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 Spacetime interval question involving a Pendulum in a Space Ship

    Hello everyone, I was working on a thought experiment and came upon an inconsistent result that I thought maybe I could get some help with. Firstly I am using natural units (c = unitless 1 i.e 1 light second per second) and I chose v = 3/5 to make gamma come out nicely (gamma equals 5/4 when v...
  2. M

    B A thought experiment: Making astronomical measurements from different floors of a skyscraper

    Bob lives on floor 500 in a 1000 meter high skyscraper. Bob has measured the time it has taken a star to orbit 1 round in the Milky Way relative to Earth. He started by measuring at the time when a single photon from the star was received exactly at the center of his telescope, and ended the...
  3. C

    B A thought experiment of the relativity of light

    A light source in uniform motion emits a flash of light. A spherically symmetric wavefront propagates from a central point, the source, or the “origin” of emission. The wave front remains at c relative to the origin as measured by all observers. How fast do these origins move? Am I missing...
  4. R0dr1go

    B Multiverse and Fictional Ideas

    I'm a 16 year old who has wondered this question for a while. If the multiverse theory is correct, and I understand correctly, there would be infinite universes in which each universe would be different from another in some form. If this is the case, would that mean that places and worlds that...
  5. MrSimp

    B Does Force Travel Faster Than Light?

    Imagine a plane 1 light minute across. Now imagine 1 person on either and of that plane. Between them, is a thin indestructible bar that is 1 light minute in length. In the center of this plane, a simple device has a wire that leads to a motion sensor on the left side of the bar. If the left...
  6. G

    A A double slit thought experiment leading to a paradox

    Consider a Double Slits experiment in which the light source is monochromatic and each slit very narrow. There will be many fringes visible on either side of the axis. 1) If the light source is pulsed with very short, randomly spaced, pulses it will produce spectrally broadened radiation and...
  7. P

    B Does this thought experiment show that Space must be curved?

    I'm hoping this is basic and obvious, but assume it's not to the general public. Ignore quantum mechanics and diffraction and assume a gun that can fire photons that each hit the center of a remote target. Place one of these and a conventional gun that shoots bullets at 1000 m/sec inside of a...
  8. Christian Thom

    A Another thought experiment about spin 1/2

    Let's take a beam of spin 1/2 particles prepared in the state |up> in the Z direction, let's pass it through a Stern& Gerlach apparatus in the X direction to get two beams of spin |right> and spin |left>, and then redirect these two beams directly in another inverted S&G to reunite them in one...
  9. T

    Venturi Effect based thought experiment

    The drawing above is regarding a thought experiment of mine. Air at ambient pressure is being blown through a convergent-divergent nozzle and in the throat section, a water filled enclosed container is attached through a tube. The water container has sufficient surface area for supplying...
  10. Lynch101

    I Quantum Ball and Cup - Thought Experiment

    I'm sure most will be familiar with the well-known ball and cup trick. The dynamics of the game itself are unimportant, we just need to have the image of 5 cups with a single ball being revealed when the relevant cup is lifted. The Set-up Imagine a machine which has a conveyor belt coming out...
  11. M

    B Measuring Light Velocity Correctly: Thought Experiment

    Let there be a track 450,000 km long and a rocket 300,000 km long with a laser attached to the bottom of it's back end with a clock beside it, and a second synchronized clock attached to bottom of its front end. Both clocks were also synchronized with a track clock while the rocket was parked...
  12. J

    B Thought experiment (debate with friend)

    Thought experiment. Let's suppose I lay bricks around the entire Earth going up 50 miles in height. I am assuming this way there's going to be more bricks at the top than at the bottom right? Well... What If I was to get a football/soccer pitch (100meters in length and concreted with spirit...
  13. S

    I A thought experiment concerning determinism in quantum mechanics

    According to the uncertainty principle, when we measure a micro-object with a measuring device, we cannot predict what value the device will show. But if we knew exactly the wave function of this device, together with the wave function of the micro-object, could we exactly predict the result of...
  14. C

    B Thought Experiment: Does Gravity's Compression Warm an Object?

    Thought experiment: (1.1) You have a homogenous object, made of one element, floating in space. (1.2) Gravity has completed the process of accelerating its atoms and molecules into the final state, such that the object is a crystalline sphere. (1.3) Gravity continues to act on the object, even...
  15. B

    B When do marbles behave like a liquid in a beaker?

    If we place a cork in a beaker then filled the beaker with normal-sized marbles, the cork will remain stationary at the bottom of the beaker. However, if we continuously decrease the size of the marble, at a certain point, the marbles will behave like a liquid and the cork will rise to the top...
  16. K

    I Information traveling faster than light? (thought experiment)

    Let us consider a hypothetical scenario, where we are able to translate any mass at a constant speed of 10m/s w.r.t to a given frame of reference. For simplicity, we are going to assume that the object is at rest initially. Case 1 - Now, consider 2 points A and B at a distance of 10m, and our...
  17. P

    I Violation of Special Relativity Principle?

