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 ...

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Bragg Scattering of Thermal Neutrons

I'm currently reviewing Bragg scattering. The particular problem below has me slightly confused on whether I'm thinking about it correctly. Questions Is the problem below referring to the kinetic ...
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16 views

potential decomposition in terms of Bloch eigenstate

Given a single particle Hamiltonian $H=-\frac{\hbar^2}{2m}\nabla^2+V(r)$, where $m$ is the electron mass and $V(r)$ is a periodic function representing the lattice potential. It is defined in the ...
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68 views

What Quantum Mechanics is really all about?

This question might sound very silly, so I'm sorry if that's the case. I'll try my best to make my point clear here. Before explaining, just to make clear, I'm not confused because of the Math ...
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1answer
18 views

What is meant by the expression “Markovian dynamics”

I know what a Markov chain is but what does it mean in physics when I say that I assume Markovian dynamics? For example in Quantum Mechanics, I read that it means that the time evolution can be ...
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18 views

Perturbation theory, eigenvalues and eigenvectors for degenerate case (1st order)

I was trying to understand the perturbation theory, but I was lost in the notation... I have understood that I have to identify the imperturbed kets that are degenerated and find the matrix V, where ...
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2answers
147 views

State of a system in Quantum Mechanics and state vectors

I'm taking a course in Quantum Mechanics and there is something I'm not being able to fully understand. On more elementary courses on Quantum Mechanics I've been told that the idea of Quantum ...
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20 views

Is there a constant decay of particles that maintains the value of the Quantum Vacuum?

By my question I am attempting to understand how the Vacuum of space remains at a constant value, (GR the universe on the large scale matches the “critical” energy density of about 8.5×10-10 J/m3) ...
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18 views

What quantum measurement formalism is easiest to implement physically?

As part of my studies and research, I have learned to work with three different measurement formalism which I define to avoid any ambiguity with the nomenclature: General measurements, which are ...
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27 views

Is the universe a Turing machine?

Reading about Computable numbers I wondered if there is any physical experiment that returns non-computable numbers or if there is any physical theory that needs non-computable numbers. Because if ...
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16 views

A wave is passing through a potential barrier

Let assume a wave is passing through a potential barrier. Like this: What will be happened to the amplitude or the shape of the wave during the movement through the potential barrier? {assuming ...
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13 views

Repulsive potential for free fermions

My question -which is probably easy to answer for a physicist- stems from trying to understand the repulsive interaction between fermions. For instance the fact that states of multifermion systems are ...
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37 views

Is this solvable? Time-dependent perturbation theory

The question is A hydrogen atom is placed in a time-dependent homogeneous electric field given by $$ \varepsilon(t) = \varepsilon_0(t^2 + \tau^2)^{-1} $$ where $\varepsilon_0$ and ...
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33 views

What is special about quantum entanglement? [duplicate]

Get two pieces of paper. In secret write the same number on both papers. Transfer one paper to the Moon. Look to the paper which is left at the Earth. Voila! We know what is on the Moon paper. The ...
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38 views

What does $(I + iδH^t)(I-iδH)$ equal to? [on hold]

$I$ is equal to 1, $δ$ is a small change. According to 43:04-43:10 of this video http://theoreticalminimum.com/courses/quantum-mechanics/2012/winter/lecture-4 \begin{align} \ i{ δ }(H^t-H)=0 ...
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3answers
51 views

Can't we consider that EmDrive is Pushing against Space itself? [on hold]

When something move through space it pushes against something else in space with equal force. However, EmDrive warps space around. So can't we say it's pushing against space itself, and thus momentum ...
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1answer
50 views

Completeness relations of eigenstates in the Heisenberg picture

I've been reading Srednicki's introduction to path integrals and I'm slightly unsure of the notation that he uses for the completeness relation of position eigenstates in the Heisenberg picture. In ...
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1answer
39 views

Oscillation of Atom

What exactly does it mean when one says 'one atom of Caesium 137 oscillates 9,192,631,770 times'? I do understand the general thing about oscillation but what exactly is the oscillation of atom, what ...
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1answer
75 views

Why is a relativistic quantum theory of a finite number of particles impossible?

