Comments about "Einstein's thought experiments" in Wikipedia

This document contains comments about the article Einstein's thought experiments in Wikipedia
In the last paragraph I explain my own opinion.




The article starts with the following sentence.
A hallmark of Einstein's career was his use of visualized thought experiments (Gedanken-Experimente) as a fundamental tool for understanding physical issues and for elucidating his concepts to others.
There is a difference between 'thoughts' and 'thought experiments'. Every one can think or have thoughts. You can also think about experiments and how to perform experiments. The issue is to what extend the results of thought experiments can be used as a validation or invalidation of real experiments or as an explanation of how the physical world operates.
Einstein considered two particles briefly interacting and then flying apart so that their states are correlated, anticipating the phenomenon known as quantum entanglement.
Here the issue is how you can explain entanglement by means of a thought experiments.

1. Introduction

They 'Thought experiments' can only provide conclusions based on deductive or inductive reasoning from their starting assumptions.
It is more than that: the starting assumptions should be clear.
Thought experiments invoke particulars that are irrelevant to the generality of their conclusions.
It is the invocation of these particulars that give thought experiments their experiment-like appearance.
Both sentences are not clear. What are particulars?
John D. Norton, a well-known philosopher of science, has noted that "a good thought experiment is a good argument; a bad thought experiment is a bad argument."
John D. Norton should explain what a good argument versus what a bad argument is.
When effectively used, the irrelevant particulars that convert a straightforward argument into a thought experiment can act as "intuition pumps" that stimulate readers' ability to apply their intuitions to their understanding of a scenario.
Intuition is a very tricky concept in science. In most cases intuition is based on actual experiments, observations and experiences.
Perhaps the best known 'thought experiment' in the history of modern science is Galileo's demonstration that falling objects must fall at the same rate regardless of their masses.
Why use the word: demonstration and not: 'thought experiment'. The two concepts are totally different.
Read the next sentence:
it was a logical demonstration described by Galileo in: Discorsi e dimostrazioni matematiche
That means it was a thought experiment and not a real demonstration.
See also: which claims that the actual experiment was performed by Simon Stevin ea.
See also: Reflection 1 - thought experiment
Einstein had a highly visual understanding of physics. etc. This included his use of thought experiments.
The problem is that thought experiments can not be used sec. The underlying reasoning should be clear and finally you need real experiments.

2 Special relativity

Rather than the thought experiment being at all incompatible with aether theories (which it is not), the youthful Einstein appears to have reacted to the scenario out of an intuitive sense of wrongness.
The problem is that it is very difficult to use a thought experiment to demonstrate that a certain physical theory is wrong. Part of the issue is that the aether theory is not clear, because it describes something that does not exist.
Einstein felt that Maxwell's equations should be the same for all observers in inertial motion.
The issue is that Maxwell's equations describe certain electromagnetical phenomena and that those phenomena have nothing to do with any observer including the mathematics that describe these phenomena.

2.1 Pursuing a beam of light

If I pursue a beam of light with the velocity c (velocity of light in a vacuum), I should observe such a beam of light as an electromagnetic field at rest though spatially oscillating.
What this sentence implies is to describe the situation when an observer has the same speed as the speed of light.
The issue is what can you learn from such ideas when you can not perform such an experiment in reality.
To study the issue what happens if an observer approaches you, from the opposite side, please select this link:
From the very beginning it appeared to me intuitively clear that, judged from the standpoint of such an observer, everything would have to happen according to the same laws as for an observer who, relative to the earth, was at rest.
IMO when that is true the description of any physical phenomena should be independent of any observer, also for an observer at the surface of the earth. To more precise the concept of an observer at rest relative to the earth has no physical implications.
Einstein felt that Maxwell's equations should be the same for all observers in inertial motion.
The problem with this sentence is what means inertial motion.
Does that mean that the observer moves in a straight line with a constant speed? (accelaration = 0) IMO the description of electromagnetical phenomena should be independent of any observer.
From Maxwell's equations, one can deduce a single speed of light, and there is nothing in this computation that depends on an observer's speed.
I think it is the otherway around.
First of all Maxwell assumed that electromagnetic phenomena can described by a single speed of light, which is reflected in Maxwell's equations.
Secondly Maxwell assumed that electromagnetic phenomena don't depend about observers speed which is also reflected in Maxwell's equations.
The question is: what does this have to do with any thought experiment.
Einstein sensed a conflict between Newtonian mechanics and the constant speed of light determined by Maxwell's equations
Newtonian mechanics has nothing to do with the speed of light.
IMO to assume that the speed of light is constant is only true when only Electromagnetic phenomena are discussed. As soon as when mass and gravity are involved this becomes questionable and is probably not true.

