Comments about "Thought experiment" in Wikipedia

This document contains comments about the document "Thought experiment" 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.

1. Overview
2. Variety
3. Origins and use of the literal term
4. Uses
5. In science
6. Relation to real experiments
10. Famous thought experiments
10.1 Physics
11. See also

Reflection

Reflection 1 - Warning
Reflection 2 - Three thought experiments - part 1
Reflection 3 - Three thought experiments - part 2

Introduction

A thought experiment or Gedankenexperiment (from German) considers some hypothesis, theory, or principle for the purpose of thinking through its consequences.
There is nothing wrong to have a theory and to try define an experiment to demonstrate that the theory is correct.: Given the structure of the experiment, it may or may not be possible to actually perform it, and if it can be performed, there need be no intention of any kind to actually perform the experiment in question.
If you can not perform the experiment in practice than the thought experiment has no meaning. For example it makes sense to first make different designs of bridges on paper and finally select the one to build which is the nicest or the most expensive. It does not make sense to design bridges on paper if you know that they will collapse.
: Famous examples of thought experiments include Schrödinger's cat, illustrating quantum indeterminacy through the manipulation of a perfectly sealed environment and a tiny bit of radioactive substance,
The only thing that Schrödinger's cat demonstrates is the interdeterminacy of a radioactive substance nameley its half life time.

1. Overview

Perhaps the key experiment in the history of modern science is Galileo's demonstration that falling objects must fall at the same rate regardless of their masses.
If Galileo actual performed this demonstatration than it is no thought experiment.: Salviati. If then we take two bodies whose natural speeds are different, it is clear that on uniting the two, the more rapid one will be partly retarded by the slower, and the slower will be somewhat hastened by the swifter. Do you not agree with me in this opinion?
How do you know that the natural speeds are different? What are the characteristics of each of these two bodies? To know this you have to perform experiments. You can not use a thought experiment for this.: Salviati. But if this is true, and if a large stone moves with a speed of, say, eight while a smaller moves with a speed of four, then when they are united, the system will move with a speed less than eight; but the two stones when tied together make a stone larger than that which before moved with a speed of eight
Again you have to perform actual experiments if this is true. Large rain drops will fall faster than small rain drops. A package dropped will fall faster than a package connected to a parachute.: Although the extract does not convey the elegance and power of the 'demonstration' terribly well, it is clear that it is a 'thought' experiment, rather than a practical one.
As such it does not any practical value except if you clearly discuss both outcomes.

2. Variety

3. Origins and use of the literal term

Much later, Ernst Mach used the term Gedankenexperiment in a different way, to denote exclusively the imaginary conduct of a real experiment that would be subsequently performed as a real physical experiment by his students.
That is the 100% correct way.: Mach asked his students to provide him with explanations whenever the results from their subsequent, real, physical experiment differed from those of their prior, imaginary experiment.
Again: That is the 100% correct road to follow.

: In Galileo’s thought experiment, for example, the rearrangement of empirical experience consists in the original idea of combining bodies of different weight.

4. Uses

: In Galileo’s thought experiment, for example, the rearrangement of empirical experience consists in the original idea of combining bodies of different weight.
That you can do. But the characteristics (pre-depositions) of the different bodies studied have to be demonstrated based on experiments.
: Regardless of their intended goal, all thought experiments display a patterned way of thinking that is designed to allow us to explain, predict and control events in a better and more productive way.
: You have to be carefull. See Reflection - Warning

5. In science

6. Relation to real experiments

The relation to real experiments can be quite complex, as can be seen again from an example going back to Albert Einstein. In 1935, with two coworkers, he published a famous paper on a newly created subject called later the EPR effect (EPR paradox).
In this paper, starting from certain philosophical assumptions, on the basis of a rigorous analysis of a certain, complicated, but in the meantime assertedly realizable model, he came to the conclusion that quantum mechanics should be described as "incomplete". Niels Bohr asserted a refutation of Einstein's analysis immediately, and his view prevailed.
From a physical point of view a group of persons can have a certain theory or claim. This theory or claim should be clear and describe in enough detail on what it is based, the physics (laws) involved and what the predictions are. A second group can oppose this theory or claim. The next step is for each group to demonstrate that their view is correct and the other party is wrong. Of course to validate a claim that something is "complete" or "incomplete" is very vaque.: After some decades, it was asserted that feasible experiments could prove the error of the EPR paper. These experiments tested the Bell inequalities published in 1964 in a purely theoretical paper. The above-mentioned EPR philosophical starting assumptions were considered to be falsified by empirical fact (e.g. by the optical real experiments of Alain Aspect).
The "experiments of Alain Aspect" of course demonstrate something. The issue is if they clearly solved what is called the Copenhagen Interpretation.: Thus thought experiments belong to a theoretical discipline, usually to theoretical physics, but often to theoretical philosophy.
Theoretical physics only makes sense if its assertions can be experimentally verified.

10. Famous thought experiments

10.1 Physics

Thought experiments are popular in physics and include: Following is a list with "Comments in Wikipedia" about these subjects

Bell's spaceship paradox Comments
Ehrenfest paradox Comments
EPR paradox Comments. Specific:
Bell's inequality
Ladder paradox Comments
Uncertainty principle Comments.
Twin paradox Comments. Specific: Einstein's Box

11. See also

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

Reflection 1 - Warning.

