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1500 questions
46
votes
4 answers
What is the physical meaning of the connection and the curvature tensor?
Regarding general relativity:
What is the physical meaning of the Christoffel symbol ($\Gamma^i_{\ jk}$)?
What are the (preferably physical) differences between the Riemann curvature tensor ($R^i_{\ jkl}$), Ricci tensor ($R_{ij}$) and Ricci scalar…
Sklivvz
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46
votes
8 answers
How should a physics student study mathematics?
Note: I will expand this question with more specific points when I have my own internet connection and more time (we're moving in, so I'm at a friend's house).
This question is broad, involved, and to some degree subjective.
(I started out as a…
Mark C
- 1,645
46
votes
6 answers
Do people wearing glasses have different field of view than those who don't?
There is one thing I sometimes wonder about ever since I was a child.
Do people who wear eye glasses see objects in different size than those who don't?(Technically different size means different projected image size on the retina.)
For example do…
Calmarius
- 8,100
46
votes
9 answers
Applications of Algebraic Topology to physics
I have always wondered about applications of Algebraic Topology to Physics, seeing as am I studying algebraic topology and physics is cool and pretty. My initial thoughts would be that since most invariants and constructions in algebraic topology…
Sean Tilson
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46
votes
1 answer
Why are dishwasher washed glasses "squeaky clean"?
They seem to have increased friction after going through a dishwasher. The unpleasant (to me) squeaky feel is probably slip-stick friction exciting resonance in the glassware. But why? What has happened to the surface of the glass to make it like…
user56903
46
votes
6 answers
Is there a physical reason for colors to be located in a very narrow band of the EM spectrum?
The portion of the electromagnetic spectrum that is visible to humans are wavelengths between 380 and 750 nanometers.
I am aware that animals have different capacities than humans, but the EM spectrum where they see colors is very close, e.g. 300 -…
Nicolas
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46
votes
4 answers
Has gravity ever been experimentally measured between two atoms?
Has there been any experiments, or are there any references, demonstrating gravity between atoms? If so, what are the key experiments/papers? Or if not, what is the smallest thing that has actually experimentally been shown to be affected by…
Lance
- 2,128
46
votes
5 answers
Density matrix formalism
The density matrix $\hat{\rho}$ is often introduced in textbooks as a mathematical convenience that allows us to describe quantum systems in which there is some level of missing information.
$\hat{\rho} = \sum_{i=1}^N c_i \rvert\psi_i\rangle \langle…
Daniel Hogg
- 672
46
votes
2 answers
Wind generators - why so few blades?
Why commercial wind generators usually have just 2-3 blades?
Having more blades would allow to increase power OR decrease diameter.
Decreased diameter would also reduce stress due to different wind speed on different height...
But despite that…
BarsMonster
- 2,371
46
votes
7 answers
Why fermions have a first order (Dirac) equation and bosons a second order one?
Is there a deep reason for a fermion to have a first order equation in the derivative while the bosons have a second order one? Does this imply deep theoretical differences (like space phase dimesion etc)?
I understand that for a fermion, with half…
Costantino
- 985
46
votes
7 answers
Does a magnetic field do work on an intrinsic magnetic dipole?
When you release a magnetic dipole in a nonuniform magnetic field, it will accelerate.
I understand that for current loops (and other such macroscopic objects) the magnetic moment comes from moving charges, and since magnetic fields do no work on…
Joss L
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45
votes
7 answers
Is it fair to judge this speedskating race by only 3 thousands of a second?
I'm reading this article:
Zbigniew Brodka of Poland won the Olympic men's 1,500 meters speed
skating title by just 0.003 seconds at the Adler Arena on Saturday.
Brodka clocked one minute, 45.006 seconds with Koen Verweij of
Netherlands crossing…
Kasper
- 551
45
votes
2 answers
Identification of particles and anti-particles
The identification of an electron as a particle and the positron as an antiparticle is a matter of convention. We see lots of electrons around us so they become the normal particle and the rare and unusual positrons become the antiparticle.
My…
John Rennie
- 355,118
45
votes
3 answers
A "Hermitian" operator with imaginary eigenvalues
Let $${\bf H}=\hat{x}^3\hat{p}+\hat{p}\hat{x}^3$$ where $\hat{p}=-id/dx$.
Clearly ${\bf H}^{\dagger}={\bf H}$, because ${\bf H}={\bf T} + {\bf T}^{\dagger}$, where ${\bf T}=\hat{x}^3\hat{p}$. In this sense ${\bf H}$ is "formally" self-adjoint.
It…
Pricklebush Tickletush
- 3,668
45
votes
4 answers
What is a phonon?
I am trying to understand intuitively what a phonon is, but for the moment I find it quite difficult (having a limited background in quantum mechanics, an undergraduate course in non-relativistic QM). In fact, I find it hard to formulate good…
doetoe
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