Qmechanic
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 2h comment Questions about null geodesic 2h comment Is there a 3D analogue of angle? Would Mathematics be a better home for this question? 2h comment Two-point function of a free massless scalar field in Euclidean space-time Crossposted to mathoverflow.net/q/237647/13917 3h comment Can we embed 2+1 space-time of GR in a 3 Dimensional Euclidean space? To reopen this question (v1) consider 1. clarifying your definition of Euclidean space, and 2. harmonize title question and question in main body (3D vs. 4D). 5h comment How can I prove that the Euler-Bernoulli beam PDE is Hamiltonian? @AccidentalFourierTransform: Ups. Thx. Corrected. 5h comment How can I prove that the Euler-Bernoulli beam PDE is Hamiltonian? Hint: The corresponding Lagrangian density is ${\cal L}=\frac{1}{2}u^2_t - \frac{1}{2} u_{xx}^2 - F(x,u)$. 7h comment How can there be an infinite number of universes? To reopen this post (v1), consider to provide references and context, preferable from scientific sources rather than popular press. 9h comment Are there any specific examples of the application of Lewis-Riesenfeld procedure to time dependent Hamiltonians in QM? This post (v2) seems like a list question. 12h comment Derivation of Euler-Lagrange equation from principle of least action Comment to the question (v2): It seems that OP is asking for the standard integration-by-parts argument in calculus of variations that leads to Euler-Lagrange equations. This is shown in many textbooks on the subject (including the above two Wikipedia links $\uparrow$), and e.g. this Phys.SE post and links therein. 13h comment How to find the rank of the matrix $\frac{\partial ^2\mathcal{L}}{\partial \dot{X^\mu} \partial \dot{X^\nu} }$ for the Nambu-Goto string Lagrangian? Observation: It is tempting to alternatively define $\quad \chi_1 := \frac{1}{2}P^2$, $\quad \chi_0 := P\cdot X^{\prime}$, $\quad \chi_{-1} := \frac{1}{2}(X^{\prime})^2$, and $\quad \chi_{\pm 2} := 0$. Then the PB algebra almost reads $\quad \{\chi_{\alpha}(\sigma),\chi_{\beta}(\sigma^{\prime})\}_{PB} = \left[ \chi_{\alpha+\beta}(\sigma)+\chi_{\alpha+\beta}(\sigma^{\prime})\right] \delta^{\prime}(\sigma-\sigma^{\prime})$ except for one sector, which is $\{\chi_{-1}(\sigma),\chi_1(\sigma^{\prime})\}_{PB} = X^{\prime}(\sigma) \cdot P(\sigma^{\prime}) ~\delta^{\prime}(\sigma-\sigma^{\prime})$; 13h comment Rotation of Spacetime $\Rightarrow$ Change in orbit/path Comment to the post (v2): Perhaps OP is asking if the spin of a lone black hole leads to a Magnus effect or curveball, so to speak? If that's the question then the answer is no. 1d comment What is really sought when we purpose Einstein's postulates in Special Relativity? Related question by OP: physics.stackexchange.com/q/252276/2451 1d comment Are units of angle really dimensionless? 1d comment Propagator in AdS Tip: Consider adding references and context in order to get useful and focused answers. 1d comment Why is the covariant derivative of the determinant of the metric zero? In the example $f=g_{00}g_{00}$, one could apply the covariant derivative $\nabla_{\lambda}$ to the $(0,4)$ tensor $T=(g_{\mu\nu}\mathrm{d}x^{\mu} \odot \mathrm{d}x^{\nu})\odot (g_{\rho\sigma}\mathrm{d}x^{\rho} \odot \mathrm{d}x^{\sigma})$, and compare the $0000$-component before and after. 1d comment Solving Schrodinger's Equation in Bunimovich Stadium Boundary Condition Would Computational Science be a better home for this question? 1d comment Lattice theory in mathematics and physics Post discussed in chat here chat.stackexchange.com/transcript/message/29309419#29309419 and below. 1d comment Why is the covariant derivative of the determinant of the metric zero? @magma : Which simple counterexample do you have in mind? 1d comment Can the second law of thermodynamics be violated in a small enough system if tried repeatedly enough? This question has been asked many times on Phys.SE. 1d comment Quantum Mechanics: how exactly does “delta function normalization” work for eigenfunctions in 1-d free space case? Related: physics.stackexchange.com/q/47934/2451 , physics.stackexchange.com/q/89958/2451 and links therein.