# Tagged Questions

Quantum mechanics describes the microscopic properties of nature in a regime where classical mechanics no longer applies. It explains phenomena such as the wave-particle duality, quantization of energy and the uncertainty principle and is generally used in single body systems. Use the quantum-field-...

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### π , σ - atomic transitions with respect to a quantization axis

In the absence of a magnetic field, how does one physically (i.e perhaps in a thought expt) access delta_m = 0 or +-1 transitions since (as I understand it) the choice of quantization axis is ...
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### What happen if adiabatic evolution fails?

Adiabatic theorem states that: A physical system remains in its instantaneous eigenstate if a given perturbation is acting on it slowly enough and if there is a gap between the eigenvalue and the rest ...
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### Why are general wave functions expressed in terms of energy eigenfunctions?

I have read that the eigenfunctions of any hermitian operator can be used as a basis to express any function, but I have only ever really seen the eigenfunctions of the Hamiltonian used. Why is this? ...
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### Why isn't there a Time Operator in Quantum Mechanics? [duplicate]

I was wondering about a scenario where you subject a quantum particle to an intense gravitation field. Why can't we apply a sort of time operator to the particle to see how time changes for the ...
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### Mean energy of free electrons

My textbook states that the mean energy of free electrons within a solid state is given by $$E_{av} = \frac{U}{N} = \frac{\int_0^{k_f}D(k)E(k)dk}{\int_0^{k_f}D(k)dk}$$ where $D(k)$ is the density of ...
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### non-negative energies

Can someone help me with the following question: given the hamiltonian: $H=\frac{1}{2m}(iP-\hbar f(X))(-iP-\hbar f(X))$ where $f(x)$ an analytic real function. prove that the ...
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### How to construct Hermitian(quantum) operator from a physical experiment?

Suppose I want to study a quantum mechanical quantity of a single particle. I have designed an appropriate apparatus, accuracy of which is limited by relevant laws of quantum theory. I have obtained a ...
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### de Broglie's relation derivation

While going through de Broglie's relation in my textbook I was stuck by a derivation: Here $$\ E=mc^2$$ has been applied to photons. That confused me as I thought that mass of photon=0 but a ...
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### Why does the nuclear force become stronger with distance? [duplicate]

Also how factual is the explanation? Is it mostly philosophical like say Many World's 'theory' or is the explanation pretty straight forward with little to no dispute?
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### Does the entanglement depend on the basis?

Let's say, we have a composite system $A\otimes B$. We take the basis for $A$ as $|i\rangle,|j\rangle...,$ the basis for $B$ as $|\alpha\rangle,|\beta\rangle....$ Then an entangled state is a state ...
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### In handling two 1/2-spin particles, why is there only one singlet state? [closed]

Why is $\left|\uparrow\uparrow\right\rangle +\left|\downarrow\downarrow\right\rangle$ not discussed, despite having a total spin s = 0?
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### Oscillator in Energy Basis Lowering and Raising Operators

On page 205 of Shankar's Intro to Quantum Mechanics, equation 7.4.12 does not make sense to me. I understand why a|e> is an eigenvector and why e-1 is its Eigenvalue, but I don't understand how that ...
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### Two Slit Experiment with polarized filters

If I did a two slit experiment with linear polarized filters in the two paths presumably I would see not interference if they were a 90 deg to each other. If they were aligned presumably the ...
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### What is the significance of the assumption of “full energetic degeneracy” in this paper?

I'm reading the new paper "Foundations of statistical mechanics from symmetries of entanglement" (available on arxiv), in which the authors Deffner and Zurek note that the quantum microcanonical state ...
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### How do Rabi oscillations act on the relative phase of state kets

Suppose we have a system with a lower state $\left|{\downarrow}\right\rangle$ and and upper state $\left|{\uparrow}\right\rangle$, coupled by a pulsed electromagnetic field. I thought that from an ...
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### Scattering from a step potential barrier

Suppose a potential barrier of the form $$V(x) = \begin{cases} V_0 & x>0 \\ 0 & x<0 \end{cases}$$ Then, for energy $E$ such that $E < V_0$, we have that the transmission and ...
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### How did physicists conclude that 98% of mass is due to the strong force?

On a video on YouTube called 'weak and strong nuclear force (9/15)' at 0:55 a scientist said that 98% of mass is due to the strong force.
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### Hamiltonian formulation of the semiclassical Model of electrons

I'm currently reading the book Solid State Physics by Neil W. Ashcroft and N. David Mermin. In Chapter 12 they introduce the "Semiclassical Model of Electron Dynamics". In short: After having solved ...
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### Examples of Matrix Product State(s)

Matrix product states(MPS) is a way of representing a (many-body) wavefunction. The method has been described in, https://arxiv.org/abs/1008.3477 However, would it be possible to see a concrete ...
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### How to find the corresponding Hamiltonian in quantum, if Hamiltonian in classical mechanics is given? [closed]

Hamiltonian in classical mechanics is $$H=wxp$$ $x=$ position, $p=$ momentum coordinate. Find the corresponding Hamiltonian in quantum mechanics!
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### Doubt regarding scattering and bound states in quantum mechanics

I just started studying quantum mechanics from Introduction to Quantum Mechanics by D. J. Griffiths. At the beginning of the second chapter, he proves that we cannot have negative energies in the time-...
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