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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What does the quantum eraser experiment tells us?

I am a beginner in this quantum-mechanics stuff. I understand the quantum eraser only from an experimental view. So I didn't understand the formalism that describes the quantum eraser. But what does ...
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2answers
746 views

Expansion in spherical harmonics in cubic symmetry

suppose I have an electrostatic potential which I expand in spherical harmonics via $$\sum_{l,m} A^l_m r^n P_l^{|m|}(\cos \theta) e^{im\varphi}$$ and I know that the field has cubic symmetry. Is ...
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1answer
373 views

What is the spectral energy density of virtual photons around a unit charge at rest?

Given that my previous question, namely "What is the number density of virtual photons around a unit charge?" has no precise answer, here is a more precise wording: What is the virtual photon ...
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4answers
328 views

Justification of ignoring large set of entanglements

If we can think about the universe as a wave function then many particles should be entangled with many other particles in the universe. The obvious question arises why we don't see those ...
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414 views

What is the density of virtual photons around a unit charge?

It seems that virtual photons also exist in vacuum. So the precise question is: What is the additional virtual photon density due to the electric field of a unit charge? Or: How many virtual photons ...
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2answers
568 views

Probabilistic quantum cloning

I have a question with regard to probabilistic quantum cloning - see for example http://prl.aps.org/abstract/PRL/v80/i22/p4999_1. It does seems like I can use the proof for no-cloning theorem to ...
6
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1answer
571 views

There seems to be no definition of “stability” in axiomatic QFT. Is there? And, if not, is this a problem?

"stability" is invoked as the justification for the axiomatic requirement that the spectrum of the generators of the translation group must be confined to the forward light-cone. The spectrum ...
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7answers
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Quantum mechanics on a manifold

In quantum mechanics the state of a free particle in three dimensional space is $L^2(\mathbb R^3)$, more accurately the projective space of that Hilbert space. Here I am ignoring internal degrees of ...
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2answers
619 views

Scalar product of coherent states

We suppose for semplicity to have a 1D oscillator, but this is a question abaout the general CCR algebra in oscillators, second quantization, quantum field theory etc. We know coherent states are a ...
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The history and modern understanding of spin

This question was inspired by Abstruse Goose :) http://abstrusegoose.com/342 It's well known that any attempt to describe the spin (of say an electron) in terms of non-internal spatial coordinates is ...
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6answers
934 views

A die versus a quantum experiment

Let suppose you roll a die, and it falls into a hidden place, for example under furniture. Then although the experiment has already been made (the die already has a number to show), that value can not ...
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2answers
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Momentum-Representations in Quantum Mechanics

Why do we get information about position and momentum when we go to different representations. Why is momentum, which was related to time derivative of position in classical physics, now in QM just a ...
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1answer
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Why does lambda decay violate parity?

When a lambda particle decays into proton and a pion, I am told it does not conserve parity. Why?
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5answers
1k views

What physics does occur at short distances in QED?

Let us take the standard QED ($e^-, e^+, \gamma$) as a model of QFT and ask what is its "short-distance" physics? They say the UV infinities appear because we do not know the real physics of short ...
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4answers
1k views

Wavefunction collapse in relativity

It is well accepted that quantum theory has well adapted itself to the requirements of special relativity. Quantum field theories are perfect examples of this peaceful coexistence. However I sometimes ...
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3answers
795 views

What does the fine structure constant describe?

Feynman says in his book "QED" that the square root of the fine structure constant is the probability for a charged particle to emit a photon. But for which wavelength? Or is it an average over all ...
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5answers
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Why do particles move? [closed]

Is all particles moving and forever? Can the movement of particles be stoped ?
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1answer
324 views

Angular momentum

I'm given with the following problem - it's an easy one, but it's nearly 1AM, I'm tired and I need some push into the general direction to get the solution: A particle is assumed to be in the state ...
3
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1answer
406 views

Quantum death like heat death possible?

