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Questions tagged [quantum-mechanics]

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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Cooling via the quantum vacuum?

Suppose you had an isolated cloud of gas at a low temperature in a vacuum with no external sources of radiation (e.g. no CMB). The gas would clearly cool via the emission of low-energy photons. But ...
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Selection Rules in electron spectroscopy

How to derive the selection rules $\Delta L= \pm 1$, $\Delta S=0$ for electron spectroscopy?
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For which irreps can normalized spin vectors be chosen continuously, for all possible spins?

Let $(u_x,u_y,u_z)$ be a unit vector. Then a normalized vector for spin-1/2 in this direction is given by normalizing a column from the pure density matrix: $\frac{1}{\sqrt{2+2u_z}}\left(\begin{array}{...
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Why did decoherence start in the first place?

We learn that the quantum wave function $\Psi$ collapses when it interacts with a classic object (measurement). My question is: Why are there classic objects after all, how did it all start? In a ...
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Detecting coherence

Is there a way to know if a particle is acting as a wave or a particle? Alternatively, if an entangled particle was already measured? A - Yes So any experiment over an entangled particle that let ...
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Why is quantum entanglement considered to be an active link between particles?

From everything I've read about quantum mechanics and quantum entanglement phenomena, it's not obvious to me why quantum entanglement is considered to be an active link. That is, it's stated every ...
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Photon absorption probability for a given molecule in gas phase

So I'm pretty sure I'm approaching this problem in the wrong way and I need some guidance (my first hint is that I think I'm thinking about a quantum mechanical problem too classically) Suppose there ...
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Can we view quantum fluctuations as a type of quantum power?

(Note: I will let the question stand as is, it has generated a good answer, but the question is too "wishy washy" and needs to be tighter.) We know that the uncertainty principle tells us that the ...
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Can decoherence time be shorter than Planck time?

Decoherence times can be estimated and are inverse functions of mass. Since there are no upper bounds on mass, can decoherence time be shorter than Planck time?
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Why do the formulas for planck's units use h instead of hbar?

On the Answers.com page on Planck length, I see two almost-same formulas for the Planck length that differ only by the use of h and hbar. However, the constants are the same, and my calculator gives ...
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Quantum mechanics of thermal radiation

What are the quantum mechanisms behind the emission and absorption of thermal radiation at and below room temperature? If the relevant quantum state transitions are molecular (stretching, flexing and ...
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Modern and complete references for the $k\cdot p$ method?

I've recently started studying the $k\cdot p$ method for describing electronic bandstructures near the centre of the Brillouin zone and I've been finding it hard to find any pedagogical references on ...
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Virtual photon counting

How to calculate number of exchanged virtual photons per unit of time between two electromagnetically interacting objects?
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Properties of the photon: Electric and Magnetic field components

Consider an electromagnetic wave of frequency $\nu$ interacting with a stationary charge placed at point $x$. My question concerns the consistency of two equally valid quantum-mechanical descriptions ...
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How does electron move around nucleus?

I need to get a nice picture about how electron moves around nucleus? I find concept of probability and orbitals quite difficult to understand?
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How do electrons interact if one of them had just exited the two slits of the double-slit experiment?

Consider the following experiment: a double-slit set-up for firing electrons one at a time. Let's now add a second electron (orange), which is fired parallel to the first one, but in the opposite ...
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Visualization of proton's wavefunction

Visualizations of hydrogen's wavefunctions / electron orbitals are abound. I could not however locate a visualization of the wavefunction of a proton. The reason I was looking for one is to see ...
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Properties of graph of subatomic particle interactions

Say there was some situation where you have a lot of subatomic particles interacting with each other and decided to draw (say, by joining Feynmann diagrams) those interactions- so that you got some ...
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Double slit experiment near event horizon

What happens if you perform a double slit experiment near an event horizon, if one of the slits is outside, one is inside the event horizon?
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Matrix models and condensed matter physics

I am sending a couple of questions which seem a bit more specific than others on this site, partially to probe if there is a point in doing so. Not sure what is the range of expertise here, and no way ...
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Practical applications for a Bose-Einstein condensate

What are the main practical applications that a Bose-Einstein condensate can have?
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Solution for the Finite 2D Potential Well - Rotational Symmetry [closed]

I was searching for the eigensolutions of the two-dimensional Schrödinger equation $$\mathrm{i}\hbar \partial_t \mid \psi \rangle = \frac{\mathbf{p}^2}{2m_e}\mid \psi \rangle + V \mid \psi \...
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Calculation of the cross section

Why, when we calculate the total cross section, we make the average other initial states and the sum over final states?
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What is the conserved quantity of a scale-invariant universe?

Consider that we have a system described by a wavefunction $\psi(x)$. We then make an exact copy of the system, and anything associated with it, (including the inner cogs and gears of the elementary ...
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What's the exact connection between bosonic Fock space and the quantum harmonic oscillator?

