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.

In a thought experiment you to perform a physical experiment in your mind. This raises one serious question: To what extend can you perform an experiment or test in your mind. For example to what extend can you predict in your mind, what happens with an egg when you slowly increase the temperature that it starts boiling?

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: The Speed of Light , by the author
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 three 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.
A routine supposition among historians of science is that, in accordance with the analysis given in his 1905 special relativity paper and in his popular writings, Einstein discovered the relativity of simultaneity by thinking about how clocks could be synchronized by light signals.
Clock synchronization is a very difficult subject.
The Einstein synchronization convention was originally developed by telegraphers in the middle 19th century.
See for more detail about the problems involved: Article_Review_On The Electrodynamics Of Moving Bodies
However, all of the above is supposition. In later recollections, when Einstein was asked about what inspired him to develop special relativity, he would mention his riding a light beam and his magnet and conductor thought experiments.
Study this painting The Speed of Light , by the author.
He would also mention the importance of the Fizeau experiment and the observation of stellar aberration. "They were enough", he said.
He never mentioned thought experiments about clocks and their synchronization.
IMO, that is very wise. Thought experiments are a very tricky road to establish "The laws of Nature"
We therefore do not know just how important clock synchronization and the train and embankment thought experiment were to Einstein's development of the concept of the relativity of simultaneity.
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. See also: Clock and Centrifuge

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.
When one apple fall's from a tree does not mean that there is no gravitational field. The same when two apples fall, even when each apple thinks, observing the other apple, that he or she is not falling.

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 field 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 Wave-particle duality (new)

From this, Einstein concluded that radiation had simultaneous wave and particle aspects.

4.3 Bubble paradox (new)

Most of Einstein's contemporaries adopted the position that light is ultimately a wave, but appears particulate in certain circumstances only because atoms absorb wave energy in discrete units.
See Reflection 3 - Particle wave duality of photons.
He used this thought experiment to argue that atoms emit light as discrete particles rather than as continuous waves:
This sentence should be written in singular not plural. As such: He used this thought experiment to argue that an atom (at a certain instant) emits (one or more) photons each as discrete particle rather than as a continuous wave:
(a) An electron in a cathode ray beam strikes an atom in a target. The intensity of the beam is set so low that we can consider one electron at a time as impinging on the target.
Also in this case a single atom is considered. This type of processes are called Electron capture. See: for more detail. Specific as part of this process "one or more characteristic X-ray photons can be emitted".
An example of an electron capture is: p + e- -> n + nue
In the case when also a photon is emitted you get: p + e- -> n + nue + photon
(b) The atom emits a spherically radiating electromagnetic wave.
How does an atom per forms this task?
In this sentence you clearly can see the problem with thought experiments.
But maybe this is part of the paradox. You assume something, which is physical not clear.
An electric current causes an electric field. A direct current causes a stationary field and an alternating current a variable electricfield which propages at the speed of light. This field propagetes with the speed of light, but such a field is here not involved (?)
(c) This wave excites an atom in a secondary target, causing it to release an electron of energy comparable to that of the original electron.
The assumed reverse reaction should then be: n + nue + photon --> p + e-
How do you know that this actual happening? The only thing is by performing an actual experiment. What this experiment should establish is that there is a clear cause and effect issue involved (at quantum level).

The importance is, that if that is the case there should not be any energy loss between photon emission and photon capture, implying that a single photon cannot propagete in a sphere.
What this also implies that such a photon caption should be rare, because most emitted photons will not be captured.

