38
votes
Accepted
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 ...
- 12.4k
34
votes
Accepted
Is a "hot cube" (analogous to an ice cube) a physical possibility?
Yes, it's possible, but as others have mentioned, such "hot cubes" would be rather dangerous. Perhaps a self-warming coffee cup is possible, but it's still not a great idea.
To be specific, ...
- 13k
21
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
$$\rho=-\nabla^2 \...
- 1,113
19
votes
Accepted
If you were encased in an iron block and dropped from a building, what would happen?
The box stops abruptly when it hits the pavement (Let's say, for sake of argument, that it hits an outcrop of bedrock that happens to stick up right next to the building.)
The force between the box ...
- 16.2k
19
votes
Is a "hot cube" (analogous to an ice cube) a physical possibility?
These exist. They are not uranium; they are just small (~1 cm) metal blocks. They store a lot of heat, and so when heated in warm water for a while and then placed in coffee, they keep the coffee warm ...
- 3,494
18
votes
Accepted
Ground → ship Wi-Fi bandwidth in my fast moving spaceship
If you are in a perfectly circular orbit, then your received signals are affected by the relativistic transverse Doppler effect.
The signals you would receive would be blueshifted by the Lorentz ...
- 137k
16
votes
Why doesn't the gravitational energy in this system of evaporating and condensing water violate the second law of thermodynamics?
At a given temperature, in your liquid water-air system, equal numbers of water molecules will enter the air from the liquid as return to the liquid from the air. The system will be in equilibrium and ...
- 1,229
16
votes
Maxwell's Demon: Why does the entropy of the overall system decrease?
You can think of entropy as measuring the amount of phase space volume that the system could be occupying. (This is the set of all possible sets of positions and momenta for all the particles.)
It's ...
- 105k
16
votes
Accepted
Areas with anti-parallel gravity in classical physics
If we let the $y$-axis point upwards then OP's 2D gravitational field is
$$\vec{g}~=~ \begin{pmatrix} 0 \cr g~{\rm sgn}(x)\end{pmatrix}.$$
It has a non-zero curl $$(\vec{\nabla}\times \vec{g})_z~=~2g\...
- 213k
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....
- 33.4k
15
votes
Accepted
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 $\...
- 10.1k
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 ...
- 109k
14
votes
Accepted
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 ...
- 6,495
14
votes
Is a "hot cube" (analogous to an ice cube) a physical possibility?
A more practical variant, and (supposedly) more effective than controlgroup's suggestion, is an capsule of a substance who's melting temperature has been calibrated to be the same temperature as you ...
- 498
13
votes
Accepted
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 ...
- 8,296
12
votes
Accepted
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,
$$|\...
- 105k
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?
...
- 15.4k
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 ...
- 25k
11
votes
Accepted
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 ...
- 7,180
11
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\,|...
- 28.7k
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 ...
- 9,805
9
votes
Accepted
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 ...
- 16.6k
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 ...
- 6,119
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 ...
- 10.7k
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 ...
- 500
8
votes
Accepted
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 ...
- 2,040
7
votes
Accepted
Why doesn't the gravitational energy in this system of evaporating and condensing water violate the second law of thermodynamics?
Why doesn't the gravitational energy in this system of evaporating and condensing water violate the second law of thermodynamics?
this is the second law :
The second law of thermodynamics states ...
- 235k
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. ...
- 1,871
7
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 ...
- 4,635
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