Arturo don Juan
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What are the criteria for a system to follow quantum mechanics instead of classical mechanics?
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6 votes

Everything is quantum mechanical. The dynamics of any physical system will always follows the laws of quantum mechanics. It is only when you zoom out sufficiently far that the laws of quantum ...

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Planck's law question
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5 votes

You are comparing the spectral radiance in frequency $B(f,T)$ with the spectral radiance in wavelength $B(\lambda,T)$. Because these are differential distributions, you cannot simply replace $f=c/\...

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A doubt in a Wikipedia article discussing Bertrand's theorem
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4 votes

You're forgetting that for fixed $m$, $u_0$ is a function of $L$. $J(u,L)$ is function of two variables, not one, so your differential equation for "$J'(u)$" isn't so simple. If we take $m$ ...

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Value of the total spin quantum number $s$ in a system of two spin-$1/2$ particles
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4 votes

First off, a system composed of two independent spin-$1/2$ particles, with definite spin-projections, would be represented by the tensor product state $|s_1,m_1\rangle\otimes|s_2,m_2\rangle$, not ...

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How do we Know Something is a Cross Product?
4 votes

It was realized long ago that the idea of a "cross product" $(A_x,A_y,A_z)\times(B_x,B_y,B_z)=(A_yB_z-A_zB_y,A_zB_x-A_xB_z,A_xB_y-A_yB_x)$ had surprising relevance in the real world (e.g. with torque ...

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Is my book's answer on a cons. of momentum question incorrect?
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4 votes

The phrase "combined mass" is misleading here. It means "mass of everything except the stones". So the final mass of the boat (and everything else still on it) is indeed $M$. What's important is that $...

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Does dark matter exist or not?
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3 votes

The article you linked merely provided counterevidence against one single direct dark matter detection done by the DAMA/LIBRA collaboration in Italy nearly 20 years ago. Nobody has ever found a ...

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Feynman Propagator for fermionic field $S_F(x-x)=0$
3 votes

This is not true. See section 3 of this article where the full analytic expression is derived. For finite mass, we can write the $x^2\rightarrow 0^+$ fermionic propagator (the $x^2\rightarrow 0^-$ ...

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Is the Momentum Operator a Postulate?
3 votes

As a quick extension to the above answers, let me repeat that none of quantum mechanics is "derived" from any preceding theories. Yes, there are many correspondences that are quite striking - ...

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Is there a list of hydrogenic transition matrix elements online?
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3 votes

As a partial answer, I took the advice of @JonCuster and pulled the following equations from "Quantum Mechanics of One- & Two- Electron Atoms" by Bethe and Salpeter. All results are in C.G.S units....

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Angular distribution of dipole moments?
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3 votes

The Hamiltonian of a rigid rotor electric dipole is given by $$H=\underbrace{\frac{p^2}{2m}}_{\text{translation}}+\underbrace{\frac{L^2_{\theta}}{2mR^2}+\frac{L^2_{\phi}}{2mR^2\sin^2\theta}}_{\text{...

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Rate at which planet "sweeps out area"
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3 votes

The area of this wedge, swept out by a Keplerian body in a time small time $dt$, is given by $$\begin{align*} dA&=\frac{1}{2}(r)(r\sin(d\phi))\\ &\approx\frac{1}{2}(r)(r d\phi)\\ &=\frac{...

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Google interview riddle and scaling arguments
3 votes

Many simplifying assumptions of our reality must be made to arrive at the results you're looking for (and that the other answers gave). I'll try to give my own 2 cents. I hope that anything incorrect ...

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Why can't electric potential be negative?
3 votes

I believe you are confusing the work done by the electric field with the work done on the particle. By the electric field: The radial force of the electric field is always pointing outwards, and the ...

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Why are the poles of the propagator the masses of bound states and not the masses of unbound states as well?
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2 votes

The sum over states that you've written is supposed to be literally the sum over all states, with the restriction that the total spatial momentum of each state should be $\vec p=0$. You are in ...

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Why are the tops of mountains covered with snow, but not at the bottom?
2 votes

To answer the question in your title, it's because it really is much colder up there. It's colder up there because atmospheric pressure decreases as you increase altitude, which is a result of gravity....

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Two different definitions for current?
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2 votes

Current is amount of charge passing through a cross-sectional area per unit time. Consider a cross-sectional area $A$. Suppose in a time period $\Delta t$ you measure a total amount of charge $\Delta ...

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What is polarisation, spin, helicity, chirality and parity?
2 votes

Polarization is, generically, a normalized vector in spin-space, i.e. the spin-state of a particle (When I say spin, I actually mean total angular momentum, spin+orbital angular momentum). Usually we ...

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Where can I find the mesonic decay constants in the particle data group listings?
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2 votes

With the help of @Slereah, I found the PDG article regarding the mesonic decay constant. Leptonic Decays of Charged Pseudoscalar Mesons In that document, you can find a summary of all the known ...

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Spontaneously broken linear sigma model in Peskin & Schroeder: where is the miracle?
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2 votes

I believe they are making a pedagogical point here that for you may be intuition/obvious. The point of this section is that there may be more distinct terms in a Lagrangian than there are free ...

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Electromagnetic wave within medium (penetration depth)
2 votes

Maxwell's (local) equations in a linear, homogeneous, and isotropic medium read $$\begin{align*} \nabla\cdot\mathbf{E}&=0\\ \nabla\cdot\mathbf{B}&=0\\ \nabla\times\mathbf{E}&=-\frac{\...

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What laws/assumptions are considered fundamental/axiomatic to the widest range of fields of physics?
2 votes

First off, just to set entropy straight, classical (Boltzmann) entropy is an emergent phenomenon in the study of multi-component systems. We may know the basic dynamical principles of each individual ...

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Rindler motion constant acceleration proper time
2 votes

Velocity is defined as $dx'/dt'$, which in your case comes out to be: $$v'=\frac{dx'}{dt'}=c\tanh\left(\frac{g\tau}{c}\right)$$ where $g$ is the proper acceleration. Notice that as $\tau\rightarrow\...

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Why external chemical potential?
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2 votes

The energy (per particle) is defined up to a global constant/reference. If we scale the energy per particle (of all particles!) by a constant $\mu_{\text{ext}}$, then if we add another particle to our ...

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I want to calculate the electric field of a charged solid sphere without using Gauss's law
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2 votes

First let me clarify something. I think what you mean by "Coulomb's law" is the solution to the electrostatic Poisson equation with the assumption (boundary condition) that it vanishes at spatial ...

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Does Inertia "conflict" Entropy?
2 votes

Entropy is a measure of our ignorance of a thermodynamic system, or in its precise mathematical form (Boltzmann's entropy formula), $S=k\log \Omega$, where $\Omega$ is the number of microstates of the ...

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I wanna learn Quantum Physics
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2 votes

As others have stated, it really depends on why you want to learn quantum mechanics, and how deeply you want to learn it. (1) If you want to learn it as badly as you want to watch a movie at the ...

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What are the gravitational binding energies of giant planets?
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2 votes

This is an answer to the point I brought up in the comments. To begin, I'd like to point out that the quoted formula for the gravitational binding energy of a spherically symmetric body isn't quite ...

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Does the Equivalence Principal account for inverse square law?
2 votes

The equivalence principle is local (as the comments indicate), only applicable for sufficiently small regions. Another example that highlights the necessity of the word "local" is the following ...

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1D Scattering Phase Shift (Finite Well) - Unphysical?
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2 votes

I figured it out. The problem was that I was simply adding the two phase shifts, ignoring the periodicity in the tangent and arctangent functions. In order to get the proper result/plot, one can ...

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