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. R

    Einstein thought experiment, different outcome of a same event

    One person on the ground and the other traveling in a train. Two light detector X and Y are placed in the train at equal distance from the observer in the train. If only one is hit first by the light, only that detector will display its name else if both detector are hit simultaneously...
  2. StevieTNZ

    Does Ghirardi's Thought Experiment Challenge Quantum Mechanics Predictions?

    Anyone who follows some of my posts knows I've fallen in love with GianCarlo Ghirardi's thought experiment. (apologises for the equations - I don't know how to work LaTeX). The experimental set-up is a 45 degree polarised photon goes through a birefringent crystal. On the other side of the...
  3. P

    Thought experiment regarding artificial gravity (Newton's third law in space)

    I was recently reading about artificial gravity and generating it in space, especially by centripetal force using Stanford Tori and Bernal sphere (only Wikipedia and these to articles, but if anyone has any more resources, I would be glad to read them, I couldn't find anything else in my...
  4. S

    A thought experiment regarding the nature of space expansion

    I devised this using a setup that I found easiest to imagine, while trying to be clear on how modern physics considers space to expand. We have a nifty device that can cause spatial distortion (no, you cannot has one!). Before we use it, we lay down two meter rulers, end to end. They both...
  5. marcus

    Distance expansion and escape velocity ( thought experiment )

    Distance expansion and escape velocity ("thought experiment") If you care to, it would help if you would check my arithmetic. I may have made one or more mistakes. Thanks to anyone who can show the conclusion here is wrong. The question came up if you have a pair of kilogram masses and you...
  6. D

    Thought Experiment regarding Time

    Consider this scenario involving twins. The twins, A and B, are some unspecified distance apart and are placed inside a container. They are told that one of them will be traveling to the other at a given fixed speed, but things will be done so that neither twin will know which one did the...
  7. E

    Thought experiment for general rel., I can't remember how it goes

    There is a thought experiment pertaining to general relativity that involves a signal traveling from the front to the back of an accelerating space ship, if I remember correctly. I'm sorry for being so vague, but this is something I saw browsing in a library years ago, and I really wanted to...
  8. C

    Free Energy Thought Experiment Question

    I have a thought experiment I don't understand. Imagine there's a grain of salt that weighs 5 mg and it's 1mm above above a pool of water and it's on a very thing piece of flimsy material that takes about 1*10^-5 Newtons of force to break. Using Newton's 2nd law, I get the grain of salt is...
  9. R

    Can a Car's Stereo Still Function at the Speed of Light?

    Hello all! I was not sure were to post this, but I was asked a very interesting thought experiment on Facebook today and was wondering if I could get everyone's opinions on it. Ok, so suppose we could really annoy Einstein and manage to get a car to go the speed of light. The question is...
  10. Z

    Acceleration thought experiment

    In the thought experiment about the laboratory at rest in a gravitational field and another at a constant acceleration, could one not tell the difference by firing light of a known frequency from a height above you, in the gravitational laboratory the light would get blue-shifted, while in an...
  11. M

    Einstein's original 'thought experiment'

    I'm in the middle of reading 'The Great Equations' by Robert P. Crease (good book) and I've just got to the chapter on E=mc^2. The chapter covers Einstein's original thought experiment. You know, the one with the mirrors and the stationery / moving observers. I've worked through the maths...
  12. B

    Thought Experiment on the speed of light

    OK, so I am not a physics major, but I think that I understand that matter or energy cannot travel faster than the speed of light. Today I was using a laser pointer, and I noticed that it traveled rather fast, and it traveled faster as it went farther away from me. So if you imagined the laser...
  13. G

    Does Traveling Affect Age? A Thought Experiment

    Let's say two people are born in a hospital at the same time. Now person A spends his entire life sitting around his house and not moving much. Person B spends most of his life traveling around in high speed cars/planes/and even takes a few trips to space. Considering time slows down...
  14. A

    Quantum Entanglement FTL Thought Experiment

    I am not sure if I understand quantum entanglement so I will propose a thought experiment which you can tell me if it is correct Say I entangle two quanta of matter, such that by observing one I collapse the wave function of the other immediately. Then I give one of the particles still in...
  15. L

    Time dilation thought experiment clarification

    Sorry for yet another time dilation question. I have a couple example problems that I can't seem to explain. 1) A ship leaves the Sun and heads for Earth at 0.5c. An observer on Earth would see the the light from the sun take 8 min, and the ship take 16 min. The pilot's clock would show it...
  16. M

    What is expansion ? A thought experiment

    what is "expansion"? A thought experiment Dear Cosmologists, I am eager to understand the concept of "expansion". I've heard and read a lot about red shift, background radiation and the fact that distances between galaxies are increasing. I think I understand the basics of special...
  17. S

    A thought experiment with theory of relativity.