Quantum decoherence is an irreversible process which is the result of interaction of the system with its environment. It prevents interference due to lack of coherence. Environment acts just like a ...
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2answers
2k views

Symmetric potential and the commutator of parity and hamiltonian

In one dimension - How can one prove that the Hammiltonian and the parity operator commute in the case where the potential is symmetric (an even function)? i.e. that [H, P] = 0 for V(x)=V(-x)
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4answers
913 views

Dirac equation on general geometries?

I have a numerical method for computing solutions to the Dirac equation for a spin 1/2 particle constrained to an arbitrary surface and am interested in finding applications where the configuration ...
2
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1answer
227 views

Is the exponential of the distribution $i\Delta^+(x)$, the 2-point function of a free quantized Klein-Gordon field theory, a distribution?

From answers to a previous question, a finite degree polynomial in the distribution $i\Delta^+(x)$, with Fourier transform $2\pi\delta(k^2-m^2)\theta(k_0)$, is a distribution, even though a product of ...
3
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1answer
659 views

Time evolution in quantum mechanics

We know that an operator A in quantum mechanics has time evolution given by Heisenberg equation: $$ \frac{i}{\hbar}[H,A]+\frac{\partial A}{\partial t}=\frac{d A}{d t} $$ Can we derive from this ...
3
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2answers
856 views

Why is it necessary to represent Schrodinger's equation as a partial differential equation?

The Schrodinger equation governs the possible time evolution of a wave function, expressed as a partial differential equation. Isn't this equivalent to the simpler equation $$\omega = \hbar k^2/2m$$ ...
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1answer
721 views

What is the status of applying numerical analysis to QM/QFT problems

This is something I don't ever seem to hear about, except regarding QCD ("lattice QCD"). What about QED? Is numerical integration always inferior to hand-calculating Feynman diagrams in perturbation ...
4
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2answers
384 views

What is a good reference for the quantum mechanical description of lasers?

I am currently taking a graduate level class on lasers. The primary focus of the class is on the design and engineering aspects of lasers, e.g. resonator design. However the first portion of the class ...
3
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1answer
400 views

Electron-hole symmetry in H and He

I'm contemplating particle-hole symmetry, and as an example I am looking at either an electron moving along a hypothetical lattice of hydrogen ions, or a hole moving along a hypothetical lattice of ...
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4answers
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Mathematical background for Quantum Mechanics [duplicate]

What are some good sources to learn the mathematical background of Quantum Mechanics? I am talking functional analysis, operator theory etc etc...
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3answers
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trying to understand Bose-Einstein Condensate (BEC)

I am a computer scientist interested in network theory. I have come across the Bose-Einstein Condensate (BEC) because of its connections to complex networks. What I know about condensation is the ...
11
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3answers
961 views

Is there a relativistic (quantum) thermodynamics?

Does a relativistic version of quantum thermodynamics exist? I.e. in a non-inertial frame of reference, can I, an external observer, calculate quantities like magnetisation within the non-inertial ...
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5answers
5k views

What does it mean for a Hamiltonian or system to be gapped or gapless?

I've read some papers recently that talk about gapped Hamiltonians or gapless systems, but what does it mean? Edit: Is an XX spin chain in a magnetic field gapped? Why or why not?
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10answers
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What is the wavefunction of the observer himself?

I am aware about different interpretations of quantum mechanics out there but would mostly like to see an answer from the perspective of Copenhagen interpretation (or relative quantum mechanics if you ...
5
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3answers
627 views

Does particle indistinguishability and quantised enery levels (in bound states) violate the Pauli Exclusion Principle?

First, I should point out that this question was raised by a particle physics Professor whose lessons I attended last year. I don't recall exactly how the question was phrased so if anyone would like ...
4
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1answer
399 views

Is there a fully quantum-field theoretic treatise of Planck's Law for black-body radiation?

I recall from my undergraduate statistical mechanics and QM classes that Planck's Law may be derived fairly straight-forwardly by considering the density of states of EM radiation in thermal ...
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5answers
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What is the usefulness of the Wigner-Eckart theorem?