In Dyson's book Advanced Quantum Mechanics , he said "These two examples (the discovery of antimatter and meson) are special cases of the general principle, which is the basic success of the ...
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1answer
44 views

De Broglie's Matter wave equation dividing by zero

I was just thinking about De Broglie's matter wave equation: $\lambda=\frac{h}{p}$ where $p$ is the momentum of the object. But what if the object is at rest? Won't we be dividing by zero? What if we ...
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55 views

Dr. Pierre-Marie Robitaille: On the Validity of Kirchhoff's Law

Lately I've been researching about the black-body spectrum and the historical development of Planck's Law. I mainly wanted to understand a little bit more why many different objects (Stars, Hot ...
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1answer
39 views

Meaning of expectation value of product of non-commuting operators

Let $\hat{A}$ and $\hat{B}$ be Hermitian observables with spectra labeled by $a$ and $b$. Then we can write \begin{equation} \hat{A} = \sum_a a\, \hat{P}_a \end{equation} \begin{equation} \hat{B} = ...
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1answer
37 views

Books on waves with Fourier Transforms

There are many waves and oscillations books out there that also include Fourier analysis but very few give the subject a thorough treatment, they just pass it in a few pages. If anybody has any ...
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1answer
57 views

Lippmann-Schwinger equation and $T$ expansion

Lippmann-Schwinger equation, in operator form, is: $$ T=V+V\frac{1} {E-H_0+i \hbar \varepsilon} T=:V+V\Theta_0T, $$ where $H_{tot}=H_0+{V}$ is the hamiltonian ($H_0$ is the free particle hamiltonian ...
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19 views

Can there be a link between non-locality and non-Markovianity?

Non-locality is a curious feature that is linked to the violation of Bell inequality of any form. It arise from the impossibility of simultaneous joint measurements of observables. The ...
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3answers
52 views

Problem with tracing over the environment and obtaining reduced density operator

I'm reading an article by Bas Henson given here. On page 25 he deals with the double slit experiment Given the wavefunction which is his equation 1.75, he claims he has traced over the environment ...
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Why is local realism not possible with QM?

In Mermin's thought experiment (pdf) particles are sent to two detectors A and B with 3 settings each. If the particle passes the test, we get a green (G) light, else red (R). Now in QM we could send ...
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197 views

Is entropy a meaningful concept on a quantum level?

My naive assumptions, as I really am at a pretty basic stage in QM, are as follows: Classically, entropy gives us a practical measure of the direction of time, as opposed to our physical laws which, ...
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Is there free will? [on hold]

From what I've understood about the answer to this question quantum physics doesn't contradict determinism, but instead it simply isn't achievable only because of our universe's nature: we are unable ...
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1answer
32 views

Fourier expansion and transform - what about the phase of the waves that i am adding?

Say we have a wave on the surface of the water and we want to describe it as a sum of other waves. So we use Fourier expansion to add waves of different wavelengths. For simplicity, say we have to ...
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28 views

How to form a density matrix? [on hold]

forming a density matrix when two independent states are there
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9 views

Can QKD nodes use a constantly evolving key?

In QKD we know that we can generate random secret keys using quantum entangled photons, measuring their qubits and then using noise reduction to negotiate probably similar bits between Alice and Bob. ...
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3answers
65 views

Is there an example of a situation where you need a continuous spectrum?

If you had a hydrogen atom you could say that you want to be able to ionize them. But if you then add the potential due to the earth, e.g. ...
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70 views

Is there a proof that the number of eigenstates is countable for a bound system?

