The Fiber-Optic Michelson-Morley Experiment (FOMMX) is based upon an interferometer. There are many types of interferometers, and they are applied to many measurement tasks in physics and elsewhere. This article focuses on the interferometer that is used in this experiment.

An instrument in which the interference of two beams of light is employed to make precise measurements. Precise means a fraction of the wavelength of light. The wavelength of light used in FOMMX is one micron, $10^{-6}$ meters.

The word interference is used in describing the combination or superimposition of two beams of light. The terms constructive interference or destructive interference describe whether the two combining light beams are in phase or out of phase with each other.

Here's a common activity that can be described in terms of phase. Consider a bigger person pushing a child on a swing. The person does not push constantly, except perhaps when starting the swinging. Once the child is swinging back and forth, the person pushes in the same direction that the swing is moving at that instance. The child and the person are not beams of light, but it does illustrate the meaning of the word phase. The phase of the swinging cycles between moving forward and moving backward. The phase of the pushing cycles between pushing forward and not pushing at all. If the pushing forward coincides with the swing moving forward, then the swinging motion increases. In this case the pushing and swing motion are in phase and the height of the swinging increases due to the constructive interference. If the pushing forward coincides with the swing moving backward, then the swinging motion decreases. In this case the pushing and swing motion are out of phase and the height of the swinging decreases due to the destructive interference.

In a Michelson interferometer, a beam of light is split in two by going through a half-silvered mirror. The two beams travel along two different paths and then are recombined. Suppose the paths are exactly the same length. (It is no small feat to make this happen.) Then the two beams are in the same phase when recombined and the combined beam is bright. Suppose one beam goes through warmer air that causes it to speed up a little. Then the phase changes and the light becomes dimmer.

In the photo-finish of a horse race, the photo shows how far back the second place horse is behind the winner. Note that it does not tell how fast either horse is going, only how much more ground the winner covered than the second horse.

This is a direct analogy to the interferometer. It does not measure the speed of light, only how much faster one beam is compared to the other beam.

The phrase “interference of two beams of light” could be restated as “the combination of two beams of light”.

The Michelson interferometer The Michelson-Morley interferometer is a Michelson interferometer …

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• Descriptions of interference
  • Pedaling a Bicycle,
    • apply forces in phase, and you go nowhere if the forces are out of phase on the pedals when you go forward.
    • If you can move the chain and pedal and wheel in both directions, if you have the reverse phase the bicycle goes backwards.
  • Paddling a boat.
    • If you pull on both the panels at the same time, you go forward for a moment. When you lift the paddles from the water for another stroke, you coast.
    • The net process is going forward.
    • If you call one paddle back and push the other one forward, and then reverse with the paddles out of water, you won’t go forward it but your turn in the circle.
    • x.
    • If you pull on the paddles out of phase with each other, then you’ll turn a little bit to the right and go a little bit forward and then you’ll turn a little bit to the left and go a little bit forward.
    • If you pull on the paddles out of phase with each other then you’ll turn a little bit to the right and go a little bit forward,
    • and then you’ll turn a little bit to the left and go a little bit forward.
    • y.
    • Also, you’ll be using your arms only, not your bag. If you pull in sync when you’re using your whole body, especially your back and even legs.
• There is a similar situation for electrons in an atom and, which by the way are standing waves, and electrons moving in space which are traveling waves. The quantity of charge is fixed by Schrodinger's wave mechanics. The electrons traveling in space might not necessarily have a charge fixed by the wave, but it might actually be the other way around that the charge of the electron somehow controls the traveling wave parameters.

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