Why did Michelson and Morley expect different light speeds in different directions?

[avito009]

By measuring the speed of the light beams through the inferometer michelson morley tried to explain the existence of ether. This was done by measuring light beams's speed. The beams were in different direction. Did they want to say that the beam in the direction of the movement of the Earth would have more speed than the beam that was in the other direction?

 

[Janus]

The two beams( or more correctly a single beam was split to follow two paths) were at right angles to each other. The argument was that if there was an aether which light moved relative to, the Earth's motion through the aether would cause the round trip time for the beam aligned with the motion to differ from that the beam at a right angle to it.

 

[tech99]

If the aether was stationary, the speed of light in the aether would be unchanged by movement of the source, but the beam traveling in the same direction as Earth would have further to travel and would arrive later. This is because the "receiver" has moved further away during the time of flight. It's like sending a water wave from stern to bow of a moving vessel.

 

[Charles Link]

As I recall, (I studied this a number of years ago), what they did is they then subsequently rotated the interferometer 90 degrees. In the presence of aether, one path would be longer than the other for a given set of fringes. Rotating it would then have the faster route (whose mirrors would thereby be further apart) take even longer and a definite change should occur in the fringe pattern. No perceptible change was observed in the fringe pattern upon rotation. Thereby, there was no faster path and no longer path and thus no aether.

 

[avito009]

But how do you make sure that one of the beams is going in the same direction as the Earth's motion?

 

[Drakkith]

But how do you make sure that one of the beams is going in the same direction as the Earth's motion?

If you know a bit about Earth's rotation and orbit about the Sun then this becomes a trivial matter. For example, if I hold my arm straight out towards the west around noon, or the east around midnight, I will be pointing in roughly the direction of Earth's motion around the Sun.

 

[avito009]

Correct me if I am wrong. The beam that moves in the direction of the Earth's rotation should travel faster?

 

[Drakkith]

Correct me if I am wrong. The beam that moves in the direction of the Earth's rotation should travel faster?

Not necessarily. At noon the Earth's rotation puts it at a slower velocity than the Earth's orbital velocity. Twelve hours later it's the reverse.

 

[Charles Link]

The aether can be assumed to be stationary with the Earth moving through it. When the Earth is moving through it, the beam in one arm will first travel in the direction of the Earth's motion with the beam traveling upstream through the aether, but upon its return it travels downstream. (If you look carefully at the Michelson interferometer, you will see the beam on one arm first travels in one direction but upon reflecting off of the mirror it travels over the same path in the reverse direction.) The other beam at right angles travels both times across the stream. Travelling across the stream puts some extra distance also, but if my quick calculations are correct, the upstream path adds enough extra time to the path than is saved by the downstream path, so that the cross stream path is somewhat shorter in time. (I saw this derivation many years ago when I studied the topic. I don't recall the final answer, but I think I have it correct here.)

 

[nasu]

It is true that you can determine the direction of Earth's rotation in a reference frame attached to the Sun.
But this is not relevant for the M-M experiment. They did not know how the Sun (or anything else) moves through ether, in what direction.
This is why their experiment was based on comparing the diffraction fringes for two positions of the system, 90 degrees apart. They found no difference between the two situations and this can be explained only if there is no motion through ether.

Edit. Oh,@Charles Link had explained it already. Did not see it.

 

[avito009]

If the Earth moves in east direction then which direction will ether move in? assuming there is ether?

 

[Drakkith]

If the Earth moves in east direction then which direction will ether move in? assuming there is ether?

Not sure. Here's what I found from the wikipedia article on the Michelson-Morley Experiment:

The Earth is in motion, so two main possibilities were considered: (1) The aether is stationary and only partially dragged by Earth (proposed by Augustin-Jean Fresnel in 1818), or (2) the aether is completely dragged by Earth and thus shares its motion at Earth's surface (proposed by Sir George Stokes, 1st Baronet in 1844).[A 5] In addition, James Clerk Maxwell (1865) recognized the electromagnetic nature of light and developed what are now called Maxwell's equations, but these equations were still interpreted as describing the motion of waves through an aether, whose state of motion was unknown. Eventually, Fresnel's idea of an (almost) stationary aether was preferred because it appeared to be confirmed by the Fizeau experiment (1851) and the aberration of star light.[A 5]

 

[Charles Link]

Not sure. Here's what I found from the wikipedia article on the Michelson-Morley Experiment:

The Earth is in motion, so two main possibilities were considered: (1) The aether is stationary and only partially dragged by Earth (proposed by Augustin-Jean Fresnel in 1818), or (2) the aether is completely dragged by Earth and thus shares its motion at Earth's surface (proposed by Sir George Stokes, 1st Baronet in 1844).[A 5] In addition, James Clerk Maxwell (1865) recognized the electromagnetic nature of light and developed what are now called Maxwell's equations, but these equations were still interpreted as describing the motion of waves through an aether, whose state of motion was unknown. Eventually, Fresnel's idea of an (almost) stationary aether was preferred because it appeared to be confirmed by the Fizeau experiment (1851) and the aberration of star light.[A 5]

Your response is somewhat unclear and/or doesn't concur with the Michelson-Morley Experiment "link" that you provided. The reason the Michelson-Morley experiment is so important is that it refutes the idea of any aether. The conclusion that is reached from it and special relativity is that there is no aether.

 

[Drakkith]

Your response is somewhat unclear and/or doesn't concur with the Michelson-Morley Experiment "link" that you provided.

I'm not sure what's unclear about it. That's a direct quote from the article. It says that the aether was assumed to be "almost stationary" prior to the experiment, which seems like an answer to avito's question. Obviously SR put an end to the aether theory.

 

[Charles Link]

I'm not sure what's unclear about it. That's a direct quote from the article. It says that the aether was assumed to be "almost stationary" prior to the experiment, which seems like an answer to avito's question. Obviously SR put an end to the aether theory.

@Drakkith Your input with your "link" now makes more sense. The Michelson-Morley experiment was performed in 1887 and I think when it first came out they thought the aether might somehow be following the Earth's travel including rotation. Einstein's Special Relativity Theory wasn't presented until 1905 and then the null result of the Michelson-Morley experiment was finally explained without any aether and it was consistent with Einstein's theory. It was no longer necessary to have the Earth pull the aether along in some strange manner.