Let's suppose I have a Hilbert space $K = L^2(X)$ equipped with a Hamiltonian $H$ such that the Schrödinger equation with respect to $H$ on $K$ describes some boson I'm interested in, and I want to ...
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Pauli exclusion principle and Entangled pairs

It is true for fermions in the same potential that the total wavefunction of two particles must be antisymmetric with respect to exchange of electrons. Which means the spin wavefunction is given by $...
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Time travel and nuclear decay

Reading a previous closed question an interesting variation has come to my mind. Suppose that time travel to the past was possible: I wait for an atom to decay and measure the time, $t_{1a}$ I ...
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Maximum theoretical data density

Our ability to store data on or in physical media continues to grow, with the maximum amount a data you can store in a given volume increasing exponentially from year to year. Storage devices continue ...
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What is the definition of colour (the quantum state)?

I heard somewhere that quarks have a property called 'colour' - what does this mean?
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Is there such a thing as “Action at a distance”?

What ever happened to "action at a distance" in entangled quantum states, i.e. the Einstein-Rosen-Podolsky (EPR) paradox? I thought they argued that in principle one could communicate faster than ...
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How can one find the energy eigenfunctions of a particle in a finite square well via the Klein-Gordon equation?

It is said that Klein-Gordon equation is a relativistic version of the Schrodinger equation. In Schrodinger equation, it is straightforward to include potential energy. But for K-G eqn things seem to ...
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Measurement problem in the orthodox interpretation

Let's look at the measurement problem in the orthodox interpretation of quantum mechanics as an inconsistency between inner and outer treatment of the measurement apparatus. You can always push your ...
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Why are von Neumann Algebras important in quantum physics?

At the moment I am studying operator algebras from a mathematical point of view. Up to now I have read and heard of many remarks and side notes that von Neumann algebras ($W^*$ algebras) are important ...
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To calculate the correlation functions of an XX spin chain, Wick's theorem is used. But is it valid for a chain of any size?

The correlation functions found in Barouch and McCoy's paper (PRA 3, 2137 (1971)) for the XX spin chain use a method which uses Wick's theorem. For the zz correlation function, this gives $\langle \...
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Path integral vs. measure on infinite dimensional space

Coming from a mathematical background, I'm trying to get a handle on the path integral formulation of quantum mechanics. According to Feynman, if you want to figure out the probability amplitude for ...
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What does an atom radiate: a wave packet or a single photon?

What does an atom radiate: a wave packet or a single photon?
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What is the range of validity of Fermi's Golden Rule?

It is well known that to calculate the probability of transition in the scattering processes, as a first approximation, we use the Fermi golden rule. This rule is obtained considering the initial ...
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Electron transitions in an infinite square well

Consider an electron in an infinite square well. The expectation values of momentum and angular momentum are all zero for energy eigenstates. An electron transition is accompanied by the emission or ...
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Total number of subatomic particles in the universe. Are they finite ? assuming any of GR or QM or even ST

Total number of subatomic particles in the universe. Are they finite ? assuming any of GR or QM or even ST.
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Separation of variables, eigenfunctions of the Dirac operator

Disclaimer: I am not a physicist; I am a geometer (and a student!) trying to learn some physics. Please be gentle. Thanks! When solving the Schrödinger equation for a particle in a spherical ...
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Is it possible to determine timescales of electron dynamics from the natural linewidth of an electronic transition?

A lot of work has been done recently on electron dynamics using attosecond pump-probe techniques; for instance in this paper. In this particular paper, the authors photoionized the neutral ...
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What is an analog to QM's Hilbert space in GR?

I've read that QM operates in a Hilbert space (where the state functions live). I don't know if its meaningful to ask such a question, what are the answers to an analogous questions on GR and ...
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Appearance of atoms

I was watching a documentary entitled "The Atom" and one of the statements made was that Atoms behave differently when we look at them. I wasn't too sure about the reasoning behind this and i'm hoping ...
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Teleportation by quantum isolation

This is a thought experiment, so please don't treat it too harsh :-) Short: If we could isolate two places A and B in the universe from all and any interaction with the surroundings, is there a ...
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Is there a difference between observing a particle and hitting it with another particle?

First, let me state that I'm a lot less experienced with physics than most people here. Quantum mechanics was as far as I got and that was about 9 years ago, with no use in the meantime. A lot of ...
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Madelung transformation

What is the Madelung transformation and how is it used?
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Why is it hopeless to view differential geometry as the limit of a discrete geometry?

This is a follow-up question to Introductions to discrete space-time: Why is this line of thought hopeless? Classical mechanics can be understood as the limit of relativistic mechanics $RM_c$ ...
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Is there a method for differentiating fractional quantum Hall states aside from finding Chern numbers?

The ground state for a quantum Hall system on a torus with fractional filling factor can be classified by the Chern number, which is why the Hall conductance is quantized. Is there another method or ...
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Correspondence principle

Is there a precise mathematical derivation for the correspondence principle for which I can replace $E \rightarrow i \hbar \frac{\partial}{\partial t}$ and $p \rightarrow -i \hbar \nabla$?
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Could teleportation ever really happen?

Could any form of matter teleportation every really happen? If so, what form would it take? Some science-fiction talks about literally sending the atoms from one location to another; others talk of ...