Questions tagged [thought-experiment]

Hypothetical situation in which, for simplification, only some physical aspects are considered in order to further understand or reveal contradictions between the laws governing them.

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What happens if a traveling sound wave encounters vacuum?

Suppose a sound wave is emitted by an object in a medium like a gas so it travels in some direction. If the wave meets a rigid object, for example a wall, it reflects back as one should expect; and if ...
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Implications of an infinite and no-information lost universe [closed]

Assume, for the sake of the question, that the universe is infinite, and that no quantum information is ever lost (no-hiding theorem). Does this mean that every piece of information we generate today ...
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Ambiguity in applying Newton's shell theorem in an infinite homogeneous universe

Newton's shell theorem has two corollaries: The gravitational attraction of a spherically symmetric body acts as if all its mass were concentrated at the center, and The gravitational acceleration ...
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Pressure in giant ball of water floating in space

In this question: Swimming in a ball of water in space They seem to have reached the conclusion that the pressure inside a giant ball of water in space is zero. Quoting the second answer: As a ...
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How much motion in fluids is random motion?

Say that the only pressure differences in the atmosphere was due to the gravitational pull of the earth. That is, say pressure only changed as a function of height. So we're assuming there's no wind, ...
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What is exactly/literally meant by observing in Schrodinger's cat thought experiment? [duplicate]

As far as I know, Erwin Schrodinger used this thought experiment to show the absurdity of the interpretation of the quantum mechanics. But many times I encounter on web even famous physicists say ...
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A thought experiment about time dilation

Suppose you are on am infinitely long train traveling on tracks and an atmosphere that has no resistance whatsoever. No this train is traveling at fractionally below the speed of light, and you are on ...
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If the only two empirical measurements we can make are change in length and passage of time, how are dimensions and units of quantities formulated?

So this is something really confusing to me. Recently a professor of mine gave a talk and said something that I thought was very interesting and kind of crazy. "The only two quantitative measurements ...
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What does Earth feel like when squeezed?

he average density is about 4g/cm^3 so if the earth was in the size of an orange, it would feel like a rather heavy ball. The crust is relatively thin, and the earth is said to be solid land masses ...
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Two spheres falling through a pipe

Imagine a pipe of radius $a$ perpendicular to the ground. A liquid flows in the pipe. Now you throw (instantaneously) two balls with radius $b$ (where $b << a$) through the pipe: one through the ...
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Does the EPR paradox assume that entangled particles leave each other's light cones, or not?

So I was reading this post by Ethan Siegel, which introduces a recent talk by Lee Smolin. In the article, Siegel outlines the EPR paradox (or something like it) and mentions that it is based on the ...
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Would protons in a straight line ever break their alignment? [closed]

Let any amount of arbitrarily spaced protons appear along a line segment of length $x$ in an otherwise empty universe at $t+0$ What mechanism, if any, could cause the breaking of said alignment ...
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Infinite Electricity from Photon Inside Closed Box with Magnet Attached in a Coil? (Thought-experiment)

Let's say I have a very large box with a magnet attached to the outside. Inside the box there is a perfect mirror on each side. I open my box and quickly shoot in one photon before shutting it. After ...
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How to prove that shadows in the cave aren't talking? [closed]

If a physicist (who have never seen people) is constantly watching on shadows and hear voices behind, will he make a statement that shadows are talking? I'm talking about physical experiment in terms ...
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Isn't mass from pure topology in the absence of matter a contradiction?

Consider the 3 dimensional projective space minus the infinity point, and empty of matter. We see that it has ADM mass. In other words, a perfectly fine geometry, orientable and asymptomatically flat,...
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Gravity breaks Newton's third law?

Consider a thought experiment where there is a point mass M present in empty universe. Then another point mass m materializes (This is ONLY a thought experiment) at a distance of 1 light year from M. ...
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“Plushness” of a simple mechanical damper?

