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1500 questions
42
votes
6 answers
Why are planets not crushed by gravity?
Stars can be crushed by gravity and create black holes or neutron stars. Why doesn't the same happen with any planet if it is in the same space time?
Please explain it in simple way. Note: I am not a physicist but have some interest in physics.
NotPhysicist
- 505
42
votes
5 answers
Why is a black hole black?
In general relativity (ignoring Hawking radiation), why is a black hole black? Why nothing, not even light, can escape from inside a black hole? To make the question simpler, say, why is a Schwarzschild black hole black?
user8784
42
votes
5 answers
Is information entropy the same as thermodynamic entropy?
Context
In one of his most popular books Guards! Guards!, Terry Pratchett makes an entropy joke:
Knowledge equals Power, which equals Energy, which equals Mass
Pratchett is a fantasy comedian and every third phrase in his book is a joke, therefore…
grochmal
- 539
42
votes
5 answers
Would you hear a gravitational wave, if its amplitude and frequency were suitable?
If there was a source of a continuous gravitational wave at (say) 50hz, and amplitude of say a micrometer (a typical sound wave displacement, I think), and you were nearby (standing happily on a planet in an atmosphere), with your ear pointing to…
GreenAsJade
- 1,272
42
votes
4 answers
Why do most office chairs have 5 wheels?
(Inspired by Why are four legged chairs so common?)
I've been wondering for a while... Why do most wheeled office chairs have 5 wheels?
My guess would be that while stability vs. simplicity results in 4 legs, adding mobility to the equation may…
42
votes
4 answers
Is Earth really a magnet?
I am a student of class 9. When I was going through magnetism and read that an earth is a magnet I got some doubts. My question is: is earth really a magnet? Doesn anyone have any proof that earth is a magnet? Is there a magnetic core at the center…
42
votes
3 answers
What is a mode?
The word mode pops up in many fields of physics, yet I can't remember ever encountering a simple but precise definition.
After having searched fruitlessly on this site as well, an easy to find place with (one or more) good answers seems in…
Janosh
- 1,244
42
votes
4 answers
How far out from the Sun is visible light still sufficient to read a book?
Recent pictures from the New Horizons spacecraft, shown below, seem to indicate that, at Pluto's distance, we are entering a twilight zone, with a distinct lack of colors, although that could be due to the planet terrain itself or the camera used to…
user81619
42
votes
5 answers
Why do physicists trust black hole physics?
Based on popular accounts of modern physics and black holes (articles, video lectures), I have come to understand the following:
Black holes are predicted by General Relativity, a classical theory of gravity.
We know that the universe is inherently…
FlagCapper
- 585
42
votes
5 answers
Why does medium not affect the frequency of sound?
I read in various places that frequency does not change with medium. Instead, wavelength changes in different mediums due to a change in speed. I understand why speed changes with medium, but I'm not sure why wavelength, not frequency, changes. One…
carbenoid
- 543
42
votes
7 answers
Ice skating, how does it really work?
Some textbooks I came across, and a homework assignment I had to do several years ago, suggested that the reason we can skate on ice is the peculiar $p(T)$-curve of the ice-water boundary. The reasoning is that due to the high pressure the skates…
Lagerbaer
- 14,836
42
votes
7 answers
Why do we need to know the shape of the slide to find the time to slide down it?
In my physics book after this solved example:
A child of mass $m$ is initially at rest on top of a water slide at height h = 8.5m above the bottom of the slide. Assuming that the slide is frictionless because of water, find the speed of the child…
pcforgeek
- 1,406
42
votes
2 answers
Equivalence of canonical quantization and path integral quantization
Consider the real scalar field $\phi(x,t)$ on 1+1 dimensional space-time with some action, for instance
$$ S[\phi] = \frac{1}{4\pi\nu} \int dx\,dt\, (v(\partial_x \phi)^2 - \partial_x\phi\partial_t \phi), $$
where $v$ is some constant and $1/\nu\in…
Greg Graviton
- 5,157
42
votes
1 answer
Understanding Poisson brackets
In quantum mechanics, when two observables commute, it implies that the two can be measured simultaneously without perturbing each other's measurement results. Or in other words, the uncertainty in their measurements are not coupled.
But in…
user929304
- 4,635
42
votes
5 answers
How does gravitational lensing account for Einstein's Cross?
Einstein's Cross has been attributed to gravitational lensing. However, most examples of gravitational lensing are crescents known as Einstein's rings. I can easily understand the rings and crescents, but I struggle to comprehend the explanation…
Dale
- 6,014