Comments about "Copenhagen interpretation" in Wikipedia
This document contains comments about the article "Copenhagen_interpretation" 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.
Contents
Introduction
The article starts with the following sentence.

According to the Copenhagen interpretation, physical systems generally do not have definite properties prior to being measured, and quantum mechanics can only predict the probabilities that measurements will produce certain results.

Generally speaking physical systems always have definite properties, except we humans do not know what they are. In fact our human involvement is inrelevant.
We perform a measurement we interact with the physical reality. Each measurement gives a result. By performing the same experiment 1000 times we can calculate the probabilities of the results. QM describes these results.

The act of measurement affects the system, causing the set of probabilities to reduce to only one of the possible values immediately after the measurement. This feature is known as wave function collapse.

The first sentence is not clear.










1. Background

n the early work of Max Planck, Albert Einstein, and Niels Bohr, the occurrence of energy in discrete quantities was postulated in order to explain phenomena such as the spectrum of blackbody radiation, the photoelectric effect, and the stability and spectrum of atoms.

This makes a lot of sense.

These phenomena had eluded explanation by classical physics and even appeared to be in contradiction with it.

For example?

While elementary particles show predictable properties in many experiments, they become thoroughly unpredictable in others, such as attempts to identify individual particle trajectories through a simple physical apparatus.

In principle all properties of al elementary particles are unpredictable. The reality is that by performing more and more experiments slowly different properties or parameters become known.
To describe the directions of the particles involved is a very difficult branch of science.
The word simple in this context is a misnomer.

Classical physics draws a distinction between particles and waves. It also relies on continuity and determinism in natural phenomena.

The last sentence should be: It also relies on physical laws in natural phenomena.

In 1925–1926, quantum mechanics was invented as a mathematical formalism that accurately describes the experiments, yet appears to reject those classical conceptions.

Quantum mechanics is also a physical law. Quantum mechanics describes the laws of the elementary particles.

Instead, it posits that probability, and discontinuity, are fundamental in the physical world.

In classical physics there is also probability, in the sense that experimental results are not always exactly the same.
The disctinction between continuity and discontinuity is "marginal".

Classical physics also relies on causality.

All of physics depents on causality.

The standing of causality for quantum mechanics is disputed.

All reactions in the universe are a result of other reactions.






2 Origin of the term












3. Current status of the term

According to an opponent of the Copenhagen interpretation, John G. Cramer, "Despite an extensive literature which refers to, discusses, and criticizes the Copenhagen interpretation of quantum mechanics, nowhere does there seem to be any concise statement which defines the full Copenhagen interpretation."

IMO that is correct. There does not exist one clear definition. Also it is impossible to answer the qusetion: who is right: The Copenhagen interpretation or Einstein.










4. Principles

1. A wave function Psi represents the state of the system. It encapsulates everything that can be known about that system before an observation; there are no additional "hidden parameters".

There only exists a reality.

2. For example, if a particle at a particular instant has a definite location, it is meaningless to speak of its momentum at that instant.

At any instant a particle has a specfic location, speed and momemtum. It is impossible to measure each simultaneous.

3. During an observation, the system must interact with a laboratory device. When that device makes a measurement, the wave function of the systems is said to collapse, or irreversibly reduce to an eigenstate of the observable that is registered

When "you" claim that the wave function collapses, "you" also have to describe what that means. When I look in the air, photons continuously hit my eyeball. Each of this hits is a measurement.

4. The results provided by measuring devices are essentially classical, and should be described in ordinary language.

This is the first sensible sentence.

5. The description given by the wave function is probabilistic.

The outcome of any experiment is probabilistic.

6. The wave function expresses a necessary and fundamental wave–particle duality. This should be reflected in ordinary language accounts of experiments.

Why?
When you perform any experiment, both the details and the results should be described as exactly as possible. The interpretation of the results should be as clear and logical as possible.

7. The inner workings of atomic and subatomic processes are necessarily and essentially inaccessible to direct observation, because the act of observing them would greatly affect them.

To understand the inner workings of atomic and subatomic processes many experiments have to be performed, starting from the most simple ones and slowly going to more complex reactions.










5 Metaphysics of the wave function

n metaphysical terms, the Copenhagen interpretation views quantum mechanics as providing knowledge of phenomena, but not as pointing to 'really existing objects', which it regarded as residues of ordinary intuition. This makes it an epistemic theory.
This may be contrasted with Einstein's view, that physics should look for 'really existing objects', making itself an ontic theory.

