Michelson–Morley experiment and the velocity of the ether wind

[Ahmed1029]

How was it justified before conducting the MICHELSON–MORLEY experiment to assume that the sun was at rest with respect to the ether? Also, was the ether assumed to have the same velocity with respect to the Earth throughout space at one instant in time, or like wind, with different velocities at different locations?

 

[anuttarasammyak]

How was it justified before conducting the MICHELSON–MORLEY experiment to assume that the sun was at rest with respect to the ether

I have not heard the experiment needs that assumption.

 

[Dale]

How was it justified before conducting the MICHELSON–MORLEY experiment to assume that the sun was at rest with respect to the ether?

That was not assumed.

Also, was the ether assumed to have the same velocity with respect to the Earth throughout space at one instant in time, or like wind, with different velocities at different locations?

The original concept of aether was a very rigid material. So it was assumed that the center of the Earth was moving through the aether at some unknown velocity. Then the motion of any point on the surface of the Earth through the aether would be the motion of the center of the Earth through the aether plus the motion of the point on the surface with respect to the center.

 

[DaveC426913]

Also, that is not the only time it was tested; it is merely the first and most famous one.

 

[Demystifier]

How was it justified before conducting the MICHELSON–MORLEY experiment to assume that the sun was at rest with respect to the ether?

It wasn't assumed. But it was assumed that ether is not dragged by the Earth, which of course was dubious.

 

[Ahmed1029]

So what happens if the sun is assumed to be at rest wrt the ether? My book uses that assumption at the outset of the experiment, that's why I was confused. So is that assumption okay?

 

[PeterDonis]

My book

What book? Can you give a reference?

 

[Ibix]

So what happens if the sun is assumed to be at rest wrt the ether?

Then you expect daily variation in light speed due to the Earth's rotation adding or subtracting from its orbital velocity, but no annual variation because the Earth's center has constant speed (or nearly so) wrt the ether. If the Sun moves through the ether then you get annual variation as well because the Earth's speed varies wrt the ether.

I guess your book is simplifying a bit because who cares? We only need to understand that the experiment could have detected ether drift and that it didn't detect anything and I believe a Sun centered ether model is enough for that. Letting the Sun move wrt to the ether doesn’t really change anything except to make the maths messier, and why worry about adding details to a model that doesn't work?

 

[jbriggs444]

Then you expect daily variation in light speed due to the Earth's rotation adding or subtracting from its orbital velocity, but no annual variation because the Earth's center has constant speed (or nearly so) wrt the ether. If the Sun moves through the ether then you get annual variation as well because the Earth's speed varies wrt the ether.

But Michelson-Morley does not purport to measure light speed. It purports to measure any discrepancy between fore-and-aft light speed and transverse light speed.

That said, yes, the expectation would be for an ether at rest relative to the sun to display zero annual variation in the magnitude of the observed discrepancy but to nonetheless always display a discrepancy. A discrepancy with an associated direction which would correlate with the position of the Earth in its orbit about the Sun.

 

[Ibix]

But Michelson-Morley does not purport to measure light speed. It purports to measure any discrepancy between fore-and-aft light speed and transverse light speed.

True. You would get variation in light speed in the direction of the ether wind but not perpendicular to it, and the variation in the difference is what M&M were trying to detect.

 

[Mister T]

So what happens if the sun is assumed to be at rest wrt the ether? My book uses that assumption at the outset of the experiment, that's why I was confused. So is that assumption okay?

If your book is a work of fiction that you wrote, then of course it's okay. The ether is a fiction, so having the sun at rest relative to it would also be a fiction.

 

[Rene Dekker]

How was it justified before conducting the MICHELSON–MORLEY experiment to assume that the sun was at rest with respect to the ether?

Stellar aberration was already known prior to the M-M experiment. The observed stellar aberration implies that, if there is an aether, that it is stationary with respect to the stars or the Sun.