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1500 questions
73
votes
10 answers
Should zero be followed by units?
Today at a teachers' seminar, one of the teachers asked for fun whether zero should be followed by units (e.g. 0 metres/second or 0 metre or 0 moles). This question became a hot topic, and some teachers were saying that, yes, it should be while…
Vidyanshu Mishra
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73
votes
1 answer
On the Coulomb branch of ${\cal N}=2$ supersymmetric gauge theory
The chiral ring of the Coulomb branch of a 4D ${\cal N}=2$ supersymmetric gauge theory is given by the Casimirs of the vector multiplet scalars, and they don't have non-trivial relations; the Casimirs are always independent.
Also in Gaiotto's class…
Yuji
- 3,682
73
votes
8 answers
Why not use the Lagrangian, instead of the Hamiltonian, in nonrelativistic QM?
Undergraduate classical mechanics introduces both Lagrangians and Hamiltonians, while undergrad quantum mechanics seems to only use the Hamiltonian. But particle physics, and more generally quantum field theory seem to only use the Lagrangian, e.g.…
Revo
- 17,504
73
votes
6 answers
Why is the sky never green? It can be blue or orange, and green is in between!
I, like everybody I suppose, have read the explanations why the colour of the sky is blue:
... the two most common types of matter present in the atmosphere are
gaseous nitrogen and oxygen. These particles are most effective in
scattering the…
user59485
72
votes
7 answers
How come neutrons in a nucleus don't decay?
I know outside a nucleus, neutrons are unstable and they have half life of about 15 minutes. But when they are together with protons inside the nucleus, they are stable. How does that happen?
I got this from wikipedia:
When bound inside of a…
Aria
- 1,097
72
votes
7 answers
Does gravity bend gravity?
Let's say that there is a large mass $M$ a light-year or so away from a black hole merger, which causes a very large gravitational wave to be produced. When the gravitational wave reaches $M$, does it bend like light bends when it comes into a…
Tachyon
- 2,120
72
votes
9 answers
Why do fans spin backwards slightly after they (should) stop?
Today, I've decided to observe my PC fan as I shut the computer down. The fan slowly lost angular momentum over time. What I've found really interesting is the fact that the momentum vector change did not stop at the zero vector, but instead flipped…
Captain Trojan
- 813
72
votes
8 answers
Why doesn't the frequency of light change during refraction?
When light passes from one medium to another its velocity and wavelength change. Why doesn't frequency change in this phenomenon?
Self-Made Man
- 2,005
72
votes
3 answers
Why is dry soil hydrophobic? Bad gardener paradox
When I forget to water my plants, and their soil becomes very dry, during the next watering I can see that the soil becomes hydrophobic. I can even see pockets of air between the repelled blob of water and the soil.
On the contrary, when the soil is…
user46147
- 3,114
72
votes
9 answers
Why can't we see images reflected on a piece of paper?
Why can't you see a reflected image on a piece of paper? Say you put a pen in front of the paper, even when light rays are coming from other sources, hitting the pen, reflecting back, and hitting the paper, there is no reflection.
What's wrong with…
katana_0
- 1,195
72
votes
5 answers
Why do we need large particle accelerators?
The LHC is much larger than its predecessors, and proposed successors much larger still. Today, particle accelerators seem to be the main source of new discoveries about the fundamental nature of the world.
My lay interpretation is that particle…
Lemma
- 733
72
votes
5 answers
Conformal transformation/ Weyl scaling are they two different things? Confused!
I see that the weyl transformation is $g_{ab} \to \Omega(x)g_{ab}$ under which Ricci scalar is not invariant. I am a bit puzzled when conformal transformation is defined as those coordinate transformations that effect the above metric transformation…
vishmay
- 1,156
72
votes
6 answers
Could a computer unblur the image from an out of focus microscope?
Basically I'm wondering what is the nature of an out of focus image. Is it randomized information? Could the blur be undone by some algorithm?
user273872
- 2,623
- 2
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72
votes
7 answers
Why are protons heavier than electrons?
Our teacher told us that protons are nearly 1800 times heavier than electrons. Is there any known reason as to why this is so?
Or is this just an empirical value, one we do not know the reason to?
Shivay Vadhera
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72
votes
17 answers
What is the simplest way to prove the Earth is round?
Assume you've come in contact with a tribe of people cut off from the rest of the world, or you've gone back in time several thousand years, or (more likely) you've got a numbskull cousin.
How would you prove that the Earth is, in fact, round?
tQuarella