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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Simple Quantum Mechanics question about the Free particle, (part2)

Continuing from my first question titled, Simple Quantum Mechanics question about the Free particle, (part1) Griffiths goes on and says, "A fixed point on the waveform corresponds to a fixed ...
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1answer
929 views

Simple Quantum Mechanics question about the Free particle, (part1)

I am reading Introduction to Quantum Mechanics by David Griffiths and I am in Ch2 page 59. He starts out writing the time dependent Schrödinger equation and the solution for $\psi(x,t)$ for the free ...
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4answers
953 views

How to calculate the quantum expectation of frequency of a particle?

I know how to calculate the expectation of < $\Psi$|A|$\Psi$ > where the operator A is the eigenfunction of energy, momentum or position, but I'm not sure how to perform this for a pure frequency. ...
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493 views

Quantum Mechanics [closed]

Quantum Mechanics. Could anyone help me to understand the concept involved? Double slit experiment can be easily understood by Wave nature of light, but while explaining it with photons, it required ...
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4answers
8k views

Does a magnetic field arise from a moving charge or from its spin, or both?

I learned that a moving charge creates a magnetic field perpendicular to its direction of motion. I also learned that charged particles like electrons have spin and they also create a magnetic field ...
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Should I begin to study quantum mechanics or wait until I have a stronger base on easier topics? [closed]

I apologize if this is off topic since it does not relate directly to the study of physics, but I could not think of a better place to ask. I am a high school student graduating in a few days and ...
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0answers
108 views

Can space-like fields not commute and still forbid faster-than-light signalling? [duplicate]

Possible Duplicate: Is microcausality necessary for no-signaling? If we try to measure two non-commuting observables, only the earlier measurement would be valid. The later measurement ...
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7answers
3k views

What is an observer in quantum mechanics?

My question is not about (pseudo) philosophical debate; it concerns mathematical operations and experimental facts. What is an observer? What are the conditions required to be qualified of observer, ...
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4answers
733 views

Where is the particle during a tunneling event?

If, say, a particle with energy $E<V_0$, approaches a finite potential barrier with height $V_0$, and happens to tunnel through, where would the particle be during the time period when it is to the ...
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1answer
3k views

Wavefunction normalization

How do we normalize a wavefunction that's a linear combination of sines and cosines (or of $Ae^{ikx}+Be^{-ikx}$ -- they're the same, right)? One you square it, wouldn't the integrand be oscillating ...
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2answers
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Degeneracy of states in mixed infinite square well, harmonic oscillator

I'm trying to determine the degeneracy of states given by $g(\epsilon)=g_{0} \epsilon$ for a system that is trapped in a quite specific potential. In two dimensions, the particle has a potential as ...
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2answers
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Scattering vs bound states

Why are these states called as such, and how do they differ? I vaguely understand that when $E > 0$ you obtain a scattering state, but when $E < 0$ you have a bound state.
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Why is $\frac{dx}{dt}=0$ in this average momentum calculation?

In the following excerpt from S. Gasiorowicz's Quantum Physics, he derives an expression for the average momentum of a free particle. $\psi(x,t)$ is the wave function of a free particle, $\psi^*$ ...
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2answers
942 views

Weyl exponential form of the Canonical Commutation Relations

What is the physical meaning of the $c$-numbers $Q, P\in \mathbb{R}$ in the exponent of the Weyl system $\exp\left[\frac{i}{\hbar} Q \hat{p}\right]$ and $\exp\left[\frac{i}{\hbar}P\hat{q}\right]$? ...
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2answers
441 views

German book on introductory quantum mechanics [closed]

I'm looking for an originally German introduction to quantum mechanics. Is there such a canonical book used in German QM undergraduate courses?
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2answers
4k views

How is the Breit-Rabi formula derived?

