This document contains comments about the article "Speed of light" in Wikipedia
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• Immediate followed by some comments
In the last paragraph I explain my own opinion.

### Introduction

The article starts with the following sentence.
The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact value is 299792458 metres per second, since the length of the metre is defined from this constant and the international standard for time.
One critical remark: There is a hugh difference the "speed of light in a vacuum" and the more general concept of the "speed of light".
When we measure the "speed of light in a vacuum" we always do that each time under almost the same conditions, implying that each time we get the same answer. To try to measure the "speed of light" in general is a very different exercise. See Reflection 1

Next:
According to special relativity, c is the maximum speed at which all matter and hence information in the universe can travel.
The problem with special relativity is that SR has nothing to do with movements in space in general. When you want to study the behaviour of objects in space you have to use GR because accelerations are involved.
In the theory of relativity, c interrelates space and time, and also appears in the famous equation of mass–energy equivalence E = mc^2
The problem is that in GR the speed of light (=c) is not a constant because c itself is influenced by gravity.
The equation E = mc^2 is already tricky itself because how is mass measured? using SR? using GR?
After centuries of increasingly precise measurements, in 1975 the speed of light was known to be 299792458 m/s with a measurement uncertainty of 4 parts per billion.
It is very important to understand exactly how this measurement is performed in detail because this immediate defines the claim that the "speed of light in a vacuum".

### 2. Fundamental role in physics

The speed at which light waves propagate in vacuum is independent both of the motion of the wave source and of the inertial frame of reference of the observer.
The problem is that the propagation of light is a physical process which has nothing to do with the "state" of any observer. Comparing the same conditions the speed will be the same. The issue is if the speed is always the same. See Reflection 1
This invariance of the speed of light was postulated by Einstein in 1905, after being motivated by Maxwell's theory of electromagnetism and the lack of evidence for the luminiferous aether
The problem is that the speed of light is very difficult to measure.
In principe the concept of an eather has nothing to do with this. In fact all the photons around us also "form" an aether.
It is only possible to verify experimentally that the two-way speed of light (for example, from a source to a mirror and back again) is frame-independent, because it is impossible to measure the one-way speed of light (for example, from a source to a distant detector) without some convention as to how clocks at the source and at the detector should be synchronized.
What do they mean with frame-independent?
The word because is wrong.

### 7.4 "Luminiferous aether"

It was thought at the time that empty space was filled with a background medium called the luminiferous aether in which the electromagnetic field existed.
In fact what people thought was that space around us, which seems to be empty was not empty at all. That there was something.
In fact that still can be considerd as true, because all space is filled with radiation and photons.
In fact there exists an important analogy when you compare our existance with a fish. For a fish the aether is the water in with they live.
Some physicists thought that this aether acted as a preferred frame of reference for the propagation of light and therefore it should be possible to measure the motion of the Earth with respect to this medium, by measuring the isotropy of the speed of light.
You can only do that by performing measurements in one way and no clocks should be used. The use of clocks is tricky because there functioning also depents on the speed of light.
Modern experiments indicate that the two-way speed of light is isotropic (the same in every direction) to within 6 nanometres per second.
Again it is in the details.

### 7.7 Defining the speed of light as an explicit constant

Following is a list with additional information:

### Reflection 1 - speed of light

In relation to the "speed of light" there are two important issues:
• Is the speed of light always the same or can the speed of the photons change?
This is a physical question about the behaviour of the photons, specific about the speed and if there is something that influences this behaviour i.e. speed. For a futher discussion see: Reflection 3 - Speed of light in a gravitation field.
• What is the speed of light?
This is a quatifying question and is much more difficult to answer.
What you want is to know the speed from A to B but what you get in most cases is the average speed going from A to B and back to A.
But also this average speed is not so easy. It is easy but, less accurate, when both A and B are situated on Earth. In that case the distances are fixed. What you get is than a local average speed. It is more complicated when A is on Earth and B is on the Moon. In that case the exact position of both Earth and Moon have to be known in order to calculate the total distance travelled by the light signal.

