# Michelson–Morley experiment?

Did this experiment show that there was no ether . Because one way the light had to move up stream and it would take longer. But both beams arrived at the same time , so does this show there is no ether . I'm just trying to make sure I understand this experiment or am I way off .

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russ_watters
Mentor
It does not show that there is no ether, it shows that the earth is not rotating through an ether. There are other possibilities such as an ether that is dragged along with earth that would show the same result, but these are mostly considered superfluous.

Born2bwire
Gold Member
I think it would be more accurate to say that it showed that there was no ether wind (or if there was it was on an order far smaller than any theories at the time). The presence of an (a)ether stipulated a set of consequences like ether wind, ether drag, and I also seem to recall that movement through a medium affects the index of refraction. MM was just one of the many experiments that chipped away at the properties of various ether theories until you arrive at the point where an ether theory must be exceedingly complex to comply with experimental results.

ok so the MM experiment does not prove that light is always perceived at c.
Thanks for your replies by the way .

russ_watters
Mentor
ok so the MM experiment does not prove that light is always perceived at c.
No!!!! That is a much different question than your first question - and a poorly phrased one at that. One can never prove anything absolutely, particularly for all situations, so it is nonsensical for that to be the conclusion of an experiment. But the MMX did prove (to a reasonable certainty) that light is always perceived to travel at C.....in an MMX.

ok im just trying to make sure I understand the MMX. Now Im not quite sure why it proves that light is perceived at c. Is it because that one of the light beams is moving toward the mirror and then away from it and the other one is moving parallel to the motion. And did both light beams travel the same distance because of length contraction. I really appreciate you helping me understand this .

Born2bwire
Gold Member
Basically the idea, at least from what I recall (which is getting less and less these days it seems), is that we have a beam of light split off and travel the same distance but along paths of normal incidence. We then recombine the beam and if the time of travel is different then this manifests itself as an interference due to the phase shift. So with the idea of an ether, if we were moving through an ether then speed of light would be dependent upon the path through the ether and our relative motion with respect to the ether. So we run the MM experiment and (ideally) we run this throughout the course of a year. The Earth's orbit moves around the Sun and thus the idea being that our relative motion through any ether will change as the year passes. So we would expect to see a change in the interference pattern between observations but this was not observed.

So, as russ puts it, the experiment only proves that the speed of light is invariant (and c) in the MM experiment which was meant to test a specific set of ether theory ideas. But it was an indicator that the speed of light may be a universal constant.

As for length contraction, I do not think that length contraction came into the original experiment. The apparatus and its results are observed in the same reference frame. The point was that the apparatus (and its observer) may be moving relative to some ether.

ok , But when we take into account the earth turning the one light beam has a longer path to take , But the light beams end up at the interferometer at the same time . And isn't this what gave Hendrik Lorentz his idea for length contraction.

russ_watters
Mentor
ok im just trying to make sure I understand the MMX. Now Im not quite sure why it proves that light is perceived at c. Is it because that one of the light beams is moving toward the mirror and then away from it and the other one is moving parallel to the motion. And did both light beams travel the same distance because of length contraction. I really appreciate you helping me understand this .
The argument is that the light from one arm should move a longer distance because the ether causes it to move at an angle. It's a geometry issue similar to the speed of a boat going across a river: A boat takes longer to go across a faster moving river because it has to angle upstream.

But when we take into account the earth turning the one light beam has a longer path to take , But the light beams end up at the interferometer at the same time.
If you do the Michelson and Morley experiment you will find that relative to the spinning earth the "ether wind" is not moving.

The experiment of itself does not explicitly imply that there is no "ether wind", rather it the experiment implies that the "ether wind" is at rest relative to the earth frame of reference.

When Lorentz came up with his theory of length contraction he believed there was still an "ether wind" it was just that light happened to become length contracted. In fact, the theory was developed to save the concept of an "ether wind".

However the idea that the "ether wind" is constant relative to the earth appeals to us as scientists in a different way. It appeals to the idea that we have no reason to be special. If the "ether wind" exists why does it just happen to be here on earth that it is constant? In much the same way that Copernicus absolved that idea that the earth was the center of the universe it appeals to the rational mind that in the case of an "ether wind" there is no reason that the earth should be considered so special as to have an "ether wind" be completely at rest relative to the earth.

ok i think i get it now , thanks for your replies

MM experiment proves that the speed of light is constant in a specific medium and there is no relativity for it and this was the main principle which lead to Einstein's special theory of relativity. Before the MM experiment, speed of light was thought to be relative to the observer's motion. I don't think it proves that ether DOES NOT exist at all.

