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Oct
19
answered How can one derive Schrödinger equation?
Oct
19
comment How quickly does gravity extend from created mass?
@LDC3: it's a theoretical prediction consistent with the orbital decay of binary pulsars, most famously PSR 1913+16 (cf Nobel price 1993, arXiv:1011.0718)
Oct
19
revised Reconciling Units in Classical System Analogies: Why Does Torque Have Units of Energy?
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Oct
19
answered How quickly does gravity extend from created mass?
Oct
19
revised Reconciling Units in Classical System Analogies: Why Does Torque Have Units of Energy?
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Oct
19
revised Reconciling Units in Classical System Analogies: Why Does Torque Have Units of Energy?
added 148 characters in body
Oct
19
answered Reconciling Units in Classical System Analogies: Why Does Torque Have Units of Energy?
Oct
16
comment Are Lockheed Martin nuclear fusion claims realistic?
see also en.wikipedia.org/wiki/Magnetic_mirror
Oct
16
revised What is divergence?
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Oct
16
revised What is divergence?
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Oct
16
answered What is divergence?
Oct
16
comment What is divergence?
The divergence of a vector field is the infinitesimal relative change in volume when transporting a volume element along its flow. The abstract definition in terms of Lie derivative and volume form is quite intuitive here.
Sep
30
awarded  Explainer
Aug
21
awarded  Announcer
Aug
20
awarded  Nice Answer
Aug
10
comment QM without complex numbers
@Problemania: $U(n)=Sp(2n,\mathbb R)\cap O(2n)\cap GL(n,\mathbb C)$; however, the intersection of any 2 of the groups on the RHS is sufficient, and in particular $U(n)=Sp(2n,\mathbb R)\cap O(2n)$; complexity arises naturally when we deal with compatible symplectic and orthogonal structures; of course it's equally valid to say that symplectic structures arise naturally from compatible orthogonal and complex structures or orthogonal ones from compatible symplectic and complex ones; but complex structures are arguably less well motivated from a physical (or perhaps 'philosophical') point of view
Aug
3
comment Why can't we do some basic algebra in tensor calculus?
assuming a background in basic linear algebra, think in terms of matrices (which are rank-2 tensors): not all matrices are invertible, and those that are generally aren't orthogonal (ie $A^t\not=A^{-1}$)
Jul
31
comment Which function denotes the energy of thermal motion within a system?
$\frac32 NkT$ for translational motion as well as appropriately weighted multiples of $NkT$ for vibrational and rotational motion as long as quantum effects can be neglegted (ie the spacing of the discrete energy levels is much smaller than $kT$)
Jul
27
comment Can statistical mechanics explain the second law completely?
Let us continue this discussion in chat.
Jul
27
comment Can statistical mechanics explain the second law completely?
I also never understood how your 'logical arrow of time' isn't killed by Loschmidt's paradox; in case of Markov processes, it apparently is, but I have yet to read the cited paper in detail