the motion of the earth would have a longer path than light travelling in the same direction. The extra path length - or which ever way you would turn the table - which with the aether theory would mean a change in the path length. Which would cause a phase change in the light, which would be visible in the interference pattern.

There was no change in the path length regardless of which way the table was turned.



Neither the wavelength nor the frequency changes. It's meant to be the path length - the distance the light has to travel. In the experiment there was no change in the path length. They were expecting a change, and didn't get one.

Cheers krd, this was exactly what I was trying to understand.

I know that the MMX didn't return a perfect null result, but do contemporary interferometry experiments return a perfect null result?

EDIT: do you know about the construction of interferometers, then and now, by any chance? Are the arms of the interferometer a vacuum, or did I read somewhere that the MMX interferometer had gas, or something, in the arms?

krd

roosh wrote: »

I know that the MMX didn't return a perfect null result, but do contemporary interferometry experiments return a perfect null result?

He may have not have had perfectly null results but that could just have been error. He didn't find what he was expecting - which was the aether.

EDIT: do you know about the construction of interferometers, then and now, by any chance?

I haven't see one up in a long time. They're not that difficult to set up if I remember.

The interference pattern is quite large in comparison to the wavelength of the light - you can clearly see them with the naked eye. Interferometers are used in things like the guide laser for a those cutting lasers that do eye surgery. The interferometer is accurate to a quarter of the wavelength of the guide laser.

Are the arms of the interferometer a vacuum, or did I read somewhere that the MMX interferometer had gas, or something, in the arms?

No the arms weren't in a vacuum. Here's a picture of the apparatus.





It doesn't matter that air is there and that it's not in a vacuum. Because which ever way you turn the table, the light will have to go through the same air - so if there was any delay through the air, it would be equally in all directions. For the sake of the experiment the air would be cancelled out.

Interferometry is incredibly accurate. Because even a tiny change in the path of light, will make a massive difference to the pattern - you can see it with the naked eye. If the aether had been there, as soon as he turned the table he would have seen something.

Morbert

roosh wrote: »

I'm still wondering about the possible deductions from interferometry experiments, such as the MMX.

From my understanding, the speed of light is not measured in the sense that the distance the light travels along the arm of an interferometer, isn't divided by the time it took to complete the journey. Instead, what is measured is the interference patterns, or fringe shifts of the re-converging light waves.

My understanding is that the light reflected from the mirrors in the interferometer converge at slightly different angles, which results in a fringe; if the speed of light, along any of the arms of the interferometer were to change, then it would result in a fringe shift.

I'm just wondering, what is it that would cause the fringe shift; I know we say that a changing speed of light would, but because we don't actually measure the speed, in terms of distance/time, what is it that would cause the fringe shift - if one were to occur; would it be a change in wavelength; a change in frequency of the light wave; or some other phenomenon?

A change in frequency, wavelength, or phase would produce a different interference pattern.

roosh

krd wrote: »

He may have not have had perfectly null results but that could just have been error. He didn't find what he was expecting - which was the aether.

Could it have been due to a small change in the path length of the light? I'm not suggesting an aether is present, but to try and elaborate, using an example:

If we imagine standing in the middle of a train, which is in a vacuum, and sending a laser pulse toward a mirror at the end of the train, which reflects it back to us; if the train is not moving, then the distance that the light has to travel in each direction is equal.

If we imagine that the train is moving though, such that the mirror is at the front end of the train; the distance that the light has to travel towards the front of the train, and the mirror, is longer than the return journey, because the mirror is moving away from the laser pulse, while we are moving towards the returning pulse; this would mean that the path length is different in both cases.


Could it be a similar scenario in an interferometry experiment, where the rotation of the table causes a very minor difference in path length which would cause a non-null result, but seemingly insignificant.

krd wrote: »

I haven't see one up in a long time. They're not that difficult to set up if I remember.

The interference pattern is quite large in comparison to the wavelength of the light - you can clearly see them with the naked eye. Interferometers are used in things like the guide laser for a those cutting lasers that do eye surgery. The interferometer is accurate to a quarter of the wavelength of the guide laser.

