• 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 Stern–Gerlach experiment demonstrated that the spatial orientation of angular momentum is quantized.
That is the interpretation.
The Stern–Gerlach experiment demonstrates that a stream of silver atoms, travelling through a magnetic field can be can be divided into two specific parts.
Particles with non-zero magnetic moment are deflected, due to the magnetic field gradient, from a straight path.
The path of the particles which travel through the magnetic field and which are magnetized, is bended, compared with a straight path.
The screen reveals discrete points of accumulation, rather than a continuous distribution, owing to their quantized spin.

### 1 Description

The results show that particles possess an intrinsic angular momentum that is closely analogous to the angular momentum of a classically spinning object, but that takes only certain quantized values.
That is the case when iron filings are studied.
Another important result is that only one component of a particle's spin can be measured at one time, meaning that the measurement of the spin along the z-axis destroys information about a particle's spin along the x and y axis.
What is not mentioned, and what is important, what happens when the magnet rotates slowly in a continuous manner. What is expected that the two dots also follow this rotation pattern. That means that the process is symmetric.
The experiment is normally conducted using electrically neutral particles such as silver atoms . This avoids the large deflection in the path of a charged particle moving through a magnetic field and allows spin-dependent effects to dominate.
These types of experiments are also important, because they can more insight what is physical involved
If the particle is treated as a classical spinning magnetic dipole, it will precess in a magnetic field because of the torque that the magnetic field exerts on the dipole (see torque-induced precession).
This is the case when iron filings are involved.
If the particles were classical spinning objects, one would expect the distribution of their spin angular momentum vectors to be random and continuous.
It should be mentioned that the word classical spinning objects, is a misnomer. What should be mentioned what the particle physical is: iron filings, an atom, an electron, a proton or a neutron.
Each particle would be deflected by an amount proportional to the dot product of its magnetic moment with the external field gradient, producing some density distribution on the detector screen.
Also in this sentence first should be mentioned what is observed and then what the explanation is.
Instead, the particles passing through the Stern–Gerlach apparatus are deflected either up or down by a specific amount.
In this case it is very important what these particles are.

### Video explaining quantum spin versus classical magnet in the Stern–Gerlach experiment

The video shows two experiments:
• clasisical magnets
• The device with North and South poles creates a magnetic field which is larger near the upper tip.
• A magnet is sent with North pole up and South pole down. The device creates a force which deflects the magnet upward
• When poles are inverted the magnet is deflected downward.
Deviation depends on the orientation of the poles.
When many magnets are sent with random orientation, they arrive anywhere verically
• quantum magnets
• When quantum electrons are sent through this magnetic setup, they are deflected
But they reach the screen only upward or downward, never in the middle.
• Each electron behaves as a magnet but with only two vertical directions: North South or South North. This quantum properties is called the << spin >>
• The video shows two experiments: one with small magnetized individual iron particles (iron filings) and one with individual electrons.
• Iron filings is distributed on a straight vertical line
• The electrons are distributed in two horizontal lines.

### 2 Sequential experiments

If we link multiple Stern–Gerlach apparatuses (the rectangles containing S-G), we can clearly see that they do not act as simple selectors, i.e. filtering out particles with one of the states (pre-existing to the measurement) and blocking the others.
More detail is required. Accordingly to the displays showed at the right hand side the experiment is performed with neutrons.
Instead they alter the state by observing it (as in light polarization).
To call, what happens inside the Stern-Gerlach apparatuses: 'observing' is wrong. What happens inside the SG apparatus is a physical process, and the result is that the original beam is split in two.

### Reflection 1 Stern-Gerlach experiment

The purpose of physics is to understand the internal structure of atoms and the elementary particles that are the building blocks of atoms. In the case of the Stern-Gerlach experiment this is the 3D structure of a neutron, and specific the magnetic component. The experiment consists of a beam of individual neutrons which move through a SG-apparatus which consists of two magnets. Those two magnets create a magnetic field which influences the path of the neutrons, and split the path in two.
The Stern-Gerlach experiment is considered a measurement. That by it self is not important. What is important to what extend the experiment influences the 3D structure of the neutron.
This is important because the final experiment consits of a sequence of three SG-apparatuses, The purpose of the Stern-Gerlach experiment is to understand the internal structure a physical process, the behaviour of a neutron based as based on its internal structure.
A second part is to agree upon that each measurement is also a physical process and finally to study the interaction between the experiment and the measurement i.e. between two processes. This interreaction has nothing to do with the concept of imaginary numbers. Imaginary numbers can be used to describe what is mathematical happening but they cannot be used to explain what is physically involved.

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Created: 11 January 2022

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