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. C

    B Quantum Mechanics vs. Pilot-Wave thought experiment

    Hello. I want to share a thought experiment that could tell Quantum Mechanics apart from Pilot-Wave interpretation. It goes like this: Quantum Mechanics vs. Pilot-Wave: Quantum Mechanics: Waves collapse to particles. Waves disappear when particles are detected. Pilot-Wave: Waves are real but...
  2. J

    Quantum antenna thought experiment

    If you have a classical antenna absorbing an electromagnetic wave, the charged particles inside the antenna will be given momentum in the directions perpendicular to the direction of propagation of the EM wave (because the E and B fields are perpendicular to propagation). If just a single...
  3. T

    Confused about EPR Thought Experiment

    In "Einstein's Moon" by F. David Peat there is a description of the EPR thought experiment,but I am confused by Bohr's response to Einstein given in Peat's book. In the EPR as described by Peat, particle A and particle B move in opposite directions after the entangled particle (AB)...
  4. L

    Miscellaneous wonders: The Carbon Nanotube Earth Veil

    Okay so to start, this is just about the most random inapplicable proposal ever, so bear with me. Its just a cool question to consider. So, here goes: What would happen, (again, this is highly theoretical) in the instance that an enormous screen - much like the ones that rests behind most...
  5. Dan Allred

    Thought Experiment: How would life on Earth have developed if....

    What would have happened to to the evolutionary process had the Earth's axis of rotation had no tilt? What comes to me first would be a total lack of anything based on the year calendar. There would be no deciduous trees or dormant grasses, certain animal species would have no particular mating...
  6. M

    B FTL Paradox: Grandfather Paradox Explained

    So here it goes: 2 points in space A & B. 1 superluminal object (fixed at 10c, no acceleration) that periodically transmits photons as soon as it starts moving. 1 observer with a very high accuracy photon counter removed a sufficiently large distance from the 2 points for light to take a small...
  7. S

    B Speed of Light & Thought Exp: How to Send Info 186K Miles Instantly

    I've built a rod 186K miles long, of ultra-light, ultra-rigid material. I am on one end of the rod, and an observer is at the other end (we're both in the near vacuum of space). I want to send a signal to the observer on the other end of the rod signaling whether the Packers have won the...
  8. choopie

    A fun thought experiment: the science of electric superpowers

    Context: I'm an artist working with a writer on a superhero comic. One of the characters we were thinking of adding is a person with electricity powers. However, the writer wants the electricity powers to be somewhat more "realistic" (at least, aside from the whole electric super-powered human...
  9. Gaz

    Exploring the Constant Speed of Light in a Thought Experiment

    Hi I want to create a thought experiment involving waves of any kind. I am going to create a thought experiment I am on a boat that is at rest in the ocean. I am standing on the boat measuring the size of the waves and the frequency. So I measure the wave along the side of my boat to have a...
  10. S

    Changing Momentum p -> L -> -p?

    Ok, so as far as I understand it, it is impossible to turn linear momentum (p) into rotational momentum (L), but I don't quite understand why. The main thought experiment I have in my head is this: A ball in space is traveling with a momentum mbVb, and gravity and friction are assumed to be...
  11. F

    Can s oribtal electrons only drop to other s orbitals?

    Just a thought: The momentum quantum number given for s states is l = 0. If an S state electron dropped in energy, and emitted a photon with l = 1, what would take place? Am I correct that the electron could only drop to a lower l = 0 (an S state) orbital?
  12. T

    A thought experiment paradox

    I am pretty familiar with the theories of special and general relativity. I know how to add velocities, I know the main postulates and the experimental confirmation. However, I thought of the following thought experiment: There are 3 experimenters (at the same point in space), who adjust their...
  13. Erik Ayer

    Stimulated emission and polarization - a thought experiment

    Imagine that one has a single photon of 632nm. It enters the back end of an open-ended HeNe laser tube - one with no mirrors - and along its path, causes the emission of another photon. According to the wikipedia entry on stimulated emission (https://en.wikipedia.org/wiki/Stimulated_emission)...
  14. F

    A Thought Experiment: Photons Passing through Detectors

    Pretend we have a multi-slitted grating whose slits are infinitesimally small. We On each slit is a detector. If we shine (UPDATE) light monochromatic light through the grating, what distribution pattern occurs on the screen past the grating? I understand the train of thought to a slight...
  15. F

    Torque on a fixed reference of falling masses

    I had a little thought experiment, in which there are two objects with the same masses near each other (same height) on freefall. If I set up a point that is on an instant besides the two masses and call it the center of torque, I get that the torque produced by the nearest one's weight is...
  16. patrickbotros

    Gas or liquid to adsorb an explosion

    Obviously we don't have the technology to make this work now but I would like to know how plausible this is as a concept/thought experiment. Basically the idea is to discover a gas or liquid with a very high activation energy (for some reaction) so that it can absorb an explosion or control a...
  17. votingmachine

    Who would be interested in an EPR thought experiment?

