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 17h awarded general-relativity 1d comment Why can an electric field escape from a black hole? @Marijn - It's not because of encouragement or what not. It helps keep the site organized by keeping track of what has already been answered and what not. For the users who are looking through questions to decide what to answer, this is a really helpful process. 1d answered Relation of conformal symmetry and traceless energy momentum tensor 1d comment Can black holes with the same mass evaporate with different speeds? Eternal black holes only exist in classical general relativity. Once quantum mechanics is introduced (which needs to be done in order to talk about black hole evaporation), then eternal black holes do not exist. 1d comment Meaning of $R=0$, $R_{ab}=0$. $R_{abcd}=0$. No, $R_{ab} = 0$ does not imply $R_{abcd} = 0$. 2d answered How do I derive geodesic equation using variational principle? 2d comment Are objects like $a^{\mu \nu} a_{\mu \nu} b^{\mu \nu}$ consistent with Einstein summation? 2d comment Are objects like $a^{\mu \nu} a_{\mu \nu} b^{\mu \nu}$ consistent with Einstein summation? You could define a diagonal metric $g_{\alpha\beta}$ whose diagonal components are $g_{\alpha\alpha} = c_\alpha$ (no sum here). Then, the quantity you want to calculate is $g_{\alpha\beta} a^\alpha b^\beta$. 2d comment Are objects like $a^{\mu \nu} a_{\mu \nu} b^{\mu \nu}$ consistent with Einstein summation? BTW, the reason you almost never come across such objects is because the norms used in GR and relativity are square norms. The inner product is a map $V \times V \to {\mathbb R}$. If we were working with vector spaces with cubed norms where the inner product is a map $V \times V \times V \to {\mathbb R}$ then maybe we would have use for such a notation. 2d comment Are objects like $a^{\mu \nu} a_{\mu \nu} b^{\mu \nu}$ consistent with Einstein summation? as I said earlier, no. The Einstein summation convention does not allow for more than two repeated indices. Only objects like $a^\mu b_\mu$ or $T_{\mu\nu} h^{\mu\nu}$ are allowed. $a^\mu b_\mu c^\mu$ is not well-defined within the summation convention. 2d comment Are objects like $a^{\mu \nu} a_{\mu \nu} b^{\mu \nu}$ consistent with Einstein summation? well, of course, this is a notation and you could define it to be whatever you want. However, since this kind of sums never appear in tensor analysis (mostly because we use square norms), it is not a useful one in this context. 2d comment Are objects like $a^{\mu \nu} a_{\mu \nu} b^{\mu \nu}$ consistent with Einstein summation? No. It is not. Indices can be repeated only twice -- no more. The type of object you are talking about never comes up in relativity. 2d comment Expectation value position of sine wave in infinite square well Apart from the issues already mentioned, your integral should be only from $-a$ to $a$. 2d reviewed Close meson decay in Yukawa theory 2d reviewed Close If quarks are fundamental, and mass is a form of energy, then can quarks/leptons be pure energy? Feb 8 reviewed Close Is light relative? Feb 6 comment Orbital angular momentum eigenstates in the $|\mathbf{r}\rangle$ representation The author is talking first and foremost of the eigenfunction of $\nabla^2$ in spherical coordinates which involves all 3 coordinates $(r,\theta,\phi)$. This eigenfunction he is denoting as $\psi(r,\theta,\phi)$. Next, he is using the explicit form of $\nabla^2$ in spherical coordinates and showing that it can be broken up into a piece that's purely $r$ dependent and a piece that's purely $(\theta,\phi)$ dependent. The latter turns out to be simply $L^2$. He concludes that the full eigenfunction $\psi$ can be constructed out of eigenfunctions of $L^2$, namely $Y^m_l$. Feb 6 comment Coulombs Law on Quarks Feb 6 reviewed Approve entropy tag wiki excerpt Feb 6 comment Coulombs Law on Quarks I suspect you have a much bigger confusion that has nothing to do with quarks or anything. A 5.33C force is just like a 1C force except now with a different (greater) magnitude. Everything else is the same. What exactly worries you here? PS - I have no idea where you got this 5.33 number from and why this number seems to be throwing you off. If you can explain then maybe we can clarify the issue you are having.