I know the more modern ones are supposed to be more accurate, but is there any notable differences in their construction?

krd wrote: »

No the arms weren't in a vacuum. Here's a picture of the apparatus.


It doesn't matter that air is there and that it's not in a vacuum. Because which ever way you turn the table, the light will have to go through the same air - so if there was any delay through the air, it would be equally in all directions. For the sake of the experiment the air would be cancelled out.

But would this not lead us to conclude that the speed of light is constant, relative to the medium i.e. air.

- That would be a separate conclusion to the one above -

Could the rotating of the table cause a minor difference in the air in the arms of the interferometer, which cause the non-null result?

krd wrote: »

Interferometry is incredibly accurate. Because even a tiny change in the path of light, will make a massive difference to the pattern - you can see it with the naked eye. If the aether had been there, as soon as he turned the table he would have seen something.

Cheers krd; it's more the conclusions drawn from such experiments that I'm looking into.

roosh

Morbert wrote: »

A change in frequency, wavelength, or phase would produce a different interference pattern.

Of course, I should have thought of that; cheers Morbert.

citrus burst

roosh wrote: »

Could it have been due to a small change in the path length of the light? I'm not suggesting an aether is present, but to try and elaborate, using an example:

Yes, this is what an interferometer in essence does. It measures the difference in the optical path difference of light to a very high degree of accuracy. A very small change would be very noticeable. They produce patterns similar to this http://farm3.static.flickr.com/2761/4256685428_b03743eb11.jpg

Any change in the path length of the light will alter the image.

roosh wrote: »

If we imagine standing in the middle of a train, which is in a vacuum, and sending a laser pulse toward a mirror at the end of the train, which reflects it back to us; if the train is not moving, then the distance that the light has to travel in each direction is equal.

If we imagine that the train is moving though, such that the mirror is at the front end of the train; the distance that the light has to travel towards the front of the train, and the mirror, is longer than the return journey, because the mirror is moving away from the laser pulse, while we are moving towards the returning pulse; this would mean that the path length is different in both cases.

Could it be a similar scenario in an interferometry experiment, where the rotation of the table causes a very minor difference in path length which would cause a non-null result, but seemingly insignificant.

I don't think this is the case, since there would be no noticeable difference due to the motion of the train.

roosh wrote: »

I know the more modern ones are supposed to be more accurate, but is there any notable differences in their construction?

Nope, this type of interferometer is known as a Michelson interferometer. The components may be arranged differently but in essence it consists of a light source, a beam splitter, two mirrors (one fixed and the other movable), a compensator and a detector.

roosh wrote: »

But would this not lead us to conclude that the speed of light is constant, relative to the medium i.e. air.

The air makes no real difference, it would cause a small change in the wavelength of the light passing through it so can be ignored. Air has a refractive index of 1.000293, vacuum has a refractive index of 1, so they are approximately equal.

roosh wrote: »

- That would be a separate conclusion to the one above -

No not really.

roosh wrote: »

Could the rotating of the table cause a minor difference in the air in the arms of the interferometer, which cause the non-null result?

Well I assume they waited long enough to let the air settle. The original experiment was done over the space of a year.

Essentially the MMX was set up to determine not if the aether existed, but what type existed. There were two main theories to explain the aether, one were it moved, the "aether wind" and the other were it didn't. Michelson did the experiment twice, the first time ruled out the stationery aether. However it was shown that it had too much experimental error to be valid.

The second time he did the experiment with Morley. They improved their experiment. This time it showed a null result for the aether wind, there was no difference in the interference pattern as it was rotated. Although it conflicted with the previous results, this experiment ruled out the aether wind. Later experiments would rule out the stationary aether.

krd

roosh wrote: »

Could it have been due to a small change in the path length of the light? I'm not suggesting an aether is present, but to try and elaborate, using an example:

Yeah, changes in the path length but not related to the aether. When you're looking for really small things, your measuring equipment needs to be very sensitive. Small temperature changes in the room effect the equipment - vibrations - things getting a little out of whack. But even with that, he would have still been able to see something. So doing any experiment like that, you're going to see noise. But if the noise is the same in all directions, then either you're theory is wrong or the equipment isn't sensitive enough. I think the there were theoretical estimates of the effect of the aether. They did not pan out.


