Michelson and Morley experiment (apparatus and light in same frame)

Question

Central idea of Michelson and Morley experiment was that if single speed of light predicted is speed of light relative to ether, then as the earth moves through the ether, light travelling along the direction of motion must travel at a different speed relative to the earth and light moving at right angles to the motion.

If I get the argument then something like saying say velocity of ball is fixed in air. Then someone watching the ball from a moving train will see the velocity of ball different in the direction of motion. But in the experiment, the measuring apparatus ( equivalent of person measuring speed of ball in train) and the what is being measured i.e. light are in the same frame (frame of earth). Why should the velocity come different in that experiment as they thought? The argument could have worked if someone to look at this experiment outside earth frame?

I know I am missing something just not able to pin point.

Answer 1

If I get the argument then something like saying say velocity of ball is fixed in air. Then someone watching the ball from a moving train will see the velocity of ball different in the direction of motion.

Not quite. The argument is that if the velocity of a ball is fixed in air, then a ball thrown on a windy day will have different velocities in different directions.

Answer 2

When you say 'the frame of the Earth' what you are referring to is a frame that is continually changing. Through the day, the orientation of the device changes as a consequence of the Earth's rotation about its axis, and through the year as a consequence of rotation about the Sun etc. If there were an ether permeating the Universe, the chance of it being at rest w.r.t the Earth would be virtually zero, and it could not be at rest relative to the Earth if the Earth is moving relative to the universe. One counter argument that was considered historically was that the Earth could somehow be dragging part of the ether along at the same speed, so that locally the ether would be stationary relative to the Earth. However, aside from the obvious conceptual objections to such an idea, there have been experiments of various sorts that rule it out, so if there were an ether, a Michelson Morely interferometer would be moving through it in different directions at different times of the day and different days of the year.