Thought experiment Definition and 345 Threads

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. I

    Question about Einstein's train thought experiment

    Hi all. I'm a run-of-the-mill layman—not a student. I've just developed an interest in physics lately and have been watching/reading a lot pop physics. So hopefully my question's not too stupid and hopefully the answer's not too far over my head. My question is about the thought experiment...
  2. S

    Rephrasing the train and platform thought experiment

    Hi all, I'm sure that things similar to this have been covered before. Also, I'm sure that a Minkowski diagram would probably clear things up no end but, anyway, please bear with me. Let's assume that we have our familiar railway carriage packed with observers who are satisfied that they...
  3. S

    Thought Experiment: The flow of time at different parts of the univers

    Hey all, Just a thought experiement here. With time dilation caused by gravity and relative velocity being a confirmed phenomena, do you think it's possible that other worlds that are extremely far away from each other, and are moving at a very high speeds relative to one another with...
  4. N

    Einstein's train thought experiment

    https://scontent-a-ams.xx.fbcdn.net/hphotos-prn1/601539_10201811069735094_388925769_n.jpg Why did they define the lightning event to be simultaneous for Stanley? Why can't it be simultaneous for Mavis, so that the lighting hits A first?
  5. F

    Thought experiment with electron and proton

    Suppose a universe with only one electron and one proton some distance apart from each others. Suppose that the motion of the particles is governed only by Coulomb's law and strong interaction. What would happen ? The electron and the proton would keep going back and forth through each others...
  6. N

    Just a thought experiment on fluid mechanics.

    I am thinking that... if a tank of water which contains a small wooden block is kept in an elevator which is accelerating upwards from rest, will the block get more dipped or remains constant position with respect to water??
  7. K

    Thought experiment with one photon

    Assume you have observer #1 who emits one photon (can that even be done?) and an observer #2 who zips (at .01 c relative to observer #1) past observer #1 at the same moment that observer #1 emits the photon (observer #2's direction is the same as the direction of the photon). One year later is...
  8. N

    Normal modes of a string thought experiment

    Hey! So If I have a stretched string of length L fastened at one end, and I am moving the other end sinusoidally, will a standing wave appear ONLY if I move the other end with one of the normal-mode frequencies of the string? If not, will the resulting wave be a moving wave which is a...
  9. T

    This is a thought experiment in relativity, can you try to solve this

    Consider two guys A & B who are separated by a distance of 1 light hour (they are at rest to each other) Then A decides to fire a bullet at B. He took his gun and fires. The bullet moves with a velocity of 90% speed of light (here the bullet is hypothetical therefore it moves at that speed at...
  10. Ookke

    Gradual acceleration with converging series (a thought experiment)

    Let's imagine a rocket that travels straight path through space, accelerating gradually (or being boosted by outside force). The speed of rocket is such that 1st light year takes 1/2 hours in rocket's own frame 2nd light year takes 1/4 hours 3rd light year takes 1/8 hours 4th light year...
  11. Low-Q

    Question of conservation of energy in this thought experiment

    Hi, I have a somewhat silly question about something I cannot figure out properly. I have a cylinder with a piston inside. The cylinder is sealed and filled with air. The piston is free to move back and forth inside the cylinder, but are also sealed so no air can escape no matter how much...
  12. R

    Thought experiment of relativity

    please explain einsteins thought experiment of relativity
  13. G

    Thought experiment about fluid compressibility

    I'm a physics noob but this is something I was thinking about when I read in my textbook that fluids are generally incompressible. Let's say I had 1000 L of water enclosed in a sphere several meters thick (say, 2 meters thick) made of an extremely hard, dense metal with 0 outlets or holes of...
  14. B

    Exploring the Possibility of Converting Mass into Energy: A Thought Experiment

    Suppose in the distant future, converting mass into energy is something as that common and easy as heating food with microwaves are today. Now, suppose we built one of these energy to mass converters in space and another mass to energy converter here on earth. Additionally, we connect the two...
  15. S

    Thought Experiment: Should Atheists End all Metabolism in Their Brain?

    Hello, I'm an atheist: a mentally disabled one with depression and no future. So perhaps this may bias my point of view: When we atheists die, we will have no memories and conscious awareness of our success and failures in life because our consciousness and memories are lost for eternity...
  16. tomwilliam2

    Double-slit experiment, thought experiment

    I'm trying to understand the implications of the double-slit experiment, and I have been considering what QM theory would predict as the outcome of the following experiment: Consider an electron gun firing electrons towards a detection screen two metres away (gun perpendicular to screen), and...
  17. B

    A thought experiment of wavefunction collapse

    I propose a simple thpught experiment; A scientist A measure the spin of an electron and finds it "up". After that he removes all evidence of his experiment and goes away, only to die in a road accident. Another scientist, obviously unaware of the experiment done by A come and measures the...
  18. T

