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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2answers
79 views

Can you demonstrate the Twin Paradox using a Schwarzschild spacetime?

Short Version: If we want to solve a problem where a particle is forced off a geodesic, can we obtain identical results if we instead solve the problem where the particle is in a different spacetime ...
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94 views

A game of boxes with rotating gears

Setup: Suppose you have 3 (identical looking) boxes named A,B,C [furthermore they have identical mass]. You are told the following about the boxes: One of the boxes actually is a uniform mass. We ...
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Is the nuclear force the cause of nucleus surface tension?

At first it looks like an easy question. If we imagine a stright row of three protons and two neutrons connected in the way one neutron comes after one proton how they can spontaneously form a ...
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124 views

At what speed does mass warp spacetime?

It is well understood that Einstein’s General Theory of Relativity explains how gravitational effects appear to occur instantaneously at a distance. Mass warps spacetime and so objects simply follow ...
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1answer
112 views

Modification of Einstein's thought experiment; time dilation and simultaneity in moving frame

Einstein imagined a man in a moving train who observed lightning which were on the same side of the track and at some distance from each other. But try to modify the experiment. Let us imagine two ...
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1answer
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Einstein's mirror in train thought experiment

I'm a bit confused about one of Einstein's thought experiments. In his experiment, he sits in a train travelling at the speed of light and holds up a mirror. From what I've read and researched, he ...
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5answers
257 views

Time dilation in a moving frame of reference

According to the theory of relativity, time slows down in a moving frame of reference. But I am confused in the example given below. Suppose there are two men $A$ and $B$. $A$ is standing in moving ...
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1answer
96 views

Gravitational field from virtual photons; does $\rho c^2 = \frac{1}{2} \epsilon_0 E^2$?

Answers to Is Energy Localised in space? got me thinking about answer(s) to Sun constantly converts mass into energy, will this cause its gravity to decrease? and the contribution to the Sun's gravity ...
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Thought experiment: Has this egg time traveled? [closed]

I was thinking about time travel and what it means to "go backwards in time" and eventually came to the conclusion that it must ultimately come down to some collection of particles at a ...
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1answer
72 views

Why does a light source emitting photons seem to violate conservation of energy?

Consider the following scenario : From here on I would like the word "laser" to be treated as just a source of monochromatic beam that emits light . You can take any other light source(if you do not ...
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1answer
40 views

Why does a sphere get uniformly charged when placed at the center of a superconducting spherical shell?

Take a superconducting spherical shell. Then place a spherical non-uniformly charged ball at its center. Then I know that the spherical (non-uniformly charged) ball will get uniformly charged. Why ...
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Intuition behind time dilation and length contraction from relativity of simultaneity

I'm trying to build intuition behind time dilation and length contraction from the following experiment: Image from "Fundamentals of Physics, 10th edition" Here, Sally's ship has velocity $...
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Pressure=Heat? Else, Heat=Radiation?

A few days ago, I came up with this paradoxical conundrum: Let's say we have a gas enclosed in a container. Supplying heat to it will cause an increase in the (magnitude of?) vibration of gas ...
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How would Schroedinger's cat “see” the world? [closed]

My question is about a "philosophical" implication of the copenhagen interpretation: Schroedinger's cat sits in a box in a superposition state, until it interferes with the rest of the universe and ...
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1answer
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Where the object exist in reality? [closed]

Suppose the object O exists at some point(x,y) in space, and two observers A and B watching object O at the same distance d, they will observe at the same distance as they have same focal length in ...
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2answers
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Length contraction of distances between two unconnected point particles moving at same velocities

The first part of my question is meant to confirm my understanding of length contraction in the context of the following simple thought experiment: Imagine three particles O(the observer), A and B in ...
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1answer
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Einstein's Train Paradox [duplicate]

Suppose there is an assassin at each end of the train, and they each use their own laser guns to assassinate a passenger $A$ located in the center of the train. At the same time, $A$ will die, ...
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3answers
140 views

Is motion really relative?

I had a question that had been bothering me for some time. I tried to come up with an explanation but I wanted to know whether it is right. Imagine two different observers say A & B floating ...
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1answer
109 views

Why not use this experiment to test gravity's quantum properties?

If a heavy object $X$ is in superposition, let's say, at two places "at the same time", to which point is the gravitational pull of that object directed to? This can probably not be answered without ...
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1answer
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Yet Another Relativity Question [closed]

Intro Thank you for reading. I realize this looks like a long question, but its a homework-like/thought-experiment question, and I've tried to put as much work into it as possible before posting it, ...
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Information propagating at less than the speed of light

Which speed does information propagate at in a medium? For example, if we live in a pool which is one light-year cube and somebody dewaters of the pool from an observer which is at 1 light-year ...
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2answers
171 views

Does gravity “flow” through wormholes (if such can be constructed)?

