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location Belgium
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visits member for 2 years, 7 months
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Jun
13
comment Is sunlight truly white?
Check out en.wikipedia.org/wiki/Black-body_radiation. Of course, the specific spectrum of the Sun is the reason why the visible spectrum is located where it is. Those wavelengths most strongly emitted by our star are good ones to use in our senses.
May
21
awarded  Popular Question
Mar
1
comment How can we redirect light from any direction to one direction
I'm doubtful about this sentence: "Once you've used this work and turned it into heat, you can put the heat back into the box and turn it into work again, forever." That last word in particular bugs me. You won't have a 100% efficient heat engine, so even if you can get the rest of the setup to work, this shouldn't be a perpetual motion machine, right? I'm thinking non-linear optics by the way, I'd say you could get a fair amount of randomly oriented light redirected using higher order effects, which can be made highly directional. They definitely won't be 100% efficient either, though.
Feb
23
awarded  Necromancer
Feb
16
comment Eigenvalues of a nearest-neighbour tight-binding Hamiltonian in (Mahan, 2003)
Alright, so actual diagonalization it is. Thanks, I was wondering if maybe it was obvious in some other way, but I guess not. Though I should probably have been able to see the result of this diagonalisation without having to actually perform it. Oh well, takes a while for the fingerspitzengefühl to return I guess. :) Thanks again!
Feb
16
accepted Eigenvalues of a nearest-neighbour tight-binding Hamiltonian in (Mahan, 2003)
Feb
16
asked Eigenvalues of a nearest-neighbour tight-binding Hamiltonian in (Mahan, 2003)
Dec
19
comment Inclined pulley with unknown hanging mass: when won't the system accelerate?
The same way you always solve this kind of problem: choose axes and write down Newton.
Dec
7
awarded  Yearling
Nov
29
comment Why is the Moon considered the major cause of tides, even though it is weaker than the Sun?
Related: physics.stackexchange.com/q/111685/16660.
Nov
17
revised How does the force between two charges becomes zero?
Added link to wikipage of limits and a note concerning the values I used in my original example, with a new example (to please Elpmaxe, god of examples))
Nov
17
answered How does the force between two charges becomes zero?
Nov
5
comment Expectation value expression Quantum Mechanics
Exactly. You're welcome :)
Nov
5
comment Expectation value expression Quantum Mechanics
This is incorrect (probably just due to going too quickly in your enthusiasm to answer though). And more importantly (in my opinion): we don't like to just give the answer to this kind of question here. It's much better to let the asker come to the answer themselves, for their own benefit.
Nov
5
comment Expectation value expression Quantum Mechanics
When you've sorted out the brackets in the derivation, note that the second term in the second line is not $-2X\langle X \rangle$. (if you don't see that, look carefully at that line and remember that taking the expectation value is linear, i.e. $\langle a A + b B \rangle = a\langle A \rangle + b\langle B \rangle$ with $a,b$ constants and $A,B$ operators)
Nov
5
comment Expectation value expression Quantum Mechanics
Look closely at your brackets in that derivation again. In LaTeX, you can use the '\langle' and '\rangle' commands for braket notation.
Oct
27
comment Why to write the Navier-Stokes equation with dimensionless quantities?
I don't have time to write up a full answer right now, but when you write an equation in terms of dimensionless quantities, the relative importance of each part of the equation becomes immediately clear. For example if $R\gg1$, then the second term on the right hand side becomes exceedingly insignificant as compared to the other terms.
Oct
27
comment How would you go about evaluating $\langle \psi \mid 100 \mid \psi \rangle$?
Try answering this: what is the (physical) meaning of $\langle\psi|\hat{A}|\psi\rangle$? (where $\hat{A}$ is an arbitrary, hermitian operator)
Oct
16
comment Mathematical understanding of Quantum Mechanics
The key idea here is that the result of $F|r'\rangle$ is another ket which may be expressed as a combination of position kets. We might e.g. have $F|r'\rangle = a|r''\rangle + b|r'''\rangle$, similar to what Alfred wrote but a little more general. Then the matrix elements of $F$ are zero except when $r''=r$ or $r'''=r$. (since they're given by $a\langle r | r''\rangle + b\langle r | r'''\rangle$)
Sep
30
awarded  Explainer