    If a thought experiment with hidden variables gave the same results as EPR experiments, would that be a publishable (even if minor) contribution?
  18. A

    Exploring Quantum Mechanics: A Thought Experiment Reveals Surprising Results

    I've been reading some cursory stuff about quantum mechanics, and I want to understand it better - so I have a little thought experiment that I'm curious about. Let's say there are two rooms that are connected by a thin wall with no window. A person is told to stand in one of the rooms for a...
  19. Logan Johnston

    How Is This Quantum Analogy Incorrect?

    Homework Statement "Sometimes the idea of the quantum is compared to the units we use for money. A dollar can be divided into smaller units, where the cent is the smallest possible unit. How is this analogy incorrect? Homework Equations E=nhf The Attempt at a Solution My thought is that...
  20. 0

    Energy and Momentum Thought Experiment

    Suppose two balls A and B are connected by a compressed massless spring initially, and they are not under any other forces. The spring is then released and the result is observed. Scenario 1: Ball A has the same mass as Ball B. Scenario 2: Ball A has greater mass than Ball B. Define the...
  21. aladinlamp

    Can a Sealed Device Increase Weight Continuously in the Direction of Gravity?

    Imagine red box as a fully enclosed device powered by electricity when turned off, it weighs 5kg, it is pushing down with force cca 49 Newtons when turned on, it would weight more, let's say 6kg, pushing down with force cca 58.86 Newtons is it in theory possible to make such device...
  22. Justice Hunter

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    Hey so i have a naive question that i always had since i was young, but i never really could find an answer to it. Lets say you have a black hole isolated in a vacuum, in a closed system. Dark matter is supposed to be a sort of weakly interacting massive particle, which exerts a gravitational...
  23. newjerseyrunner

    If space is curved, could it be detected

    I was contemplating the size and shape of the universe. Our observable universe is perfectly 3D spherical, but that's a result of a finite speed of light and a beginning of time, and I was wondering if there was a way to deduce the size of the universe. I imagined the universe as a 4D sphere...
  24. CaptDude

    Thought experiment on movement

    Question: If there were only a single elementary particle in the entire universe - could it move, and if so, how would its movement be measured?
  25. T

    Silly current path thought experiment

    Hi, I want to varify something that I intuitively know (I think), that might not seem intuitive. If an electrical current is circulating and then it's path splits in two, where there is a circulating current. The larger current (black) will have to pass through this loop; what I want to varify...
  26. G

    Thought experiment; what happens if you alternate an image

    like a person with a background and just the background really fast, like a strobe effect. If you cycle back and forth fast enough would the person appear like normal or 50% see through?
  27. A

    Thermodynamics thought experiment

    There is some ideal gas in a container moving with some velocity on a smooth surface and you suddenly stop it( say by using your hands) , will the temperature of the gas increase? It seems to me that since you're suddenly stopping it there is no work done (because of no displacement ) so the...
  28. L

    Thought experiment on magnetic field propagation

    Some 30 years ago I learned in the elementary school, that if I move a permanent magnet in a coil which ends are somehow electrically closed, than I need to apply force on the magnet, because the generated current creates a magnetic field which is against the magnet's field. However if the...
  29. I

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  30. Moriarty

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    My SO showed me this on reddit and we had fun with it. If you were sent back to the early 19th century naked and alone how would you convince someone that you were from the future and not crazy? No tricks allowed (meaning you can't pull a raygun from your colon). Let's hear it.
  31. S

    A thought experiment general relativity

    Let's assume the Earth was perfect sphere with radios of 6,750,000 meters. There are 3 clocks , one will not move one will move west in velocity V and the other will move east in the same velocity. The Earth is still rotating. So one clock is going with the Earth rotation and the outher is going...
  32. B

    Twin Paradox: Will Car A Age Slower?