Thought experiments can be very misleading. Specific if thought experiments are used to unravel the laws of nature, if they are complex and not very clear or if two thought experiments are used to demonstrate the same theory.

Thought experiments can never be used just by thinking to discover the laws of nature.
It is for example impossible, just by using your brain to discover that water freezes or can boil. Much more is necessary. You must read, observe, measure and perform real experiments to discover how the results of experiments change as a function of certain input parameters.
a complex thought experiment is Schrodiger's cat which uses radioactive material to demonstrate that the cat can be in two states (life and dead) at the same time or simultaneous.
A situation based on two thought experiments is the Ladder paradox. See Wikipedia Commets: Ladder paradox
The Ladder paradox is a thought experiment to validate Length Contraction which itself is also a thought experiment and very difficult to demonstrate.
Thoughts experiments specific related to SR are tricky. It is very difficult to unravel how objects move through space just by thinking, because our mind has nothing to do with this. Ofcourse you can certain thoughts about the issues involved, but the final tests are in observations of the reality and in performing experiments. Infact what we see gives a very biased point of view. When we observe an approaching train, the train becomes larger, but that is a visible illusion.

Reflection 2 - Three thought experiments - part 1

This reflection is used to challenge thought experiments. Three thought experiments are investigated.

The first thought experiment starts from a 3 D reference frame at rest. Inside this frame there is a 3D grid. At each point of the grid is an identical clock. All the clocks are counters, all the clocks are synchronised and all the clock show the same count. How this is performed is no issue,
At the beginning of the experiment all the clocks are reset.
In this experiment we have a set of spaceships which fly from clock to clock.
- We have one spaceship which travels in a staightline and then back. The readings are from start to finish: 10, 20, 30, 40*, 50, 60, 70, 80.
- A different spaceship also in a straightline. The readings are: 15,30,45,60*, 75, 90, 105, 120.
  The point * is the furtest distance. When the clocks are the same clock (or point in space) then the second spaceship travels slower.
- A third spaceship travels in a circle. The readings are: 15,30,45,60*,75,90,105,120. That means the speed of #2 and #3 is the same, but the clocks visited are different.
The second thought experiment is exactly the same for each spaceshap as in the first experiment. The difference is that in each spaceship there is an identical clock.
- The readings for the first space ship are: 8,16,24,32*,40,48,56 and 64. That means when the space ship returns the final reading 64 as staying at home clock (80). That means the moving clock runs slower. How come?
  To explain that we assume that the speed of light in the rest frame in all directions is the same.
  In fact there are two different explanations because they depent how the clock functions i.e. about the direction of the parallel mirrors.
  1. When the clock operates with two parallel mirrors in the direction of motion then the time of the moving clock is t1 = gamma * t0. See Reflection 1 - Lorentz Contraction Specific Figure 3
  2. When the clock operates with two parallel mirrors perpendicular to the direction of motion then the time of the moving clock is t1 = gamma^2 * t0. See Reflection 2 - Moving Observer Specific Figure 8
The third thought experiment is rather different. We start with a train with has a length L. All the clocks are in the x direction and the difference between two clocks is also L. At the train there are two observers: One observer A at the front and one observer B at the back. That means the train starts between two clocks and both observers are at the same position as a clock. The train will also move in that direction.
The length of the train is 1000 units and the speed of the train is v=1 unit per count.
when the train starts we assume instantaneous speed v=1 and the readings of the Observer A at the front of the train, are for clock 1: 0, for clock 2: 1000, for clock 3: 2000, for clock 4: 3000, etc. The Question is will the observer B at the end also make the same observations?
train B-------->A --X---------X---------X---------X---------X clock 0 1 2 3 4
The issue is length contraction which is a function of the speed of the train. There are two possibilities?
- There is no length contraction. That means when Observer A is at clock 2, her readings will be 1000 and so will observer B at clock 1. When Observer A is at clock 3 the readings are 2000 and so for Observer B at clock 2
- There is length contraction. That means when Observer A is at clock 2, her readings will be 1000, but observer B has already passed clock 1 and is somewhere between clock 1 and 2. When there is a clock at that position B should read 1000, but there is no clock in between 1 and 2 but only at position 1 which B passed earlier. That means the reading of clock 1 should be less then 1000, suppose 800.
  When Observer A is at clock 3 her readings are 3000 but Observer B has already passed clock 2 earlier. Again the same logic as before but now her clock reading of clock 2 should be 1800.
  When Observer A is at clock 4 her readings are 4000 and observer B at clock 3 earlier her readings are 2800

Reflection 3 - Three thought experiments - part 2

In Reflection 2 - Three thought experiments - part 1 3 thought experiments are discussed. The issue is can you realistic perform the three experiments as a thought experiment solely in your mind and predict the outcome? The first thought experiment is not very special. The second thought experiment is more interesting. In that case you try to predict the final readings of two clocks, each on board a spaceship, of which one stays at home (point A). The second spaceship travels from A to B and back to A. The prediction is that the readings are different.
The third thought experiment also requires investigation. In that experiment the issue is length contraction. The problem is there is now simple way to predict that there is length contraction involved. IMO the simplest solution is: there is no length contraction observed.

On the other hand when there is length contraction observed this will have consequences for the outcome on experiment specific based how the clock in the moving train functions: i.e parralel mirrors in the direction of movement or perpencicular on the movement. In that case the distance between the mirrors will decrease which will make the clocks slightly faster but still slower as the clocks at rest ?

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Created: 13 Februari 2015

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