But..........recently ...this whole thing of the Higgs field...Could it be a return to the aether - and be completely wrong as well.

Could it be a similar scenario in an interferometry experiment, where the rotation of the table causes a very minor difference in path length which would cause a non-null result, but seemingly insignificant.

It's a long time since I did an interferometry experiment - but when I did we had a room set up - buckets of sand for some stuff (stops vibrations)...And of course these experiments were painfully easy to f-up.

I bet they repeated the experiment again and again and again. In those results a bias in the table, depending on which way it was turned would eventually show itself. And it's the same with the noise - go long enough, you eventually see a signal, or traces of a signal.

But would this not lead us to conclude that the speed of light is constant, relative to the medium i.e. air.

- That would be a separate conclusion to the one above -

Could the rotating of the table cause a minor difference in the air in the arms of the interferometer, which cause the non-null result?

Air, at that distance as far as light is concerned is so close to vacuum, it would be hard to notice its effect - and it will be the same effect in each direction. The same noise in every direction.

Not saying I'd love to have been there - but I'd say it was interesting to watch them turn the table back and forth a few thousand times - recalibrate etc.

roosh

krd wrote: »

Yeah, changes in the path length but not related to the aether.

Exactly, the changes in the path length wouldn't be caused by the aether.

For example, if you are on a moving body, in linear motion, and send a laser pulse from the back of the body to a mirror in the front [in the direction of motion] then the path length of the laser pulse from the back to the front will be longer than the return journey, from front to back; this would be despite the fact that the distance between the emitter and mirror remains constant. A change in path length wouldn't necessarily represent a change in the speed of light either, as the light would be traveling at the speed of c from the emitter to the mirror, and from the mirror back to the emitter; it would just have different distances to travel.

krd wrote: »

But..........recently ...this whole thing of the Higgs field...Could it be a return to the aether - and be completely wrong as well.

Haven't heard much about that; must have a look into it.

krd wrote: »

It's a long time since I did an interferometry experiment - but when I did we had a room set up - buckets of sand for some stuff (stops vibrations)...And of course these experiments were painfully easy to f-up.

I bet they repeated the experiment again and again and again. In those results a bias in the table, depending on which way it was turned would eventually show itself. And it's the same with the noise - go long enough, you eventually see a signal, or traces of a signal.

I wouldn't so much suggest that it is a bias in the table, but if the earth was in motion, not necessarily linear, then the rotation of the table could cause a very minoar change in path length, as per the example above, using linear motion; the change in path length would be a lot more complicated to discern than if it were linear motion, but the orientation of the table with respect to the motion of the earth would affect the path length, I would imagine.

krd wrote: »

Air, at that distance as far as light is concerned is so close to vacuum, it would be hard to notice its effect - and it will be the same effect in each direction. The same noise in every direction.

It would still act as a medium though wouldn't it; would it perhaps behave the same as what one would expect with an entrained aether say?

That's not to say that air is the aether, rather that, with the air being dragged along with the motion of the Earth, would it, perhaps, behave as one would expect an entrained aether to behave?

krd wrote: »

Not saying I'd love to have been there - but I'd say it was interesting to watch them turn the table back and forth a few thousand times - recalibrate etc.

It have would been, indeed.

krd

roosh wrote: »

For example, if you are on a moving body, in linear motion, and send a laser pulse from the back of the body to a mirror in the front [in the direction of motion] then the path length of the laser pulse from the back to the front will be longer than the return journey, from front to back; this would be despite the fact that the distance between the emitter and mirror remains constant. A change in path length wouldn't necessarily represent a change in the speed of light either, as the light would be traveling at the speed of c from the emitter to the mirror, and from the mirror back to the emitter; it would just have different distances to travel.

