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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42 views

Why quantum field theory is never used in engineering?

I love physics, but I really don't like engineering! Reasons: Physics has a lot more mathematical stuff; mathematical stuff is the most interesting Physics is a lot more general than engineering I ...
2
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0answers
30 views

Identical particles and topological phases

I've been wondering about the argument that is always put forward for why the only possible identical particles in three dimensions or more are bosons or fermions. My question is related to a very ...
0
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0answers
31 views

Sakurai Exercise 2.17 (Harmonic Oscillator, Ladder operators)

I the field of the harmonic oscillator and ladder operators I am trying to solve exercise 2.17 from Sakurai and want to proof the following relation $$ \langle x^{2n} \rangle = (2n - 1)!! \langle ...
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0answers
9 views

Compton Scattering problem verification [on hold]

A photon emergre in an angle $\theta=\frac{\pi}{4}$ in a Compton scattering the kinetic energy of the photon is $K_i=2K_f$. I must find $K_i$ in $MeV$. $\lambda_f -\lambda_i= \lambda_c$. Then: ...
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1answer
41 views

What is the Quantum Mechanical Operator for Electric Potential?

I understand that charge and electric potential are conjugate observables in QM. See https://en.wikipedia.org/wiki/Conjugate_variables The quantum mechanical operator for charge, q, is simply equal ...
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2answers
15 views

Compton Effect, why does electromagnetic radiation lose less energy when hitting particles with greater mass?

One of the problems in Rae's Quantum mechanics book, at the end of the first section, relates to x-ray's scattering off either an electron or a carbon atom. Using the equation given to you in the book ...
1
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0answers
7 views

Writing Breit-Pauli spin-spin-coupling Hamiltonian as a sum of irreducible spin tensor operators

The spin-spin interaction part of the Breit-Pauli Hamiltonian for two electrons is essentially (omitting constants and the contact term) \begin{equation} \frac{3(\mathbf S_1 \cdot \mathbf r)(\mathbf ...
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0answers
33 views

Is all energy quantised?

I'm wondering if all energy is quantized.Can a particle with kinetic energy have any value of kinetic energy or is kinetic energy also quantized?My reason for asking this is that if a particle is ...
0
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2answers
39 views

Can two different objects or system of molecules have different temperatures, but having same internal kinetic energy?

If I take an extreme case, where a body has only an internal potential energy with zero internal kinetic energy, does this body have a temperature? Another question related to it: if two objects A and ...
0
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1answer
17 views

How do we acquire c-coefficients (“weights”, “probabilities”), when dealing with superposition of stationary states?

I searched quite a lot textbooks to answer this question, but I couldn't find anything satisfactory. This could be more general question, but it will be easier to just restrain to particle (say ...
0
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2answers
35 views

Confusion over basis sets in quantum mechanics

In a sense, I feel like I understand every step in the following (fairly typical) calculation, but I don't understand the conclusion. I consider a vector space spanned by two basis vectors ...
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0answers
33 views

The question is related to quantum mechanics [on hold]

The energy gap in germanium is 0.72 eV. Find the maximum wavelength of electromagnetic radiation which can cause transition from the valance band to the conduction band.
2
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1answer
41 views

flux quantization in superconducting ring

I am trying to understand SQUID microscopy from the ground up, so I am starting with flux quantization in superconducting rings. I found a nice presentation that covers some of the details, but I am ...
0
votes
1answer
25 views

Basic question about valence/conduction bands

In most introductions to solid state physics that I've read, they explain the origin, basically, of the band gap as follows: By doing a superposition of the standing waves inside a BZ, one can find ...
0
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0answers
49 views

Why the quantization of things is “forbidden” by classical physics?

I would like to know why we say that what gave the idea of Quantum Physics was the fact that things appeared to be quantized Why is it impossible to quantize things in classical physics? Why ...
0
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0answers
53 views

Can black holes with the same mass evaporate with different speeds?

