# B Ether and Michelson-Morley experiment

1. Mar 27, 2016

### rakhi israni

why ether medium fail in michelson morley experiment?

2. Mar 27, 2016

### drvrm

If ether medium would have been there the experiment expected a fringe shift.
due to the change in the speed of light in traversing identical path lengths in the interferometer in two ways -

first along the velocity of ether wind (as the earth is moving through the ether and carry the ether along with it -producing a wind) and
second in the direction opposite to the ether wind but the results did not provide that shift .
perhaps thats why the statement above is quoted.
however Einstein said that one does not require a medium for light to travel and it always travel with a constant value in our universe.

3. Mar 27, 2016

### FactChecker

They thought there was aether. The Michelson-Morley experiment sent a light beam in the direction of the Earth's motion, "upstream" against the aether, and bounced it back ("downstream" with the aether) to the starting point. That should always take longer than if the Earth was not moving in the aether. To see this, imagine the extreme case where the Earth is moving so fast that it takes an hour for the light to go one foot "upstream". Then it could never make up the lost time when it goes back downstream. With the actual Earth speed, the light would still take a tiny bit longer than it should. But it did not. They compared that light beam with a split beam that was sent sideways and back the exact same distance. The two beams came back at the identical time.

4. Mar 30, 2016

### carl susumu

But if the test beam propagated in the direction of the ether drift then is reflected back in the opposite direction does not that cancel the test for the ether. And what about light propagating through a glass vacuum tube, does that not count? What's the purpose of Michelson's experiment. Oh. Lorentz..transformation-->>>relativity.

5. Mar 30, 2016

### Staff: Mentor

No. On the assumption that the speed of light depends on ether drift, light traveling parallel to the ether drift will take longer to make the round trip that light traveling perpendicular to the ether drift.

What does this have to do with the Michelson-Morley experiment?

6. Mar 30, 2016

### rootone

It's one of those experiments which intended to prove a theory to be correct, but the result showed that the theory must be wrong.
That's OK.

7. Mar 30, 2016

### FactChecker

Don't forget that the light would go in the "slow" direction for a longer time than it would go in the "fast" direction. So the effects would not cancel out. The round trip would take a longer total time.

8. Mar 31, 2016

### Janus

Staff Emeritus
Let L be the distance between emitter and the mirrors. The time it would take for the light to make the round trip perpendicularly to the motion of the device is
$$T\perp = \frac{2L}{c}$$
With c being the speed of light relative to the ether.
In the parallel direction, the one way trip time in one direction will be
$$T_1\parallel = \frac{L}{c-v}$$
and
$$T_2\parallel = \frac{L}{c+v}$$
in the other, where v is the velocity of the device with respect to the ether.
This makes the total round trip time
$$T \parallel = \frac{2Lc}{c^2-v^2}$$
The only way this gives the same time as the perpendicular light path is if v=0 (no ether drift)

9. Apr 1, 2016

### carl susumu

Are you saying that there is an ether, composed of matter, or there is not. I'm confused.

10. Apr 1, 2016

### Janus

Staff Emeritus
I'm saying that if there were an ether, the M&M apparatus would be able to detect its own motion with respect to it.

11. Apr 1, 2016

### carl susumu

So, definitively are you saying that there is no optical ether, composed of matter?

12. Apr 1, 2016

### Staff: Mentor

The hypothesis being tested by the Michelson-Morley experiment is that there is an ether and that the velocity of light measured by a measuring device depends on the velocity of that device relative to the ether. The ether being hypothesized is not composed of matter; it is a fundamentally different kind of substance.

13. Apr 1, 2016

### Ibix

...and the result of the test was null. So we don't believe in it any more, whatever it actually would have been made of if it were real.

14. Apr 1, 2016

### Staff: Mentor

Per my previous post, nobody ever hypothesized an "optical ether, composed of matter", so it didn't take the Michelson-Morley experiment to rule it out. But, as Ibix has just pointed out, the M-M experiment also ruled out an ether even if it was not composed of matter (which, as I said, the ether hypothesized by M-M before they did the experiment was not).

15. Apr 1, 2016

### Ibix

The ether that Michelson and Morley proposed to detect (or, more precisely, find the Earth's velocity relative to) had to fulfil certain criteria. It had to be able to pass through normal matter more or less unhindered (otherwise radio waves couldn't go through walls). It had to have some kind of internal structure, since light waves were polarisable and, hence, transverse.

That's a fairly straightforward and simple model, as these things go.

But Michelson and Morley didn't detect any velocity with respect to the ether, ever. So it's a wrong model. People began to add bells and whistles to the theory, such as "ether entrainment". That's the idea that ether does in fact interact with normal matter. It is dragged along with normal matter - so the Earth drags ether around and we are always at rest with respect to the near-Earth ether (or close enough for Michelson and Morley to miss the difference). People began to try things like Michelson and Morley experiments with one arm shielded by a lead tube to provide more entrainment in one arm than the other.

But, along came Einstein with a complete, simple (at least as Occam's sense of the word) theory that explained Michelson and Morley's experiments - and Fizeau, etc. There is no need for complicated ether-drag models for which there is no experimental evidence. In fact, the GPS strongly suggests that there's no such thing (since the satellites are very precise clocks a long distance from the Earth, and they still act as if there were no ether).

Strictly, the Lorentz Ether was never ruled out. However, as I understand it, the Lorentz Ether has no physical properties - it just is. In other words it's a "presentist" interpretation of special relativity, as opposed to the "eternalist" picture (the block universe) that is (I think) the more popular view. It is not the ether that Michelson and Morley tried to find, however, and is most definitely not made of matter.

16. Apr 2, 2016

### secur

By the way Lorentz Ether Theory is mis-named. The key difference from Einsteinian relativity is, L. assumes an absolute spacetime reference frame, which the word "ether" conveniently represents. In the light of modern discoveries, think of that absolute frame as the one in which CMBW is exactly isotropic, and forget about ether.