This is where relativity comes in. The mirror, and the emitter are both in motion. But that motion is in the same direction at the same velocity. So, for all intents and purposes, the mirror and the emitter, and the light beam, are in an inertial frame - in relation to each other they're not moving. The path length of the laser is not effected. However. If you were an observe in a different frame - say in position in space watching the earth spin by, you would see a difference in the laser's path length - but the people standing in the room wouldn't. It's not that the people in the room are missing something.

There's a clip somewhere on Youtube explaining Einstein's relativity, with people on a train playing ping pong. To an observer watching the train pass, when balls are hit down the carriage, they seem to travel slowly over a long distant, when the serve is returned they seem to travel very fast over a short distance. For the people on the train, the ping pong balls seem to travel at the same velocity and over the same distance in both directions.

This is a really uncanny thing to get your head around. When we look at a distant star travelling away from us, the light is red shifted (Doppler effect). To us, it's wavelength appears to be longer - its' path seems to be longer - BUT that's only as it appears to us. It hasn't changed. Our relative velocity and position in space makes it look like its' path is longer.relative to us the path is longer - relative to the light and it's emitter the path never changes.

I wouldn't so much suggest that it is a bias in the table, but if the earth was in motion, not necessarily linear, then the rotation of the table could cause a very minoar change in path length, as per the example above, using linear motion; the change in path length would be a lot more complicated to discern than if it were linear motion, but the orientation of the table with respect to the motion of the earth would affect the path length, I would imagine.

No. As far as the observation in the experiment goes, since all the parts of the apparatus are all travelling in the same direction, as far as the parts of the apparatus are concerned there is no change in the path length.

The crux of the experiment was the belief that there was something there - the aether - that was actually static. That everything in space could move freely through it, and that light propagated itself through it. But that it was static.

And the simple reason behind this belief was that it was hard to believe anything else.

It would still act as a medium though wouldn't it; would it perhaps behave the same as what one would expect with an entrained aether say?

No. The whole thing about the experiment is it showed light to be propagating itself as if there was no medium. Never mind that air in the room. Most of what makes up air is pure vacuum. This is also the thing. They knew light could travel through a vacuum (sound can't it needs a medium). So, the only way they could explain light being able to travel through a vacuum was that there was a very hard to detect medium there, the ether. The truth is, light doesn't need a medium and propagates itself.

That's not to say that air is the aether, rather that, with the air being dragged along with the motion of the Earth, would it, perhaps, behave as one would expect an entrained aether to behave?

The air in the room is moving at the same velocity and in the same direction as the rest of the apparatus. So, there is no drag.


Empty space - the vacuum - is a very hard idea to get your head around. It is actually empty (apart from the vacuum fluctuations but forget them for a minute).

And not only is the vacuum empty, but it's size is relative to the velocity of its observers. It's pure nothingness. It doesn't even have a constant size. It's dimensions only seem constant to an observer - different observer at a different velocity will measure different dimensions (it will seem bigger or smaller, longer or shorter, depending on which direction the observer is travelling and at what speed). ................Now....That idea sounds crazy, like science fiction, but that is reality of how it works.

roosh

krd wrote: »

This is where relativity comes in. The mirror, and the emitter are both in motion. But that motion is in the same direction at the same velocity. So, for all intents and purposes, the mirror and the emitter, and the light beam, are in an inertial frame - in relation to each other they're not moving. The path length of the laser is not effected. However. If you were an observe in a different frame - say in position in space watching the earth spin by, you would see a difference in the laser's path length - but the people standing in the room wouldn't. It's not that the people in the room are missing something.

