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38 votes
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Why is the photon clock equivalent to all clocks?

Invoke the principle of relativity. An inertial observer carries both a light clock and a mechanical wristwatch, which agree when all are at rest. If they don't agree when the inertial observer is ...
robphy's user avatar
  • 11.9k
20 votes

Ambiguity in applying Newton's shell theorem in an infinite homogeneous universe

The problem lies in the boundary conditions. Ignoring factors of $G$ and $\pi$, gauss's law of gravitation relates the gravitational potential $\Phi$ to the mass density $\rho$ by $$\rho=-\nabla^2 \...
user105620's user avatar
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19 votes
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If you were encased in an iron block and dropped from a building, what would happen?

The box stops abruptly when it hits the pavement (Let's say, for sake of argument, that it hits an outcrop of bedrock that happens to stick up right next to the building.) The force between the box ...
Solomon Slow's user avatar
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18 votes
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Ground → ship Wi-Fi bandwidth in my fast moving spaceship

If you are in a perfectly circular orbit, then your received signals are affected by the relativistic transverse Doppler effect. The signals you would receive would be blueshifted by the Lorentz ...
ProfRob's user avatar
  • 132k
16 votes

Maxwell's Demon: Why does the entropy of the overall system decrease?

You can think of entropy as measuring the amount of phase space volume that the system could be occupying. (This is the set of all possible sets of positions and momenta for all the particles.) It's ...
knzhou's user avatar
  • 103k
16 votes

Why doesn't the gravitational energy in this system of evaporating and condensing water violate the second law of thermodynamics?

At a given temperature, in your liquid water-air system, equal numbers of water molecules will enter the air from the liquid as return to the liquid from the air. The system will be in equilibrium and ...
bpedit's user avatar
  • 1,229
15 votes

Photon thought experiment

The easiest way of seeing what happens is to draw a space-time diagram. The stationary box walls form two vertical parallel lines with a 45 degree zigzag path for the photon as it bounces between them....
Anders Sandberg's user avatar
15 votes
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A bar that appears straight in one reference frame appears bent in another?

Your calculations are basically correct, for calculating the angles between light rays going forwards or backwards. Angle $\beta'$ between the forward going light rays should indeed be smaller than $\...
KDP's user avatar
  • 5,345
14 votes

One-way speed of light experiment, no clocks or mirrors (with simulation)

Unfortunately, there is simply no possible way to measure the speed of light independently of your synchronization convention. In this case, if you use the standard isotropic synchronization ...
Dale's user avatar
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14 votes
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Stuck on thought experiment about light

As @JonCuster already pointed out in the comments, you are assuming an incompressible fluid. Under unrealistic assumptions you of course get unrealistic results. In reality, you will get a simple ...
rfl's user avatar
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13 votes
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How can the weight of a container be affected by an object's buoyancy?

The flaw in your logic comes from neglecting the water's acceleration after cutting the string, and forces on the water before cutting the string. The ball is accelerating upward, yes, and that would ...
AXensen's user avatar
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12 votes
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Why not use this experiment to test gravity's quantum properties?

This is a perfectly good idea, and there are definitely experiments that test this. The issue, however, is that there are really three possibilities. Suppose that we have a mass in superposition, $$|\...
knzhou's user avatar
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11 votes
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Why does the 'sticky bead argument' for (gravitational waves carrying energy) work?

I would guess that the argument applies to the case where the frequency of the gravitational wave is low compared to the natural frequency of the stick, or put another way, the rate of change of ...
John Rennie's user avatar
11 votes
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Will heat flow by conduction oscillate until thermodynamic equilibrium is reached in a solid object?

