Date: Summer 2013
Our physics teacher last year, while teaching a unity on light and electromagnetic radiation, brought up the subject of the luminiferous aether and how it has been thought to not exist and such. But it made me think: If light travels nearly the same exact speed everywhere throughout the universe, should that not point to the existence of such a medium? Much like how the speed of sound in air does not differ very much (unless of course you drastically change elevation).
Thanks for the question. One of the postulates of special relativity is that the speed of light is constant in all reference frames. The speed of light can actually vary quite a bit depending on the material through which it passes. The Michelson-Morley experiment was the best evidence for the lack of any aether. I encourage you to read more about this interesting experiment.
I hope this helps.
Would it suggest a uniform medium, or would it only suggest that propagation conditions are the same throughout the universe. What could be more uniform than a vacuum?
Richard E. Barrans Jr., Ph.D., M.Ed.
Great question, but your argument actually does not take into account motion with respect to the aether. If the aether is motionless everywhere, then we would expect light to travel the same velocity as the conditions everywhere are the same in this case. How is this possible though? Earth has motion through the aether, unless of course the aether is somehow motionless around the earth, i.e., the aether is dragged by the earth. Move away, though, and what about the motion of the sun? The other stars with respect to one another? The galaxies? Somewhere you will find relative motion to the aether, and in this case, if the aether is the conducting medium, light must be dragged by it. If you are moving in air, and you measure the time it takes sound to get from one location to the next will be different if you measure it in different directions. If you are going towards the sound, it will take less time to travel from your front to your back than if you are standing still. “But,” you say, “I see that in the air its speed stays the same. I take the air speed into account.” Here is where the light argument breaks down. How do you measure the speed of the aether? You do it by measuring the time it takes light to go from point A to point B. If you are pointed in the direction the aether is traveling, light will move faster than if you measure in the opposite direction. Light should be dragged, in other words, by the “aether wind.” Experiments to see this change in speed were originally performed by Michelson and Morley and have been performed many times in different situations since the original experiment. Somewhere, if the aether exists, these experiments should have seen a shift in light speed. None has been observed. You could argue that the aether exists but that it does not affect light, but simply carries it. If it has no effect (measurable that is), than why do we need it? This was Einstein’s argument. No need to postulate an aether when it cannot be proven.
Kyle J. Bunch, PhD, PE
The basis of the Michelson-Morley experiment was that the earth was moving through the ether.
It would be analogous to shouting into a wind and then shouting across the wind. The apparent speed of sound would be less shouting into the wind that shouting at 90 degrees to the wind.
unfortunately, M&M found no difference in velocity of light regardless of direction.
The only way the ether could exist and NOT show this directional asymmetry would be if the ether was moving exactly at the same speed and direction as earth's movements. That is highly unlikely, so we reject the existence of the ether.
Hope this helps.
If I understand what you (and your teacher) mean by the term luminiferous aether?, ?it does not exist. There are several counter experiments that support its non-existence. Some examples:  The speed of light IS constant everywhere in the Universe, so far as we know.  Unlike sound and mechanical waves, and electrons in a wire, all of which require a medium to transmit. Light does not. Have this limitation.  The photoelectric effect, which by the way won Albert Einstein a Nobel prize, is inconsistent with the existence of a ?luminiferous aether?. You can find the photoelectric effect explained on any number of websites.  If light required a medium to transmit, its speed should differ if you shine a light in the direction, and the opposite direction of the Earth?s rotation (Doppler effect). Careful numerous experiments have shown this is not the case.
The speed of light has a maximum valve in a vacuum (300 million meters per second) but can be slowed while passing through another medium, such as water or glass. Light travels the speed it does because it has no mass. The theory of luminiferous aether has been replaced in modern physics by special relativity and quantum mechanics.
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