Hey krd, the point I was trying to get at was that the path length of the photon, could be different, while the measured distance between the emitter and the mirror would remain the same; this, however, wouldn't be determinable over a round trip, because the difference in the path length - from the measured distance between the emitter and the mirror - of the forward journey would be offset by the difference in the path length of the return journey. This would have the same effect as if the path length was the same in both directions.

krd wrote: »

There's a clip somewhere on Youtube explaining Einstein's relativity, with people on a train playing ping pong. To an observer watching the train pass, when balls are hit down the carriage, they seem to travel slowly over a long distant, when the serve is returned they seem to travel very fast over a short distance. For the people on the train, the ping pong balls seem to travel at the same velocity and over the same distance in both directions.

cheers krd, I'm familiar with the Einsteinian explanation, I'm just trying to consider what alternatives there might be.

krd wrote: »

This is a really uncanny thing to get your head around. When we look at a distant star travelling away from us, the light is red shifted (Doppler effect). To us, it's wavelength appears to be longer - its' path seems to be longer - BUT that's only as it appears to us. It hasn't changed. Our relative velocity and position in space makes it look like its' path is longer.relative to us the path is longer - relative to the light and it's emitter the path never changes.

How can we say that, relative to the light and it's emitter, the path never changes, if we observe it to change?

krd wrote: »

No. As far as the observation in the experiment goes, since all the parts of the apparatus are all travelling in the same direction, as far as the parts of the apparatus are concerned there is no change in the path length.

It makes sense that there is no change in the distance between the emitter and the mirror, but the actual path length that the photon travels could be different to the measured distance between the emitter and the mirror.

If all parts of the apparatus are traveling in the same direction, then the only explanation for a constant path length would be an addition of velocities, which would treat light as a ballistic. Alternatively the explanation would be that the apparatus aren't moving at all, as opposed to not moving relative to each other.

krd wrote: »

The crux of the experiment was the belief that there was something there - the aether - that was actually static. That everything in space could move freely through it, and that light propagated itself through it. But that it was static.

And the simple reason behind this belief was that it was hard to believe anything else.

But we know that light doesn't need an aether, right?

Can we then consider the experiment without the concept of the aether, without necessarily adopting the Einsteinian explanation, and without considering light as a ballistic?

krd wrote: »

No. The whole thing about the experiment is it showed light to be propagating itself as if there was no medium. Never mind that air in the room. Most of what makes up air is pure vacuum. This is also the thing. They knew light could travel through a vacuum (sound can't it needs a medium). So, the only way they could explain light being able to travel through a vacuum was that there was a very hard to detect medium there, the ether. The truth is, light doesn't need a medium and propagates itself.

But where there is a medium, such as water, glass, or even air, it will affect the speed of light, will it not?

krd wrote: »

The air in the room is moving at the same velocity and in the same direction as the rest of the apparatus. So, there is no drag.

Empty space - the vacuum - is a very hard idea to get your head around. It is actually empty (apart from the vacuum fluctuations but forget them for a minute).

And not only is the vacuum empty, but it's size is relative to the velocity of its observers. It's pure nothingness. It doesn't even have a constant size. It's dimensions only seem constant to an observer - different observer at a different velocity will measure different dimensions (it will seem bigger or smaller, longer or shorter, depending on which direction the observer is travelling and at what speed). ................Now....That idea sounds crazy, like science fiction, but that is reality of how it works.

Sorry, what I meant was that, wouldn't the air behave as an entrained aether, for that very reason, because it is moving at the same velocity and in the same direction as the apparatus?

krd

roosh wrote: »

Hey krd, the point I was trying to get at was that the path length of the photon, could be different, while the measured distance between the emitter and the mirror would remain the same; this, however, wouldn't be determinable over a round trip, because the difference in the path length - from the measured distance between the emitter and the mirror - of the forward journey would be offset by the difference in the path length of the return journey. This would have the same effect as if the path length was the same in both directions.

The beams are being split and then mixed at right angles. So, not all the beams are making a return journey - so the ether drag is not being cancelled out.

I think if you did the MMX under fast flowing water you would see what MM expected to see. The water will act an ether.

There are other reasons why it's observable that there is no aether. Astronomical observations. If there was an aether, it would need to have a refractive index. Instead of seeing the stars as we do now, they would appear as rainbow coloured spheres - they don't. The night sky would be full of rainbows - which it isn't.

The other thing. On an astronomical scale ripples in the aether would be noticeable. But even if there was no ripples, we would see refraction.

