• 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.
The Doppler effect (or the Doppler shift) is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source.
There is a physical difference if the Doppler shift is considered with or without an observer. Reflection 1 - Doppler effect versus an observer
For waves which do not require a medium, such as light or gravity in general relativity, only the relative difference in velocity between the observer and the source needs to be considered.
How do we know that the movement of photons does not involve a medium? The medium are the millions of photons surrounding us.
This is a physical issue and has nothing to do with general relativity.

### Reflection 1 - Doppler effect versus an observer

There is a clear physical distinction with a Doppler effect with or without an observer.
Starting point is a process which creates waves at a certain fixed frequency f when the process is at rest relative to the medium that propagates the waves. The wave pattern is symetric.
• The Doppler effect means that the wave pattern becomes physical a-symetric when the source has a speed v relative to the medium.
That means the wave length decreases infront of the source and increases behind the source.
To measure this pattern an observer at rest with the medium is required.
• A different type of Doppler effect is involved when the observer is not at rest with the medium. In that case when the observer moves towards the source the observer measures a decrease in wavelength or an increase in frequency. When the observer moves away from the source this is the opposite.
It is important to consider the two different types because in the first case this is a physical effect, in the second case not.

### Reflection 2 - Doppler effect with photons.

The Doppler effect with photons is physical very different compared to water waves and noise propagation. With water waves it is the combined behaviour of millions of water molecules which causes a wave. With photons it is the combined behaviour but also of individual photons.
A photon is emitted when inside an atom, an electron jumps from an higher energy level to a lower energy level. One photon can be considered as one energy packet (with a certain frequency). That same energy packet can be used that within one atom an electron jumps from a lower to a higher energy level.
For water to produce an interference patern your starting point are parallel waves which are guided through two holes. Behind the holes forms such an interference patern.

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Created: 22 January 2019

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