# Why did Michleson assumed that ether is moving ?

## Main Question or Discussion Point

Why did Michelson assume that ether is moving ?

I mean why didn't he assume that Ether doesn't move and that is why speed of light doesn't change ? .. If Earth is moving in a direction, speed of light is "c".. if it is moving in the other direction, speed of light is "c" too .. because the ether is standing still so there is nothing that the beam of light go with or against ?

Thank you

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They assumes the medium would be fixed with respect to background stars, or the galaxy, or whatever, but not fixed in the frame of reference of the Earth.

That is, they did not presume the frame of reference of the Earth was unique above all others.

They assumes the medium would be fixed with respect to background stars, or the galaxy, or whatever, but not fixed in the frame of reference of the Earth.
Thank you

ghwellsjr
Gold Member
Why did Michelson assume that ether is moving ?

I mean why didn't he assume that Ether doesn't move and that is why speed of light doesn't change ? .. If Earth is moving in a direction, speed of light is "c".. if it is moving in the other direction, speed of light is "c" too .. because the ether is standing still so there is nothing that the beam of light go with or against ?

Thank you
Your question is confusing to me. Before Michelson did his experiment, he assumed that the ether was not moving but that his location on the surface of the earth was moving through it at different speeds in different directions at different times of the day and at different seasons of the year. He also assumed that the speed of light was c relative to the ether and that he could measure an ether wind as his apparatus moved through the ether.

When he failed to measure any ether wind no matter when he did his experiment, he concluded that the earth must be dragging the ether with it just like it drags the air with it as it rotates and so there is not a 1000 mile per hour wind caused by the rotation of the earth at the equator. He concluded his paper with the suggestion that maybe if the experiment were performed again at the top of a high mountain, it might be able to detect some ether wind where presummably the earth would be dragging the ether less.

When you said "because the ether is standing still", did you mean it is standing still with respect to the earth or standing still with respect to the sun or other stars?

Regarding Michelson Morley Experiment

The speed of light (relative to the ether) is the hypotenuse of a right triangle with sides of length (speed of light on Earth) and (Earth's velocity relative to Ether)

Can anyone clarify that point ?

Regarding Michelson Morley Experiment

The speed of light (relative to the ether) is the hypotenuse of a right triangle with sides of length (speed of light on Earth) and (Earth's velocity relative to Ether)

Can anyone clarify that point ?
You realize that the ether of light is a defunct theory, right?

yes I do know that .. but that is what they thought about when they were doing the experiment .. it seems to be a simple idea but i can't get it ..

They thought that if they would measure the speed of light (with the earth motion) and (against the earth motion) and (perpendicular to the earth motion) .. The first two are simple addition and subtraction but the third one is not understandable to me

russ_watters
Mentor
If light travels at a constant speed through the ether and the ether moves perpendicular to it, then the light moves at an angle. Consider a boat traveling straight across a river: it has to angle upriver in order to move straight across and as a result takes longer to get across the river than if the river wasn't moving.

Ich