Does anyone know how the Breit-Rabi formula (energy dependence of the hyper-fine splitting in presence of any external magnetic field for alkali atoms when $F=I \pm 1/2$) is derived, or where to find ...
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4answers
2k views

Applications of the Spectral Theorem to Quantum Mechanics

I'm currently learning some basic functional analysis. Yesterday I arrived at the spectral theorem of self-adjoint operators. I've heard that this theorem has lots of applications in Quantum ...
3
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1answer
424 views

Seiberg Witten theory

I'm currently reading the Seiberg-Witten paper on $N=2$ supersymmetric Yang Mills pure gauge theory (i.e. no hypermultiplets). I have the following question: How does one understand that the metric ...
2
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1answer
396 views

Tunneling Rate Constant

I am trying to "decode"/derive an expression for the macroscopic rate constant for the tunneling of protons through a potential energy barrier that I read in a journal article: $$ k_{\rm ...
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4answers
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Why do electrons occupy the space around nuclei, and not collide with them?

We all learn in grade school that electrons are negatively-charged particles that inhabit the space around the nucleus of an atom, that protons are positively-charged and are embedded within the ...
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3answers
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What makes matter “solid”?

I'm a non-physicist with a basic high-school understanding of physics. I've always wondered what it is that makes things "solid". Why do molecules rebound from each other? There's just a bunch of tiny ...
6
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1answer
501 views

Generalization of spin coherent states for an arbitrary group?

My question is inspired by the analogy of the Berry phase in the spin coherent state representation of a rotator and the Aharonov-Bohm phase of a magnetic monopole (see e.g., Section 1.8.3 in ...
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3answers
6k views

How to construct the radial component of the momentum operator?

I'm having trouble doing it. I know so far that if we have two Hermitian operators $A$ and $B$ that do not commute, and suppose we wish to find the quantum mechanical Hermitian operator for the ...
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2answers
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Relation between total orbital angular momentum and symmetry of the wavefunction

My question essentially revolves around multi-electron atoms and spectroscopic terms. I understand the idea that the total wavefunction for Fermions should be antisymmetric. Consider as an example, ...
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1answer
443 views

Time evolution of quantum state after an observation

As described in the wikipedia article on wave function collapse, the mathematical formulation of quantum mechanics postulates that wave functions change according to two processes: When not being ...
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2answers
911 views

Has quantum entanglement been demonstrated to be able to take place over infinite distances?

In my poor understanding of quantum physics, quantum entanglement means that certain properties of one of two 'entangled' quantum particles can lead to change over infinitely large distances when the ...
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1answer
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Light waves and Schrödinger probability waves

Ok, bearing in mind that I only have a brief understanding of quantum mechanics (no formal education, only from reading about concepts in books), so I could be way off here, I have a question ...
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2answers
291 views

What happens if up quarks are replaced by down quarks and down quarks are replaced by up quarks?

I believe the quarks would flip the proton's internal charge, reverse the spin of the proton, transform into another flavor, and then become unconfined and free.
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3answers
630 views

Can spin be infinite?

Can spin of a particle or a group of particles become infinity? Explain plz. Is there any representation for spins like dot(for $S=0$) and arrow (for $S=1$)? If so what for $S= \infty$?
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1answer
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Quantum Entanglement: how to generate 2 entangled particles?

I get quantum entanglement but I don't quite get how one would go about generating two complementary particles that are entangled (a photon and its entangled sibling, an electron and its entangled ...
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6answers
775 views

How low can an electron go?

Title says it all - recently I encountered a strange homework exercise on de Broglie dual theory with an electron wavelength of few millimeters - which implies the velocity lower than 1 m/s. I ...
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7answers
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What is the Physical Meaning of Commutation of Two Operators?

I understand the mathematics of commutation relations and anti-commutation relations, but what does it physically mean for an observable (self-adjoint operator) to commute with another observable ...
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2answers
1k views

Can quantum communication really replace electromagnetic waves for telecommunication medium in future?

Currently I am planning to get masters degree. So I am thinking about a subject in which I have to get masters degree. Following are my questions to leading physicists.. Which technology is the ...
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3answers
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Once a quantum partition function is in path integral form, does it contain any operators?