    From the michelson-morley experiment, if a clock were to measure the time period of light hitting the mirror and returning back, it would be 2L/c, where L is the distance between the laser nd the mirror. For a moving observer, the time period would have a factor of *gamma*, the boost factor...
  18. Boltzman Oscillation

    B Deriving formula for force by thought experiment

    Hi all, I was thinking punching a round ball on a flat surface and seeing how I could determine a formula for force from it. I thought the following: 1. The ball will go further the harder I punch and thus force must be proportional to displacement d. 2. Ball will go further if it is lighter...
  19. Justice Hunter

    B Thought Experiment : Try and get Alice to "0%" the Speed of light

    Someone asked a really interesting question on a comment thread somewhere's, and ever since, I could never really stop thinking about what the proper answer to it could be. It's a really basic question, but it unpacks a can of worms. The exact question I read was the following This question...
  20. StevieTNZ

    I Another Wigner's Friend Thought Experiment Paper

    https://phys.org/news/2021-05-future.html The paper in question can be found at https://www.nature.com/articles/s42005-021-00589-1 This thread is for discussion of the said paper.
  21. Sophrosyne

    B Twins Paradox Thought Exp: What Happens When Reunited?

    I have been looking through some of the threads about the twins paradox in relativity. It’s clear there’s a lot of confusion on this, and I am yet one more person very confused on this. So I was thinking about a hypothetical experiment, and I will lay out my hypothesis of what might...
  22. C

    Wall made of Pendulum -- thought experiment

    If your house wall is made of pendulum or a ball attached to a string on top. It won't attract seismic forces. Whereas if the wall is fixed solid. It can attract seismic forces. What is the physics explanation of it? Is it because you try to deflect momentum or inertia? What is the right...
  23. L

    A Thought Experiment: Aether Medium

    I would like to conduct a thought experiment. If some form of medium exists, what would it be made of? What form would any constituent parts take? Would those parts exhibit any properties? How would they function as a medium? What relation would the medium have with atoms and other particle...
  24. John_Dee

    Thought Experiment - Centripetal Force

    Ok, so we know that if one were inside a donut-shaped spaceship that is rotating around it's axis, that the passengers will experience centripetal force. It seems obvious to say that the ship is rotating relative to the nearby stars and planets. So far, so good. But... what if we removed all...
  25. B

    B Measuring a change in gravity - thought experiment question

    Imagine I hang a mass from the ceiling of a building using a rope. The mass is hanging 2m above the floor. Also tied "in line" with this rope, and between the weight and the ceiling, is a scale that measures the weight of the mass. Let's say the scale reads 10 Newtons. Now, between the...
  26. Someone_physics

    A Thought experiment in relativistic quantum mechanics?

    Background --- Consider the following thought experiment in the setting of relativistic quantum mechanics (not QFT). I have a particle in superposition of the position basis: H | \psi \rangle = E | \psi \rangle Now I suddenly turn on an interaction potential H_{int} localized at r_o =...
  27. R0dr1go

    B Is the Universe Expanding Due to Spinning?

    Hello all, I recently did a thought experiment and thought, "what if the universe as a whole is spinning?" This could solve the dark energy problem, as if the universe was spinning, there would be outward pull, and therefore keep the universe expanding. And, I don't know if this means anything...
  28. AN630078

    Thought experiment: How would ocean levels change?

    I am not too sure how to approach this, initially I thought it may be more of a calculus and related rates of change problem, i.e. finding an expression how the volume and height of water change with respect fo time. I do not know whether this is the right idea or how to progress any further...
  29. starkreactor

    I Thought Exp: Gen Relativity & Time Dilation

    I am trying to understand a thought experiment I just posed, which is: if an observer is traveling near the speed of light, and sends out two photons; one in the direction of travel and one in the opposite direction, how does general relativity account for time dilation? The photon "in front"...
  30. J

    B Thought Experiment: Exploring Violations of Bell Inequalities

    I have been thinking about the Violation of bell inequalities , trying to justify how non locality can be determined from violation of bell tests. I have been through Dr. Chinese page which has partially convinced me that there can be no hidden variables , but I need to understand what...
  31. Hamiltonian

    B A thought experiment about the Sun vanishing

    suppose suddenly the sun disappears at a time t. at this arbitrary time t, the Earth should fling off tangentially to the point in its orbit at time t as there is no centripetal force keeping it in orbit. we know light takes about 8 minutes to reach the earth. so will humans on Earth experience...
  32. P

    A Thought Exp: Transform Laws & Feedback Needed

    I'd like to get some feedback on the following argument. Gallilean mechanics, with the Gallilean transformation laws, is a perfectly consistent theory. Special relativity, with the Lorentz transformation laws, is another perfectly consistent theory. The question is - can we have some physical...
  33. quasar987

