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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-theory tag for the theory of many-body quantum-mechanical systems.

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When we learn quantum mechanics, we are told that the only way to extract information from a system is to conduct measurements. We are told that if two observables commute then performing one measurem …
asked May 30 '17 by user1379857
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The dispersion curve for superfluid helium-4 is given above. To my knowledge, the first paper that was able to argue that the curve should take this shape from first principles was Feynman's 1954 pa …
asked Feb 7 '18 by user1379857
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As you probably know, there are two different subjects, often referred to as "quantum mechanics" and "quantum field theory." They are closely related (i.e., QFT is just what happens when you quantize …
answered Jul 9 '17 by user1379857
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Physical theories have dimensionful constants. Each constant can be found via measurement, by fitting some equation to data. Mathematically, you would expect each constant to be "defined" in this way …
asked Dec 22 '17 by user1379857
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Sometimes this is claimed without much explanation. The time evolution operator is given by exponentiating the Hamiltonian: $$ U(t) = \exp(-i t\hat H / \hbar ). $$ For concreteness, when we think abo …
answered May 3 by user1379857
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I don't think its as serendipitous as you think If I understand you correctly, you're imagining the group action of $PSL(2, \mathbb{C})$ acting on your two-level system just as a $2 \times 2$ matrix …
answered Aug 4 '17 by user1379857
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No. When we say that two particles "attract," we usually mean that there is some intermediary field causing the attraction, i.e. a "force carrier." You can easily make theoretical models of bosons whi …
answered Nov 26 '18 by user1379857
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If you're talking about the non relativistic Schrodinger equation $$ i \hbar \frac{d}{dt} \psi = - \frac{\hbar^2}{2m} \nabla^2 \psi + V(x) \psi $$ then a gravitaional field effects the particle by cha …
answered May 12 by user1379857
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That's a very good question, but is actually very difficult to answer. The problem is that to understand the quantum mechanics of light, you really have to understand quantum field theory, not just qu …
answered Jun 11 '18 by user1379857
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You are quite right. The path integral is (mostly) useless in non-relativistic quantum mechanics. The main use of the path integral comes from quantum field theory. In QFT, the path integral is taken …
answered Mar 5 '18 by user1379857
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The relationship between "orbital angular momentum" and "intrinsic spin" is an interesting one. I will assume you know something about representation theory. A good place to start is here. Think abo …
answered Jul 25 '17 by user1379857
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I think what you wrote is fine, and interpretation matters a lot to answer this question. However, I think the notion of "wave particle duality" is still meaningful. There are two ways a wave function …
answered May 26 '18 by user1379857
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Lets say we have a closed system with states in a Hilbert space $\mathcal{H}$. Every state can be expressed as a sum of energy eigenstates. In a closed system, like a box of atoms, entropy will increa …
asked Aug 28 '17 by user1379857
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I'm not sure I totally understand your question. Quantum mechanics and classical mechanics are just mathematically completely different. The statement $[q, p] = 0$ in classical mechanics is pretty ton …
answered Jan 29 by user1379857
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The harmonic potential is not the only potential with evenly spaced energy levels. Consider a potential $V(X)$ that is $\frac{1}{2} m \omega_1 x^2 - \frac{1}{2} \hbar \omega_1$ for $x > 0$ and $\frac …
answered May 18 '18 by user1379857

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