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 this experiment successfully be used to find out one-way speed of light? [duplicate]

As with almost every question on this topic, this question is also inspired by the Vertiasium video Why No One Has Measured The Speed Of Light. This experiment tries to measure the one way speed of ...
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10 votes
2 answers
373 views

What is the Frauchiger-Renner experiment? Can it be described more simply?

Several question on the Physics SE have mentioned a 2018 paper by D. Frauchiger and R. Renner with the title "Quantum theory cannot consistently describe the use of itself" (doi, arXiv). ...
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Ball collision with a wall

I understand that when a ball collides with a wall it exerts a force. By Newton's 3rd law, the wall exerts an equal and opposite force back on the ball. As a result in an ideal world, the ball will ...
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-3 votes
1 answer
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What will be the path of light coming from torch that is lit vertically in a train that is moving at the speed of light compared to the ground?

Assume a train is moving at the speed of light and a person is sitting in the train with a torch. Say the person lit the torch vertically upward comparing to the ground of the train then after the ...
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2 votes
2 answers
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Does mass affect distance travelled by a toy car?

Say, I conduct an experiment with a toy car and a ramp (Fixed angle). If I were to use the same toy car but place different weights on it (assuming the same size and shape), would the car with more ...
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1 answer
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Kinetic and chemical energy analysis in a moving reference frame - thought experiment with a car [duplicate]

Suppose a 1000 kg car and an observer are both moving to the right at 20 m/s and 10 m/s, respectively. The driver steps on the gas, leading to acceleration equal to 5 m/s² (the moving observer ...
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Why isn't this a way to transfer information faster than light using quantum entanglement?

Suppose Alice and Bob have entangled particles A and B that are very far apart. Alice uses Particle A to send information through its spin along the x axis, and Bob uses Particle B to receive this ...
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Is this an example of Heisenberg's Uncertainty principle?

I recently read about an example of the correlation between position and momentum measurements and how one's precision leads to inaccuracy in the measurement of the other variable. Suppose there is an ...
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4 votes
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Is motion un-observable? and if so how can we know it exists?

Let's say I show you two pictures of the same desk, the only difference between them being the position of a red pen. Our intuition tells us that the pen must have been in motion at some point after ...
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1 vote
4 answers
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Do perpendicular lights interfere on a surface?

In the following experiment, we have an observer looking at a material illuminated by a light. Imagine that the lights are similar to a laser (coherent?) so that all of the photons follow the ...
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2 votes
2 answers
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A relativity paradox on gravitational force

Imagine an infinite number of point masses $m_0$ moving with velocity $V_0$ on $y$-axis towards $+y$ (with equal distance $L_0$ between each two consecutive masses). Scenario 1: A mass $m_1$ is moving ...
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How much water is needed for its weight to cause fusion?

Assuming we have a cylinder with an interior volume equal to one milliliter per centimeter of cylinder height and it is filled with water. Also assume earth gravity and the cylinder is unbreakable. ...
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1 vote
3 answers
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Would light bend the other way, if I use antimatter instead?

Imagine the following setup: an antimatter straw, an antimatter glass filled with antimatter water and we have antimatter atmosphere just in case. My question is: does Snell's law still apply here as ...
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1 vote
2 answers
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SR time dilation in a centrifuge dilema

Picture a giant centrifuge gyrating close to the speed of light (indulge me). Person A is sitting in it close to the axis, person B further away on the radius and they are facing each other. To ...
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2 answers
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Are there any yet unresolved SR paradoxes?

I'm asking apart of the famous twins-paradox of special relativity theory and the ladder-paradox which have been resolved in the literature if there are still some paradoxes or variations of old ones ...
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Measuring the speed of light using Doppler Effect? [duplicate]

Could it be possible to measure the speed of light using the Doppler effect of light? I would propose an experiment where a light source is moving towards a photo detector at a constant speed, due to ...
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1 vote
1 answer
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Which energy is lost by an oscillating charge?

Let us imagine a charged block connected to a fixed spring in gravity free space and in a complete vacuum (to avoid air drag) . Now if an external agent stretches the spring from its natural length ...
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3 votes
1 answer
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How does a balloon expanding at relativistic speed look like?

