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1500 questions
83
votes
5 answers
What causes ballpoint pens to write intermittently?
After a while, a ball point pen doesn't write very well anymore. It will write for a little distance, then leave a gap, then maybe write in little streaks, then maybe write properly again. It seems to be worse with older pens, but I have observed…
Olin Lathrop
- 13,084
83
votes
4 answers
What causes the water in this fountain to reverse direction?
In a children's museum, I ran across this fountain. You can adjust the flow rate with a valve, visible at the bottom. At low flow rates, the sheet of water does more or less what you'd expect: it curves downward, eventually falling more or less…
Ben51
- 10,037
83
votes
13 answers
Turbulent spacetime from Einstein equation?
It is well known that the fluid equations (Euler equation, Navier-Stokes, ...), being non-linear, may have highly turbulent solutions. Of course, these solutions are non-analytical. The laminar flow solutions (Couette flow for example) may be…
Cham
- 8,015
83
votes
5 answers
Understanding the Bloch sphere
It is usually said that the points on the surface of the Bloch sphere represent the pure states of a single 2-level quantum system. A pure state being of the form:
$$
|\psi\rangle = a |0\rangle+b |1\rangle
$$
And typically the north and south poles…
user098876
- 1,631
82
votes
9 answers
How to treat differentials and infinitesimals?
In my Calculus class, my math teacher said that differentials such as $dx$ are not numbers, and should not be treated as such.
In my physics class, it seems like we treat differentials exactly like numbers, and my physics teacher even said that they…
Ovi
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82
votes
2 answers
Why doesn't a nucleus-like body made up of just neutrons exist?
We know that neutrons exert short ranged nuclear forces over other nucleons in a nucleus, and these forces are only attractive in nature. Also this force is universal and doesn't differentiate between charged or uncharged bodies. So why doesn't a…
Mehul
- 793
82
votes
21 answers
Comprehensive book on group theory for physicists?
I am looking for a good source on group theory aimed at physicists. I'd prefer one with a good general introduction to group theory, not just focusing on Lie groups or crystal groups but one that covers "all" the basics, and then, in addition, talks…
Lagerbaer
- 15,136
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82
votes
10 answers
Why is light called an 'electromagnetic wave' if it's neither electric nor magnetic?
How can light be called electromagnetic if it doesn't appear to be electric nor magnetic?
If I go out to the sunlight, magnets aren't affected (or don't seem to be). And there is no transfer of electric charge/electrons (as there is in AC/DC…
makerofthings7
- 1,057
82
votes
17 answers
How exactly does curved space-time describe the force of gravity?
I understand that people explain (in layman's terms at least) that the presence of mass "warps" space-time geometry, and this causes gravity. I have also of course heard the analogy of a blanket or trampoline bending under an object, which causes…
Zac
- 923
82
votes
2 answers
Why did Feynman's thesis almost work?
A bit of background helps frame this question. The question itself is in the last sentence.
For his PhD thesis, Richard Feynman and his thesis adviser John Archibald Wheeler devised an astonishingly strange approach to explaining electron-electron…
Terry Bollinger
- 22,299
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82
votes
12 answers
What is time dilation really?
Please will someone explain what time dilation really is and how it occurs? There are lots of questions and answers going into how to calculate time dilation, but none that give an intuitive feel for how it happens.
John Rennie
- 367,598
82
votes
7 answers
Does a charged particle accelerating in a gravitational field radiate?
A charged particle undergoing an acceleration radiates photons.
Let's consider a charge in a freely falling frame of reference.
In such a frame, the local gravitational field is necessarily zero, and the particle does not accelerate or experience…
Sergio
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82
votes
6 answers
Why do we think there are only three generations of fundamental particles?
In the standard model of particle physics, there are three generations of quarks (up/down, strange/charm, and top/bottom), along with three generations of leptons (electron, muon, and tau). All of these particles have been observed experimentally,…
Scott Carnahan
- 3,259
82
votes
8 answers
Moon's pull causes tides on far side of Earth: why?
I have always wondered and once I even got it, but then completely forgot. I understand that gravity causes high and low tides in oceans, but why does it occur on the other side of Earth?
Muhammad Umer
- 2,733
81
votes
6 answers
Why does water stop boiling immediately after turning off the heat?
When I am heating water on a gas stove, it begins to boil after some time and bubbles of air can be seen escaping out.
However, as soon as I increase the amount of heat in the stove, the rate of escape of air bubbles increases immediately, and as…
Mrigank Pawagi
- 1,131