Let's start by assuming that a transversible wormhole can be constructed in the most "likely" way we can currently imagine... Say, we created two entangled black holes and use "negative matter" to ...
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3answers
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Einstein's light clock [duplicate]

In Einstein's thought experiment involving light moving on a train, the mirrors the light is bouncing between are perpendicular to the train, and it is these mirrors that the formula for time dilation ...
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1answer
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Entanglement thought experiment with redshift

Thought experiment: 2 entangled photons, one escape from galaxy to galaxy without hitting an electron and another one got caught between 2 perfect mirrors. Would the redshift (wavelength becomes ...
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6answers
170 views

Relativity Of Simultaneity Confusion

I have currently been introduced to relativity of simultaneity with the following thought experiment: Consider a train in which there are two presidents, one sitting at the front and the other at the ...
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Can black holes prevent heat death?

I recently learnt that black holes possess among the highest amounts of entropy in the universe. I might have stretched the idea preposterously far, but is it possible that this might be one of the ...
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Ground → ship Wi-Fi bandwidth in my fast moving spaceship

I have a hypothetical question that hopefully makes sense considering only the rudimentary amount of knowledge I have on relativity. Assume I am in a spaceship orbiting around a spherical satellite, ...
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1answer
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Einstein light clock with mirrors but what if mirrors outside the train?

Einstein's light clock with mirrors experiment involves mirrors on top and bottom of a moving train and light reflecting back and forth. To a stationary observer, it would seem that the light is ...
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Einstein box : an entering photon

I was wondering if anyone considered a photon entering the Einstein-box, say because outside is 'hotter'. Would an increase in mass-energy be observable? Some may consider this a trivial question but ...
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Gravitational wave thought experiment: changing circumference?

I just started learning about gravitational waves and how they can cause the distance between 2 neighbouring particles to increase and decrease in an oscillatory manner. I made up a thought experiment ...
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1answer
52 views

Would the Michelson-Morley Experiment have Discovered our Atmosphere?

We all know the earth is surrounded by an atmosphere, and we know that it is the medium through which sound travels. If the Michelson and Morley experiment was modified to find sound’s medium would ...
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How to add a specific amount of energy to a system? [closed]

Say I can measure mass, temperature, and geometrical data over time. Are there any processes that are known to add a specific amount of heat so I could model the conjugate variables (I am thinking of ...
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4answers
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Moon paradox 2.0

Can anyone help me with what goes wrong with the following Moon paradox thought experiment ? You all know the faster than light (FTL) Moon "paradox": send a beam to the moon through a "hand laser," ...
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Magnetic field of a charged cloud

Assume a perfectly insulated hollow sphere containing a gas of particles that has a temperature $T$, where each particle has a charge $q$. As T changes, will the magnetic field around the sphere ...
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What would torsion in spacetime look like? [duplicate]

Christoffel symbols explain the gravity we observe in our daily lives. Say I have a region of spacetime with high torsion, so general relativity isn't a good description of the region. In that region, ...
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Wigner's Friend in the many worlds?

Background In the thought experiment of Wigner's friend the many-worlders claim: "Everett claims to solve the Wigner's Friend paradox by only allowing a continuous unitary time evolution of the wave ...
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2answers
220 views

Could entropy be reversed with enough energy? [closed]

Could a vast amount of energy (such as all the energy in the accessible universe) be intentionally used to pull matter in the universe back together, thus reversing entropy? (edit: the previous ...
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1answer
40 views

Splitting of charge in a symmetry argument

Suppose we have some object that has charge $q$ and we take an identical object and we bring the two objects together. One asks what the charge on either object would be after separating them. ...
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1answer
29 views

Can induced emf reverse direction of current in a circuit?

Suppose that my increase in the current is so high that emf developed across inductor just exceeds the potential difference across the battery in LR circuit .So can the direction of current be against ...
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Is there a loop of inductions when we change magnetic field near a conductor?

What I am basically asking is when there is a change in magnetic field ,a current is induced in the coil of wire to oppose that field .Now since field of induced current is decreasing the original ...
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1answer
99 views

A simple though experiment: molecular sieve + buoyancy = perpetual motion?

The picture below is a low-effort mock-up I made showing a closed loop with uniform ambient room temperature and top-down gravity containing two species of gas—although the idea may apply equally to ...
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2answers
485 views

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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5answers
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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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2answers
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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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2answers
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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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1answer
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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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1answer
77 views

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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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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