    Energy thought experiment (oldschool very)

    This might seem a bit weird at first but it raised an interesting question to me about power, work and energy. I watched this show about how the trebuchet is so much more efficient than previous designs and then I sort of thought of it as a way to store energy and I wondered to myself, if you...
  19. M

    Time Dilation Thought Experiment

    A ninja the size of solar system has a sword that could slice through something the size of the sun. For the purpose of the thought experiment let's imagine a black hole and the time dilation effects around it. Now take away the black hole but keep the time dilation. What would happen to the...
  20. V

    Thought experiment involving spacetime curvature:

    Peter Donis and Nugatory taught me a lot about spacetime curvature yesterday, but it has left me with so many questions. It sounds like mass slows down time as it warps spacetime. So, I suppose this means: more mass = more spacetime curvature = less time elapsing. Okay, in addition to...
  21. A

    Hypothetical thought experiment about time dilation

    Let's say that you leave Earth at 75% the speed of light (c). You travel out and back for a minute. And when you come back an hour has passed on Earth. (I understand this is not accurate; I'm just using it for the sake of this hypothetical) You leave again at 99% of c, again for a minute. When...
  22. N

    Quantum eraser experiment - another thought experiment

    First of all, I am new to this forum but I already appreciate the great efforts invested here and the quality of many of the discussions! This is all pretty neat! My mind has just been blown by reading about the "Quantum Eraser" experiment.. It shows that it is the availability of the...
  23. B

    Exploring Boson Behavior in a Thought Experiment

    Hi guys, Let us consider this thought experiment of having say Avogadro number of bosons in a box. According to statistical mechanics, it is equally probable to find every distribution of bosons in the box. But, say we wait really long enough to find that at one point of time, we find all the...
  24. edpell

    Hydrogen molecule in a small box, thought experiment

    What would happen if we placed a hydrogen molecule in a box with sides less than the Bohr diameter. That is, a box so small the electrons could not orbit around the protons at a distance of a Bohr radius. What state would the two electrons and two protons take?
  25. S

    Thought experiment to test Relativity of Simultaneity

    The thought experiment starts with the basic experiment shown here: http://en.wikipedia.org/wiki/Relativity_of_simultaneity "A flash of light is given off at the center of the traincar just as the two observers pass each other. The observer on board the train sees the front and back of the...
  26. H

    Thought Experiment: Fuel Consumption in Outer Space

    My brother and I were discussing fuel consumption which lead to this thought experiment. Let's say you have two identically shaped spaceships of different masses traveling in outer space where they are not effected by any forces. No wind drag, no friction, no gravity, etc. Let's assume...
  27. V

    A thought experiment on relativistic Electrostatics

    Here is a thought experiment on Special Relativity involving charges which are rest with respect to each other. Consider a configuration of charges as shown in the image. Four identical charges (q) are placed at the corners of a square with an opposite charge at the centre (Q). The value of...
  28. C

    Thought Experiment: Exploring GR Foundations

    Hello! In order to deepen my understanding of GR foundations, I tried to create something like thought experiment. I would like to post it so you criticize it and tell me if this a correct thinking or just a delusion I have created in order to fill my mind gap. Here it is: Suppose there is an...
  29. X

    How can I determine the moment of inertia of an irregular object?

    Hello everyone, (This might be a little long winded but I would very much appreciate some help on this!) Today while in psychology class, I was thinking about a much more important subject, namely a project I want to start working on. Basically what I want to do is build a motor from the...
  30. H

    A Thought Experiment in the HUP

    A number of threads in this forum oscillate between pure mathematical descriptions of the HUP and QM generally, while others *try* to provide a physical, visualisable causal model to explain them. Heisenberg's original quantum microscope explanation for the HUP was such an example which...
  31. J

    Coriolis force thought experiment

    Let's say I build a 500 m long circular road around the north pole. Then I drive on the road at speed 20 m/s, to the east. There will be a noticeable inertial force to the south. Is that inertial force a Coriolis force?
  32. N

    Thought experiment about time dilation

    My knowledge toward physics stay on high school level, and all i know about relativity are from wiki, so please make the explanation simple~ In wiki, it said it is possible and logical that B's time runs slower relative to A while A's time runs slower to B. But i still can't understand how...
  33. I

    Thought experiment about energy

    I have a thought experiment for physicists and thermodynamicists and the like. Those who are always looking for 2+2=4 but for some reason avoid discussing certain issues that are very clear. Simply put the picture where we have an object of mass m in the ground. The physics math show that we...
  34. L

    Who came up with the Light Clock Thought Experiment

    Everyone is familiar with the Light Clock, a thought experiment which forms the entire basis of Special Relativity. Through the Light Clock, the Lorentz factor can be derived. So who was the genius who first came up with the Light Clock? Was it Lorentz himself? Or someone else? When did it...
  35. M

