• 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 quantum mechanics, the measurement problem is the problem of how, or whether, wave function collapse occurs.
Why is it then not called: The wave function collapse problem?
The answer is rather simple: A (water) wave is something physical. Waves slowly die out and disappear.
A wave function like a sinus function is something mathematical. The best way to simulate a wave is on an analog computer. Using an AC the wave will disappear when you turn down the volatage. In that case the wave will become flat, if that is what you want.
The inability to observe such a collapse directly has given rise to different interpretations of quantum mechanics and poses a key set of questions that each interpretation must answer.
That amplifies the reasoning, because the concept is not clearly defined, not to use it.
The wave function in quantum mechanics evolves deterministically according to the Schrödinger equation as a linear superposition of different states.
You need a clear physical experiment to demonstrate what is involved. Part of the problem is to calculate the schrödinger's equation and to demonstrate, involving actual measurements, that the schrödinger equation is a correct description of the physical reality.
As a general question: How can one establish a correspondence between quantum reality and classical reality?[3]
It is important to agree upon that there only exists one physical reality. To describe that reality by any form of a mathematical equation, accurately, at particle level, is impossible.

### 1 Schrödinger's cat

A thought experiment often used to illustrate the measurement problem is the "paradox" of Schrödinger's cat.
Thought experiments should never be used as a starting point to understand the physical world and all the processes involved.
A mechanism is arranged to kill a cat if a quantum event, such as the decay of a radioactive atom, occurs.
To understand this event many experiments have to be result. The results will give a probabilty when the event is supposed to happen. Implying there exist no paradox.

### 2 Interpretations

Generally, views in the Copenhagen tradition posit something in the act of observation which results in the collapse of the wave function.
When the act of observation causes the collapse of the wave function, the whole concept of a wave function actual collapses i.e. disappears and has no meaning.
This concept, though often attributed to Niels Bohr, was due to Werner Heisenberg, whose later writings obscured many disagreements he and Bohr had had during their collaboration and that the two never resolved.
That is a pity, because as such you will never know who was right. Maybe what is closer to the truth, neither were right.
In these schools of thought, wave functions may be regarded as statistical information about a quantum system, and wave function collapse is the updating of that information in response to new data.
Statistical information can only be collected by performing actual measurements. Again when these measurements destroy the wave function that whole concept destroys itself.
De Broglie–Bohm theory tries to solve the measurement problem very differently: the information describing the system contains not only the wave function, but also supplementary data (a trajectory) giving the position of the particle(s).
You can never solve a problem if the problem is not clearly defined.
Home work: How do you solve the problem that water can physical boil and can freeze?
How do you solve the problem that water can boil and freeze, using the Broglie_Bohm theory?

### Reflection 1 - The measurement Problem

The only way to understand the physical world or any process is by performing experiments. The most important part is to perform observations and measurements. Observations are important to describe as detailed as possible the evolution of the process. Measurements are important to quantify what is involved. A second step is to repeat the process as often as possible. The third step is to make small modifications.

Consider a reactor with only water. You turn on the heater. Next measure the temperature with a thermometer. You will observe that the temperature of the water slowly rises. Turn the heater off the temperature of the water will slowly decrease. Place your hand against the reactor wall you will feel that the outside temperature follows this same pattern. This makes the whole process slightly more complicated.

Next consider the following chemical reaction: A + B -> C. For example, H2 + O2 => H20. This is a reaction between hydrogen and oxygen to produce water. In this reaction you immediate produce a final product, which can be removed, to let the reaction continue.
However also the reverse is true: A => B+C. For example, H20 -> H2 + O2. In this case your final product is a mixture of two gasses which have to be removed to let the reaction continue. After that the two gasses have to be separated.

The next reaction we are going to study is this one: A + B -> C + D + E + F.
A typical case is this reaction in order to produce the Higgs boson: https://cds.cern.ch/record/2759490/files/Feynman%20Diagrams%20-%20ATLAS%20Cheat%20Sheet.pdf
Input in the reaction are two gluons. The output is electron-positron pair and a muon-antimuon pair.
My estimate is that this reaction was not recognized immediate. You need many tries in order fully demonstrate (or prove) what is really happening.
What is the next step? What the diagram does not show are the actual measurement devices. My estimate is that in order to demonstrate each of the four 'particles' you need a special measurement device in which a specific reaction takes place. That means a total of 4 special designed measurement devices and these 4 as often as can be fitted in a sphere around the point where the reaction takes place. What this means that when the reaction actual takes place it is possible that between 0 and 4 particles are measured.

Suppose we measure the particles C, D and E, what is the chance that the missing particle is F? IMO, the chance is high. In the indicated document more Feynman are possible which involve the Higgs boson. IMO they require different measurement devices, which are not considered.
That still means also the following two reaction are possible: A + B -> C + D and A + B -> E + F

The final reaction we are going to study is this one: A + B -> f1 + f2.
In this reaction there is a collision of two particles A and B which produces two photons: f1 and f2.
In the reaction studied each of the photons have a specific frequency also called f1 and f2.
That means when the reaction takes place and both photons are measured than always one has the frequency f1 and the other has frequency f2. That means at first side that the photons are correlated. That in turn means, in a specific reaction, when the first photon measured is f1 that the other photon, still to be measured should be f2. And vice versa. The simplest explanation is that these frequencies are established at the point of collision. This follows the same reasoning, with what happens, inside the reaction which produces the Higgs boson.

This is NOT how Quantum Mechanics explains this behaviour. Accordingly, to QM this correlation, called entanglement, is created when the photon is measured. This measurement also involves action at a distance and the distance involved can be large when the positions measurement devices used to measure both photons are 'far away'.
The central issue is when you perform any experiment which includes any reaction between certain elements or products all the changes happen in a certain sequence of individual steps. That means the final product is not reached instantaneous but slowly. A reaction is like baking a cake, which primarily exists reading the recipe. You follow all the steps, exactly as described. You buy or borrow all the required ingredients. You mix them, using the required quantities, in the order as required. Finally, you put everything in oven, at the predefined temperature, for the fixed time. At the end you open the oven and you enjoy eating the cake with your friends. And what is important, during this whole process, no measurements are required, about the internal changes taking place inside the mixer and oven. From a mathematical point no mathematics is involved. From a quantum mechanical point, don't worry about the collapse of the wave function when you break an egg or when you open the door of the oven to look inside. At that critical moment almost nothing physical happens, except you observe the results of maybe 4 hours hard labour.

In order to study the details of 'entanglement' please read this document: https://escholarship.org/uc/item/1kb7660q "Polarization correlation of photons emitted in an atomic cascade" by Carl Alvin Kocher, May 1967
At page 1 is written: "An isolated atom, optically excited, returns to the ground state by way of an intermediate state, with the spontaneous emission of two successive photons. Quantum theory predicts that a measurement of the linear polarization of one photon can determine precisely the linear polarization of the other photon."
My interpretation of this reaction is, that even if no measurements are made both photons are polarised in opposite directions. That means the measurements as such are not the cause of the correlations. The correlations are part of the process described in figure 1.

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Created: 21 January 2022

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