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 interferometer is 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 on the order of 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, or any waves, 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 instant to swing higher. 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 alternates between moving forward and moving backward. The phase of the pushing alternates 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 are then 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. It measures the difference in the speed of light between two beams.

In the 1800s, light was thought to be a wave in some sort of medium. By analogy, sound is known to be a wave in air and other mediums. The words Luminiferous Æther were intended to mean light-bearing medium. It was presumed that the speed of light through this medium was fixed. If such a medium existed, it was therefore reasoned that by combining the speed of light measured on Earth and the known speed of the Earth in the solar system, then the speed of the Luminiferous Æther in the solar system could be deduced.

Michelson calculated that the difference in the speed of light would be a detectable if the two arms of his interferometer were oriented so that one arm moved parallel to the Æther motion and the other moved perpendicular to it. He and Morley built such an interferometer and tested it in 1887. Based on all the assumptions involved, Michelson concluded that the small readings they obtained indicated that there was no such æther.

The interferometer used in my experiment has the basic configuration as the original Michelson-Morley apparatus. The most important difference between that interferometer and mine is that the light path in my interferometer is formed by optical fiber rather than the mirrors used in the original.

I claim that the mass density in the light path is a key condition determining the outcome of a Michelson-Morley experiment.

The difference between conventional experimental results and my results is explained by the difference between their mass density parameter and my mass density parameter. They chose a mass density near-zero, and I chose the mass density of glass, the core of optical fiber. Given that the claimed, previously unrecognized, experimental parameter is different, one should allow that the results might be different. This means that Einsteins relativity and the associated experiments and observations remain valid when the mass density is low, as it is in a vacuum.

This description is a great oversimplification. For one thing, there is a second previously unrecognized parameter. Also, various assumptions related to the expected and actual results need reexamined.