The energy of the secondary electron depends only on the energy of the original electron and not at all on the distance between the primary and secondary targets.
This is the same as above.
Far more plausible would be to say that the first atom emitted a particle in the direction of the second atom.
Although Einstein originally presented this thought experiment as an argument for light having a particulate nature,
A much more realistic reverse reaction is: n + nue --> p + e-
i.e. that there is no photon involved.
it has been noted that this thought experiment, which has been termed the "bubble paradox", foreshadows the famous 1935 EPR paper.
In his 1927 Solvay debate with Bohr, Einstein employed this thought experiment to illustrate that according to the Copenhagen interpretation of quantum mechanics that Bohr championed, the quantum wavefunction of a particle would abruptly collapse like a "popped bubble" no matter how widely dispersed the wavefunction.
The quantum wavefunction is much more a mathematical solution. I expect that Einstein was in the favour of a more realistic physical interpretation.
The transmission of energy from opposite sides of the bubble to a single point would occur faster than light, violating the principle of locality
When you use the concept "principle of locality" you have to describe what it means and that this principle can be applied in this situation.
A much simpler aproach is to write:
The instantaneous transmission of energy from all sides of a sphere to a single point, is physical impossible.
In the end, it was experiment, not any theoretical argument, that finally enabled the concept of the light quantum to prevail. In 1923, Arthur Compton was studying the scattering of high energy X-rays from a graphite target.
"In the end, it was experiment, not any thought experiment etc". That is the 100% preferred way to do science.

4.4 Einstein's light box

The Solvay Debates between Bohr and Einstein began in dining-room discussions at the Fifth Solvay International Conference on Electrons and Photons in 1927.
Einstein's issue with the new quantum mechanics was not just that, with the probability interpretation, it rendered invalid the notion of rigorous causality.
The fact that the outcome of an experiment shows a probability distribution is a valid result. The fact that each experiment shows a (slightly) different result also has a cause.
After all, as noted above, Einstein himself had introduced random processes in his 1916 theory of radiation.
There is nothing wrong to consider that certain processes are random i.e. assuming that they cannot be described rigorous by any law or equation.
Rather, by defining and delimiting the maximum amount of information obtainable in a given experimental arrangement, the Heisenberg uncertainty principle denied the existence of any knowable reality in terms of a complete specification of the momenta and description of individual particles, an objective reality that would exist whether or not we could ever observe it.
It is tricky to make a difference be the concepts reality and knowable reality by means of a principle.
There exists, almost by definition, a physical reality, completely independent of us humans.
At the same time there exists also a hugh limitation for us humans to understand this physical reality, in total, at all its details.
For example it is impossible to calculate the speed of a single photon.
Over dinner, during after-dinner discussions, and at breakfast, Einstein debated with Bohr and his followers on the question whether quantum mechanics in its present form could be called complete.
First you need a definition of what means complete.
Secondly why this discussion?
Einstein illustrated his points with increasingly clever thought experiments intended to prove that position and momentum could in principle be simultaneously known to arbitrary precision.
Doubts are in the details of the experiments.
The only prove exits when you perform the experiment.
For example, one of his thought experiments involved sending a beam of electrons through a shuttered screen, recording the positions of the electrons as they struck a photographic screen.
You need the details of this experiment
On the final day of the conference, Einstein revealed that the uncertainty principle was not the only aspect of the new quantum mechanics that bothered him.
That is completely understandable.
The well-known relationship between mass and energy E = m*c^2 would allow the energy of the particle to be precisely determined.
That is only true if the mass m is precisely determined, experimentally.
Bohr was shaken by this thought experiment.
Consider the illustration of Einstein's light box:
1. After emitting a photon, the loss of weight causes the box to rise in the gravitational field.
We assume that before emitting the photon the needle of the light box is in a certain equilibrium state
If after emitting a photon we actual observe that the needle moves, then you have performed an important experiment.

A simpler lightbox experiment

In the picture shown, it is easy possible that the photon hits the shutter, resulting that there is no mass loss.
That is not what we want and what we should try to prevent.
To do that remove the shutter and the clock and make the cage as open as possible. The photon detector should be outside the box, such that it is not weighted any more.

I understand that maybe this makes all what is wanted impossible, but if we can clearly establish item 1 i.e. that photon emission reduces mass, we have performed a huge task.

2. The observer returns the box to its original height by adding weights until the pointer points to its initial position. It takes a certain amount of time t for the observer to perform this procedure. How long it takes depends on the strength of the spring and on how well-damped the system is. If undamped, the box will bounce up and down forever. If over-damped, the box will return to its original position sluggishly
Exactly the same problem should also happen under item (1), but it is not mentioned. Strange
6. Hence, c^2*dm*dt = dE*dt > h .
The accuracy with which the energy of the photon is measured restricts the precision with which its moment of emission can be measured, following the Heisenberg uncertainty principle.
The problem is that you should actual perform this experiment, and investigating what the outcome is (without using the Heisenberg Uncertainty principle)
After finding his last attempt at finding a loophole around the uncertainty principle refuted, Einstein quit trying to search for inconsistencies in quantum mechanics.
To use the Heisenberg Uncertainty principle, to make apoint or to prove something, does not make sense.