I am doing some self-study in between undergrad and grad school and I came across the beastly Wigner-Eckart theorem in Sakurai's Modern Quantum Mechanics. I was wondering if someone could tell me why ...
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1answer
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Why must the deuteron wavefunction be antisymmetric?

Wikipedia article on deuterium says this: The deuteron wavefunction must be antisymmetric if the isospin representation is used (since a proton and a neutron are not identical particles, ...
4
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2answers
474 views

Glass melting at near absolute zero?

I read this report and summarise here but my question is - if quantum mechanics will make glass melt at temperatures near absolute zero and it is near absolute zero then wouldn't this be a huge issue ...
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3answers
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Green functions in Quantum Mechanics

How the Green's functions and the Quantum Mechanics are related? Do they can be used to solve the Schrödinger equation of an particle subjet to some potential that is not a Dirac's delta? And the ...
3
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2answers
3k views

How does electron excitation relate to atomic orbitals?

Taking off from the Rutherford-Bohr model, Figure 1. Rutherford-Bohr model. when an electron absorbs energy (from light, heat, or otherwise) it jumps to a higher energy level, or shell. Then when ...
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3answers
2k views

Partition function of a hydrogen gas

Hi I have a doubt (I'm not very expert in statistical mechanics, so sorry for this question). We consider a gas of hydrogen atoms with no interactions between them. The partition function is: $$ ...
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3answers
510 views

Formalization (and meaning) of Heisenberg Cut

In discussions around a recent Stack answer which covered axiomatic quantum mechanics, Bell's Theorem and Random Fields (related to axiomatic QFT) the phrase "Heisenberg Cut" was used. The basic ...
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3answers
637 views

Local measurement of entangled particle pairs and interpretation of state

This just started to bother me after reading yet another entangled particle question, so I hate to ask one myself, but... If we have two entangled particles and take a measurement of one, we know, ...
4
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3answers
536 views

Entangled particles

So we have two particles (A and B) that are entangled. From what I understand, entanglement isn't destroyed, it is only obscured by subsequent interactions with the environment. Particle A goes ...
4
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1answer
313 views

What are the current (popular(ish)) approaches to modelling the quantum nature of spacetime at the Planck scale?

My guess at a list of them would be: spin foams, casual sets, non-commutative geometry, Machian theories, twistor theory or strings and membranes existing in some higher-dimensional geometry... ...
12
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2answers
2k views

Are we living in a false vacuum? Is there any way to tell?

I was thinking of the noted 1980 paper by Sidney Coleman and Frank de Luccia--"Gravitational effects of and on vacuum decay"-- about metastable vacuum states that could tunnel to a lower energy "true ...
3
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2answers
214 views

Does boundedness of observables in the Haag-Kastler axiomatization rule out interactions?

In an interacting theory I expect there to be caustics, resonances, and other situations in which some observables would give an infinite experimental result. Of course, these are idealized states and ...
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5answers
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Is Gravity an entropic force after all?

Recently, there was a rapid communication published in Phys.Rev.D (PRD 83, 021502), titled "Gravity is not an entropic force", that claimed that an experiment performed in 2002 with ultra cold ...
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3answers
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A tutorial explanation of decoherence?

Is there a tutorial explanation as to how decoherence transforms a wavefunction (with a superposition of possible observable values) into a set of well-defined specific "classical" observable values ...
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Is Stephen Wolfram's NKS, an attempt to explain the universe with cellular automata, in conflict with Bell's Theorem?

Stephen Wolfram's A New Kind of Science (NKS) hit the bookstores in 2002 with maximum hype. His thesis is that the laws of physics can be generated by various cellular automata--simple programs ...
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What does John Conway and Simon Kochen's “Free Will” Theorem mean?

The way it is sometimes stated is that if we have a certain amount of "free will", then, subject to certain assumptions, so must some elementary particles."(Wikipedia) That is confusing to me, ...