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1500 questions
36
votes
7 answers
Why can’t gravitons distinguish gravity and inertial acceleration?
If gravitons mediate the gravitational force, couldn’t the detection of gravitons by an observer be used to distinguish whether they are experiencing gravitational acceleration vs. inertial acceleration, contradictory to general relativity? If this…
Jack Edwards
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36
votes
5 answers
Why is the sky *uniformly* blue?
I've read a lot of answers to the questions why the sky is blue. However all the answers I found contain mostly qualitative analysis: Rayleigh scattering is changing the direction of blue light, so there is more blue light coming to the eye along…
Vlad
- 479
36
votes
11 answers
Can mathematics lead to a result which is physically untenable?
Consider some known physical fact, e.g. $\nabla \cdot \mathbf B = 0$ for the magnetic induction $\mathbf B$. Now, is it possible that a mathematical theorem exists, which yields a wrong prediction?
E.g. a hypothetical - correctly proven - theorem…
Javi
- 1,272
36
votes
13 answers
Why don't we use rapidity instead of velocity?
In school we learn that we can add velocities together, and then later on we learn that it's not correct and that there is a speed limit. Why create all this confusion when we could just use rapidity to begin with?
Rapidity is defined as $w =…
Fax
- 511
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36
votes
4 answers
Is the speed of sound almost as high as the speed of light in neutron stars?
Have you ever wondered about the elastic properties of neutron stars?
Such stars, being immensely dense, in which neutrons are bound together by the strong nuclear force on top of the strong gravity that “presses” them together, one would think they…
JKL
- 3,890
36
votes
5 answers
Second law of Newton for variable mass systems
Frequently I see the expression
$$F = \frac{dp}{dt} = \frac{d}{dt}(mv) = \frac{dm}{dt}v + ma,$$
which can be applied to variable mass systems.
But I'm wondering if this derivation is correct, because the second law of Newton is formulated for point…
yarnamc
- 1,281
36
votes
3 answers
Can a wormhole be created if it has not always existed?
I know there are solutions to Einstein's field equations that give a wormhole geometry. But they are time independent. They are static. Is there a process where empty flat spacetime can evolve into a wormhole by an appropriate flow of matter and…
JeffK
- 715
36
votes
4 answers
Dimension of Dirac $\gamma$ matrices
While studying the Dirac equation, I came across this enigmatic passage on p. 551 in From Classical to Quantum Mechanics by G. Esposito, G. Marmo, G. Sudarshan regarding the $\gamma$ matrices:
$$\tag{16.1.2} (\gamma^0)^2 = I , (\gamma^j)^2 = -I \…
36
votes
5 answers
Why don't we define potential due to a magnetic field?
We define electric potential and gravitational potential and use them quite often to solve problems and explain stuff. But I have never encountered magnetic potential, neither during my study (I am a high-schooler), nor during any discussion on…
user243267
36
votes
5 answers
How do we know neutrons have no charge?
We observe that protons are positively charged, and that neutrons are strongly attracted to them, much as we would expect of oppositely charged particles. We then describe that attraction as non-electromagnetic "strong force" attraction. Why posit…
MacThule
- 421
36
votes
4 answers
What is the difference between center of mass and center of gravity?
What is the difference between center of mass and center of gravity?
These terms seem to be used interchangeably.
Is there a difference between them for non-moving object on Earth, or moving objects for that matter?
DarkLightA
- 1,442
36
votes
3 answers
Is there a frame of reference in which I was born before I was conceived?
I'm struggling to understand the relativity of simultaneity and position.
If my conception and birth are separated by time but not space, a frame of reference in which my birth and conception are simultaneous should exist right?
If another observer…
IchVerlore
- 760
36
votes
2 answers
Can light be compressed?
What if we take a cylindrical vessel with an inside surface completely reflecting and attach a piston such that it is also reflecting. What will happen to light if we compress it like this?
user204283
- 487
36
votes
5 answers
Intuition for multiple temporal dimensions
It’s easy, relatively speaking, to develop an intuition for higher spatial dimensions, usually by induction on familiar lower-dimensional spaces. But I’m having difficulty envisioning a universe with multiple dimensions of time. Even if such a thing…
Jon Purdy
- 487
36
votes
2 answers
In nuclear bomb explosions, witness describe their hands becoming transparent. How does that happen?
Witnesses of nuclear explosions have described their hands becoming transparent, and that they could see the bones. For example, see here. How does that happen?
radon
- 385