In 1887, two physicists created an experiment to measure the Earth with respect to the so-called "preferred" reference frame that Maxwell's equations seemd to suggest.

Albert Michelson and Edward Morley's experiment was asking whether the transit time would be equal for the two roundtrips of the light beams in their experiment (See below for a description of the experiment).

They measured this time for the light beams by recombining the beams when they arrived back at their starting point. The light waves would then be superimposed, one on top of the other. If the light beams had different travel times, then their phases would be out of line when recombined. Light waves with different phases produces a pattern of light and dark when viewed by an inferometer (See below).

Interference "fringes" showing no change as the interferometer is rotated. (G. Joos, Lehrbuch der Theoretischen Physik, Akademische Verlags., Leipzig, 1930)


George Fitzgerald was the first physicist to accept the idea of the experiement, he wrote:

"I have read with much interest Messrs. Michelson and Morley's wonderfully delicate experiment....Their result seems opposed to other experiments....I would suggest that almost the only hypothesis that can reconcile this opposition si that the length of material bodies changes, according as they are moving through absolute space or across it, by an amount depending on the square of the ratio of their velocities to that of light."

But when the experiment was completed, they observed no difference in the light travel times. They tried the experiment many more times and at different times of the year, but eventually conceded that light's speed is the same for any direction. Even with their experimental limitations recognized, the speed of light had to be equal in both directions. This result was important because it meant that Newtonian physics were valid -- there was nothing wrong its foundations. Physicists could trust Newton's laws and not worry about there being a preferred reference frame that might make their measurements and calculations, and therefore theories, invalid.


The Michelson-Morley Inferometer was an experimental setup in which a beam splitter split a light beam into two parts. One half moved in one direction, struck a mirror, and then was reflected back to another mirror. The other half of the beam did the same thing, but in a different direction.

The setup they used could be rotated. Two different arms (which the light beam halves traveled) could be moved in the direction the Earth's motion. So the apparatus had a light source, a beam splitter, and several angled mirrors to reflect the light back.

The original apparatus was mounted on a block of stone that floated in a mercury pool. They could rotate the block to see changes relating to the Earth's motion in orbiting the sun. One set of light beams was set to travel parallel to the direction of the Earth through space, while another set of light beams was set to travel cross this motion.


James Maxwell had a theory showing that electricity and magnetism were actually only part of one electromagnetic force. He was then later able to show that electromagnetic radiation was actually light itself. But his theory and equations were dependant on the speed the light travels. But velocity is relative, so his equations were not invariant -- they were relative.

Yet the lack of invariance did not bother physicists of the time, at first that is. The early reasoning was that light traveled through a medium, the luminiferous ether. They reasoned this because waves in matter required a medium in which to propagate (move). But one should keep in mind that this is not the same as Aristotle's ether. This luminiferous ether apparently only existed to provide the expected medium for light; it had no mass and could not be seen. It was wondered why the ether should exist for so special or specific a function, becuase air does not exist solely for sound waves to move through. But at the time, the view of waves was so set that no other alternative was taken seriously.

Electromagnetic waves, traveling disturbances in the electromagnetic spectrum, do not need to move through anything to propagate (see previous Page). Because electromagnetic waves share the same nature, they differ only by wavelength. Groups of frequencies, or bands, divide the spectrum. Radio waves occur at low frequencies, and then moving to the higher frequencies are microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and at the very shortest frequency, gamma radiation. But the only reason these waves have different names is because of their separate discoveries before it was known they were of the same nature. The band division has so signifigance. Visible light, that which we can all see, is not fundametally different from any of the other waves.

The speed of the motion of waves in a vacuum is the speed of light. All electromagnetic waves move at this speed, but only in a vacuum. When moving through a medium of some sort, a group of electromagnetic waves will travel through the medium at different speeds. This is called dispersion.

All the wavelengths in the visible band of the electromagnetic spectrum compose white light. When white light passes through a prism, the different wavelengths move at slightly different speeds through the glass and make them refract differently at the two surfaces they cross. The prism splits the white light into its "colored" components.




But...

Under Galilean transformations, however, Newton's laws of an object's motion are invariant (please see the link if you need a recap of invariance and transformations). All frames are equal and there is no specific rest frame. The transformations are supposed to link observations in one frame to how the observations should look in another frame. Yet, with the transformations, the equations associated with electromagnetism are not invariant. The speed of light in electromagnetism is not an invariant quantity / measurement.

Maxwell, the creator of the electromagnetism equations, thought there should be a specific frame where his equations would be correct, corresponding to the idea of the specific rest frame. If so, then all measurements in any other frame would be related to the rest frame because an observer would have to recognize the relative motion between the rest frame and whatever other frame the measurement are being taken from.

But, again, the Michelson-Morley experiment could not detect or find evidence for motion with respect to a specific rest frame. In the absence of this frame, the speed of light didn't have a special frame. Two possibilites that physicists didn't want to consider arose:
But the problem still existed that Newtonian equations were just as successful at explaining mechanics as Maxwell's equations were at explaining electromagnetism.

Explaining the Result

In 1889, George F.FitzGerald made one of the first attempts to explain the Michelson-Morley result. His theory was that an object moving through a frame would shrink in the direction of its motion because of the ether. If an arm on the inferometer was shortened in the direction parallel to the Earth's motion, the travel distance for the light moving that direction would be shortened by the amount needed to make up the change in the speed of the light's propagation.

If this was done, the trip time for both light beams would be equal, and not produce the pattern of light and dark fringes. But there was no theory explaining why objects would contract this way. One hypothesis was that because intermolecular forces are electromagnetic in nature, then maybe matter's structure was affected by motion.

But a lot of scientists clung to a conservative view, that moving objects drag the ether along with them -- near the Earth's surface then, no relative motion and drag would be detected. Part of this theory was due to the knowledge that light moving through a moving fluid was different than if the fluid was at rest. Some of the fluid's velocity taken up by light. But if light is dragging the ether, it should lose energy and slow down in orbit, ultimately falling into the sun. Yet no effect like this was observed.

Ernest Mach had a braver proposal for the Michelson-Morley result. He said that no ether-relative motion was seen because the ether didn't exist. Because the ether was not detected after an elaborate experiment to test its existence, he said, then the theory was proved wrong.

With Fitzgerald's theory, there was not any known force that could make a moving object shrink along its direction of movement. Hendrick Lorentz believed the result of the Michelson-Morley experiment, but at the same time he took Fitzgerald's suggestion of contraction seriously. Fitzgerald, Henri Poincare, and Joseph Lamour reexamined electromagnetism and found something peculiar. If Maxwell's equations were expressed by electric and magnetic fields measured at rest, the equations took on a simple, pleasing mathmeatical form. However, if the equations were expressed in terms of different fields experienced by someone moving through them (the ether), then the equations took on a more complicated appearance. The laws or equations could be made to appear simple by imagning that all moving objects shrink by the amount Fitzgerald's theory suggested. This thought woulde plain the Michelson-Morley result. But this wasn't enough, because to make the equations simple, it would also have to be imagined that time moves more slowly as well.











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