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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
Sep
28
comment Time-ordered operator in Srednicki
This would be helpful to more people if you explicitly included all the equations you reference.
Sep
11
revised Frequency Of Light
Improved and expanded
Sep
10
comment Why does the Metropolis algorithm allow changes even for ∆E > 0?
Basically: you want the global minimum and if you don't allow going up in the energy landscape, you can't get out of a local minimum to get to the global one.
Aug
24
comment About solitons, what is the difference between kinks and vortices?
I want to add another outstanding book with introduction to solitons, instantons and the like: "Gauge field theories: an introduction with applications" by M. Guidry (1991). This is a personal favourite of mine because Guidry's style of rhetoric clicks well with my internal thought process. Another good reference I know is "Solitons and instantons: An introduction to solitons and instantons in quantum field theory" by R. Rajaraman (1982).
Aug
19
comment How much energy would the Human Torch need?
You don't have to take the mass of the average person, Human Torch weighs 170 lbs. :P
Aug
16
comment Unknown letter ℑ used in an equation
This isn't really the kind of questions we answer here but Detexify can often help, since the LaTeX command to write a symbol usually has a somewhat telling name (in this case \Im, meaning "the imaginary part of"). You might have guessed that from the equation itself based on Euler's formula. Actually, I just read the last part of the accompanying text and it says so there as well, so now I'm confused as to why you asked...
Aug
15
awarded  Nice Answer
Aug
9
comment Finding the mass of pure matter
@celtschk and the people who voted earlier: I've edited my answer. I can't comment more on the implications of having a Planck density right now, but sufficit to say this is quantum gravity territory.