# Michaelson Morley experiment.

1. Jun 22, 2008

### mtworkowski@o

I think I read somewhere that there's a flaw in the Michaelson Morley experiment. Is this true?

2. Jun 22, 2008

### Staff: Mentor

The Michelson Morley experiment is over a hundred years old. It has quite a few flaws! And it has, of course, been refined considerably since then. But at the time that it was done, it was done correctly and as well as could be expected and provided groundbreaking evidence of the constancy of the speed of light and lack of existence of an aether.

3. Jun 25, 2008

### Nickelodeon

Did they test for an aether travelling vertically or just horizontally?

4. Jun 25, 2008

### cristo

Staff Emeritus
What, you mean into the earth?

5. Jun 25, 2008

### maverick_starstrider

What's 'vertically' vs. 'horizontally' on a rotating planet rotating a star rotating in a galaxy, etc.

6. Jun 25, 2008

### mtworkowski@o

when I think of IR frames, the one thing I think of is this. Is the speed of light dependent on the speed of the source. Here's the thought. Tell me if this is amusing. "Which is the worst of the collisions? A: 2 cars head on at 60 mph each. (both same mass). B: 1 car, 60 mph hits a bridge abutment. Oh and can I just add this? Somebody told me that because the MME used light going in two directions that it averages out the delta v. What do you all think?

Last edited: Jun 25, 2008
7. Jun 25, 2008

### Staff: Mentor

I'm not sure that example works because cars are designed to be crushed while bridge abutments are not. How about this: a head-on collision between two cars going at 60mph is the same energywise as a collision between a car moving at 120mph and a stationary car. In any case, yes, kinetic energy is relative to the reference frame. I'm not sure what that has to do with the speed of light, though.
That's a very common misconception about the MM experiment, and I can never understand why: If you read any halfway decent description, of it, the usual analogy is to compare the MM experiment to measuring the speed of a boat moving sideways across a river -- not up and down the river.

So if the point of your initial question was to ask 'was the MM experiment capable of measuring what it claimed to measure?' - The answer is yes.

Last edited: Jun 25, 2008
8. Jun 25, 2008

### HallsofIvy

Yes, the speed of light is independent of both the speed of the source and the speed of the "observer". That's the whole point of the Michaelson-Morley experiment (and its many refinements).

If two cars, each going 60 mph, hit head on, that is equivalent to a car hitting a parked car at (approximately) 120 mph. While it's not quite equivalent to hitting a bridge abutment (which isn't going to "give") at 120 mph, I think it would be much worse than hitting an abutment at only 60 mph.

Yes, the only way to measure the "speed of light" in two different directions (at right angles to one another) at the same time and same place was to use light that goes "out and back". In his original paper on Special Relativity, Einstein clearly stated that he was only assuming that average speed of light "here to there and back to here" was the same in all reference frames.

9. Jun 25, 2008

### Staff: Mentor

Clarification on the last point - I read an extra piece into that question that wasn't actually there. Usually the question is meant to ask: 'because the speed of light is averaged, doesn't that mean it can't really measure the speed of the ether?' The answer is no.

10. Jun 25, 2008

### mtworkowski@o

Last edited: Jun 25, 2008
11. Jun 25, 2008

### Nickelodeon

Yes - if you wanted to test for an aether then it would be worth checking that direction too.

12. Jun 25, 2008

### mtworkowski@o

No Russ I don't know if thats true. If you say it is I'll believe you but it doesn't help the analogy.

13. Jun 25, 2008

### mtworkowski@o

[That's a very common misconception about the MM experiment, and I can never understand why: If you read any halfway decent description, of it, the usual analogy is to compare the MM experiment to measuring the speed of a boat moving sideways across a river -- not up and down the river.

I'll draw the analogy the way I see it and you can clear it up. wind is west to east at 50 mph. Plane is traveling at 150 air speed west to destination and then back. Ground speed out is 200 and back is 100. Average is what? That's the problem these people are seeing.

14. Jun 25, 2008

### mtworkowski@o

Sorry, I was distracted. Fix the directions. you know what I mean.

15. Jun 25, 2008

### Nickelodeon

I don't think they were expecting the average speed of light in the respective directions to be different. I imagine they were hoping to see a wavelength variation. With your plane analogy above, the interference pattern would be apparent. I think that is the general idea.

16. Jun 25, 2008

### Staff: Mentor

And not stationary plus one moving at about 85 mph? Or have I misunderstood "energywise"?

17. Jun 25, 2008

### paw

Why? Just wait 6hrs and the Earth plus apparatus will rotate 90 degrees.

Furthermore the Earth couldn't be moving through the ether in two orthoganal directions at the same time so the two arms of the interferometer are enough to detect an ether if it was there. A third arm is completely unnecessary.

18. Jun 25, 2008

### mtworkowski@o

Borek, what did you say?

19. Jun 25, 2008

### mtworkowski@o

Doesn't an interference pattern indicate a slight change in speed? I'm not getting this I apologize. I think if the leg that's parallel to the aether is containing the light going out and back we have an average. Is there a fringe on this? Oh the leg parallel is reading only incoming. What a dope I am.....still?

20. Jun 25, 2008

### mtworkowski@o

You know, this still sounds fishy.