What is Thought experiment: Definition and 345 Discussions

A thought experiment is a hypothetical situation in which a hypothesis, theory, or principle is laid out for the purpose of thinking through its consequences.
Johann Witt-Hansen established that Hans Christian Ørsted was the first to use the German term Gedankenexperiment (lit. thought experiment) circa 1812. Ørsted was also the first to use the equivalent term Gedankenversuch in 1820.
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. Physical and mental experimentation could then be contrasted: 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.
The English term thought experiment was coined (as a calque) from Mach's Gedankenexperiment, and it first appeared in the 1897 English translation of one of Mach's papers. Prior to its emergence, the activity of posing hypothetical questions that employed subjunctive reasoning had existed for a very long time (for both scientists and philosophers). However, people had no way of categorizing it or speaking about it. This helps to explain the extremely wide and diverse range of the application of the term "thought experiment" once it had been introduced into English.
The common goal of a thought experiment is to explore the potential consequences of the principle in question:

"A thought experiment is a device with which one performs an intentional, structured process of intellectual deliberation in order to speculate, within a specifiable problem domain, about potential consequents (or antecedents) for a designated antecedent (or consequent)" (Yeates, 2004, p. 150).
Given the structure of the experiment, it may not be possible to perform it, and even if it could be performed, there need not be an intention to perform it.
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, and Maxwell's demon, which attempts to demonstrate the ability of a hypothetical finite being to violate the 2nd law of thermodynamics.

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  1. moe darklight

    Squaring The Circle (thought experiment)

    OK, so I understand mathematically why one can't square a circle, but when I do the following thought experiment I can't see how one could not square the circle: - I tie a string at both ends and lay it on a table so that it forms a perfect circle. - Now I place four pins, at equal distances...
  2. A

    Opinion/discussion/advice on a simple thought experiment please

    I am not a physics expert but have been having a discussion about the effects of the Special Theory of Relativity with a friend. I came up with an example where time does not actually alter at high speeds, although it can be perceived to by a stationary observer because of the time it takes...
  3. DaveC426913

    Why is Schrodinger's Cat only a thought experiment?

    The explanation I've heard is that quantum superposition is an atomic-scale phenomenon, that it makes no sense to apply it to a macro-scale object such as a cat. But that's not what the experiment is doing. The radioactive isotope is subject to quantum effects, and it seems to follow that the...
  4. snoopies622

    Thought experiment on potential energy

    If I lift an object, does its mass increase due to my investing it with potential energy? If I then drop it, does its mass increase further as it falls due to the acceleration? I don't see a contradiction here, but my intuition tells me that something is amiss.
  5. Y

    Long planet and Galileo thought experiment

    A long time ago, Galileo stated the vertical motion of a falling body is not influenced by its horizontal motion. Is that still true in moderns terms, taking relativity into account? For example, if a particle is fired horizontally at 0.8c, will it hit the ground at the same time as a...
  6. H

    Thought experiment: Implosion reaction

    I have question. If we take an ordernary nuclear bomb and inject anti-matter into the core, and find a way to isolate the matter/antimatter reaction from the rest of detonation. Would it be possible to use force of the nuclear bomb to generate a black hole and thereby vaporize an entire...
  7. C

    Why did Einstein postulate the theory of the speed of light in an at-rest frame?

    You know the 'thought-experiment'...the spacehip traveling at a speed close to the speed of light...where a photon is emitted upwards to a mirror, bounces off and returns to its source. The space traveller (observer 1) standing by the emitter observes a simple up and down motion. A second...
  8. S

    What Happens to My Wave-Like Nature When Driving at Half the Speed of Light?

    Say I was to somehow get in a car and drive at half the speed of light or so... my wave-like nature would show up right? What do you think this would look like from my perspective? from an observers perspective? What does it even mean to acquire a wavelength. What kind of wave do I become...
  9. J

    Problem with thought experiment

    Please be patient, as you are dealing with a relativity newbie here! I've read Einstein's book, "Relativity: The Special and the General Theory", and I can't seem to figure out chapter nine, "The Relativity of Simultaneity". To refresh your memory, there is a train moving at a constant...
  10. N

    Thought Experiment on Dark Matter

    Thinking about the hypothetical dark matter, I came to the following (primitive) thought experiment. Consider a large sphere (say radius R) filled uniformily with matter and a small test mass located at radius R1. This test mass will experience a force to the centre of the sphere and the...
  11. P

    Einstein's Train Thought Experiment: Exploring Lack of Simultaneity

    why we do not observe any “lack of simultaneity” when we substitute sound signals for the light flashes in the “thought experiment” with a train moving past a platform (Einstein's lightning strike-train "thought experiment")? this has troubled me for 5hours now. i don't know whether it is too...
  12. K

    Thought Experiment on ToR postulate-1

    'All laws of physics hold good in all inertial frames of reference.' observer A - standing still on earth observer B - in jetplane-1 flying at mach-2 jetplane-2 is moving at mach-1 co-directional to jetplane-1. A observes in accordance with Newton's 3rd law. B observes, fuel level...
  13. M

    Help with a thought experiment.