Suppose that we could observe from outside a system expanding radially at relativistic speed ($\frac{dr}{dt}$ about $c$, constant and/or accelerated). It can be an ideal balloon, or a spherical ...
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1 vote
2 answers
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An overwhelming thought experiment regarding Newton's Third Law and motion of two bodies in an ideal condition

I came up with this experiment in my head. Suppose there are two blocks $A$ and $B$ in an ideal situation(no friction, gravity, air resistance whatsoever). Velocity of $A$($v_A$)=$10 ms^{-1}$ Velocity ...
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1 vote
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Hello! How would the rotating chair experiment (which explains the conservation of angular momentum) look like in vacuum or space?

Would it look any different from what we observe in daily life?
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2 answers
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Necessarily of gravitons in gravitational thought experiment

The following is strictly a thought experiment, so please forgive the implausability of the scenario. I've read that if an object in space is moving in a relatively straight line, it will begin to ...
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9 votes
4 answers
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One-way speed of light experiment, no clocks or mirrors (with simulation)

As can be seen in the many creative questions here, it is hotly disputed whether you can measure the speed of light in only one direction to the point where a infamous postulate was made that the ...
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Could electrons exert pressure on a gas?

What would happen if in a sealed chamber with a gas one injects electrons (by any means of emission)? Would the pressure in the chamber rise? What would happen to the gas (say H)? What would ...
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1 vote
5 answers
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Thought experiment on relative motion

Imagine two persons (A and B) in a vacuum in outer space that are in front of each other and are not in motion. Say that suddenly A starts moving away with a constant velocity from B. In this case, ...
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1 answer
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One hydrogen atom universe. Is it possible to free the electron?

Let's assume a vast universe with only one hydrogen atom inside and some (unlimited amount of?) electromagnetic radiation to excite the atom. Is it ever possible to free the electron according to the ...
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2 answers
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Is general relativity required from quantum mechanics?

One of the paradoxes against quantum mechanics was claimed by Einstein during one of the Solvay conferences: a paradox called the Einstein box also explained in this question. The solution proposed by ...
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1 answer
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Thought experiment and $E=mc^2$

Suppose we have an indestructible box that doesn't let through any matter or radiation or whatever. In the box, there is matter which is evenly distributed (state A). The energy content of the box ...
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2 votes
0 answers
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Looking for books (or info) on the Thought Process of great scientists [closed]

I have always wondered how great scientists made their discoveries, but none of my textbooks describe their thought process. I wonder if anyone knows books that explains the Phenomenon and spark that ...
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What happens when a wormhole mouth moves over a smaller object?

This is really lots of questions in one, but I'll start by expanding on the title. Say that we have an infinite, euclidian universe (4d spacetime) with nothing in it but a single object (1m diameter ...
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-1 votes
2 answers
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If an object had an acceleration only in the time dimension relative to an observer, what would it look like to an observer? [closed]

The other day I was thinking about how an object moving away from an observer in spatial dimensions appears to get smaller as it gets further away. That made me wonder- if there was an object that was ...
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Thought experiment regarding destructive interference

Considering the following thought experiment: Starting with a cavity with perfect mirrors and a monochromatic electromagnetic wave which matches the distance of the mirrors so that constructive ...
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1 vote
1 answer
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How sparse does air have to get before the speed of sound slows dramatically? Or ceases altogether?

Supposedly, at low altitudes, the increase in the speed of sound due to the increased pressure of air (and therefore, compressibility?) almost exactly cancels out the decrease in sound velocity due to ...
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3 votes
1 answer
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Thought experiment on Special Relativity (SR)

Hi I have been reading up and trying to understand special relativity. I've thought of this question and I am not quite sure what I am not understanding right. Disclaimer: I'm quite new to this topic ...
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1 vote
2 answers
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What force is acting on the container?

Foreword: I have been trying to solve this problem for several days: [Does diffusion cause the bottle to move to the left?] But the answers presented there are contradictory. Some say the bottle will ...
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1 vote
1 answer
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Does Bohr's solution to Einstein's Light-box experiment work without gravity?