2.2 Magnet and conductor

Maxwell, for instance, had repeatedly discussed Faraday's laws of induction, stressing that the magnitude and direction of the induced current was a function only of the relative motion of the magnet and the conductor, without being bothered by the clear distinction between conductor-in-motion and magnet-in-motion in the underlying theoretical treatment.
The last part should be: in the underlying physical treatment.
Yet Einstein's reflection on this experiment represented the decisive moment in his long and tortuous path to special relativity. Although the equations describing the two scenarios are entirely different, there is no measurement that can distinguish whether the magnet is moving, the conductor is moving, or both.
What is exactly the thought experiment in the "Magnet and conductor thought experiment" ? Against a fixed background, in a laboratory, it easy to monitor who is moving.
Expressed in contemporary physics vocabulary, his postulates were as follows:
  • 1. The laws of physics take the same form in all inertial frames.
  • 2. In any given inertial frame, the velocity of light c is the same whether the light be emitted by a body at rest or by a body in uniform motion.
These postules should be compared with the following two more physical oriented postulates: What these postulates exclude is to consider the surface of the earth a rest frame.

2.3 Trains, embankments, and lightning flashes

The essence of the thought experiment is as follows:
  • Observer M stands on an embankment, while observer M' rides on a rapidly traveling train. At the precise moment that M and M' coincide in their positions, lightning strikes points A and B equidistant from M and M'.
This whole experiment raises a serious problem. First of all there are two events.
  • Light from these two flashes reach M at the same time, from which M concludes that the bolts were synchronous.
Based on what sort of reasoning can M conclude that the events tA and tB happenend simultaneous?
This immediate raises an issue about the validity of thought experiments.
The only thing that is sure that the second and third event each happened before the first event tM.
  • The combination of Einstein's first and second postulates implies that, despite the rapid motion of the train relative to the embankment, M' measures exactly the same speed of light as does M. Since M' was equidistant from A and B when lightning struck, the fact that M' receives light from B before light from A means that to M', the bolts were not synchronous. Instead, the bolt at B struck first.
First of all you cannot use the concept of a train in rapid motion. You should perform this exeperiment with different speeds of the train. This immediate raises an issue: what is validity of this thought experiment, because of inaccuracy reasons.
The only thing that is for sure is that assuming that M observes the two flashes simultaneous, M' will not observe the two flashes simultaneous.
Because M' moves away from the event tA at A, M' will observe the event tA, after M does.
Because M' moves towards the third event at B, M' will observe the event tB, before M does.
See also Reflection 2 - Who or what is true.
In all of this reasoning the speed of light c is not involved/measured.
Einstein discovered the relativity of simultaneity by thinking about how clocks could be synchronized by light signals.
Clock synchronisation is a very difficult subject.
We do know, however, that the train and embankment thought experiment was the preferred means whereby he chose to teach this concept (relativity of simulataneity) to the general public.
The most important thing what the train thought experiment teaches you is that two observers, in relatif motion, will not agree if two events (light signals) are simultaneous or not. To decide who is right is tricky and requires a grid of external clocks.

3 General relativity

Because for an observer in free-fall from the roof of a house there is during the fall—at least in his immediate vicinity—no gravitational field. Namely, if the observer lets go of any bodies, they remain relative to him, in a state of rest or uniform motion, independent of their special chemical or physical nature. The observer, therefore, is justified in interpreting his state as being "at rest."
That is the question. IMO the observer is not at rest in a larger reference frame including the earth surface and the house.