    Hi, I had a question on theory of relativity. It is more of a thought experiment. As per the theory of relativity, as the speed of an object increases time slows down. If any particle travels faster than the speed of light, theoretically it has to travel back in time. So from this we can...
  18. M

    Red/blue shift in classic train/platfom thought experiment?

    To avoid confusion, I'm talking about: "A flash of light is given off at the center of the traincar just as the two observers pass each other. The observer onboard the train sees the front and back of the traincar at fixed distances from the source of light and as such, according to this...
  19. C

    Some thought experiment on intelligence

    Hey! This occurred to me in a conversation with a friend, so nothing overly serious. Suppose that through some means, for example a surgery, we could amplify the intelligence of people without altering the memories they accumulated throughout life, including how fast things proceed, like...
  20. T

    1/2 kilogram black hole: thought experiment

    Greetings! Would this be a fair estimate to the radius of a black hole? Assumptions: The equation for escape velocity is adequate for calculation of black hole radius. \upsilon = \sqrt{\frac{2M}{r}} Working from there, setting the desired velocity faster than light would mean nothing could...
  21. A

    SR/LET Thought Experiment Help - Three clocks problem

    Firstly, hello. This is my first post at Physics Forums, and I want to thank everyone in advance for contributing their thoughts and expertise here. I'd like some help with the following thought experiment. My apologies if I'm posing too basic a problem, but I haven't been able to find...
  22. M

    Flash of light thought experiment, need explanation

    So suppose in frame K at x_1 a photon is emmited and its absorbed at x_2= x_1+L so, the photon in K has traveled a total distance of L. Now suppose we have frame K' moving at some velocity Beta*c. Why is it that in K' the distance traveled by the photon is different than L if the speed of light...
  23. R

    A simple hands on hypotesis / thought experiment / question /

    A simple hands on hypotesis / thought experiment / question / :) Hi, first I'd like to say that I've been trying to understand the way electricity works for some time now and thanks to the really great articles of William J Beaty, I really think I'm starting to grasp this stuff now.. Now, so...
  24. Orion1

    Quantum Fluctuations and the Origin of the Cosmic Microwave Background Radiation

    Consider a thought experiment for a moment that hopefully does not violate Physics Forums rules... A sphere is coated with a thin unexposed photographic film, the sphere is placed in a dark room and exposed to an external environment of point sources of light. The film is developed and has...
  25. soothsayer

    Electrodynamics thought experiment

    So I was thinking about E&M today, and specifically, how E&M relates to the ideas of relativity. Specifically, that magnetism is induced by a change in an electric field and vice versa. This got me thinking of some scenarios. In each, you have a particle emitting a constant magnetic field and...
  26. T

    Maxwell's Demon Thought experiment

    I recently just heard of this thought experiment where two rooms/resoviors whatever you want to call them are adjoined by a door that a little demon controls. Both rooms have the same temperature, The demon then opens this door when particles with higher kinetic energy are nearby to let them...
  27. R

    Thought experiment on the nature of light

    There have been a number of very interesting posts lately around the nature of light. So I devised a simple thought experiment to try and understand it a bit more. Say John and I are next to each other in rocket ships, at rest wrt to Jane, who is 1 light minute away in her rocket ship. All...
  28. J

    What is the ratio of radii for a black hole and steel ball in equilibrium?

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  29. I

    Question on the reasoning behind the light clock thought experiment.

    I read that the speed of light is independent on the relative velocity of the source. So why is it that the beam zig-zags if there is relative motion between an observer and a light clock. This zig-zagging implies the vector addition of velocities. I know that the speed c is kept constant during...
  30. S

    How Fast Does My Friend Appear When We Travel in Opposite Directions at 0.7c?

    If I travel away from Earth at 0.7c, relative to the Earth and my friend travels in the opposite direction at 0.7c, relative to the Earth, what speed to I measure him to be traveling at, relative to myself?
  31. M

    Traveling at Light Speed Through Space: A Thought Experiment

    My brother-in-law proposed the following thought experiment: Suppose a person was sitting on the nose of a spaceship traveling at the speed of light through outer space. Now suppose that person pushed off against the spaceship launching himself ahead of it. Is it the case that that person will...
  32. K

    A thought experiment for surpassing the speed of light

    You have a straight, light, hard, inelastic rod at hand. It's pretty long, long enough so that you can hold it out of your window and into your friends window at the other side of the street. The way you and your friend communicate is through morse code. You move the rod a distance x so that...
  33. Q

    An interest thought experiment

    Suppose we have a detector which can detect the electromagnetic wave. If there exists the electromagnetic wave, the detector will flash. Now, A holds this detector and runs at a constant velocity relative to a charged particle. Will the detector flash? I think it will. In Chapter 11.10 of...
  34. V

    Thought experiment: Observation via Recording

    I'm just a physics dilettante, so if I make no sense, please forgive me. Imagine any setup that creates macroscopic effects from quantum events. The setup is isolated from observation. Schrödinger's cat would work, but I happen to like cats so we'll just say a light either goes on or doesn't...
  35. R

    Understanding B and H Fields: A Thought Experiment on Magnetic Field Propagation

    Hey everyone, I'm learning about B and H fields in electromagnetic waves and propagation class. Trying to wrap my head around the idea that magnetic fields wrap around current carrying wires. Consider this: If you were to take a clear, hollow cylinder, drill a hole at both ends, run a...
  36. Y

    What happens in this thought experiment?