This type of a discussion does not make sense. When you perform science the subject is the reality.




6 Born rule

For the Copenhagen interpretation, it is axiomatic that the wave function exhausts all that can ever be known in advance about a particular occurrence of the system.

You can only know something in advance about a particular experiment if you have performed the same experiment many times.

It only goes as far as saying that on every occasion of observation, some actual value of some property is found, and that such values are found probabilistically, as detected by many occasions of observation of the same system.

This sentence is vaque.
When you perform the same experiment many times, you become familiar with its probabilistic nature or characteristics. 
For presentday science, the experimental significance of these various forms of Born's rule is the same, since they make the same predictions about the probability distribution of outcomes of observations, and the unobserved or unactualized potential properties are not accessible to experiment.

The probability distribution is a result of many experiments. You can call this the Born rule. The parameters of the wave function should also agree with the results of the same experiments.


7 Nature of collapse

Those who hold to the Copenhagen interpretation are willing to say that a wave function involves the various probabilities that a given event will proceed to certain different outcomes.

The whole concept of the wave function is very tricky. Specific the wave function of what.






8 Nonseparability of the wave function








9 Wave–particle dilemma

The term Copenhagen interpretation is not well defined when one asks about the wave–particle dilemma, because Bohr and Heisenberg had different or perhaps disagreeing views on it.

Starting point is a proper definition what the waveparticle dilemma is, on which both (all) parties agree.

According to Camilleri, Bohr thought that the distinction between a wave view and a particle view was defined by a distinction between experimental setups, while, differing, Heisenberg thought that it was defined by the possibility of viewing the mathematical formulas as referring to waves or particles.

The experimental setups come first. Mathematical formulas to describe the results come second.
This common practice has nothing to do with the Copenhagen Intrepretation.
IMO Einstein will agree about this.

Bohr thought that a particular experimental setup would display either a wave picture or a particle picture, but not both.

I fully agree with him. This is very good starting point.

Heisenberg thought that every mathematical formulation was capable of both wave and particle interpretations.

In theory it is possible to describe both in one mathematical formulation.
If it is possible to describe both more accurate in two formulations then I prefer that.

Alfred Landé etc. Eventually this led to his being considered unorthodox, partly because he did not accept Bohr's oneortheother view, preferring Heisenberg's alwaysboth view.

It is very important to study the actual experiments. See Reflection 1: Bohr versus Heisenberg.




10 Acceptance among physicists

In a 2017 article, physicist and Nobel laureate Steven Weinberg states that the Copenhagen interpretation "is now widely felt to be unacceptable."

I agree with him.
The only way to under stand the physical world at the atomic or subatomic level is by performing experiments.
Using these experiments you can define the quantum mechanical laws.
These experiments also define the boundaries and probabilities involved.






11 Consequences

1. Schrödinger's cat
This thought experiment highlights the implications that accepting uncertainty at the microscopic level has on macroscopic objects.

It is generally speaking impossible to unravel the laws of nature by means of a thought experiment. Only real experiments can be used

A cat is put in a sealed box, with its life or death made dependent on the state of a subatomic particle.

It is specific the state of this subatomic particle that counts. The state is the physical situation if the radioactive particle has decayed, Yes or No. This decaying process defines the half live time, which has to be established my multiple experiments.

But this can't be accurate because it implies the cat is actually both dead and alive until the box is opened to check on it.

The cat is actual alive before the particle has decayed and dead after the particle has decayed. This has nothing to do if the box is open or closed, from the point of view of an observer.

How can the cat be both alive and dead?

Physical the cat is never both alive and dead. When the observer performs a continuous observation this is easy to check.

2. Wigner's Friend
Wigner puts his friend in with the cat. The external observer believes the system etc.

When you put both the cat and your friend in the box, the state of both is the same. That is first both are alive, secondly (after the event) both are dead.

3. Doubleslit diffraction
Light passes through double slits and onto a screen resulting in a diffraction pattern. Is light a particle or a wave?

Light in fact is a stream of photons. Photons can interfere which each other. A photon is a photon.

The same experiment can in theory be performed with any physical system: electrons, protons, atoms, molecules, viruses, bacteria, cats, humans, elephants, planets, etc.