What heuristics generally define the "plushness" of a mechanical damping or braking system? For example, if a person or some mass is to collide with a fixed wall or the ground (e.g., after a vertical ...
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Thought experiment - Universe that follows $f \propto \frac{1}{r}$ law [closed]

In our universe, due to reasons usually attributed to the 3D geometry of it, the forces of nature seem to mostly be $\propto \frac{1}{r^2}$, inversely proportional to the distance of the source of the ...
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Do Events Conditional to Simultaneity Occur in Every Reference Frame?

Imagine a train with a table that contains a light on both ends (so that the line between the two sources of light is parallel with the line of motion of the train) and a light sensor in the perfect ...
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1answer
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Einstein's Simultaneous Lighning and the Lorentz Transformation

I am reading Einstein's book, "Relativity: the Special and General Theory" translated by Robert W. Lawson, and I have some questions concerning Special Relativity -- specifically, the thought ...
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Proposed experimental considerations of a micro-mechanical oscillator in a superposition state

This question asks for the experimental result that is theoretically expected or is known already from experience, when one considers the following two scenarios for the micro-mechanical oscillator ...
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1answer
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Clarification on Heisenberg thought experiment

Heisenberg imagined a high energy photon was absorbed by an electron, but in order to make a measurement on that electron it will have to absorb another photon which means it velocity gets higher but ...
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Relativity thought experiment - spaceship travelling at speed of light [closed]

I have basis knowledge in special relativity, but I haven't studied it for too long and I'm stuck in a simple thought experiment. Assume that there is a spaceship departing from the earth at speed of ...
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Compression of gas within a partitioned cylinder

I have been doing some thought experiments, and would like some help in thinking about two situations: a) There is a (metallic) container on a weighing machine. This container has an upper chamber ...
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1answer
91 views

Einstein Elevator thought experiment

Were there any background theories for Einstein's elevator thought experiment which developed the equivalence principle. Can Newton's equivalence principle (the equality of inertial and gravitational ...
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Special and general relativity

Is the light postulate in special relativity (that the speed of light is constant in all frames of reference) correct if the equivalence principle shows that light slows down in gravitational field? ...
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I have a thought experiment that defies the uncertainty principle - what's wrong with it?

There's a particle somewhere on the x-axis (this whole thing is 1-dimensional) at x0, which we do not yet know. I fire a photon at it from the origin with a known speed (c) and a known wavelength λ1 ...
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How did Einstein use the conclusions of special relativity to reach the equivalence principle (using the Elevator thought experiment)?

Could someone please explain how Einstein used the Elevator thought experiment and the principles of special relativity to conclude that uniform accelerates creates uniform gravitational field (...
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A twisted triple paradox

As far as I know when a person travels with some velocity relative to other person time runs slower to him relative to the other person (Because the velocity of light is constant in all frame of ...
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Can we theoretically guide cold air from high altitude down to the needy cities as free AC?

If you had an insulated tube (approximate size say 15k feet (5km) tall, 50ft (15m) diameter) with the top end open and bottom end closed, would the air in the tube eventually become as cold as the ...
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Rigid bar on a pivot

Say we have a solid bar in space. It is on a pivot, the pivot being right at the bar's center of mass There is a massless rocket pushing on one end of it, making it spin, faster and faster. Suddenly,...
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Do we need infinitely many objects to make one have kinetic energy?

We have one ball at rest. How can we give it energy to make it move? One way is by striking it with another moving ball, for which we need another ball with energy. Then how can we make the other ball ...
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Can you short out eddy currents using opposing magnetic fields?

Suppose you put a varying magnetic field source on one side of a thin metal sheet. You would get eddy currents in the corresponding direction. Now you take another similar source and place it exactly ...
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For a laser that changes color discretely between each ultra fast pulse, what color would the laser appear to the naked eye?