    A Relativity-Related Thought Experiment that has Given me Trouble

    Hello everyone, I'm not a physics major or anything but I have an interest in math and physics. I do know the basics of relativity and with what I know I can't make sense of this. (I came up with this on my drive home after a long calculus lecture, in case you're wondering.) Imagine that you...
  36. I

    Average Randomness Thought experiment

    "Average Randomness" Thought experiment Please bear with my explanation, I am, by no means a Physicist. During a recent theoretical thought experiment, I thought of a strange factor that I feel I should let out before it becomes forgotten by the constant wave of thought experiments that flow...
  37. A

    Pushing a stick thought experiment

    "Pushing a stick" thought experiment Hello, I'm new here and I wanted to ask a question that has been bugging me for a while. I did a quick search in the forum but frankly not sure what to search for. Imagine you have a long stick that stretches from the Earth to the sun. If I push on one...
  38. K

    Thought experiment on Simultaneity

    I am having a variation of the classic simultaneity experiment (Explained in http://en.wikipedia.org/wiki/Relativity_of_simultaneity#The_train-and-platform_thought_experiment) From the midpoint of a train 2 laser beams are sent to both sides of the train. The beams reach the 2 end-points of...
  39. T

    A thought experiment (excuse me if I have wrong information, i'm new)

    So, let's think of a world where all the decisions are made by probability, e.g. flipping a coin, and quantum physics was never discovered. Now, let's say they use a device, a contained version of the double slit experiment, except that the particles are supposed to land in one of two...
  40. P

    A thought experiment on gravitation

    A thought experiment on gravitation... Let us suppose we have a very large planet X which is assumed to have some acceleration 'a'. Suppose we keep on digging a well and afterwards we drop a small mass 'm' then due to the force there will be an acceleration. As acceleration is change in...
  41. T

    What is the explanation behind the binary result in the EPR thought experiment?

    Hello dear readers, I have a problem. I'm giving a lecture tomorrow on the EPR experiment and Bell's theorem and so on. I had the feeling I really understand the essence of the EPR thought experiment. However, a few hours ago I just had a disturbing thought (while practicing the lecture in...
  42. E

    A time dilation thought experiment

    Let's say we have two objects in space, far from any gravitational source. In each "object", there's an observer, and a clock. Each observer is constantly looking at both clocks(his and the other object's). Now, for observer A, observer B is moving at 0.9c, and for observer B it's the exact...
  43. I

    Probability basics - Heads or Tails thought experiment.

    Right, This is very confusing for myself, I'm pretty sure it will be so for everyone else. My question is really related to quantum theory, but I find it most appropriate to spell it out here as it's more of a maths question on the basics of probability. Here my experiment: I stand at a...
  44. J

    Gravity, Pressure and forces - a thought experiment

    Hi, I just wanted to get some peoples opinions on a thought experiment I've had. If you can provide references to your input this would be helpful. If you only want to pass comment then keep that to yourself. I'm really looking for constructive feedback in terms of science, learning and...
  45. T

    Need help with a thought experiment about point of view,motion, and gravity.

    Ive been thinking about orbits. We have plenty of points of reference(stars) that help us determine our orientation within the milky way ect. So we can effectively determine orbital paths, rotation, ect. So I've been thinking about the Earth and the moons orbital interaction independently of any...
  46. P

    Thought experiment with two point sources of light

    You are in a dark room with no other sources of light The walls are painted black, so no light is scattered from walls You have two point sources of light of equal power + same height about ground They are 10 feet apart You stand a thick rod vertically in middle of two, and 4 feet in...
  47. J

    Thought experiment: Rising bubble in rigid container

    I came across a blog which posted a paradox and it really got me thinking. This is what it read: You have a perfectly rigid jar, which is sealed and filled with a perfectly incompressible liquid. Within the liquid, at the bottom of the jar is a small bubble filled with an ideal gas. Since...
  48. M

    Heisenbergs thought experiment

    Heisenbergs "thought experiment" In two different quantum mechanics books I read about "thought experiments" involving using a microscope to determine the position of an electron, which were both presented as evidence to back up Heisenbergs uncertainty principle. Are there no real experiments...
  49. J

    Please Explain to me this thought experiment

    Suppose a rocket is moving away from Earth at 0.5c. The rocket is moving towards a beam of light. There are two clocks on the rocket placed 1m apart. Each clock stops ticking when the beam of light hits it. The speed of light is then calculated as d/t = 1/(t1-t2). What effects occur for the...
  50. J

    Thought experiment: Origin of inertia is gravitational?

    Hi, What do you think of this thought experiment that demonstates a situation where the origin of inertia must be gravitational (Mach's Principle)? Imagine an empty box with perfect mirrors on its walls. Now let us pump in a large amount of photons into the box. Let us assume that...
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