3.1 Falling painters and accelerating elevators

A powerful "being" of some sort begins pulling on the rope with constant force.
Unlucky description. A better is: The chamber should be envisioned as the inside of a rocket, which motor is started, at a constant trust.
Within the chamber, all of the man's perceptions are consistent with his being in a uniform gravitational field.
You can never use an human experience, his perception, as a scientific fact.
Objects have "gravitational mass," which determines the force with which they are attracted to other objects. Objects also have "inertial mass," which determines the relationship between the force applied to an object and how much it accelerates.
The issue is how each is measured. In fact you should compare in two different experiments the same gravitational force and the same inertial force to the same object and compare the speeds observed.
See also Equivalence Principle Reference 1 and 2.
But until Einstein, no one had conceived a good explanation as to why this should be so.
Did Einstein realy give a good explanation?
From the correspondence revealed by his thought experiment, Einstein concluded that "it is impossible to discover by experiment whether a given system of coordinates is accelerated, or whether...the observed effects are due to a gravitational field."
When you cannot demonstrate (backup) a certain statement, declaration or proposition by an experiment, how can you than claim that it is correct?
An extension to his accelerating observer thought experiment allowed Einstein to deduce that "rays of light are propagated curvilinearly in gravitational fields
Why did not he at the same time als proclaim, that the speed of light in a gravitational filed is not constant?

4 Quantum mechanics

Therefore, Einstein before 1925 originated most of the key concepts of quantum theory: light quanta, wave-particle duality, the fundamental randomness of physical processes, the concept of indistinguishabity, and the probability density interpretation of the wave equation.
Physical processes are not fundamental random. At least much less from the human point of view than from the physical point of view. The uncertainty principle more or less describes the limitations of human observations but that does not mean that this limitation is valid from the physical point of view.

4.1 Background: Einstein and the quantum

4.2 Einstein's light box

4.3 EPR Paradox

Einstein's thought experiment involved two particles that have collided or which have been created in such a way that they have properties which are correlated.
You can not perform such an experiment as a thought experiment. You cannot perform a thought experiment about particles that have been collided. You can also not perform a thought experiment about particles that have been created. This does not have any scientific value.
The figure depicts the spreading of the wave function from the collision point.
And this is a thought experiment? strange.
However, observation of the position of the first particle allows us to determine precisely the position of the second particle no matter how far the pair have separated.

5 Notes

6. See also

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

Reflection 1 - thought experiment

A thought experiment is the performing of any experiment in your mind. For example in order to play chess you do not need a chessboard and the actual chess pieces. An alternative is to play it in your mind and telling your oponent which piece you are going to move. This requires a certain practice.
The starting point of any thought experiment in the physical realm is that you must have a certain theory or idea and using the thought experiment you explain the experiment and predict the outcome. What is important that you need a real experiment to validate your theory or idea.
Using a thought experiment you can also explain that certain theories are in contradiction with each other. Again you need a real experiment to decide that this is true and which theory prefers.

Reflection 2 - Who or what is true.

A slightly modified train experiment.
              Figure 1
What this indicates is that both observer M and M' observe exactly the same and that tM = tM'. The point is that this can be true. In case M observes the two light signals simultaneous also at the same time, when the two observers can touch each other also M' can observe the two light signals simultaneous, but that does not mean that the equation AM = MB = A'M' = M'B' is true
When you compare events than the events tA and tA' are the same. Also tB and tB' are the same. Also the two events tA and tB are both before tM.
Figures 2 and figure 3 shows a better indication of what is happening.
              Figure 2

              Figure 3
During the time that the light signal moves from B to M, point B' on the train moves from B to B'. The same for point A' which moves from A to A'. That means A'M' <> M'B'. The logic behind this reasoning is the idea that the speed v of the embankment is considered zero and that the point B' with a speed v > 0 moves a way from point M.
The most important question to answer: is this true?. To be more precise: does this thought experiment decide what is true? Or to raise a different question: does this thought experiment decide if figure 1 is correct or if something like Figure 3 is at stake?
The assumption is that the speed of the embankment is zero. But is this correct? Are the two events tA and tB simultaneous?
The most important lesson to learn is that a thought experiment can not answer this question A second lesson is IMO that it is very difficult to repeat the same experiment with the same outcome, because the earth turns around its axis, except if the speed of the photons in horizontal direction is controlled by the gravity field of our earth.

Reflection 3


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Created: 10 May 2018

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