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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Can we use this experiment to distinguish between two inertial frames?

I saw a statement in my HC Verma's physics book. One should take smaller steps while walking on ice since it ensures smaller friction force. Now let us say there is a man M of mass 50kg standing on ...
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Why can't particles “remember” the past?

The question pertains to a point in Feynman Lectures, Vol III, 5 - 3, which is : We want to know whether all those that get through $T$ also get through $S′$. They do not. Once they have been ...
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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 | \...
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What's wrong with this experiment to measure the one way speed of light? [closed]

It's pretty simple so there must be something obviously wrong with it but what is it? Starter is an electric signal emitter that sends a pulse to the laser source and the detector, starting a counter ...
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Will this spaceship collide with the star? (time dilation)

I thought of the above thought experiment and arrive on 2 conflicting conclusions. I can't seem to identify the flaw in my reasoning. Suppose there is a star 4 light years from earth that has will ...
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How to read voice clips off a glass plate?

This question comes from the context of two great works on popular science, one a book on cognition and artificial intelligence Gödel, Escher, Bach (GEB) And the other a fictional TV series Fringe. In ...
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Gravitational Time Dilation Paradox?

I can't get this around my head it seem a paradox to me. In my thought experiment. Suppose there are two guys John and Jack. John start throwing a ball at near speed of light Meanwhile Jack stays very ...
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Einstein's train thought experiment

I know similar questions have been asked multiple times, hence I'll get straight to my doubt which I couldn't find cleared in the other ones. The earth frame observer sees the lightning flashes ...
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Does an angular acceleration applied to a large body affect any bodies orbiting it?

Imagine the Earth (large body) and a single satellite, let's exclude all other bodies for this example. The satellite is orbiting the Earth parallel to the equator and is maintaining a constant ...
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I just saw the Einstein's thought experiment about train

In the experiment, two lightning bolts strike at the same time and the stationary observer at the train sees the lightning simultaneously but the one in the middle of the train station would see at ...
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Are quantum fluctuations observable?

I know I am asking out of my limits, but i felt relaxed thinking about this. Now I read somewhere about quantum fluctuations (that energy can be generated in randomly at two positions where total sum ...
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Yet another question about Schrödinger's cat

I had always believed that the point of the (in)famous Schrödinger's cat thought experiment was pretty obvious: people, back then, were puzzled with the state space being (basically) a vector space. ...
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Feynman Lectures Chapter 4.2: Understanding the obtainment of free power

Excerpt: Call this reversible machine, Machine A. Suppose this particular reversible machine lifts the three-unit weight a distance X. Then suppose we have another machine, Machine B, which is not ...
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Feynman Lectures Chapter 4.2: Add or remove weights in a non-ideal machine?

Excerpt: A very simple weight-lifting is shown in Fig. 4-1. This machine lifts three units "strong". We place three units on one balance pan, and one unit on the other. However, in order to ...
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Feynman Lectures Chapter 4.2: Why is this a machine that only lifts three units?

I am having some trouble with the following passage in section 4.2 of vol. I of the Feynman Lectures on Physics: A very simple weight-lifting is shown in Fig. 4-1. This machine lifts three units &...
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Feynman Lectures Chapter 4.2: Understanding the weight-lifting machine visually

Why a non-reversible weight lifting machine cannot lift higher that a reversible one is related to my current post, but everyone in that thread seems to have understood how Feynman's simple machine ...
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What is wrong with this definition of ordered state?

It is written in my book that a disorederd state is more probable than an ordered state and hence every system tends to move spontaneously to a state of higher disorder or higher probability. But I ...
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Some doubts regarding electric field inside a hollow shell made of charges

This question is a follow up to this question. Here my doubt is about a shell made up entirely of charges. I am asking this question because all physics and electrostatics/electrodynamics books talk ...
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Electric field inside a non-conducting shell with a charge inside the cavity

I have read about conducting charged shells and spheres and know somewhat about the electric fields associated with them. But I have never found anything on non-conducting shells. I have searched ...
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Special relativity compounding transformations doesn't work? (thought experiment)

Take an earth-star and a spaceship frame. The spaceship is heading at near the speed of light towards the star from the earth. Now let's say that in the earth-star frame, there are three events the ...
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Implication of a third observer in time dilation derivation

View from A View from B View from C In the familiar experiment to derive time dilation we have observer A traveling with a light clock. The light clock sends a light pulse from the roof of the ...
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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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