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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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Time reversal in case of adsorption

We know that the entropy of gas molecules being adsorbed at the surface of a catalyst decreases. Also, the arrow of time is accounted for by an increase in entropy. Then can we say that if we fit a ...
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38 views

“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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What would a universe which follows an inverse force law look like?

If the gravitational force obeyed an inverse law $f_\text{g} \propto \frac{1}{r}$ rather than an inverse square law $f_\text{g} \propto \frac{1}{r^2} ,$ the amount of work needed to separate two ...
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45 views

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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68 views

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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70 views

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
29 views

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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2answers
87 views

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
69 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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1answer
66 views

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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1answer
67 views

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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1answer
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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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3answers
105 views

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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2answers
56 views

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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1answer
72 views

A rotating solid bar in space [closed]

Say there is a solid bar in space. Imagine it vertically, with the top point labeled $A$, and the bottom point labeled $B$ (these aren't points on the bar, but points in space between which the bar ...
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55 views

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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1answer
23 views

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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1answer
24 views

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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Could wormholes be theoretically manipulated to travel through time? [duplicate]

If one were to assume that mouths of a wormhole could be moved: one mouth would be oscillating at a very high speed and the other on an unaccelerated path. By placing clocks at either end,the one ...
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36 views

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
46 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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2answers
44 views

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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1answer
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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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2answers
81 views

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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1answer
365 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
54 views

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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1answer
39 views

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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1answer
68 views

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
82 views

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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1answer
1k views

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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4answers
730 views

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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3answers
134 views

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 ...
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1answer
59 views

Relativity of Simultaneity - confusion regarding the ordering of events when taking different inertial frames as the origin

I'll admit my question steals a graphic from this user's post here, but it is a different question regarding the same scenario that struck confusion of mine. Here is the graphic, illustrating the ...
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2answers
282 views

Sound Waves + Closed Box --> What happens under movement?

Setup: We have a closed, and from the outside isolated, box with two sound-emitting-generators placed at opposing ends, and one microphone placed in the middle (--> picture). The sensors are all ...
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1answer
57 views

Solar system sized air sphere black hole, and some questions regarding it [closed]

First of all, I'm not a physicist, I'm just a curious computer science guy, so pardon my non-technical views of this. Now, suppose we somehow manage to fill a solar system sized spherical space with ...
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1answer
98 views

Quick check: relativity, rockets, clocks and the equivalence principle

One common thought experiment that introduces relativity on gravitational fields is the "clocks on an accelerating rocket": Paraphrasing Mr. Feynman: Suppose a rocket, with two clocks, one on each ...
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Can I compute mass of an object in a box based on the sound the object makes when shaken? (Barton Fink movie)

In this movie called "Barton Fink" the main character, Fink, is given a mysterious box with an object inside it, he shakes the box up and down, is it possible to calculate the mass of the object ...
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3answers
2k views

Light clock with mirrors on a moving train [duplicate]

I am reading a new popular science book where a thought experiment proposed by Albert Einstein is explained. It is a thought experiment with a light clock. The light clock is emitting light straight ...
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3answers
78 views

Basic special relativity thought experiment problem

I've been interested in a thought experiment that a friend proposed to me. If we take a huge block of glass through which a beam of light passes, it results that the velocity of the beam in the medium ...
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2answers
299 views

Feynman's disk paradox used as a gyroscope?

I have recently learned about Feynman's disk paradox, where angular momentum is stored in the properties of the electric field. One example would be a coil with current going through whose axis is ...
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4answers
121 views

Experimentally verifying some facts

What are some thought experiment/real life experiment to convince me these two facts, so that I can test them actively: Fact-1: a. The speed of a falling body is non constant, and is increasing. b. ...
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150 views

kinetic energy in photoelectric effect

the formula of kinetic energy in photoelectric effect is given by $$K = h\nu - w$$ where $K$ is kinetic energy of released electron, $\nu$ is frequency of light and $w$ is work needed to release ...
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103 views

Communication range inside the photon sphere of a Black hole

Alice and Bob are positioned diametrically opposite to each other, just a little bit inside the photon sphere of a huge (great mass) non-rotating black hole (BH). Both of them can communicate via ...
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121 views

How do different reference frames give different results (such as in Bell's Paradox)?

In these previous questions (Lorentz contraction and magnetism), in order to understand how Lorentz contraction works, I was suggested to first understand “the Bell’s paradox”. But I found the ...
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1answer
49 views

If a bullet was fired above a spherical atmosphere-less Earth at LEO orbital velocity would it orbit for many years? [closed]

If I fired a hypothetical bullet from 1000m above the surface of a perfectly spherical Earth with no atmosphere and it was going 28,080 km/h when it left the barrel, would it be in orbit above the ...
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6answers
388 views

From the speed of light being an invariant to being the maximum possible speed

As I understand it, the special theory of relativity is based on two principles - that there is no preferred inertial frame (which is common sensical once we realize that all motion is relative motion)...