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1500 questions
75
votes
5 answers
Why is water clear?
Water appears transparent to visible light, yet most other objects are opaque. Why is that? Is there an explanation why water appears transparent?
Is water transparent at all wavelengths, or are the visible wavelengths somehow special? If it is…
D.W.
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74
votes
6 answers
Is there a way for an astronaut to rotate?
We know that if an imaginary astronaut is in the intergalactic (no external forces) and has an initial velocity zero, then he has is no way to change the position of his center of mass. The law of momentum conservation says:
$$…
David
- 1,116
74
votes
7 answers
What's the physical meaning of the statement that "photons don't have positions"?
It's been mentioned elsewhere on this site that one cannot define a position operator for the one-photon sector of the quantized electromagnetic field, if one requires the position operator have certain formal properties. This is a theorem that…
knzhou
- 107,105
74
votes
2 answers
Is spacetime flat inside a spherical shell?
In a perfectly symmetrical spherical hollow shell, there is a null net gravitational force according to Newton, since in his theory the force is exactly inversely proportional to the square of the distance.
What is the result of general theory of…
Leos Ondra
- 2,265
74
votes
7 answers
Is spacetime discrete or continuous?
Is the spacetime continuous or discrete?
Or better, is the 4-dimensional spacetime of general-relativity discrete or continuous? What if we consider additional dimensions like string theory hypothesizes? Are those compact additional dimensions…
linello
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74
votes
11 answers
Could a candle theoretically melt iron?
The title question is rather illustrative. I suppose the real question would be:
Is heat cumulative?
Put back into an example:
If I have a lit candle right beneath an iron bar, assuming the candle will remain lit indefinitely, and that the…
user1869935
- 1,209
74
votes
2 answers
Where are all the slow neutrinos?
The conventional way physicists describe neutrinos is that they have a very small amount of mass which entails they are traveling close to the speed of light. Here's a Wikipedia quote which is also reflected in many textbooks:
It was assumed for a…
Physics Footnotes
- 2,796
74
votes
5 answers
Why does a system try to minimize its total energy?
Why does a system like to minimize its total energy? For example, the total energy of a $H_2$ molecule is smaller than the that of two two isolated hydrogen atoms and that is why two $H$ atoms try to form a covalent bond. According to the classical…
SRS
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74
votes
6 answers
Why don't choir voices destructively interfere so that we can't hear them?
Sound is propagated by waves. Waves can interfere.
Suppose there are two tenors standing next to each other and each singing a continuous middle-C.
Will it be the case that some people in the audience cannot hear them because of interference?…
chasly - supports Monica
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73
votes
3 answers
Idea of Covering Group
$SU(2)$ is the covering group of $SO(3)$. What does it mean and does it have a physical consequence?
I heard that this fact is related to the description of bosons and fermions. But how does it follow from the fact that $SU(2)$ is the double cover…
SRS
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73
votes
2 answers
Why does dry spaghetti break into three pieces as opposed to only two?
You can try it with your own uncooked spaghetti if you want; it almost always breaks into three when you snap it. I am asking for a good physical theory on why this is along with evidence to back it up. Or, a reference to a good study previously…
David Ball
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73
votes
5 answers
Could the observable universe be bigger than the universe?
First of all, I'm a layman to cosmology. So please excuse the possibly oversimplified picture I have in mind.
I was wondering how we could know that the observable universe is only a fraction of the overall universe. If we imagine the universe like…
A. P.
- 3,408
73
votes
6 answers
Why do we still need to think of gravity as a force?
Firstly I think shades of this question have appeared elsewhere (like here, or here). Hopefully mine is a slightly different take on it. If I'm just being thick please correct me.
We always hear about the force of gravity being the odd-one-out of…
ejrb
- 1,669
73
votes
5 answers
What does it mean for a Hamiltonian or system to be gapped or gapless?
I've read some papers recently that talk about gapped Hamiltonians or gapless systems, but what does it mean?
Edit: Is an XX spin chain in a magnetic field gapped? Why or why not?
Jordan
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73
votes
3 answers
How strong are Wi-Fi signals?
My family members dislike the idea of having many devices communicating wirelessly in our house, arguing that the signals have negative effects on our physical health.
I would like to tell them the EM signals are in fact weaker than the light from…
Chong Lip Phang
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