By Don Lincoln, Fermilab
Physicist James Clerk Maxwell’s equations predicted the speed of light with respect to the luminiferous aether. It meant that we might be able to measure the motion of Earth through the aether. It brought us to an effort in 1887 when physicists Albert Michelson and Edward Morley performed what is now known as the Michelson-Morley experiment. What was it?
When Thomas Young sent light through two different slits, it made a series of bright and dark spots on a distant screen. Whether the spots were bright or dark depended on a shift between the waves coming from each of the two slits, which physicists call a phase shift. If the peak of one wave coincided with the peak of the other wave, the result was a bright spot. If the peak of one wave coincided with the trough of the other wave, the result was a dark spot. The important thing is the shift between two waves.
So, Albert Michelson and Edward Morley decided to do a similar thing, although the details were pretty different. They conducted the Michelson-Morley experiment at what is now Case Western Reserve University in Cleveland, Ohio. They started with a beam of light that they shot at a half-silvered mirror. A half-silvered mirror reflects half of the light and lets the other half pass through.
The half-silvered mirror was positioned at a 45-degree angle with respect to the beam. That means that half the light passed through in a straight line, while the other half was reflected at an angle of 90-degrees compared to the original light. After passing through the half-silvered mirror, the two beams were travelling at right angles to one another.
They then placed two full-strength mirrors to reflect those two beams of light back to the half-silvered mirror. The reflected beams both passed through and were reflected by the half-silvered mirror and this recombined the two beams onto a screen which they could see.
Similar to Thomas Young’s experiment, depending on whether the two beams of light arrived in phase, with the peaks enhancing one another, or out of phase, with peaks and troughs cancelling, the result would be either a bright spot or a dark spot.
It is important to note that this experiment was extremely sensitive to vibrations. After all, the wavelength of light is less than a millionth of a meter—less than a thousandth of a millimeter. Vibrations that would make one light path a tiny bit longer or shorter would cause the pattern to flicker between bright and dark spots. In fact, their equipment was so sensitive that someone stomping their foot at a distance away the length of a football field would ruin their measurements.
Hence, to minimize the vibrations, they not only put their apparatus underground but they also mounted their entire apparatus on a heavy sandstone block and floated the block and their equipment in a pool of mercury. Their apparatus did suppress vibrations and, because the sandstone block was floating in a pool of mercury, one could also spin it, making this a critical feature.
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Aether and the Speed of Light
While this experiment was being conducted, it is important to remember that the thinking of the time was that aether permeated the entire universe. But we knew that the Earth rotates and orbits the Sun and it was likely that the Sun moves through the cosmos. That meant that it was very probable that the Earth moves through the aether. Alternatively, the aether could be thought of as a kind of a wind blowing over the Earth.
Because the Michelson-Morely apparatus shot two beams of light at 90-degrees with respect to one another, it meant that they could rotate their apparatus in such a way that one arm was in the direction of the wind, while the other was perpendicular to it. And, if the speed of light is defined with respect to the aether, in Michelson and Morley’s lab, light would travel at different speeds through the two arms. Furthermore, if light travelled at different speeds, it would affect the phase of the waves passing through the two arms, and that would affect the expected pattern on the screen.
Killing the Idea of the Aether
Of course, the experimenters didn’t know the direction the aether was passing through the Earth, but here’s the beauty of their experiment. They could rotate their apparatus, moving the arms to be parallel to the movement of the aether or perpendicular. And, even if they didn’t know which was which, they would expect the speed of light between the two arms to change as they rotated it.
It meant that they should expect to see changes in the patterns of brightness and darkness on the screen. If they saw these changes, they would have also discovered the aether.
So, what did they see? In a moment totally worth a drum roll, what they found was, exactly zero. Nothing. There was no change at all in the pattern of light and dark seen on their screen. And the only realistic explanation for this is that the velocity of light in both paths didn’t change with the orientation. And, from that, it logically follows that the Michelson-Morley experiment killed the idea of the aether.
Now, the idea of the aether didn’t die immediately. People thought of possible explanations, ones like the idea that the Earth was dragging along the aether as it orbited the Sun, so the aether in the vicinity of the Earth was stationary as far as the Earth was concerned. But the Michelson-Morley experiment was certainly the beginning of the end.
A series of researchers thought about a variety of possible explanations, but slowly, one by one, they were all falsified until we were left with Einstein’s conjecture, which is that the aether doesn’t exist and that all observers measure light as travelling at a single and unique speed, which is about 300,000 kilometers per second.
Common Questions about the Michelson-Morley Experiment
The Michelson-Morley experiment was conducted at Case Western Reserve University in Cleveland, Ohio.
To minimize the vibrations, Albert Michelson and Edward Morley not only put their apparatus underground but they also mounted their entire apparatus on a heavy sandstone block and floated the block and their equipment in a pool of mercury.
Albert Einstein’s conjecture was that the aether doesn’t exist and that all observers measure light as travelling at a single and unique speed, which is about 300,000 kilometers per second.