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  33. R

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    (from my limited understanding) In our observable universe a photon could travel say, 5 billion light years in a straight line if unperturbed. Call the points X and Y traveling down-up. If point X was moved one Planck length to the left, would it still be able to travel exactly to point Y?
  34. Zack Davis

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  35. Solvay

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    I'm curious about time dilation sizes of below 4 situations. Please assume I'm observing the situations far away in zero gravity. And please ignore all SR effects, just focus on GR. http://dishdev.me/data/timedilationq.png Equivalence Principle says acceleration and gravitational fields can...
  36. W

    Gravitational attraction thought experiment

    Hi all. New to physics forum and glad to be here. I have been referencing the site for a while now and have finally come up with a question i haven't been able to find on here. So I guess i thank all of you who are already on here for the help. Assumptions: -Imagine an infinitely...
  37. J

    Train thought experiment confusion

    I'm seeing examples where for the person in the train the offset of both lights at the end of the train are not seen as simultaneous, while for the observer on the platform it is seen as simultaneous - and vice versa from other sources. What's going on? Furthermore, if my school textbook is...
  38. J

    Quantum eraser thought experiment?

    http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser From the DCQE experiment above , assuming we could , 1. Send the idler photons on a long round trip that too a year or so before going through the double slit which is in the same lab as the detector D0. 2. Take...
  39. T

    Thought experiment of Special relativity

    I have a little bit confuse on the Simultaneity I know if there are 2 coordinates. One coordinate S' is moving V relative to S. We all in inertial reference frame. like the following graph...
  40. A

    Thought Experiment: Where Does the Missing Energy Go?

    Hi, I figured out a thought experiment/problem, here it is: A friend of mine is living in a galaxy far away. He has a machine which can make individual photons with a definite frequency (lets say blue light) and can emit them toward me. Here I have machine which can detect these photons...
  41. blintaro

    Spring Energy Thought Experiment

    Hi guys, I work at the tutoring center of my university, and recently a student came in with a question that has been troubled me ever since. Say you have a certain mass M a distance Δχ above a massless spring. When asked how much energy this system has, you would say it has only...
  42. M

    Acc. due to earth's rotation - Thought Experiment

    Hi All, I have always been haunted by this basic question and will be grateful to anyone who can shed light on this! Please consider this experiment - A cube is kept on a plane horizontal surface on the equator (where the Earth's rotational speed is the maximum). If the friction between...
  43. T

    Information Faster Than Light - Thought Experiment

    Assuming it were possible to build a sturdy enough rod like structure, say equal the length it would take light to travel in two minutes. If you were to push one side of said rod and move it, would the far side of the rod move before light were able to travel the length of the rod? If so...
  44. S

    Einstein's thought experiment about the speed of light

    Please excuse this question from a lay person but if I don't ask I will never know. I know that the speed of light is a constant, that no matter how fast an object travels, light travels away from that object at the same speed. Hence nothing can travel at or faster than the speed of light, time...
  45. D

    Moving fairground light, thought experiment

    Ok, I have an experiment, well actually it is more of a question than an experiment. I have a very elementary background when it comes to relativity. So anyway, here is what I am wondering: Picture an extremely long line of those lights you see on the fair grounds (the ones that are timed to...
  46. M

    Heavy question; the gravity of a thought experiment

    Just thinking about gravity. How is the gravity effected by the surroundings of a location? Suppose we have a giant farris wheel with very good bearings and we place it in a location; like the Grand Canyon - somewhere with a very steep dropoff; such that say 40% of the wheel is over the...
  47. ecoo

    Einstein's Train Thought Experiment

    I recently watched a video in Einstein's train though experiment. www.youtube.com/watch?v=wteiuxyqtoM From what I got from it, events can appear to be at different times when compared to each other depending on the observer. But isn't there an absolute event timing of when the events...
  48. H

    Alan Guth thought experiment about negative energy and gravity

    Recently, I watched a lecture with Alan Guth. He made an interesting statement. He said that to create an electric field requires work, because the like-charges prefer to repel one another, therefore this requires energy to bring them together. However; because gravity is attractive, it requires...
  49. F

    Just a simple thought experiment

    When I first learned the Work-Energy Theorem, I was a little confused. After all, it implied that no work was done when you push against a wall that doesn't move, but I know that I still feel tired after pushing against a wall. So, suppose you have an electro-hydraulic piston bolted to the...
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