It does not show that there is no ether, it shows that the earth is not rotating through an ether. There are other possibilities such as an ether that is dragged along with earth that would show the same result, but these are mostly considered superfluous.
Why is this considered superfluous.There are similar circumstances whereby the enviroment is dragged along and not considered as such.Take the air surrounding the earth it is dragged along with it but when someone shouts from a distance we don't experiance a doppler shift.
Or when a stone is dropped from a height into a still pond.The concentric waves produced are nice and round there is no deformation formed on the side the earth is being dragged through space.So why was there an expectation that the aether if it existed should behave any different than other substances that are dragged along.

The argument is that the light from one arm should move a longer distance because the ether causes it to move at an angle. It's a geometry issue similar to the speed of a boat going across a river: A boat takes longer to go across a faster moving river because it has to angle upstream.
Nonsense when you consider that it would take a boat longer when it angles upstream in a fast moving river but quicker by exactly the same amout when it travells downstream.
Jump on a moveing walkway and walk in the direction of travell till you get to the end of it.
Consider that the end is where the mirror would be in the MMX.
When you get to the end turn round and walk back at the same pace to where you jumped on.
Time yourself for the journey.
Now walk the same distance without a walkway and time yourself.
The time taken will be equal for both.

Born2bwire
Gold Member
Nonsense when you consider that it would take a boat longer when it angles upstream in a fast moving river but quicker by exactly the same amout when it travells downstream.
Jump on a moveing walkway and walk in the direction of travell till you get to the end of it.
Consider that the end is where the mirror would be in the MMX.
When you get to the end turn round and walk back at the same pace to where you jumped on.
Time yourself for the journey.
Now walk the same distance without a walkway and time yourself.
The time taken will be equal for both.
No. Assume that you need to travel a distance d and you move with a speed v. Normally, to travel back and forth it will take,

$$t = \frac{2d}{v}$$

Now if you are slowed down by a speed of x on your outward journey and sped up by x on your inward journey, then the time will be

$$t = \frac{d}{v-x} + \frac{d}{v+x} = d\frac{(v+x)+(v-x)}{v^2-x^2} = \frac{2dv}{v^2-x^2}$$

Thus, it will always take you a longer time to traverse the distance. And since the two paths of the MM experiment were orthogonal to each other, they would (except under specific conditions) experience different amounts of ether wind along the paths of travel.

russ_watters
Mentor
Why is this considered superfluous.There are similar circumstances whereby the enviroment is dragged along and not considered as such.
If you look for something in enough places and never find it and it doesn't have any impact on the workings of your equations, then it becomes unnecessary to continue to hypothesize its existence.
Nonsense when you consider that it would take a boat longer when it angles upstream in a fast moving river but quicker by exactly the same amout when it travells downstream.
You misunderstand the geometry: the boat never travels downstream, it only angles upstream in both directions, otherwise it couldn't travel straight across the river and straight back.
Jump on a moveing walkway and walk in the direction of travel....
Perhaps you are unaware of what the experimental setup look like: the goal is to compare the speed of light "along the moving walkway" with the speed of light perpendicular to "the moving walkway".

If you look for something in enough places and never find it and it doesn't have any impact on the workings of your equations, then it becomes unnecessary to continue to hypothesize its existence.
You would not accept that that air or water did not exist because of the lack of impact on the workings of your equations from the examples I gave though.
You misunderstand the geometry: the boat never travels downstream, it only angles upstream in both directions, otherwise it couldn't travel straight across the river and straight back.
Probably did though I took that into account.
Perhaps you are unaware of what the experimental setup look like: the goal is to compare the speed of light "along the moving walkway" with the speed of light perpendicular to "the moving walkway".
Yes I am aware that it is to compare the speed of light perpendicular to the moving walkway.
If you take it that light travells at it's minimum speed in both directions when it is moving perpendicular to the walkway and that any deviation from the perpendicular would result in a loss or gain depending on which direction the light was travelling.
It is possible that the gains and losses cancell out to equal it's minimum speed.

