• The text in italics is copied from that url
• Immediate followed by some comments
In the last paragraph I explain my own opinion.

### Introduction

The article starts with the following sentence.
It attempted to detect the relative motion of matter through the stationary luminiferous aether ("aether wind").
The problem is that when something is not clear than it is very difficult to describe. That is the case with every that is called eather and "eather wind".

### 1. Detecting the aether

Earth orbits around the Sun at a speed of around 30 km/s. The Earth is in motion, so two main possibilities were considered: (1) The aether is stationary and only partially dragged by Earth, or (2) the aether is completely dragged by Earth and thus shares its motion at Earth's surface.
• The problem is we also have something what is called Earth atmosphere which is dragged by the earth.
• What the eather concept more or less claims that space is not empty i.e. there exists no vacuum.
• To make things more complicated we now consider that there is also something what is called the Higgs field.

### 2.1 Michelson experiment (1881)

They then recombined on the far side of the splitter in an eyepiece, producing a pattern of constructive and destructive interference whose transverse displacement would depend on the relative time it takes light to transit the longitudinal vs. the transverse arms.
Which again is a function of a speed v. When v=0 this displacement is constant, I assume.

### 2.2 Michelson–Morley experiment (1887)

Because of the motion of the Earth around the Sun, the measured data were also expected to show annual variations.
This idea is clever.
When the speed of light is constant in all directions throughout the universe it should be a chalenge to detect our speed relative to the speed of light.

### 3.1 Observer resting in the aether

This is a very unlucky header because there is no aether.
The description is much more relevant for an observer at rest from the center of the earth.
Light is sent from the source and propagates with the speed of light c in the aether.
This is again a very unlucky sentence because the word aether is used.
A very important issue is how the speed of light is measured or calculated.
The reflecting mirror is at that moment at distance L (the length of the interferometer arm) and is moving with velocity v.
What is this speed v? How is it measured.
At this time, the mirror has traveled the distance v*T1. Thus c*T1 =L+v*T1 and consequently the travel time T1=L/(c-v).
That is all correct in an "abstract" frame in which the Observer is at rest and in which the speed of light is the same in all directions. In that frame for a light source which moves at a speed v these equations are correct. The problem is for a real earth based situation the speed v is unknown.

### 3.2 Observer comoving with the interferometer

This is an observer very close to the interferometer at the top of the surface of the earth.
This is a rotating reverence frame.

### 3.4 Length contraction and Lorentz transformation

According to this law all objects physically contract by L/gamma along the line of motion (originally thought to be relative to the aether), gamma=1/ sqrt{1-v^2/c^2} being the Lorentz factor.
• What is the definition of an object.
• What is the definition of the speed v
• How is the speed of c measured.
The problem is that all objects is a too general concept. The earth is also an object.
The issue is not so much the Lorentz factor. First of all you have to demonstrate that length contraction is a physical process. See also Reflection 2 - The simplest experiment to detect Length Contraction

### 3.5 Special relativity

Albert Einstein formulated the theory of special relativity by 1905, deriving the Lorentz transformation and thus length contraction and time dilation from the relativity postulate and the constancy of the speed of light, thus removing the ad hoc character from the contraction hypothesis.
• Einstein assumed that the speed of light has the same value in each reference frame which has a constant speed relative to an other reference frame.
• In such a moving reference frame clocks tick slower and there is length contraction. The result of the two is that the speed of light is the same.
• IMO that does not mean that the speed of light is constant in one reference frame.
This allows a more elegant and intuitive explanation of the Michelson-Morley null result.
Understanding has nothing to do with elegancy. A better word could be simpler.
Understanding also comes by starting from a simple case with a minimum number of variables or parameters and explaining which changes are observed. By introducing more variables more complex cases can be treated.
We should be carefull that explaining becomes the solely use of mathematics.

### 3.6 Incorrect alternatives

Also terrestrial tests using particle accelerators have been made that were inconsistent with source dependence of the speed of light
The issue is not so much source dependency of the speed of light but source (frequency) dependency about the speed of the source.
In a comoving frame the null result is self-evident, since the apparatus can be considered as at rest in accordance with the relativity principle, thus the beam travel times are the same.
You first have to prove what the relatity principle is based on experiments.
In a frame relative to which the apparatus is moving, the same reasoning applies as described above in "Length contraction and Lorentz transformation", except the word "aether" has to be replaced by "non-comoving inertial frame".

### Reflection 1 - general

What can you learn from the Michelson-Morley experiment?
The idea behind the MM experiment was that based on "a model" the predicted outcome of the experiment should be that the position of the interference pattern should change as a function of the speed of the apparatus.
The result of the experiment was negative in the sense that the interference pattern was constant.
The leaves two options:
• The model is wrong. period
• You can modify the model, such that the result of the experiment is in agreement with the "new" model
This is what happend. By introducing the Lorentz factor the model was saved.
The Lorentz factor introduces physical length contraction.
The question to answer is: what is exactly physical changed.

Maybe a whole different explanation of the Michelson-Morley experiment is possible.
When you perform the experiment in an absolute restframe the outcome of the experiment should be negative. The reason is simple because in this rest frame both the speed of light and the frequency of the monochromatic light used should be the same.
What happens if you move the whole apparatus towards the right in such a situation?
One effect will be that the frequency of the monochromatic light will change in the direction of movement.
If this change is the same as the difference in length in the two directions than the netto result will be zero.

### Reflection 2 - aether

The Michelson-Morley experiment is supposed to demonstrate that there is no aether. The aether is something invisible like water for a fish which influences the propagation of light. As such the MM experiment demonstrates that light propagates in all directions with the same speed. However that does not mean that there is not something like an aether, however it does not influence the speed of light.
The question is what are photons? Photons are IMO packages of energy which propagate through space. The question is energy of what? I would say package of something that you can call aether. This aether is all around us and invisible to us in the sense that we cannot detect it. It is the same (of course not completely) as water for a fish. The biggest problem difference is that it does not move (any way that is what we assume) and it is at rest. Maybe throughout the whole Universe.

This is only a thought.

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Created: 18 January 2015

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