Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: Interpreting Michelson-Morley

Learn more physics!

Most recent answer: 08/23/2013
I don't understand how the Michelson-Morley experiment was supposed to detect an ether wind in the basement of a building in the envelope of the earth. wouldn't just the atmosphere be enough to deflect such an ether from affecting those things on the surface of the earth? How much more would a building the experiment is done in also deflect such an ether wind? Wouldn't it be better to conduct such an experiment out in space, far from any gravitational bodies (or perhaps all around a gravitational body), yet perhaps dragged through the ether by one of those bodies (via a graphene elevator bridge?) Then we would see the effect outside of the atmosphere, and witness if light was being deflected one way or another to any appreciable degree based on where in (or outside) of the earths gravity, and in relation to the sun, the experiment takes place? It seems so much is based on the results of the experiment, yet I don't think the experiment measured what they claimed it would.
- Jim Holten (age 36)
Albuquerque, NM, USA

Actually, the entire structure of relativity can be derived based on any of a number of different results (relativistic Doppler shifts, time dilation measurements,...). Michelson-Morley played a role in its time. It's been repeated in all sorts of different ways, always giving the constant-speed result. One sort of interpretation you're suggesting was already ruled out before the experiment was done by a variety of "ether-drag" experiments, which fit a specific  "ether-drag" formula that becomes unimportant for low indices of refraction, like that of air. Another type of interpretation you seem to be suggesting, a sort of adjustment of the light propagation rest frame to the frame of large nearby masses, had been ruled out by stellar aberration experiments.

None of this has any bearing on the current status of special relativity, which predicts a variety of effects to extraordinary accuracy. Just to pick one example, as part of quantum field theory it predicts the gyromagnetic ratio of an electron to about one part in 100 billion.

Mike W. 

(published on 08/21/2013)

Follow-Up #1: experiments with sound and light

Have they tried either the Michelson-Morley experiment or the Double Slit experiment with sound to see if they get similar or different results?
- Jim Holten (age 36)
Albuquerque, NM, USA

The double-slit experiment gives similar results with sound and light, so long as you keep the appaaratus for the sound at rest with respect to the medium. With light, there is no medium so you don't have to do that. You can do the sound experiment yourself with a pair of speakers fed the same pure tone. Move your head around and you'll hear the interference pattern. 

The Michelson-Morley experiment is way fancier than what you need to see the difference between sound and light. Take a light source and a viewer on a platform. Set them in "motion". The viewer sees exactly the same source, no matter how fast the alleged motion. Try the same for sound. If the motion with respect to the medium is at the speed of sound in the direction away from the source toward the listener, the listener won't hear anything at all. The  sound can't reach him.  This is what you get for the lead plane if two are flying in line at the speed of sound. It can't hear the trailing plane. There's no corresponding effect for light in a vacuum.

If you do want to see the sound version of M-M, it's been done and published: 

Sound Wave Propagation in a Moving Medium and the Michelson-Morley Experiment
J. L. Morack and T. D. Roberts
American Journal of Physics -- October 1971 -- Volume 39, Issue 10, pp. 1203

spoiler alert: The results are different than for light.

Mike W.

(published on 08/23/2013)

Follow-Up #2: Result of Michelson Morley Experiment

In the Michelson Morley experiment, why did they assume that the ether frame of referrence was at absolute rest. Personally, this experiment proved that there is no frame of reference that is absolutely at rest, but does not proved or disproved the excistance of an ether. They assumed a property of sumthing that could not be measured. When they try to measure it, turns out the assumption was incorrect. So instead of dismissing the assumption they dismiss the ether. Huh? Sorry nothing waves without a medium. Thank you, Very confused.
- Noel Mendez (age 47)
New York, N. Y.

Hello Noel, The sort of aether people believed in was one where the speed of light was constant with respect to the aether.  The combination of Michelson-Morley with other experiments (stellar aberration, etc. ) essentially ruled out any aether with that property. Now you could always say that there is an aether whose existence has no experimental implications, but that's just playing with words.

On the other hand, you could say that the principle of relativity and Maxwell's equations are themselves general laws. From that you draw a variety of detailed conclusions not only about electricity, magnetism and dynamics, but also about necessary features of things that were not even heard of then: the chromodynamic and weak nuclear forces, quantum spin, ... These predictions turn out to all be true, including a prediction of the electron magnetic g-factor to better than one part per trillion.

So we know what we mean by the principle of relativity and what it implies and we can test the implications. They pass the tests. What does it mean to say there's an aether? 

 Lingyi and Mike W.

(published on 04/21/2013)

Follow-up on this answer.