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No. Assume that you need to travel a distance d and you move with a speed v. Normally, to travel back and forth it will take,

$$t = \frac{2d}{v}$$

Now if you are slowed down by a speed of x on your outward journey and sped up by x on your inward journey, then the time will be

$$t = \frac{d}{v-x} + \frac{d}{v+x} = d\frac{(v+x)+(v-x)}{v^2-x^2} = \frac{2dv}{v^2-x^2}$$

Thus, it will always take you a longer time to traverse the distance. And since the two paths of the MM experiment were orthogonal to each other, they would (except under specific conditions) experience different amounts of ether wind along the paths of travel.
If we take the points of a compass North South East and West and the centre of the compass as the point where the light originates from in the MMX.
Is it not possible for ease, that when the light is travelling from the centre along the horizontal to the west it gains 1kps and when it travells from the west to the centre it looses 1kps so the value at the centre is 300000kps.
Likewise when it travells east it looses 1kps and when it travells back to the centre it gains 1kps so it's value remains 300000kps.
However when it travells north it does not gain any speed and when it travells back to the centre from the north it also don't gain any speed so it's speed remains at 300000kps at the centre.
If it were to travell to the NW it would gain 0.5 kps and when it travells back to the centre it would loose 0.5 kps.
The reverse would be true of NE and any other directions would be fractions of this.
No matter which direction the light travelled in when it arrived back to the centre the value would be the same but the speeds along the paths different.

ghwellsjr
Gold Member
ok , But when we take into account the earth turning the one light beam has a longer path to take , But the light beams end up at the interferometer at the same time . And isn't this what gave Hendrik Lorentz his idea for length contraction.
Yes, this is exactly right. I made a series of animations to illustrate generally what happened in MMX and how Lorentz could have come to his length contraction explanation and how that explanation (along with time dilation) was re-interpreted by Einstein. See these two posts:

https://www.physicsforums.com/showpost.php?p=3059029&postcount=78

https://www.physicsforums.com/showpost.php?p=3059104&postcount=79

Yes I am aware that it is to compare the speed of light perpendicular to the moving walkway.
If you take it that light travells at it's minimum speed in both directions when it is moving perpendicular to the walkway and that any deviation from the perpendicular would result in a loss or gain depending on which direction the light was travelling.
It is possible that the gains and losses cancell out to equal it's minimum speed.
Sorry I can't find the edit button.
I should have said that light travells at a constant when moving perpendicular to the walkway and that any deviation from the perpendicular would result in a loss or gain depending in which direction the light was travelling.etc.

Did this experiment show that there was no ether . Because one way the light had to move up stream and it would take longer. But both beams arrived at the same time , so does this show there is no ether . I'm just trying to make sure I understand this experiment or am I way off .
No. It did show however that the then existing ether theories were wrong. Following those theories, the experimenters took for granted (they even didn't mention it) that Newton's mechanics is exactly correct, according to which the dimensions of an interferometer are completely unaffected by inertial motion.
With the assumption of a stationary ether and by combining Newton's mechanics with Maxwell's electrodynamics, assuming both to be exactly correct, an optical effect was expected from rotating the apparatus. The non-detection of their expected effect was called a "null result", and the solution involved a correction of Newton's mechanics.

You can read their full paper here:

http://en.wikisource.org/wiki/On_the_Relative_Motion_of_the_Earth_and_the_Luminiferous_Ether

Harald

Born2bwire
Gold Member
If we take the points of a compass North South East and West and the centre of the compass as the point where the light originates from in the MMX.
Is it not possible for ease, that when the light is travelling from the centre along the horizontal to the west it gains 1kps and when it travells from the west to the centre it looses 1kps so the value at the centre is 300000kps.
Likewise when it travells east it looses 1kps and when it travells back to the centre it gains 1kps so it's value remains 300000kps.
However when it travells north it does not gain any speed and when it travells back to the centre from the north it also don't gain any speed so it's speed remains at 300000kps at the centre.
If it were to travell to the NW it would gain 0.5 kps and when it travells back to the centre it would loose 0.5 kps.
The reverse would be true of NE and any other directions would be fractions of this.
No matter which direction the light travelled in when it arrived back to the centre the value would be the same but the speeds along the paths different.
I know this is late but I forgot about this thread. I just demonstrated above that if you traverse the same distance back and forth with a gain and loss in speed then the net effect is that you traverse the distance slower than usual. Because the two paths are orthogonal, the projection of each path onto the ether wind would be different. The only case where the effects to the speeds would be identical would be when both arms are 45 or 135 degrees to the direction of the ether wind. All other cases the ether wind would cause the two beam paths to take differing amounts of time to traverse. The resulting phase delay would have shown up in the measurements of the experiment.

I know this is late but I forgot about this thread. I just demonstrated above that if you traverse the same distance back and forth with a gain and loss in speed then the net effect is that you traverse the distance slower than usual. Because the two paths are orthogonal, the projection of each path onto the ether wind would be different. The only case where the effects to the speeds would be identical would be when both arms are 45 or 135 degrees to the direction of the ether wind. All other cases the ether wind would cause the two beam paths to take differing amounts of time to traverse. The resulting phase delay would have shown up in the measurements of the experiment.
Would this still be the case if there was no aether wind and only extra speed provided by the movement of the earth through the universe.

Born2bwire