    I Thought Experiment Proving Newtonian Momentum Not Conserved

    On p.170 of French's book on special relativity there is this thougth experiment attributed to Lewis and Tolman (1909). It is about two individuals throwing identical balls of mass M at each other with identical speed. The balls bounce against each other and are caught again. See attached...
  34. G

    B Length Contraction Thought Experiment: Spot Mistake/Wrong Assumption

    Alice travels in a spaceship, which she measures to be L. The spaceship is moving with velocity v relatively to Bob. Alice makes a light beam traveling along the spaceship and measures the time interval it takes to go from one end to another, ΔtA. So, equation (1): L = c × ΔtA From Bob's...
  35. Steve Blandford

    I Thought experiment on photon entanglement

    This is a thought experiment I had with myself a couple of weeks ago. I have never seen it proposed. But please be kind I am not physicist but just a hobbyist. So this experiment is specifically for entangled photons. So here goes. I was thinking of something going faster and faster and time...
  36. S

    B Is there a thought experiment to show that the speed of light is constant?

    I know the amazing thought experiment by Albert Einstein with the two light clocks. (The observer at the train station has a light clock and the person in the train.) It's amazing because you can even deduce the formula to calculate how fast the clock in the train goes. But this experiment...
  37. just dave

    B A thought Experiment in Time Dilation

    Ok so here's my idea,it came to me while watching a record album spin on my turntable(I believe it was Dark Side of the Moon). Technically this exp. could be done now but the results would take a while. You take a disc of some super strong material attach it to a ultra high RPM motor and put...
  38. L

    Faster than light thought experiment

    Hi :) I just had a (most likely totaly unoriginal) thought and thought I should try to ask someone who's thinkings on physics are somewhat more refined than mine. So after thinking twice about private messaging Mr Tyson and Mr Cox :) I decided to google 'physics forum'. So here I am with my...
  39. K

    I Sending Signal from Planet Near Gargantua: A Thought Experiment

    The other day my friend asked me a really interesting question regarding the scene from interstellar where they go down to Miller's planet, where every hour on this planet is 7 years of Earth time. He asked me if they were to send a signal to the spaceship where Romilly was, what would happen...
  40. R

    I Special Relativity: Comparing Lengths in Parallel Motion

    In Introduction to special relativity by Resnick,there is a thought experiment to compare lengths perpendicular to relative motion as given in the below image. What if we try to perform such an experiment to compare lengths parallel to relative motion? Suppose there are two horizontal rods...
  41. Clay Gillespie

    Thought Experiment about light speed

    If we’re looking through a telescope at a craft we launched from Earth that is now passing Mars and send a radio signal to our craft telling it to turn on one of its lights on and it takes thirteen minutes for the radio waves to get from Earth to our craft how long will it take before we see the...
  42. Shay Katzir

    B Is this an adequate thought experiment?

    I've been struggling to develop an intuition about Einstein’s relativity, without luck, until i invented my own thought experiment, to help me. Now i feel i have the intuition, but it leads me to a new misunderstanding, i hope someone here might help me clarify. My thought experiment involves...
  43. B

    Parallel Plate Pulsed Proton Beam Generator (Thought Experiment)

    I am trying to come up with a design for a device with which to generate a rapidly pulsed proton beam (on the order of 1 pulse per ms). My thought was to apply a 10 kV potential between two parallel plates (each with a hole in their center) and inject a steady stream of hydrogen gas through the...
  44. D

    B Einstein's Train Thought Experiment

    So I got this from an article on wikipedia that covers Einstein's train thought experiment: A popular picture for understanding this idea is provided by a thought experiment similar to those suggested by Daniel Frost Comstock in 1910[13] and Einstein in 1917.[14][12] It also consists of one...
  45. dRic2

    B Thought experiment -- A radioactive decay detector and randomness

    Take one radioactive element and put a detector all around it so that you can immediately detect whenever it will undergo radioactive decay. Have a clock connected to the detector to note the "exact" instant at which the atom decays. Let's say that after 3min after the clock started counting the...
  46. J

    I Exploring Simultaneousness in Einstein's Thought Experiment

    Let's modify Einstein's thought experiment on simultaneousness. A person G on ground sits in the middle of two fireworks. The fireworks are wired (equal distance) to an ignitor just in front of the person G. A person T on a train of constant velocity V passes by the ignitor and pushes the button...
  47. P

    B Superposition of traps (thought experiment)

    Hi Pf Instead of placing a screen on each path of an interometer i propose to place opticz traps. in both cases it will unable the photon to go further. i wonder if one on the traps will be heavier or if there will be symmetry (superposition).
  48. Frederick T

    B Travel at 0.5c & Observe Redshifted Light Traveling at c

    It's come to my attention that if we were to travel in a spaceship at 0.5c directly away from our sun, I would observe that the light from the sun as being red shifted. What I would also observe was that this red shifted electromagnetic wave would in fact travel away from me in the spaceship...