### Reflection 2 - One way speed of light.

The problem is when you want to measure the speed of light you want to do that in one specific direction. That means from A to B where A is "my position" and B is a distance away in any direction.
To measure a speed you need a fixed distance and two clocks. One clock at position A and one at position B. In order to measure the speed you must measure the duration that the signal goes from A to B. To do that you must observe the time on Clock A when the signal is transmitted and the time on Clock B when the signal arrives at B. The difference is the duration. When you know the duration the speed can easily be calculated.
The problem is in order to do this accurate the two clocks have to run synchrone and simultaneous. Synchrone is not so difficult because it means that you have to use two complete identical clocks. Simultaneous is not so simple because it means that the two clocks should indicate at each instant the same reading.
IMO what that means is that to perform physics you have to define one reference frame as being at rest.

### Reflection 3 - Speed of light in a gravitation field.

In order to study the speed of light in a gravitational field we perform two experiments.

In the first experiment instead of light we use a ball which we drop from a tower and which bounces back.
From experience we already know that when you drop a ball from a tower, that the speed is not constant. The cause is gravity.
In reality we use two balls: A red ball and a black ball.
The black ball bounces back via the groundfloor of the tower.
The red ball bounces back via an extra floor half way the top and the ground floor.
Figure 1 below shows the path of two balls: A red ball and a black ball.
The red line in the middle, identifies half the height of the tower. Here the two paths divide. Figure 1 Both balls are dropped at the same moment, but the height that each ball falls is different. The black ball falls the full height and than bounces back. The red ball drops half the height of the tower, bounces back. This process is repeated once. The black ball is dropped at t0. Initial speed = 0 When the ball is at the bottom the black ball bounces back. The black ball is back at the top of the tower at t1. This terminates a complete jump. The red ball is also dropped at t0. The initial speed = 0 Ball "2" bounces back and is identified with the letter 2. When the ball is half way between the top and the bottom the red ball bounces back. The first jump is finished when the red ball is back at the top. There after the whole process repeats itself The red ball is back at the top of the tower at t2. This terminates a complete jump. Figure 1
What Figure 1 clearly shows that the arrival time of the two balls (indicated as t1 and t2) is different. t1 is first and t2 is later.
This means that the average speed of the black ball is larger than the average speed of the red ball . This difference is explained by the second leg of the black ball i.e. when the black ball is drops near the bottom That means there is acceleration involved and the cause is the force of gravitation or gravity.

In the second experiment in stead of two balls we use a two lightsignals. The question is the behaviour of the two lightsignals (photons) identical as of the two balls?
Lightsignal 1 is reflected at the bottom (with a mirror). Lightsignal 2 is reflected with a mirror at half distance. For Lightsignal 2 the process is repeated once.
The problem is that it is impossible to perform such an experiment in reality. The question is the outcome.

IMO the outcome is that in theory the arrival times are different. The theory is that photons have mass in the same sense that objects have mass. This leads to the same behaviour for both and that the speed of photons is influenced by gravity. This is in agreement with the observation that the path of the photons is not straight but bended close to mass.

### Reflection 4 - Speed of light in a gravitation field experiment.

The purpose of the experiment is to demonstrate that the speed of light is not constant when a lightsignal approaches the earth. The problem is that a distance like the tower of Pisa is much to short to demonstrate this effect. A cavity which different heights in which the light signal bounces back a million times is a better path to follow.

You can also think about a ring laser in vertical direction. See "https://en.wikipedia.org/wiki/Ring_laser_gyroscope#Description. What is important that in both cases the photon source is at the same height above the object (Earth) you want to use. In the first test you place the gyroscope on the floor, the vertical arms are long and the photon source is at the top. In the second test you place the gyroscope high above the floor (but below the top), the arms are short and the photon source is (stays) at the top. Simple arithmatic is enough to decide if the speed is constant, but again accuracy is not enough.
It is important that the source is always at the same height (from the center of gravity) because gravity in principle can already influence the behaviour of the photons at emission.

In a sense what the experiment does it compares the behaviour of two clocks. You have one slow ticking clock and one fast ticking clock. The path of the lightsignal is slow ticking clock is long and in the fast ticking clock is short. To claim that there is any length contraction involved does not seem obvious.

 ``` top /--->---\ | | ^ V | | Source ---> / \----> Detector ---> \ /----> | | V ^ | | \--->---/ Floor ``` The experiment at the left tries to make things simpler. The setup is symmetric. In this setup the source and the detector are at the same height, in the middle. One lightray goes via the floor and one via the top. The experiment involves with two ligtsignals at the source. Those lightsignals can be compared at the detector. The expectation is that the signal via the floor arrives earlier because the speed is higher. The cause is gravity.

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Created: 24 November 2016
Updated: 14 September 2016
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