4,513 reputation
1617
bio website
location
age
visits member for 2 years, 11 months
seen 3 hours ago

Dec
12
comment Point masses and infinite densities
@I'm-A-Crackpot-QUACK-CRANK: because it's convenient; oftentimes, we're only interested in the center-of-mass coordinate, and internal structure is an unnecessary complication, unknown or out of scope of the theory (eg quantum); of course, sometimes such a simplification doesn't work (eg point particles are problematic in classical electrodynamics)
Dec
11
revised Is position fundamentally different from other observables?
deleted 7 characters in body
Dec
11
answered Is position fundamentally different from other observables?
Dec
8
comment Legendre transformation of Entropy as a natural function of T, V, n
solving the first law for $dS$ yields $U, T^{-1}$ as conjugate variables
Dec
5
comment Do accelerated charges radiate or not?
@lurscher: of course they won't radiate if they are at rest relative to the dragged aether ;)
Dec
5
answered In QM, do we deal with basis or orthonormal sets?
Dec
5
comment Applications of analytic continuation to physics
isn't this also the basic idea behind zeta function regularization (replacing a divergent series by the finite value obtained from analytically continuing the series from a convergent parameter domain)?
Dec
4
comment $p\ dq$ is the “tautological” one-form?
supposedly, it has to do with solder forms and the tautological 1-form of the frame bundle (that one is tautological because it's a $R^n$-valued tensorial 1-form that corresponds to the identity of the tangent bundle understood as an associated vector bundle); however, I was never able to make the definitions match
Dec
4
comment What's the physical intuition for symplectic structures?
taking the bird's eye view, dynamics are given by a vector field on some manifold (it's just the infinitesimal description of the flow); if we want to derive that vector field from a potential, we need additional structure as the natural differential operator $d$ yields a 1-form
Dec
2
comment Why isn't the product rule used in the definition of mechanical work?
$\int\vec F\cdot d\vec x$ is precise if you understand it as symbolic notation for the 1-form $\sum_i F_i dx^i$
Dec
2
comment How to show that every Killing vector field is a matter collineation?
see also mathoverflow.net/questions/47332/…
Dec
2
revised How to show that every Killing vector field is a matter collineation?
added 38 characters in body
Dec
2
comment How to show that every Killing vector field is a matter collineation?
@Trimok: see physics.stackexchange.com/a/88705/6389
Dec
2
answered How to show that every Killing vector field is a matter collineation?
Dec
1
comment What are the borders of determinism?
@jinawee: I went ahead and 'fixed' it; as far as stackexchange goes, I'd say it's better to ask forgiveness than wait for permission - after all, it's easy to roll back my changes if anna disagrees...
Dec
1
revised What are the borders of determinism?
added latex and fixed some minor stuff
Dec
1
comment Why doesn't Length of cycloid depend on speed of rolling?
Thus, the arc lengths don't have to be the same - shouldn't that read Thus, the arc lengths don't have to be different?
Dec
1
comment Examples of singularities in classical physics
obligatory link to Wikipedia: en.wikipedia.org/wiki/Rational_function#Applications and de.wikipedia.org/wiki/Rationale_Funktion#Anwendungen ; the German one lists some examples, but their singularities aren't particularly interesting as far as I can see
Dec
1
comment Are all elementary particles of the same type exactly the same?
@akhmeteli: in a way, yes: in the Dirac basis, the spinor components are related to the particle/anti-particle and spin up/spin down distinction; the gamma matrices are responsible for mixing them and can be decomposed as $\gamma^0=\sigma^0\otimes\begin{pmatrix}1&0\\0&-1\end{pmatrix}$, $\gamma^k=\sigma^k\otimes\begin{pmatrix}0&1\\-1&0\end{pmatrix}$; note that the second factor of $\gamma^0$ is diagonal, consistent with the idea that the particle/anti-particle distinction is related to projection onto the time axis
Nov
30
comment Are all elementary particles of the same type exactly the same?
@akhmeteli: well, we do describe both particles and anti-particles with a single Dirac field; the idea of a single worline for all electrons is not really workable (despite conjectured proton decay involving positrons), but I like it nevertheless ;)