I am curious about the following observations: (1) For a normal Schwarzschild black hole, it evaporates according to $dm/dt=-1/m^2$; (2) We have eternal black holes which do not evaporate (though ...
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0answers
16 views

What is the mass defect of stable nucleus? [on hold]

What is mass defect of stable nucleus? A-positive B-zero C-negative D-less than unstable nucleus
1
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0answers
22 views

Converting a Bell state to a less entangled state by LOCC [on hold]

How can I convert a maximally entangled state, $ \frac {1} {\sqrt2} (|00\rangle +|11\rangle) $, to a less entangled state like, $$ cos(\theta)|u_0 v_0\rangle + sin(\theta)|u_1 v_1\rangle $$, by using ...
3
votes
2answers
79 views

Successor to Copenhagen Interpretation as Orthodox Interpreation of Quantum Mechanics

First, I read the questions FAQ for this and I hope this does not violate the rules. I am not asking for personal opinion, but for observations of hard evidence of trends on this subject. When I ...
2
votes
1answer
82 views

Can a particle have an imaginary mass?

Now, I'm not very involved in the physical sciences, beyond some high-school classes - though I did listen to some college lectures for physics. I was reading "Do Tachyons Exist?" by John Baez, which ...
0
votes
1answer
27 views

Degenerate perturbation theory

I don't understand the part about turning off the perturbation. What is meant by or what is he referring to when he says "upper" and "lower" states. Why must the "upper" state reduce to a ...
0
votes
1answer
93 views

Which non-Copenhagen interpretations of Quantum Mechanics have actually produced something useful? [on hold]

The question has most of it in the name. "Useful" may be interpreted as: measurably different from Copenhagen interpretation and true potentially measurably different and potentially true (i.e. not ...
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0answers
33 views

What is the correct setting for Kirchhoff's Law of Thermal Emission? [on hold]

For example, is the setting free space? Is it a surface? etc. So, what is the correct setting of Kirchhoff's Law of Thermal Emission?
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0answers
22 views

Why does the interference patterns in delayed choice quantum eraser cancel out? Why/how does it have a phase shift?

Talking about this experiment https://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser#The_experiment_of_Kim_et_al._.282000.29 I am assuming that anyone able to answer my question is already ...
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0answers
21 views

Multiple contemporary measures of a photon's state

From quantum mechanics, when you polarize a photon using computational base and you try to measure its state using a diagonal base, you'll get with the 50% of probability $|0\rangle_{{} + {}}$ or ...
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1answer
45 views

Expectation value position of sine wave in infinite square well

Given a neutron (mass$\approx$939 MeV/c$^2$) in an infinite square well of size $a$, the value of the expectation value for position should be in the range $[0-a]$. I know that the general form of the ...
0
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1answer
53 views

$[a,a^{\dagger}]=1$ or $[a,a^{\dagger}]=I$

$[a,a^{\dagger}]=1$ or $[a,a^{\dagger}]=I$? On every book i read $[a,a^{\dagger}]=1$. But doesn't $a$ have infinite dimension? So why does the commutator go to a number which is one dimensional?
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1answer
70 views

Why is particle superposition still part of quantum mechanics? [duplicate]

After reading an article on Schrodinger's Cat, it seems that if we take the environment as an observer, that superposition cannot occur because all atomic and subatomic entities would be observed all ...
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0answers
52 views

Does an expectation value over only part of a wavefunction have physical meaning?

Does the expression: $$\langle p\rangle_{x=a, b} =\frac{\int_a^b \Psi(x)^*\,\hat{p}\,\Psi(x)dx}{\int_a^b |\Psi(x)|^2dx} $$ have any physical meaning when $\int_a^b ...
1
vote
1answer
25 views

Fetter & Walecka's derivation of second quantised potential term in many-particle TDSE

For the potential term in the Hamiltonian, I understand that we go through the same process as with the kinetic energy term. On the RHS of the TDSE, we get something like ...
0
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3answers
35 views

How does commutation between the hamiltonian and angular momentum operator (squared) imply conservation of Angular momentum?

So we are looking at central potentials in QM; The lecturer poses the question, when is $\textbf{L}$ conserved? He then considers the commutator of $\hat{H}$ and $\hat{L^2}$. We have; ...
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0answers
25 views

Momentum Operator [on hold]

Hello Community, I'm writing a paper on quantum mechanics and have a question about following formulas and the derivation of them. It's in german but I was told that I have an error somewhere in the ...
1
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0answers
27 views

Classical Quantum or Relativistic?

An ensemble contains free electrons at 10^3 electrons per m^3 at 10^7 K. What can this ensemble be treated as: a Classical Quantum or Relativistic gas or in some overlapping domain?
2
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3answers
96 views

How bizarre can the Quantum two slit experiment get?

