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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Quantum tunnelling time experiment, what would the times be either side of the barrier?

Having read Quantum Tunnels Show How Particles Can Break the Speed of Light in Quanta; I felt the need to ask some smarter people than I a question. Consider two pairs of rubidium emitters and ...
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Given a collapsed state, can we derive the prior shape of the wavefunction?

More or less the title. Assume that we have found a box containing a completely isolated system of particles. We do not know for how long this system has been allowed to evolve. We do know what ...
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Thought Experiments

Can anyone please explain to me the importance of thought experiments? How does one arrive at a conclusion in a thought experiment. Like for example, the electron through double slit experiment is a ...
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Wigner's friend question [closed]

This is probably a dumb question, but does anyone know whether both Wigner and Wigner's friend observe the same result once both have been in the room and measured?
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Calculating the Universe's change in entropy for irreversible processes involving a system and a heat bath

Water at temperature $T_s$ is added to a lake at temperature $T_0 > T_s$ until the total system reaches equilibrium. What is the change in entropy of the Universe? This is a homework question but ...
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If I heat up a ball, is the radial temperature gradient in the middle of it going to be zero in all directions?

Assume I have a uniform ball, heat transfer is solely governed by the heat equation ($\dot{u} = \alpha \Delta u$). It has an initial temperature distribution solely dependent on $r$, ie $T=T(r)$, ...
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All the information (photons) within a unit cube of space [closed]

So here is a thought, lets fix a cube of side 1cm, it contains light passing through it from all possible angles, be it stars or insect. If we change our angle of view we can see different objects due ...
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Is the Accretion Disk of a Black Hole just an illusion?

As a matter of fact, I recently learnt that as an object is approaching a black hole, to an outside observer the object seems to be getting slower and slower and will cross the event horizon after an ...
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Hidden variable special relativity paradox

Members of this community were kind enough to contribute to my last paradox post: Special relativity paradox involving two light clocks I'm creating this new post to address the issues raised in that ...
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Special relativity paradox involving two light clocks

I came up with this thought experiment that seems to exemplify a paradox. In this paradox, one clock is ticking unevenly for one observer and evenly for another. Essentially, the clocks record a “tick”...
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If everything slowed down around you, but you were just as fast you are normally, would you jump higher? [closed]

If everything slowed down around you, but you were just as fast you are normally, would you jump higher? I am thinking velocity would increase, thus the force generated would be stronger, but I am not ...
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Clarification on an example to teach momentum but seems to give two answers

Thanks for any help. Here is the momentum example: Alice obtains a fire extinguisher which she holds firmly on her office chair. Suppose Alice opens the nozzle of the fire extinguisher so that it ...
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Hafele–Keating plane paradox

As many of you have probably known there was a famous Hafele–Keating experiment, which was about a plane traveling the globe in order to prove predictions of special relativity and general relativity. ...
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Debunking thought experiment purporting to show that gravity is not classical

As far as I know, there are no known observations that require a quantum theory of gravity. Now, that's not to say we don't need a quantum theory of gravity. In fact, I get the sense that we do, at ...
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What time does the particle reach the screen in this thought experiment?

Suppose a particle with a gaussian wavefunction moves to begin with towards a position detector screen. How do we obtain the 'Time of arrival' distribution, when time can't be an observable? Should ...
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Newtonian Mechanics Thought Experiment Gone Wrong And Needs Clarification [duplicate]

I have been doing personal study in Classical Mechanics and reading Newton's Laws. While thinking about them I had a question I haven't been able to answer. It comes from the interactions of Newton's ...
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How to calculate the bulge in a swimming pool surface caused by a local point mass?

Imagine a swimming pool of an arbitrary width, small on a planetary scale. At a height above its usual surface, a superdense mass is suspended. How do I calculate the distortion in the surface of the ...
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Double slit experiment: Single photons of red light mixed with high intensity infrared light

First this is only a presumtion. Let say we fire single photons of red light at the double slit to get a interference pattern on the screen. The distances between frindges would have certain ...
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Parabolic mirrors used in pair to produce much higher intensity light beam ready for transmission?

If a parabolic mirror is used to concentrate sunlight approximately to a focus point why can't an another smaller parabolic mirror be used to reparalelise the beams that are leaving the focal point ...
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Experimentation on “Quantum Permutations”

Consider the following hypothetical experiments in quantum physics: Experiment A: Consider four 'shells' in a line (see here for inspiration for name), and there are four different colour peas/...
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Why is the photon clock equivalent to all clocks?

I can understand why, if the speed of light is invariant, a photon clock would tick slower. I find this explanation very useful in terms of introducing the idea of time dilation (also because it ...
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World of 2-dimensional beings [closed]

In a world of two-dimensional beings, How would a two-dimensional (A) being view another two-dimensional (B) being as? Would they always perceive each other as a straight line?
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Relative time dilation at relativistic speeds

As I understand it (feel free to correct me if I'm wrong here), the core tenet of Einstein's theory of special relativity is the idea that if you were on a sealed train moving at any constant velocity ...
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Behaviour of clocks and measuring rods on a rotating frame of reference

I'm reading "Relativity: The Special and the General Theory" by Albert Einstein. In the following extract: Let us consider a space-time domain in which no gravitational field exists ...
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Why is Maxwell's demon taken seriously as a challenge to the second law of thermodynamics?

Supposedly, Maxwell's demon, a thought experiment, is a challenge to second law of thermodynamics, an experimental law. Relativity started in the same manner as a thought experiment, but the obvious ...
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A Hidden Principle in Relativity

Thought experiments are very common in Special and General Relativity (SR,GR). Usually a thought experiment is structured as follows: We present a setup in a frame of reference We apply principles of ...
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Could you reversibly control the mass of a box of self-propelled nanoballs by increasing their speed?

Exploring the nature of confined energy giving rise to "mass" or gravitation, My assumptions: Most of my own mass is from the confinement of baryonic quarks in the strong force A bunch of ...
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Parallel Stripes and the classic globe confirmation experiment [closed]

I have been taking time this summer to try and understand what "reasons" some flat earthers offer for their beliefs for a "science and the public" class. One of the oldest and most ...
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Can macroscopic objects be influenced by quantum physics?

I was wondering if macroscopic objects can be influenced by QM in this thought experiment. A die is launched on a perfectly flat surface. The interaction between surface and die is elastic (no loss of ...
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What will eventually happen to a coin spinning in space?

There were some answer comments at Is it possible to make the Sun spin a coin in space? that could be a question in their own right, so here it is. Posit that we have a coin in space, on both faces ...
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What happens when you get rid of a singularity? [closed]

There's a pretty common physics chestnut about what would happen if the sun just up and disappeared. "How long would it take for us to notice?" The answer is of course roughly eight minutes--...
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Let's suppose there is only one object in this universe. May that be a quark, an atom, etc. What will be the consequences? [closed]

I'm not a physics expert, but this question really intrigued me, so I thought "why not ask those who are"! Let's suppose there is only one object in this universe. May that be a quark, an ...
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What prevents a (theoretically ideal) railgun from being a perpetual motion device?

So, I understand the basics of how a railgun works - the current in the conducting rails and the armature sets up a magnetic field, which, via the Lorentz force acting on the current in the armature ...
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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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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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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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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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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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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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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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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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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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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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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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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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