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8h
comment Could the LIGO in fact measure .. the expansion of the universe? What specifically is the numerical ratio of effects?
Related: physics.stackexchange.com/questions/235336/…
22h
comment Redshift of merging black holes
@RobJeffries, makes sense to me. The Planck paper referenced by LIGO (1502.01589) headlines $H_0=67.8 \pm 0.9$ km/s/Mpc and $\Omega_m=0.308 \pm 0.012$, so besides not including the uncertainty, LIGO's values look to be very slightly offset from Planck's nominals). Hmmm, compared to the LIGO uncertainties these variations are very small; maybe that's why they were ignored?
Jan
19
comment Is magnetic field due to an electric current a relativistic effect?
related: physics.stackexchange.com/questions/38999/… , physics.stackexchange.com/questions/3618/…
Jan
17
comment Is equivalent resistance always lower if we add a resistor to a passive electronic circuit?
Very nice approach! I think, though, that you overreach a bit when you state: "The product is definitely positive". It is entirely possible that v1=0; consider the example of a balanced bridge mentioned by Emilio Pisanty in a comment above.
Dec
27
comment Help me understand eqn. 28.6 from Feynman
@TolgaYilmaz, (ii) When I calculate that second derivative of the unit vector (which I did a while ago, taking a very long time to work through it), the result is complicated, and in fact is non-zero even if the acceleration $a'=0$. What I wrote in my answer was only approximate (good at low velocities and large distances). However, Feynman's expression (28.6) for the electric field is exact: I was able to show its equivalence to the results in Jackson's and Schwartz's textbooks.
Dec
27
comment Help me understand eqn. 28.6 from Feynman
@TolgaYilmaz, (i) If you're referring to the units, the convention is the vector $\boldsymbol{r'} =|\boldsymbol{r'}| \boldsymbol{e}_{r'}= r' \boldsymbol{e}_{r'}$, so $\boldsymbol{e}_{r'}$ is unit-less.
Aug
24
comment Can Maxwell's equations be derived from Coulomb's Law and Special Relativity?
Chapter 12 title is "Dynamics of Relativistic Particles and Electromagnetic Fields".
Aug
24
comment Can Maxwell's equations be derived from Coulomb's Law and Special Relativity?
@Ruslan, my ref is Jackson, Classical Electrodynamics, 2nd ed. Section 12.2 heading is "On the Question of Obtaining the Magnetic Field, Magnetic Force, and the Maxwell Equations from Coulomb's law and Special Relativity."
Jul
28
comment Why does a free-falling body experience no force despite accelerating?
somewhat related: physics.stackexchange.com/questions/143406/…
Jun
24
comment Why do we must initially assume that the wavefunction is complex?
Do you have a ref for the Soler paper? 1900 seems a tad early. Thanks!
Jun
21
comment How do Cooper pairs form?
Possible duplicate: physics.stackexchange.com/q/36428
Mar
29
comment Is the Wikipedia version of the Heisenberg equation of motion correct?
Your edit is very helpful, thanks. Note: it turns out I introduced a sign error in the commutator of the equation I was trying to copy from Wikipedia. It's now fixed, but my typo may have leaked into your answer.
Mar
29
comment Is the Wikipedia version of the Heisenberg equation of motion correct?
Thank you. I'm having trouble distinguishing your answer from yuggib's edit (which has time priority). Is the $t$ vs. $t'$ distinction important? Also, in your last equation, do you mean the full (convective?) time derivative instead of the partial derivative?
Mar
29
comment Is the Wikipedia version of the Heisenberg equation of motion correct?
1) I agree that $dA_H/dt \ne (dA_S/dt)_H$, but I wrote the partial derivative wrt t, $\partial A_H/\partial t=(dA_S/dt)_H$, not the full derivative. In your example, I think the partial derivative applied to $A_H$ would only act on the explicit time dependence ($t^2$), not on $x_H^2$ or $p_H^2$, hence the equality I proposed. 2) My goal is education, not amusement. I don't know if I'm being too picky.
Mar
28
comment Is the Wikipedia version of the Heisenberg equation of motion correct?
@AndrewLedesma, Groan... Thank you. Fixed.
Mar
27
comment Is the Wikipedia version of the Heisenberg equation of motion correct?
Thank you for this answer. Unfortunately, I'm a bit confused by it. 1) On the one hand, your "true" equation looks the same as mine, and different from Wikipedia's. 2) On the other, in your example, it looks like $\partial A_H / \partial t = (dA_S/dt)_H $, which would validate the interpretation that you said "did not really make sense to write". In fact, if this equality holds in general, I would have to concede Wikipedia has it right. Thoughts?
Mar
27
comment Is the Wikipedia version of the Heisenberg equation of motion correct?
Thank you for this answer. I certainly have no problems with the classical equation of motion; it's as rigorous as physics gets, I think. For the quantum version, I've only seen derivations that start with the Schrodinger picture; your "direct replacement" procedure is intriguing. Forgive my skepticism, but do you have any references for it? Thanks.
Mar
10
comment Child-Langmuir space charge law for non-zero cathode potential (non-zero initial electron velocity)
(1) thanks but I'll pass, sorry. (2) In this formulation, the initial electron velocity is just that, an initial condition. Once emitted from the cathode, an electron is accelerated by the local electric field, which is determined by (a) the voltage difference between anode and cathode and (b) the electric field from other emitted electrons (the space charge). The value of the cathode voltage alone is insufficient. (3) Presumably there are different physical assumptions behind the two formulations, but I don't have anything intelligent to say about them.
Feb
1
comment In a positively biased PN junction, where do the injection carriers come from?
If, as you say, you like the answer to your Q1 and Q2, you should accept that post, and then ask Q3 and Q4 in a new question.
Jan
31
comment What experiments compete with BICEP 2, and when are their results expected?
@annav: No. Planck measured dust by looking at multiple frequencies, which BICEP2 couldn't do, and the two organizations have now agreed that the signal BICEP2 observed is just dust.