4.5 EPR Paradox

Einstein's fundamental dispute with quantum mechanics was not about whether God rolled dice, whether the uncertainty principle allowed simultaneous measurement of position and momentum, or even whether quantum mechanics was complete. Does a physical
All these issues can not be solved with thought experiments
It was about reality.
It is about the physical reality.
Does a physical reality exist independent of our ability to observe it?
The existance of the Universe has nothing to do with any human behaviour.
The Universe already existed before we humans started to exist.
Ofcourse in order to understand the evolution of the Universe you need observations.
To Bohr and his followers, such questions were meaningless.
Very strange.
All that we can know are the results of measurements and observations.
That is correct. Thought experiments as such are not required.
It makes no sense to speculate about an ultimate reality that exists beyond our perceptions.
When speculation is involved (and concepts like ultimate reality) you move yourself on a slippery road.
Einstein's beliefs had evolved over the years from those that he had held when he was young, when, as a logical positivist heavily influenced by his reading of David Hume and Ernst Mach, he had rejected such unobservable concepts as absolute time and space.
You can only reject something if you first clearly have defined what that "something" is.
The problem is that the concepts of time and time can not be observed.
What can be observed is the bahaviour of a clock and the distance between objects and the length between objects.
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 - Particle wave duality of photons.

The particle wave duality stems from the issue if you consider many photons or one photon.
My understanding is that wave concept is an issue when many photons are considered. In that sense a single flash of light propagates in space as a sphere. The particle concept becomes an issue when single photons are considered.

Closely related is the issue: What is the size of a photon.
This last part becomes the most visible if use one photon in a two slit experiment.
The issue is if you first perform this experiment with one slit (close the second) and then with two and you observe a difference in the (interference) patern of where the photon is detected, then the only explanation is that in the second experiment the photon (part of it) goes two both slits and the size of the photon is an issue.

Reflection 4 - Thought experiments versus hypothesis

What is the better way to understand physical processes: By means of a thought experiment or by means of an hypothesis (i.e. a supposition)?
IMO all the thought experiments explained above are rather tricky, because the gritty details are not discussed and actual tested by means of an experiment.

To answer the question if physical length contraction is involved can not be answered by a thought experiment and because of that the whole meaning of the train experiment in relation to relativity simultaneity is rather dubious.

The bubble paradox has the same problem. How can you perform this thought experiment with the idea that it is in accordance with the physical reality?
IMO considering Electron capture (as discussed in Wikipedia) you should start with a real experiment which actual happens. In fact this experiment should establish that there is a correlation between here, i.e. the place where an electron is captured, and overthere i.e. the place where in electron is emitted. Such a process cannot be performed by a thought experiment.
The next step is to answer the question? how do we explain this. Reading the text in the wikipedia you can see that there are two answers:
  1. One leg by means of the emission of a neutrino at the position here, and a second leg by means of a neutrino capture at the position overthere
  2. One leg by means of an emission of a neutrino and photon at the position here, and one leg by means of a photon and neutrino capture at the position overthere
The second one I expect is much more rare.
The question is how both possibilities realistic. The only way is to perform experiments, maybe thousands of experiments because the second option can be rare.
These experiments have to be performed after leg one, at least to detect a neutrino.
If it is possible to also detect one photon then the problem is solved?
The real issue becomes if suppose, there is definite not a neutrino (*) involved but still there is an electron emission overthere. Only in such a case you can make the hypothesis that there is a photon involved, not something in the form of a wave but more something concentrated at one 'point' i.e. a particle.

The bottom part is that you cannot establish this by means of a thought experiment.
(*) In this particle experiment there is always(?) a neutrino involved. A better experiment is an experiment where you are sure that this is not the case (i.e. something that smells like "action at a distance")


If you want to give a comment you can use the following form Comment form
Created: 10 May 2018
Updated: 14 January 2020
Updated: 23 January 2020

Go Back to Wikipedia Comments in Wikipedia documents
Back to my home page Index