You are mistaken. You seem to be assuming that there is some kind of inertia in the process of heat transfer, as in water sloshing about in a tank. There is no such inertia here, so there is no ...
sammy gerbil's user avatar
  • 27.4k
11 votes

Pressure in giant ball of water floating in space

Assuming this giant ball of water can hold itself together due to cohesion, wouldn't you still feel the pressure from...well, simply the water molecules themselves, moving randomly in all directions? ...
JMac's user avatar
  • 15.3k
11 votes

Ground → ship Wi-Fi bandwidth in my fast moving spaceship

There are two effects due to relativity. The motion of the spaceship around the satellite results in a time dilation that can be calculated using special relativity. If the satellite has ...
S. McGrew's user avatar
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11 votes
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What time does the particle reach the screen in this thought experiment?

I just finished a thesis on this subject and I'm happy to share. None of the linked papers are my own. The time of arrival in quantum mechanics is actually a subject of ongoing research. It is ...
maybe try codidact instead's user avatar
10 votes

Can we theoretically guide cold air from high altitude down to the needy cities as free AC?

Believe it or not, the air at the bottom of the tube would be just as hot at the air at the same height outside the tube. In air that's well mixed, the temperature decreases as you go up at a rate ...
Ben51's user avatar
  • 9,735
10 votes

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

Answering my own question: I'll rephrase the argument, or at least crucial parts of it, in the language of quantum computing. The experiment We start with a qubit prepared in the state $\sqrt\frac23\,|...
benrg's user avatar
  • 27.4k
9 votes

Symmetry in the Simultaneity of events in Special Relativity

There is a symmetry in the following sense : two lightning strikes which appear simultaneous to $(B)$ appear non-simultaneous to $(A)$ two lightning strikes which appear simultaneous to $(A)$ appear ...
SolubleFish's user avatar
  • 5,944
9 votes

Are there any yet unresolved SR paradoxes?

No, there are no unresolved paradoxes in special relativity. It is more than a century old and has been thoroughly studied. At this point in history any claim that there is a logical flaw in SR should ...
Eric Smith's user avatar
  • 9,276
8 votes
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Ambiguity in applying Newton's shell theorem in an infinite homogeneous universe

Updated 07.11 We can chose the model to discuss the problem and so let us chose: Model: Newtonian mechanics/Newtonian gravity, with the Universe filled with uniformly dense matter, interacting only ...
A.V.S.'s user avatar
  • 15.8k
8 votes

Why is the photon clock equivalent to all clocks?

Based on my current understanding of the topic the light clock is not a proof of time dilation but simply a clue, pointing at it. You are absolutely right in stating that the light clock is not a ...
Noumeno's user avatar
  • 4,567
8 votes

Are quantum fluctuations observable?

I believe there are cases where they are observable. Especially through their effects. In cosmology, for example, quantum fluctuations in the early inflationary epoch are considered to have seeded the ...
Stathis Artis's user avatar
8 votes
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How to read voice clips off a glass plate?

We will assume that the "molten glass" never actually became a liquid, as it would have fallen from the window and created a pool on the ground. The best we can hope for is that it became ...
Michael M's user avatar
  • 1,950
7 votes

Maxwell's Demon: Why does the entropy of the overall system decrease?

There are different kinds of entropy here. The thermal entropy due to the molecules moving faster or slower is a red herring and actually isn't important for understanding Maxwell's demon at all. ...
reductionista's user avatar
7 votes

Why is the photon clock equivalent to all clocks?

Why is the photon clock equivalent to all clocks? Stipulate that, in some inertial reference frame, there is a photon clock and some mechanical clock that are co-located and at rest in this frame. ...
Alfred Centauri's user avatar
7 votes

Is motion un-observable? and if so how can we know it exists?

You must be using the word motion in a very specific sense, because in the more general sense of the word motion is clearly observable. Motion is a change of position over time, and we can clearly ...
Marco Ocram's user avatar
  • 26.4k
7 votes
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Why isn't this a way to transfer information faster than light using quantum entanglement?

Once you measure an entangled state, the "entangledness" gets destroyed and they start behaving like regular spin particles. This is one of the biggest impediments in building quantum ...
Eli Yablon's user avatar

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