    I was working on a thought experiment to help myself understand the curvature of space when I got stuck on something (I hope) is simple. I realize this might be a basic problem but I was hoping to generate some discussion. The experiment involves a "universe" two neutrally charged masses of...
  14. X

    Thought Experiment - Measuring speed of light

    Here's my thinking: The faster you go relative to the universe, the more the universe appears to contract. However, c remains constant as measured by distance/time. Now, what about to a photon? To a photon, the entire universe would appear as a single point, correct? To this same photon, c...
  15. D

    Yet another spaceship thought experiment

    Suppose there's a spaceship accelerating at a constant rate, say g, thanks to some magic rocket engine. This engine creates a constant force on the ship, which is sufficient to accelerate its constant mass at a constant rate. Supposing that the floor is perpendicular to the line of travel, the...
  16. I

    Dumb Thought Experiment: Nonconducting Sphere with Point Charge and Conductor

    suppose you have a sphere of nonconducting material with a point charge not in the center but in one octant and around it you have a conductor. if examined outside the conductor,will there be a stronger field near the octant where the charge is in the insulator? gauss' law says no, vivid...
  17. L

    Is this thought experiment posible?

    Take a 10 m long cilinder 2 m wide, cover the top and put it on water in such way that the level inside the cilinder is 10 m above sea level Take a 1 square m float,and put it inside the cilinder from sea level it will float 10 m up Now cover the bottom of the cilinder and then uncover the...
  18. G

    What is Einstein's thought experiment about special relativity?

    I do not understand Einstein's thought experiment explaining special relativity. If I understand it correctly, you have two people. One is on the center of a moving train, and the other is on the ground next to the tracks. As the one on the train and the observer pass each other, lightning...
  19. M

    A question about a thought experiment in space

    Attached to this post is a thought experiment in space. It is only one page (38kb) in microsoft Word. At the end of the page is my question, and I hope some of you may help me about it. Thank you in advance.
  20. F

    Thought experiment in relativity of simultaneity

    My text reads thus (the part which is bolded is that which I don't understand, I just reckon that the rest is needed for the context) : This seems very wrong to me. Am I incorrect in saying that Mavis must see the two flashes simultaneously, and also must conclude that they happened at the same...
  21. A

    Clock Synchronization and the Effects of Gravity in a Two-Story Building

    My friend and I are in a two story building. I am on the lower floor, he is on the upper. We are in a constant gravitational field. Is it true that our watches cannot be synchronized? If so, what is the proof of this claim?
  22. A

    Gravitational Time Dilation: A Thought Experiment

    I stand shoulder to shoulder with a group of people, we all face the same direction. We all have synchronized watches and nobody is moving. We face a second group of people, facing us. They all have synchronized watches and nobody is moving. Now suppose that my group feels like they are in a...
  23. W

    A black hole thought experiment

    Suppose I dive head first into a black hole of size 100 light second from far away. When I reckon that I will cross the event horizon in 1 second, I throw a stone to my right at 1/100 the speed of light, which pushes me to my left at 1/10000 the speed of light. Here are my questions: a. will...
  24. J

    Oddball thought experiment with electrons

    Hello All, I was thinking about how electrostatic fields are used to accelerate electrons in different applications (like particle research) and I thought of a question I might run past you all. Let's say that we have an electron that is at on side of a set of charged capacitor plates...
  25. J

    Thought Experiment with Iron wire

    Hello All, Consider the following. Take a cylindrical magnet (say 2” long by 0.5” diameter and wrap it with iron wire in such a fashion that the windings do not touch (assuming it is uninsinuated). Wrap just enough turns of wire so that the magnet’s field is essentially short circuited...
  26. I

    Spawn of black hole thought experiment

    This thread is being started to avoid having https://www.physicsforums.com/showthread.php?t=124037" bunked down here to the realms of philosophy. We'd hit a few assumption walls, too many. I'm the OP of the parent thread, and I'm repliing to JesseM whose raised some fine point about why...
  27. I

    Hollow spherical blackhole thought experiment

    I have a thought experiment for anyone interested. All replies welcome: Imagine if you will, a large spherical body e.g. a moon, about the surface of which are placed many large thermonuclear devices. Deep inside the moon sits an intrepid/foolish experimental physicist. When the the devices...
  28. C

    Age Difference in Earth Orbit Around Another Star

    If we were to take the present Earth and set it into gravitational orbit around another star revolving at a rate of once every 92 Earth days, would the people in that orbit age faster, slower or at the same rate as those in the current Earth's orbit?
  29. L

    Thought Experiment: What Happens When Photons are Added to a Vacuum Space?