    Let us say we have a type I source of entangled photons (so that we get a positive correlation if the polarising analysers are aligned). After leaving the source both photons are passed through a vertical polarising filter and continue in opposite directions. Anne is closer to the source and has...
  37. N

    Why is there a time dilation in relativity and what are its implications?

    the source from where i read about the above experiment also mentioned that the outcome is favoured for bohr's point of view( mathematically equations by bell)... if that's true____ why relativity limit of speed of light?
  38. F

    Entangled photon polarization thought experiment disproof request

    I was trying to search for a similar topic, but as I haven't found one here it goes. Thought experiment: Periodically emitted chain of single photons of non-polarized light is split into two entangled pairs and they are traveling apart from each other. One going toward point A, the other toward...
  39. A

    Who Would You Be After Death: The Revival Thought Experiment

    OK, let's say you died. For the sake of simplicity let's say the cause of your death was just your life processes suddenly stopping. All your memories, etc., remained in your brain. Now let's say you were revived by some new tech. QuestionL would it be YOU who were now experiencing in the body...
  40. M

    A simple thought experiment | Please confirm

    Please excuse my naivete, but this thought experiment has been running through my mind all week. Suppose you have three people in room. Suppose the room is sealed such that no air can come in and no air can come out. Suppose there's enough oxygen in the room for us to conduct our experiment(...
  41. K

    Triangle GR thought experiment

    Hi all! I am still a GR noob and this example has been puzzling me for a while, so I need some help that's hopfully. Let's imagine that there are stars A and B at 10 LY from Earth (at point C) each and 10 LY from each other. A signal is sent every year from Earth (C) indicating the...
  42. D

    Multiple clock thought experiment

    Consider a large number of clocks each within some apparatus programmed to accelerate away from, remain moving for a time, accelerate toward, remain moving for a time and then return to a point of origin. A similar clock simply remains at that point of origin. The accelerated clocks will reach...
  43. Y

    Ehrenfest / rod thought experiment.

    This is new thread on an issue that was that getting slightly off topic in the original thread. Let's consider a slight variation of the Ehrenfest experiment. The fairly rigid carriages are all linked together by elastic couplings and are on a suitably highly banked track. As the...
  44. J

    Conservation of Energy in a Gyroscope Thought Experiment

    Conservation of Energy in a Gyroscope Thought Experiment I have posted a though experiment at the following address. I would appreciate anyone's thoughts on this. http://sibleysystems.pro/gyro/gyro.html" Thanks in advance. Jamie
  45. T

    Optics thought experiment and 2nd law of thermodynamics

    Imagine a closed isolated system consisting of 2 rooms separated by a window. The window is built with one layer of material that has refractive index 1.5 and one layer of material that has refractive index 2, for a wide range of wavelenghts (say 1 \mu m - 50 \mu m), and low absorption. The side...
  46. G

    Thought Experiment: Portals, Magnets, and Power

    I was surfing digg yesterday, and saw the following post: http://images.wildammo.com/2010/07/24/world-energy-problems-solved/ For those who don't recognize it, it's based off a video game called 'portal'. In the picture, the water falls towards the floor where it falls through the blue...
  47. I

    Does light must be wave-like in Einstein mirror clock thought experiment?

    Hi, In attempt to understand SR, a moving mirror clock is often given as part of a thought experiment, which leads to the dismissal of absolute time. To save people reading, the short version of my question is this: This experiment assumes light is wave-like, right? The long version of...
  48. E

    A thought experiment: Light in a mirrored box?

    There is a perfect box with with no gaps or cracks, with mirrors on the interior walls of the box. There is also a light bulb which can emit light but reflects any that goes upon it (with a super two-way mirrored surface, perhaps?). If one switches the lightbulb on, then there is light in the...
  49. T

    Sun disappearing thought experiment

    I have viewed other threads regarding this thought experiment but have not seen any that answer my question. "If" the sun disappeared, the Earth would just continue on in a straight line, I understand that. But what about our moon? Would it continue to orbit the Earth as the Earth traveled...
  50. R

    Light speed thought experiment

    I'm trying to get my head around an idea that i had and i was wondering if you people here could clarify for me i am sure some of my basic assumptions are wrong so here goes 1 objects cannot go faster than the speed of light, it take more and more energy to accelerate an object as it...
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