When you solve Schrödinger equation for a free particle with no boundary conditions your eigen states are indexed by quantum number $k \in \mathbb R $ and $\mathbb R$ isn't countable but if you add a ...
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1answer
16 views

magnet repulsion in a cylinder [on hold]

Firstly, Am doing a research project on the use of repulsion of magnets to form a car shock absorbers. Can the forces of repulsion manage to absorb the shock of a 2.0 ton car?
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66 views

Decay of excited electron from Schrödinger equation?

I'm currently studying for a quantum mechanics exam. I found some old exams which has the following question I'm trying to solve. Please tell me if my reasoning is wrong. Can you see from the ...
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51 views

Can you have more than two entangled particles? [duplicate]

Given the diagram on the wikipedia page for Quantum Entanglement we a beam of photons hitting a crystal which splits the beam into to entangled particles. Is it possible to split the beam into four ...
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1answer
60 views

How to model matter in code [on hold]

I've always had a passion for science, understanding and programming. Given my interests, I thought a cool thing to do would be to model real world theories in code (with visuals) so that others may ...
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2answers
106 views

Is there any $SU(\infty)$ gauge theory in quantum field theory?

The groups $U(N)$ and $SU(N)$ are the most important Lie groups in quantum field theory. The most popular are the $U(1),SU(2),SU(3)$ groups (these gauge groups form the Standard model). But is there ...
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74 views

Bell's inequality, “complete plans” and spooky action at a distance

What am I proposing and asking? In Mermin's thought experiment entangled particles, i.e. photons are sent to two machines A and B with 3 settings each. The idea of a "complete plan" how to react to ...
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59 views

how to solve the following question about quantum mechanical wave function? [on hold]

How to solve this question regarding quantum mechanical wavefunction ?
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1answer
13 views

Are the produced entangled photons in type II SPDC always in phase?

Are the produced entangled photons in type II SPDC (spontaneous parametric down conversion) always in phase? In Kim's delayed choice quantum eraser, two entangled photons are produced by a BBQ ...
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1answer
61 views

Children of entangled particles

Theres a photo on the wikipedia page Quantum Entanglement there's a image of a photon being split into two entangled photons via crystal. I used this picture to demonstrate the scenario below. ...
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1answer
61 views

is there an operator which measures the mass of particles?

When I studied a spin, the textbook said spin is an intrinsic quantity like mass. However, while we can calculate just expectation values $ \langle \textrm{S}^2\rangle $ or $ \langle S_z\rangle $, the ...
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71 views

Natural Philosophy [on hold]

My question is an extension of the celebrated question on the moon’s existence if unobserved. “do we still have tides on earth if the moon is unobserved?”
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84 views

Can an electron go along different paths at once in a circuit

A single electron moves along a circuit and comes to a fork in the wires. The wires separate but come back together near the end of the circuit. From what I know, the electron will travel along all ...
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1answer
52 views

Can one detect a single photon through measuring its impulse/momentum on a mirror?

Can one detect a single photon through measuring its impulse/momentum on a mirror? If the answer if YES or theoretically possible, photon path and interference fringes can be detected simultaneously ...
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1answer
59 views

How to deal with eigenvectors which are not square integrable?

In Quantum Mechanics there is one type of situation I'm still unsure on how to deal with. First of all, I want to make clear I'm trying to understand how to deal with this rigorously. What I'm talking ...
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2answers
38 views

Quantum entanglement thought experiment

One can "filter" a collection of electron pairs generated in an entangled (fixed-direction) spin state (e.g. singlet $\left|\uparrow \downarrow \right\rangle - \left|\downarrow \uparrow\right\rangle$) ...
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1answer
29 views

How to produce a given entangled state of two quantum bits?

I was watching Leonard Susskind's video series on quantum entanglement, where he looks at the spins of two electrons. In particular, there are entangled states of the form $$\alpha\left|\uparrow ...
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1answer
78 views

Does a Buckyball spin like an electron or like a baseball?

Does a Buckyball spin like an electron or like a baseball? We are often told that an electron does not really spin like a baseball. Only one (or two, if you count up and down) spin states, for ...