    I have come across this question in another forum and thought to post it here, hopefully will get some interesting views. This is a thought experiment so the setting might not be achivable with todays technology and knowledge, instead try to look at the implications of this set up rather then...
  30. E

    On this thought experiment in which kinetic energy seems to be created

    please help on this thought experiment in which kinetic energy seems to be created first i would like to set two things straight: first this is a question of something that seems very strange to me not an affirmation, second my intention is not to challenge but to try to help trying to improve...
  31. N

    Thought experiment / paradox question

    Let's say I start out with a (spin) neutral electron beam. Now, I can place an up/down Stein-Gerlach spin filter/detector in the beam so that I have a running tally of the "up" electrons, and a spin-down exit beam. Since the measuring device is accumulating spin-up electrons, it should also be...
  32. Ε

    Understanding Einstein's Thought Experiment & Question 10 of the Relativity Quiz

    I just took the relativity quiz here: https://www.physicsforums.com/quiz.php?quizid=28 and I goofed on question 10. Could someone explain it to me? Below I've posted what the review of the quiz said about question 10 along with some of the previous questions for context. This seems to...
  33. A

    Easy thought experiment imposible fro me

    i have a ball over a frictionless table tight to a cable tight to a frictionless axe in the middle of the table if the ball weights 1 kg, the rope is 1 m long and the linear speed of the ball is 100 m/s the tension on the rope will be 10000N (v*v*m/r) if i want the rope tension to reduce...
  34. A

    Thought experiment that shows hor the 3rd principle fails on spirals

    im in a 10 m tower, a 1kg cannonball is shot underneath me horizontally at 10 m from me and I am holding a cable that is tight to the ball, for the ball to make a circle the tension shown in a dinamometer put in the cable would be v*v*m/r= 1000N being the speed of the ball 100 m/s i shoot the...
  35. E

    Thought experiment that if solved will prove 3rd law to fail in spirals pls help

    im in a 10 m tower, a 1kg cannonball is shot underneath me horizontally at 10 m from me and I am holding a cable that is tight to the ball, for the ball to make a circle the tension shown in a dinamometer put in the cable would be v*v*m/r= 1000N being the speed of the ball 100 m/s i shoot the...
  36. B

    QFT thought experiment in 1+1 dimensions

    First off let me start by stating that I am aware that you must get a great amount of "snake-oil theories" on this board and that I have no interests in "peddling". I am merely a high-school student, very interested in physics, who was lucky enough to stumble across these forums. I had an idea...
  37. M

    Simple reflection thought experiment

    I am going to start out with a simple thought experiment: A particle is moving towards a wall at a 90 degree angle. The wall is perfectly smooth; therefore, when the particle bounces off, it will be traveling along the same path as it did when it came in. Does the ball ever come to a...
  38. T

    'Speed of Light' Thought Experiment

    Consider the following situation: a car is moving away from a stationary observer with velocity V. At distance S it triggers a contact in the road which sends a light signal back to the observer from the moving car. The question is how long will it take for the signal to reach the observer ...
  39. K

    EPR type thought experiment with black hole

    Assuming that I have an atom that produces two photons having opposite polarization, and I send one toward the event horizon of a black hole and the other is sent to a second observer who does not measure his photon's polarization until enough time has passed to insure that the other photon has...
  40. G

    Exploring the Electric Activity of Inert Gases in the Atmosphere

    I just recently saw a video in Physics class about Einstein's thought experiment that led to his discovery of Special Relativity. I was just curious how he knew at that time that light always moves at c even when the observer of the light is moving. I know that is the most important part of SR...
  41. petmar

    Visualizing Space: A Thought Experiment with a Rotating Rod and Flashlight

    A "Simple" Thought Experiment Imagine you're out in space with no gravitational influence. Now, you have in this space a rod, 1 light-minute long, of some super-ridgid material. At either end of the rod, there's an astronaut. Astronaut 1 also has a flashlight. Astronaut 1 rotates the rod on...
  42. F

    What happens when you turn on the light in a simple thought experiment?

    This is a thought experiment that I've done many times over the years. I like to create a nice and simple cosmos that allows me to select the variables I choose to particapate in my experiment. I always start out with a solar mass of some size. The first time I did this experiment, I just...
  43. D

    A Revolutionary Idea: Rethinking Time Measurement in Physics

    Well guys, I'm back. I thought about it some more and perhaps I have a way to reach you in spite of your utter refusal to think things out. In case anyone reading this does not know who I am, I am that idiot who has suggested that the physics community has over looked something significant...
  44. Y

    Light Clock Thought Experiment: Parallel Moving Mirrors

    I have raised this issue in a different manner before - but never got a satisfactory analysis. In the traditional parallel moving mirrors (separated by distance d) light clock thought experiment, the photon is considered to travel back and forth over the same path in the moving train (both...
  45. N

    Thought experiment for using Black Holes for cmmunication

    Let's say that we have two planets -- planet A and planet B, 100 Lightyears apart in space. Centered directly between them is a black hole with a 25 lightyear Schwartzchild radius. (The idea is that it's 25 lightyears fromt the Schwartzschild surface to each planet.) If someone on planet A...
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