# The aether

1. Nov 25, 2009

### crx

OK, i imagine that a few ensteiniens of you will roll eyes when they hear...aether! I was thinking about Michelson-Morley experiment and i have some questions. If light is thought to be an perturbation of aether (just like waves on water or sound waves) that their speed should be independent of any aether flow (in case that we suppose that earth is moving through aether, i guess), just like sound waves, their direction and speed its independent of the air movement. So the experiment i guess was not adequate to show any result...

2. Nov 25, 2009

### mgb_phys

But not independant of our movement through it.
If there is a fixed aether with a fixed speed of light relative to it we should see a different speed moving in opposite directions - exactly the same as the doppler effect

3. Nov 26, 2009

### Staff: Mentor

But the direction and speed of sound waves are not independent of the air movement. If this were not the case then it would be very difficult for passengers on a jet to carry on a conversation.

4. Nov 26, 2009

### crx

So if i shoot a focused ultrasound to a target (transducer), in no moving air i will miss the target when the air between the source and target its blowing? I think yes, if the speed of the wind its considerable high relative to the speed of the sound and the source......But how much should be the aether speed (relative to the source) to have a considerable effect on an electromagnetic wave?

5. Nov 26, 2009

### Staff: Mentor

Yes. As a matter of fact, this very technique is used to noninvasively measure the speed of water in pipes.
Well, the speed due to the earth's rotation is about 1000mph -- and that's enough we should be able to detect it.

6. Nov 26, 2009

### Staff: Mentor

That all depends on the sensitivity of your measuring apparatus. That was the whole point of the Michelson-Morley experiment. They had built a very sensitive device. Modern devices are much more sensitive, on the order of 2 cm/s or better.

7. Nov 26, 2009

### cfrogue

what do they measure?

8. Nov 26, 2009

### crx

the phase difference between the two waves....interferometer

9. Nov 26, 2009

### cfrogue

Oh, so this does not measure the contant speed of light?

10. Nov 26, 2009

### Staff: Mentor

Yes it does, the device is constructed so that if the speed of light is not constant any difference shows up in the phase.

11. Nov 26, 2009

### mgb_phys

It compares the relative speed of light in the two arms - very accurately!

12. Nov 26, 2009

### cfrogue

Frequency is a function of wavelength and speed. MMX measures frequency.

To decide that speed has been detemined, one would have to know the absolute motion of an inertial frame to eliminate the wavelength compression as a determining factor.

Modern Physics/Michelson-Morley Experiment
Walter Ritz's emitter theory (or ballistic theory), was also consistent with the results of the experiment
http://en.wikibooks.org/wiki/Modern_Physics:Michelson-Morley_Experiment [Broken]

This rules out any conceptually coherent ballistic theory of light propagation, according to which the speed of light is the vector sum of the velocity of the source plus a vector of magnitude c. Ironically, the original Michelson-Morley experiment was consistent with the ballistic theory, but inconsistent with the naïve ether theory, whereas the Sagnac effect is consistent with the naïve ether theory but inconsistent with the ballistic theory. Of course, both results are consistent with fully relativistic theories of Lorentz and Einstein, since according to both theories light is propagated at a speed independent of the state of motion of the source.
http://www.mathpages.com/rr/s2-07/2-07.htm

Walter Ritz's emitter theory (or ballistic theory), was also consistent with the results of the experiment, not requiring aether. The theory postulates that light has always the same velocity in respect to the source.[6] However it also led to several "obvious" optical effects that were not seen in astronomical photographs, notably in observations of binary stars in which the light from the two stars could be measured in an interferometer. If this was correct, the light from the stars should cause fringe shifting due to the velocity of the stars being added to the speed of the light, but again, no such effect could be seen.
http://en.wikipedia.org/wiki/Michelson-Morley_experiment

Here is the bottom line, MMX does not prove a constant speed of light.

That is why in the early 60's, some folks began to try to eliminate Ritz's theory with experiments from moving light sources since MMX could not do that.
http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html#moving-source_tests

Last edited by a moderator: May 4, 2017
13. Nov 26, 2009

### mgb_phys

True - but do you need an experiment to prove the speed of light is constant.
Don't you believe Maxwell ?

14. Nov 26, 2009

### crx

what about the Sagnac effect ?

15. Nov 26, 2009

### cfrogue

I absolutely believe the speed of light is a constant.
I just understand MMX does not prove it.

This is further proof, MMX cannot measure speed.

The Kennedy-Thorndike Experiment

R.J. Kennedy and E.M. Thorndike, “Experimental Establishment of the Relativity of Time”, Phys. Rev. 42 400–418 (1932).
This uses an interferometer similar to Michelson's, except that its arms are of different length, and are not at right angles to each other. They used a spectacular technique to keep the apparatus temperature constant to 0.001°C, which gave them sufficient stability to permit observations during several seasons. They also used photographs of their fringes (rather than observing them in real time as in most other interferometer experiments). Their apparatus was fixed to the Earth and could only rotate with it. Their null result is consistent with SR.

16. Nov 26, 2009

### cfrogue

I understand it.

17. Nov 26, 2009

### Hurkyl

Staff Emeritus
You shouldn't believe it absolutely!

It never claimed to.

18. Nov 26, 2009

### cfrogue

Yes, I should.

What are the choices.

I know all of them.

It is the case, that most of physics accepts mindless detail on this matter.

I wonder what you do.

Yes, most of physics falsely claims MMX proves a constant speed of light.

Last edited: Nov 26, 2009
19. Nov 26, 2009

### Staff: Mentor

cfrogue, you are more or less correct, although I think you are exagerating. MMX and similar interferometer experiments prove isotropy of the speed of light. This is part of the speed of light being "constant" but certainly not all of it. Other experiments were required and performed. That is why the FAQ has more than one entry.

20. Nov 27, 2009

### TMFKAN64

Even if you believe Maxwell, don't you need to check to the precision that technology allows that he was right?