If electrons can interfer which other electrons can only be demonstrated by means of an actual experiment. This can not be claimed a priori. The same for all the other examples.

4. EPR (Einstein–Podolsky–Rosen) paradox
Entangled "particles" are emitted in a single event.

Entangled (correlated) particles are always created in a single reaction.
The condition that particles are entangled can only be established by means of many identical experiments.

Conservation laws ensure that the measured spin of one particle must be the opposite of the measured spin of the other, so that if the spin of one particle is measured, the spin of the other particle is now instantaneously known.
The logic is the other way around. Only by means of many experiments a correlation can be established. This correlation implies that when you repeat the same experiment and only one is measured you know the state of the other one. It is important that no faster than light speed communication is involved.


The most discomforting aspect of this paradox is that the effect is instantaneous so that something that happens in one galaxy could cause an instantaneous change in another galaxy.

There exists no instantaneous change between galaxies. Entangled particles always involve a common origin i.e. a common reaction. Such a common origin explains the entanglement. Such a correlation does not exists between galaxies.






12 Criticism

The completeness of quantum mechanics (thesis 1) was attacked by the Einstein–Podolsky–Rosen thought experiment which was intended to show that quantum mechanics could not be a complete theory.

Thought experiments cannot be used to unravel physical laws. Only real experiments can.

Experimental tests of Bell's inequality using particles have supported the quantum mechanical prediction of entanglement.

The behaviour of entanglement can only be established by means of experiment.

Steven Weinberg in "Einstein's Mistakes", Physics Today, November 2005, page 31, said:
The Copenhagen interpretation describes what happens when an observer makes a measurement, but the observer and the act of measurement are themselves treated classically.

The act of performing a measurement is a cause of a reaction which inturn can cause other reactions. Nothing strange.

This is surely wrong: Physicists and their apparatus must be governed by the same quantum mechanical rules that govern everything else in the universe.

There exist no quantum mechanical rules that govern what happens in the universe. In some sence what you do in a laboratory only involves the surrounding of the laboratory.

But these rules are expressed in terms of a wave function (or, more precisely, a state vector) that evolves in a perfectly deterministic way.

How do you know that? Only by performing experiments.
And what do these experiments tell you? That the results are not always exactly the same.
The origin is that the wave function is not a perfect description of the reality.

So where do the probabilistic rules of the Copenhagen interpretation come from?

If there are probabilistic rules then these rules come from obeserving the results of experiments.

The problem of thinking in terms of classical measurements of a quantum system becomes particularly acute in the field of quantum cosmology, where the quantum system is the universe.

The universe is not a quantum system. This concept does not add any content. The universe is a complex system in evolution.

13 Alternatives








14. See also
Following is a list with "Comments in Wikipedia" about related subjects
Reflection 1: Bohr versus Heisenberg.
When you study 9 Wave–particle dilemma you get a glimpse about the different opinions of Bohr and Heisenberg. In some sense both are right. Bohr's opinion is based on experiments and Heisenberg's opinion is based on mathematics. In order to decide if maybe both are correct you have to evaluate actual experiments performed.
 When the experiments involve water waves and the interference paterns then you can only speak about waves.
The interference pattern reflects the wave nature.
 When the experiments involve light (Young) what you observe on the screen are individual photons which react which the screen. What you see on the screen is an interference pattern. This pattern is physical created in front of the screen. As such the mathematics involved comparing water waves with photons is the same.
 When the experiments involve multiple electrons also these interference patterns are observed. As such again the mathematics involved is the same.
In reality more is involved.
 In the book "In search of Schrödingers cat" at page 170 John Gribbin writes:
Indeed, for, photons and electrons, if we took a thousand identical experiments in a thousand different laboratories, and let one particle pass through each experiment we could add up the thousand different results and still get an overall distribution pattern in line with diffraction, just as if we let a thousand electrons through one of those experiments.

Next John Gribbib writes:

A single electron or a single photon on its way through one hole in the wall, obeys the statistical laws which are only appropiate if it "knows" whether or not the other hole is open. This is the central mystery of the quantum world.

IMO if there exists a diffraction pattern for single photon experiments when two slits are open then the single photon in some way or an other goes to both holes.
To make this opinion acceptable the same experiment with only one hole open should be performed.
My prediction is in both cases (either left or right open) a normal distribution will be observed.
This result depents about the distance between the holes or slits.
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Created: 28 January 2017
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