This document contains comments about the document "One-way speed of light" in Wikipedia
• 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.
When using the term 'the speed of light' it is sometimes necessary to make the distinction between its one-way speed and its two-way speed.
What is important to understand that light propagation is a physical process independent of how it is measured.
The second issue is to understand exactly how the speed is measured.
The "one-way" speed of light from a source to a detector, cannot be measured independently of a convention as to how to synchronize the clocks at the source and the detector. What can however be experimentally measured is the round-trip speed (or "two-way" speed of light) from the source to the detector and back again.
The question to answer is if the two-way speed of light is the same as the speed of light. Part of the problem is the distance between source and reflector and between the reflector and the detector(source).

1. The two-way speed

The two-way speed of light is the average speed of light from one point, such as a source, to a mirror and back again.
Okay. See next remarks.
Because the light starts and finishes in the same place only one clock is needed to measure the total time, thus this speed can be experimentally determined independently of any clock synchronization scheme.
There are three problems which makes this experiment complex:
1. The fact that there are three points involved which can all move in space. (source, mirror and back)
2. The exact length of the light signal travelled. The easiest situation is when the distance between source and mirror and between mirror and back is fixed.
3. The definition of: what is a clock i.e. a second
Many tests of special relativity such as the Michelson–Morley experiment and the Kennedy–Thorndike experiment have shown within tight limits that in an inertial frame the two-way speed of light is isotropic and independent of the closed path considered.
What the Michelson-Morley experiment demonstrates that at the surface of the earth when you observe the arriving events of two (perpendicular) signals of the same length that the arriving times are the same. The explanation requires that length contraction is involved. See also Comments: Michelson-Morley experiment

2. The one-way speed

Although the average speed over a two-way path can be measured, the one-way speed in one direction or the other is undefined (and not simply unknown), unless one can define what is "the same time" in two different locations.
The one-way speed of light is based on the time difference between two events and the distance between the two events.
The two-way speed of light is based on the time difference between two events and the total distance between the two events.
To measure the time that the light has taken to travel from one place to another it is necessary to know the start and finish times as measured on the same time scale.
By preference all with the same clock.
This requires either two synchronized clocks, one at the start and one at the finish, or some means of sending a signal instantaneously from the start to the finish.
Instantaneous signals are physical impossible. This part should be removed.

3.1 Einstein convention

This method synchronizes distant clocks in such a way that the one-way speed of light becomes equal to the two-way speed of light.
The concept used is "Length Contraction" See: Comments: Length Contraction See: Comments: Einstein synchronisation

3.2 Slow clock-transport

It is easily demonstrated that if two clocks are brought together and synchronized, then one clock is moved rapidly away and back again, the two clocks will no longer be synchronized due to time dilation.
Better:
It is easily demonstrated that starting from two identical clocks if one clock is moved rapidly away and back again, then the time on this clock runs behind they stayed at home clock. This is called time dilation.
If however one clock is moved away slowly in frame S and returned the two clocks will be very nearly synchronized when they are back together again.
Better:
If this is done slowly then the time difference is less.
 ``` ^ | 2---------3---------2 | . . . . X | . . . . | . . . . |. . . . 1-----------------------------1 time Figure 1 ``` The picture at the left shows three clocks. The vertical line is the X axis. The line 1 - 1 demonstrates clock 1. This is the stay at home clock. The line 1 - 2-3-2 - 1 shows clock 2. Clock 2 is a fast moving clock. The line 1 - 3- 1 shows clock 3. Clock 3 is a slow moving Clock. Time Dilution predicts that when they return the time difference between clock 1 and 3 is small and between clock 1 and 2 is large.
Figure 1 assumes that the distance between clock 2 and clock 1 is constant.
Thus the measured value of the average one-way speed is dependent on the method used to synchronize the start and finish clocks.

8.2.2 Aether theories

For an observer in motion with respect to the aether, this would result in slightly different two-way speeds of light in different directions. I
In 1887, the Michelson–Morley experiment showed that the two-way speed of light was constant regardless of direction or motion through the aether.
The Michlson-Morley experiment indicates that in order to explain a two-way speed of light experiment Length Contraction is involved, that the speed of light is independent of the observer and that there exists no aether wind.

8.2.3 Preferred reference frame

A preferred reference frame is a reference frame in which the laws of physics take on a special form.
this seems to me a very unclear definition The laws of physics are generally speaking descriptions of experiments. The behaviour (results) of experiments have nothing to do with a reference frame. A ball on earth always falls down indepent if you are in a train at rest or in a moving train. To describe the behaviour you should use the largest possible frame i.e. a global frame.
That means not from a moving frame but from a frame from which the train is moving.
Next:
The ability to make measurements which show the one-way speed of light to be different from its two-way speed would, in principle, enable a preferred reference frame to be determined.
And suppose that that is true, which law of physics has now a special form? (or laws)
Next:
This would be the reference frame in which the two-way speed of light was equal to the one-way speed.
This whole paragraph is very unclear.

May be the following text is better:

A preferred reference frame is a reference frame in which the one-way speed of light is equal as the two-way speed of light.
Accordingly to Einsteins it is not possible by any experiment make this distiction.

Reflection 1 - General

When you want to discuss the One-way speed of light the first question to answer is if the speed of light at your location is the same in all directions. One important issue is that the speed of light is the speed of the photons, the supposed building blocks of electro magnetic radiation. This speed is indepent of the observers involved.

What is also important is that if this speed is the same in all directions local at your location and independent of your speed. It does not mean that it is the same everywhere in the universe.

The first question to answer is what is the simplest experiment to establish that this speed is constant in all directions.

• One answer could be that this is impossible.
• A different answer is that this is true by definition
IMO the best strategy is the following:
1. Starting point are three identical clocks
2. Clock 1 stays at home. Clock 2 you move very slowly to the right a fixed distance.
To use Time dilation (Special Relativity) ad hoc in this experiment is wrong.
The next question is to quantify this constant speed.

Reflection 2 - The importance

When you want predict the position of the galaxies, stars and planets your starting point is a grid in which you draw the positions and speeds of a certain moment. In general all these positions are based on One-way type of observations assuming that the speed of light is the same throughout the Universe.

The center of the grid should be in the center what you want describe. IMO for all the positions and velocities using Newton's Law no length contraction nor Time dilation is involved.
The first thing you have to do is to calculate the masses of all the objects involved. Those masses are the masses at rest even when all the masses are in movement.

Reflection 3 - (Absolute) reference frame

An absolute reference frame (There exist only one) is the reference in which (absolute) simultaneous events are possible.
In such frame (at rest), the clock rate is the fastest.
In any other frame the clock rate is slower.

The problem with "one-way-speed-of-light" is that always when you want to perform such a measurement two clocks are involved. Those two clocks have to be synchronised and in order to do that a light signal has to be used. This means that the operation of tools you need are dependent about what you want to measure and that is not correct.

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Created: 25 January 2015

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