Note 1: Don't apprehend my question as 'quantum woo'. I do not mean so and hate anyone who does. Note 2: I am just a 14 year old physics enthusiast. Don't expect any background. On to the real ...
1
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1answer
51 views

Conservation of energy in quantum mechanics

In Griffiths' book Introduction to quantum mechanics (second edition, page 37) it states: The time-independent Schrödinger equation says $$\hat{H} \psi_{n} = E_{n}\psi_{n}$$ so $$\langle H ...
12
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3answers
1k views

Do electrons in an atom always have the same 'direction'?

Perhaps speaking of direction of an electron isn't quite correct. But does QM indicates a kind of way whether all electrons are going e.g. 'clockwise' or not? Of course QM just gives a probability ...
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0answers
13 views

When you make materials smaller is there a trend in the manner light is absorbed or reflected?

For instance if you take gold is shines like yellow, but if you make it smaller I've heared that it absorbes more the smaller frequencies. At a size of 100nm a piece of gold will absorb fe more blue ...
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votes
1answer
40 views

What is the correct statement of Kirchhoff's Law of Thermal Emission?

There are various quite different statements in textbooks and other science literature as to Kirchhoff's Law of Thermal Emission. So, what is the correct statement of Kirchhoff's Law of Thermal ...
3
votes
0answers
48 views

Loss of interference in single-photon Mach–Zehnder interferometer with detector in only one arm

I have read that if you have a Mach–Zehnder interferometer (doing a single-photon experiment) and put a non-destructive detector in only one of the two arms (connected to the first beam splitter), you ...
1
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0answers
24 views

density of states with changing potential floor

I wish to calculate the density of states for a 1D finite potential well, for the scattering states, meaning $E>0$. I have a potential of the form: $$ V(x)=\begin{cases} -V_0 & |x|<a\\ 0 ...
4
votes
1answer
105 views

Help needed to understand “On the reality of the quantum state”

I am having trouble to understand the reasoning in the following paper, On the reality of the quantum state. MF Pusey, J Barret and T Rudolph. Nature Phys. 8, 475–478 (2012); arXiv:1111.3328. ...
2
votes
5answers
94 views

Is QM something that belongs to all relative small particles or only to subatomic particles? [on hold]

For us human beings subatomic particles like elektrons are described by the laws of QM. But imagine their is a very huge giant looking on the universe. For him all the planets and stars are so tiny ...
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votes
0answers
37 views

Are there some explication for the wave function in Quantum Mechanics? [on hold]

I start to learn the Quantum Mechanics and I don't really understand the mechanisme behind the wave function. As what I feel and as I think the world as multivers, 1/ I see wave function like a way ...
0
votes
0answers
40 views

Spinor on an eigenstate of $J_z$ [on hold]

The state of a particle with spin 1/2 is described (in the $S_z$ representation) by a spinor $[\Psi]=\left( \begin{array}{c} \phi_+ \\ \phi_- \\ \end{array} \right)$ Its known to be eigenstate of ...
4
votes
7answers
2k views

How can a product of Bra and Ket be a scalar if they are matrices?

I am trying to teach myself Quantum Mechanics and am currently on Complex Vector Space Arithmetic. According to Wikipedia, product of a Bra and Ket is a scalar (which, I think, means a complex ...
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votes
1answer
47 views

What is the origin of the quantum operators for p and E? [on hold]

It is always stated the quantum operators for p en E are the ones we´re familiair with. But where do these operators come from?
2
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0answers
56 views

What would be the research advances if we discover the gravitational waves? [on hold]

As researchers of LIGO (USA) and VIRGO (France/Italy) say that they have discovered gravitational waves thanks to black hole collision today, what would be, apart of the important confirmation of ...
0
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1answer
57 views

Physical meaning of weight function in inner product in Quantum Mechanics

When taking the inner product of say two functions in Quantum Mechanics,we include a weight function w(x,y,z) that is usually equal to unity(in my undergraduate introductory QM course anyway). But ...
1
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0answers
25 views

Restriction to the total angular momentum of two identical particles with spin 1

I've been asked if there is any aditional restriction to obtain the possible values of the total angular momentum J by considering that the 2 particles of spin S=1 are identical. If they are not ...
1
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0answers
36 views

Proving Probability Current and Momentum relationship

I am trying to show that $\mathbf j = \frac{\hbar}{2mi}\left(\Psi^* \mathbf \nabla \Psi - \Psi \mathbf \nabla \Psi^{*} \right) \,, $ simplifies to $\mathbf j = \frac{\mathbf p}{2m} (|\Psi|^2) $. I ...