[PS: Not a science student beyond high school] Hi, I was reading a book on the debates surrounding interpretations of quantum mechanics (link) and it discussed how the solution to Einstein's Light-box ...
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1 vote
1 answer
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Free Expansion in Szilard Engine?

A crucial step in the definition of Szilard Engine is computing the work done by the single molecule in moving the piston from the middle to one end (left or right, depending on the location of the ...
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1 answer
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What would be the consequences of action-reaction pair not being anti-parallel? [closed]

In the real world, action-reaction pair always make an angle of exactly at 180ᵒ. But what would happen if instead it was something like, say 179.9ᵒ. What would be the weird effects? I started off by ...
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3 votes
2 answers
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What experiments would tell us about Wigner's friend?

I am trying to understand the Wigner's friend thought experiment and I can see what the problem is, but I'm not really understanding the implications (I've read this and related questions, but ...
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1 vote
4 answers
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Under relitivity, does apparent acceleration decrease towards the front of an accelerating object?

As I understand it, the Bell's spaceship paradox thought experiment says that if you have two identical rockets aligned in the direction of travel, which start accelerating simultaneously (in frame S) ...
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3 votes
4 answers
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Symmetry in the Simultaneity of events in Special Relativity

I have been bothered for long by this question and finally decided to ask it here. I am going to resort to the original thought experiment by Einstein, involving the moving railway carriage and two ...
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1 vote
1 answer
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What about this Hubble volume thought experiment is unphysical?

I've been considering a situation with Hubble volumes that I'm having a difficult time resolving. For simplicity, assume the expansion of space is constant, rather than accelerating or decelerating. ...
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0 votes
2 answers
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Is there a maximum speed you can travel through time? If so, could you ever reach such a speed?

I understand that everything in the universe travels at the speed of light through spacetime. As a layperson I'm going to refer to this velocity as the spacetime velocity. Some examples.. Objects ...
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0 votes
3 answers
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Consider the famous thought experiment

If we consider two people A and B who both are conducting the famous Schroedinger's Cat Experiment. Person A tries to open the box to see if the cat is alive or dead, Chances of Cat's death are 50-50 ...
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0 votes
1 answer
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How the time slows down? In thought experiment of special theory of relativity in which two mirrors are used how the time gets slow for moving person? [duplicate]

If experiment is observed from stationery frame of reference then light travels more distance than frame of reference of mirrors so to keep speed constant time for stationery frame of reference should ...
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0 answers
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Equilibrium change in the isotherms of real gasses

I am not sure I understand Isotherms of real gasses correctly (the equilibrium line to be specific): Imagine a closed container with a piston between two compartments filled with a real gas (top blue ...
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-2 votes
1 answer
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When does a photon become part of a moving system? [closed]

I was wondering when a photon becomes part of a moving system. Does it need to be generated inside e.g a moving train? Or can it be a photon generated out-side the train but reflected by mirrors that ...
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2 votes
3 answers
152 views

Time period of a simple pendulum at the centre of earth

So I thought of this hypothetical experiment and wondered whether it be possible for it to even happen at center of earth even though gravity is zero. So considering a pendulum with length much much ...
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1 vote
1 answer
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Who does all the calculation in Archimedes Principle? [closed]

Let's do an experiment. In the first half of the experiment, you ask some geometers to determine the volume of a pineapple. They would model the 3D shape of the pineapple, which is a pretty tricky ...
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2 votes
0 answers
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What's the relation of entropy with synchronisation?

2 pendulums tied to a same rod and oscillating with different frequencies and phases synchronize over time. This could be done at home just to see how interesting it is that things try to sync their ...
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1 vote
4 answers
165 views

In special relativity, how do we know that distance doesn't change in the direction perpendicular to velocity?

In the theory of special relativity it is said that the distance in the direction of the speed changes by a factor of $$\gamma=\frac{1}{\sqrt{1-\frac{v^{2}}{c^{2}}}}$$ How do we know that the distance ...
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