Once a quantum partition function is in path integral form, does it contain any operators? I.e. The quantum partition function is $Z=tr(e^{-\beta H})$ where $H$ is an operator, the Hamiltonian of the ...
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1answer
169 views

Bell Tests using position measurement

I don't know about all the details of Bell tests using methods like parametric down conversion, but at least in some versions of the EPR paradox you get two photons moving apart in opposite ...
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2answers
563 views

What are constraints on a “purity” operator in quantum mechanics?

Consider the normalized state, written in some orthonormal basis as: $$\psi = A |0\rangle + B |1\rangle$$ Let's define a "purity operator" for a basis as any operator whose expectation value gives 1 ...
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6answers
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Is there a direct physical interpretation for the complex wavefunction?

The Schrodinger equation in non-relativistic quantum mechanics yields the time-evolution of the so-called wavefunction corresponding to the system concerned under the action of the associated ...
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7answers
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Does Quantum Mechanics assume space and time are continuous?

I was confused when I was listening to a Quantum Mechanics lecture online. Are space and time assumed to be continuous or discrete in Quantum Mechanics? I can see the question is vague, but this is ...
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2answers
258 views

Bell Test statistics

I have some questions about the statistics of Bell Tests. For convenience I'm going to refer to the test described in this paper: http://arxiv.org/abs/quant-ph/0205171 They show a coincidence rate ...
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1answer
213 views

Quick question on the ionization energy and the selection rule

So I am looking through my book and it says ".... the order of the excited states is exactly the same order (3p-4s-3d-4p)". But now I am looking at a question in the book and it says "Is 3d to 4s ...
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1answer
343 views

Is a 5p to 5s transition possible in a multielectron atom?

Hey guys, I'm reading my physics book right now and I have a quick question about transitions in multielectron atom. I am posting a picture as well so you can see what I am talking about. How come the ...
4
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1answer
267 views

Uncertainly Principle in orthogonal directions

The Heisenberg Principle states that for each direction, $\Delta x\cdot \Delta p_x \ge \hbar , \Delta y\cdot \Delta p_y \ge \hbar$ and $\Delta z\cdot \Delta p_z \ge \hbar$. But, can anything be said ...
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2answers
460 views

Charge distribution in positronium

Inspired by this: Electrical neutrality of atoms If I have a wavefunction of the 'reduced mass coordinate' for a hydrogen like atom made from an electron and a positron, what is the spatial charge ...
2
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2answers
4k views

What is wrong with the De Broglie–Bohm theory a.k.a “Causal Interpretation” of quantum theory? [duplicate]

Possible Duplicate: Why do people still talk about bohmian mechanics/hidden variables I've heard of De Broglie–Bohm theory a.k.a causal interpretation of quantum theory. The predictions ...
3
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4answers
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Is it possible to know the exact values of momentum and velocity of a particle simultaneously?

I know that by Heisenberg's Uncertainty Principle that it is not possible to know the exact values of position and momentum of a particle simultaneously, but can we know the exact values of momentum ...
4
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1answer
553 views

Fourier transform and commutation of functions

The book I am reading takes the unjustified step $$e^{-\frac{i}{\hbar}\vec{p}\cdot\vec{r}}f(\vec{p}) = f(\frac{\hbar}{i}\vec{\nabla})e^{-\frac{i}{\hbar}\vec{p}\cdot\vec{r}}$$ and similarly, he uses ...
2
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2answers
2k views

Does the wave function need to be zero at the boundaries? [closed]

Recently I had a quiz in my physics class and I feel like the professor made a mistake on the solution for it. Yes, I already have the answer to this question. I am not trying to get people's answers ...
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4answers
957 views

Nature of Photons

Why is it that photons are emitted in bundles? My physics teacher's answer was "it's complicated"...
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2answers
195 views

Does the Casimir effect allow to change the lifetime of a radiating atom?

Is it true that a spontaneously light-emitting atom changes its lifetime if it is put between two parallel plates that are so near that they attract each other through the Casimir effect? Thus: does ...
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1answer
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Can observations of entangled particles affect their unobserved counterparts?

There are two experiments that are often used to explain Quantum Mechanics: the two-slit experiment and the EPR paradox. I am curious what would happen if you combined them. Imagine an experiment ...