Interferometry experiments are not that difficult to do. A lab laser - which most schools have, some mirrors and glass slides.

roosh

krd wrote: »

The beams are being split and then mixed at right angles. So, not all the beams are making a return journey - so the ether drag is not being cancelled out.

cheers krd, I can see that, but it wasn't necessarily the simplistic idea of light making a round-trip while the apparatus is in simple linear motion that I was trying to get at, but the idea that the path length could be different from the measured distance between the mirror and the emitter.

If that were to be the case, how would we expect the light to travel, given the rotation of the earth, it's orbit around the sun, and it's motion through the galaxy?

krd wrote: »

I think if you did the MMX under fast flowing water you would see what MM expected to see. The water will act an ether.

There are other reasons why it's observable that there is no aether. Astronomical observations. If there was an aether, it would need to have a refractive index. Instead of seeing the stars as we do now, they would appear as rainbow coloured spheres - they don't. The night sky would be full of rainbows - which it isn't.

The other thing. On an astronomical scale ripples in the aether would be noticeable. But even if there was no ripples, we would see refraction.

Interferometry experiments are not that difficult to do. A lab laser - which most schools have, some mirrors and glass slides.

OK, but I think we're both in agreement that there is no aether.

I might not be getting my point across clearly, or I may just be misunderstanding what you are saying.

The question I am trying to ask is essentially, how light behaves when it travels through air; when light has to travel through air it will have an affect on it's speed won't it, compared with when it travels through a vacuum? However miniscule that affect may be.

When air is present it will act as a medium won't it? Just like water will act as a medium when it is present, even though light doesn't need it? Would the air in the apparatus, then, not perform the same function as an entrained aether, even though no actual aether exists?

krd

roosh wrote: »

If that were to be the case, how would we expect the light to travel, given the rotation of the earth, it's orbit around the sun, and it's motion through the galaxy?

That's getting into relativity. It depends on your frame of reference. The table in the MMX is in the same frame of reference as the light - and which is also in the frame of reference of the rotating earth.

The question I am trying to ask is essentially, how light behaves when it travels through air; when light has to travel through air it will have an affect on it's speed won't it, compared with when it travels through a vacuum? However miniscule that affect may be.

Where air is different from glass or water is there is a lot of actual empty space. So the light is travelling through vacuum a lot of the time. The air does act as a medium, but most of the time the light is travelling through empty space - some is being absorbed and scattered by the air too - that's why the sky is blue, and we have the greenhouse effect.

Another example that might give you a better idea - Rainbows. When it's raining, little spheres of water are falling through the air. The water has a much greater refractive index than the air. Most of the light doesn't hit the rain drops - but the light that does eventually is refracted, and you see a rainbow.

And another example is coloured sunsets and sunrises. The sun is at a lower angle to the earth - so more light is transmitted through the air, and it refracts. And you can get multi-coloured horizons etc. There's a lot of water vapour in the air, and that changes with the weather.

Would the air in the apparatus, then, not perform the same function as an entrained aether, even though no actual aether exists?

The apparatus is too small for the air to make a difference. If it was a few miles wide, then it would be noticeable.

kevmy85

Michelson interferometers are real easy to setup. Give me a couple of mirrors, a beamsplitter and a laser and I could one in 10 mins (not a good one mind!!)

The basic setup is below:


Michelson interferometers are still widely used today in research and metrology especially. There are loads of different configurations and types other than the Michelson (including Mach-Zender, Twyman-Green, point-diffraction interferometers, shearing interferometers).

While it was a pain to make a good one in those days the advent of stabilised optical tables, optical design software and commercial opto-mechanical parts make it fairly easy to do. An average 3rd or 4th year physics undergrad should be able to build one from scratch in a few weeks.

In terms of uses they are used to test glasses, lenses, crystals, eyes, silicon wafers, in spectrometers, in loads of research applications...

By changing the path length difference between the two arms you change the number of fringes - this is used to measure the path length difference. By looking how the fringes change shape (but not amount) you can measure the aberrations along the path length.
By putting a specimen (like a contact lens) in the beam path you can measure the thickness (change in path length) and the optical aberration power(like focus, astigmatism) of the specimen.