## Comments about "Special relativity" in Wikipedia

This document contains comments about the article Special relativity 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.
In Albert Einstein's original pedagogical treatment, it is based on two postulates:
1. The laws of physics are invariant (i.e. identical) in all inertial systems (non-accelerating frames of reference).
2. The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.
The issue is how important are these postulates if you want to describe the evolution of the Universe using one reference frame?
Physical processes have nothing to do with a particular reference frame, as such you should select for a frame in which on average the process is at rest.
Special relativity implies a wide range of consequences, which have been experimentally verified, including length contraction, time dilation, relativistic mass, mass–energy equivalence, a universal speed limit and relativity of simultaneity.

### 2 Lack of an absolute reference frame

The principle of relativity, which states that physical laws have the same form in each inertial reference frame, dates back to Galileo, and was incorporated into Newtonian physics.
This sentence seems to indicate as it is wrong when you try to understand the processes that take place in the Universe when you only use one reference frame.
The aether was thought to constitute an absolute reference frame against which speeds could be measured, and could be considered fixed and motionless.
In order to measure a speed of something you need a reference frame and a clock. The concept of an eather is not a requirement.
Einstein's solution was to discard the notion of an aether and the absolute state of rest. In relativity, any reference frame moving with uniform motion will observe the same laws of physics.
The whole issue is if two 'identical' processes in relatif motion behave the same.
In particular, the speed of light in vacuum is always measured to be c, even when measured by multiple systems that are moving at different (but constant) velocities.
The whole issue is how the speed of light is measured.
Consider an observer who emits a flash of light. This flash is supposed to propagate in a sphere. How is this measured?
Consider a second observer which at the same when the first observer emits his flash passes with a speed v. How does this observer measures the speed of light?

### 3.1 Reference frames and relative motion

Reference frames play a crucial role in relativity theory.
Reference frames play a crucial role in any theory.
At the same time it also raises a serious question: Why do you need reference frames?
In addition, a reference frame has the ability to determine measurements of the time of events using a 'clock' (any reference device with uniform periodicity).
It is very important to describe what a clock is. A simple definition is a device which emits light flashes between two parallel mirrors.
Since the speed of light is constant in relativity in each and every reference frame, pulses of light can be used to unambiguously measure distances and refer back the times that events occurred to the clock, even though light takes time to reach the clock after the event has transpired.
The first problem is that the path of a light ray is not straight but bended which makes it more difficult to define the origin.
The second reason why this is complicated, is because when I receive a light signal I can not define its origin. Only by using concepts like doppler shifts or standard candles I can specify the origin better.

### 3.3 Measurement versus visual appearance

Time dilation and length contraction are not optical illusions, but genuine effects.
They should always be discussed separatly. What is the definition of genuine?
Measurements of these effects are not an artifact of Doppler shift, nor are they the result of neglecting to take into account the time it takes light to travel from an event to an observer.
Mesurements etc. How?
Physical effects have nothing to do with an observer. They constitute internal changes in the process itself and should be measured indepently of view point of any observer.
Fig. 1-13 illustrates a cube viewed from a distance of four times the length of its sides. At high speeds, the sides of the cube that are perpendicular to the direction of motion appear hyperbolic in shape.
Fig 1-13 shows two things: (1) Length contraction in the direction of motion (2) Rotation of the cube.
The problem is that the details how this "Length contraction" is measured and/or how observed are not mentioned.
• IMO the left picture shows a situation which can only be observed when the cube is straight in front of the observer. However in that case the observer will not observe Length Contraction.

### 4.1 Relativity of simultaneity

Two events happening in two different locations that occur simultaneously in the reference frame of one inertial observer, may occur non-simultaneously in the reference frame of another inertial observer (lack of absolute simultaneity).
The word occur is wrong.
"Two events happening in two different locations which one observer observes simultaneously, may not be observed simultaneously by an other moving observer". The issue is are there simultaneous events? Does it make sense to define simulatneous events?

### 4.2 Time dilation

The problem with Time dilation (moving clocks) is that moving clocks behave physical differently.
Time dilation explains a number of physical phenomena; for example, the lifetime of muons produced by cosmic rays impinging on the Earth's atmosphere is measured to be greater than the lifetimes of muons measured in the laboratory.
The cause that these processes behave different is not Time dilation. The only thing that you can say is that Time dilation describes this behaviour (i.e follows the same logic). To find the cause you have to investigate the process itself in more detail.

### 4.3 Length contraction

The dimensions (e.g., length) of an object as measured by one observer may be smaller than the results of measurements of the same object made by another observer (e.g., the ladder paradox involves a long ladder traveling near the speed of light and being contained within a smaller garage).
The most important issue is how is this length measured. In fact it should be measured in the same reference frame.

### 13. See also

Following is a list with "Comments in Wikipedia" about related subjects

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Created: 21 October 2017

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