Say you had a laser that pulsed faster than your eye could perceive it, so that it appeared to fire constantly. Imagine that in between each pulse the laser changed to every color of the rainbow for ...
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Does a black hole evaporate by the time an observer reaches the singularity? [duplicate]

Assume you are an invincible observer who falls into a blackhole... Your time runs slower than the universe outside, as you get closer and closer to the singularity, the ratio between your time and ...
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1answer
55 views

How to show that apparent velocity V(app) is always greater than -c/2 for an object fleeing away from you?

I am currently taking the online course on Relativity and Astrophysics provided by Cornell on EdX. There, how the apparent velocity of an object moving at a fraction of c might sometimes be greater ...
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Is it possible to see traveling light beams?

As we all know the speed of light is the thing that allows us to see Now let's imagine a beam of gamma rays from a Black hole just a few light years away passing through the solar system somehow was ...
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How to debunk this perpetual motion machine? [duplicate]

I know it's impossible. I normally find the flaw behind the principle, but this one is hard to crack. The two wheels are geared together, so that no magnet on one wheel "backlashes" the wheel in a way ...
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Density of infinite gas cloud

I am not exactly sure if this is the right place to post this, so do comment if not. Consider a universe similar to ours, filled with only a gas, distributed uniformly throughout it. What is the ...
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Is there any thought experiment that has proof? [closed]

Is there any thought experiments that are proved mathematically? Or is it thought experiments are all pure hypothesis?
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The fully-relativistic classic solution of 2 electrons starting from rest?

Has anyone seen the following problem in an article or a textbook? Consider 2 electrons initially at rest at $\pm x_0$ in the lab frame. Describe the fully-relativistic, i.e. solving Maxwell's ...
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528 views

Determining the mass of a rolling cart

If I have a cart that has a force sensor on it and then I have the available supplies: Motion sensor, computer to read the motion and force sensors, weights and weight holder, wood blocks (tilt the ...
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1answer
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Is it easier to gain energy or to lose it?

Here's an interesting question that came to my mind today: Suppose I have two containers of water. I heat Container $A$ to $60°C$ and Container $B$ is chilled to $-20°C$. If I leave them both in ...
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Why is closeness assorted with high potential energy and actually not being far away?

I know that, technically, given the standard convention $U=0$ at $r=\infty$ would imply that if $r$ is large, then $U$ would be small, hence justifying that close objects to a potential energy source ...
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Special Relativity - Does this thought experiment correctly prove time dilation and length contraction?

Assume you're in outer-space and a spaceship flies past you at a very high speed. In the ship, the pilot is shooting a light beam from the ground to the ceiling, and a mirror on the ceiling reflects ...
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1answer
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Can a signal be sent and received in the same point? What are the limitations?

I'm wondering about this scenario: START: $B$ and $A$ moving to the right with speed $u$ $A$ present in point $X$ Signal $S$ with speed $v'$ sent by $A$ in point $X$ towards $B$ $\require{AMScd}$ \...
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Photon thought experiment

If I have a closed box of length 299,792,458 metres fitted with mirrors on opposite sides inside the box that are reflecting light vertically (up down) again and again, then what will happen if I move ...
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Relativity of simultaneity - under what conditions would two events happen simultaneously for frames $S$ and $S'$ when $S'$ is moving relative to $S$?

I feel like I have a fundamental confusion with the following exercise: I'm confident with my answer in (a), which is that $B$ must be within the light cone of $A$, or that $B$ is causally connected ...
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Can you “rewind” sound messages by going supersonic? [duplicate]

I was having a discussion with a friend about supersonic flights. It was then when he proposed an idea on an interesting experiment that can be done during a supersonic flight. The experiment goes ...
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If torque has a different value depending on the set origin, how can rotational kinetic energy be absolute?

I'm going to illustrate this confusion of mine in a thought experiment (poorly): Here we have origins for $\vec r_2, \vec r_1,$ and $\vec r_0$. What's undeniable is that this wheel is spinning in ...