Tagged Questions

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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50 views

Do I understand life [closed]

I AM NEW HERE,BUT LOTS OF CONFUSION IN MY MIND, life or Death depends upon time...or other things... on earth A man lives 60 year ,,what about there life at another place of universe where time is ...
1
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0answers
28 views

Finding the expectation value of the annihilation operator with respect to a given state

Using dirac notation we were given a state vector $$|\Psi(t=0)\rangle = A\sum\limits_{q=0}^Q \frac{1}{(q+i)} |\phi_q\rangle$$ Where $\phi$ is part of a complete orthonormal set. I found the ...
1
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2answers
72 views

why do metals feel cold when electron moves at Fermi velocity?

This might be a very elementary question. Considering that electrons in typical metals have very high Fermi velocity, why is it that when you touch a metal you feel cold? My intuition tells me that ...
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0answers
18 views

space representation of a local potential

Why is it that for a local potential $V$: $$\langle x | V | x' \rangle = V(x) \langle x | x' \rangle = V(x) \delta(x - x')$$ I saw this being used in a book but without any explanation. The part with ...
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1answer
46 views

Superposition and simultaneous observation

Trying to understand superposition. Ok, so double slit experiment. The multiple paths the particle simultaneously travels interfere with each other but as it is absorbed, it chooses one "actual" ...
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1answer
66 views

Are there any known physical implementations of quantum gates?

I was wondering if there are any known implementations of a small number of quantum gates that can interact with each other. Certainly we don't have a "complete" set of quantum gates (where ...
1
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1answer
122 views

Do consciousnesses get “scattered” across the many worlds of the MWI?

According the many worlds-interpretation (MWI) of quantum mechanics, following a decision with possible outcomes $A$ and $B$, with respective probabilities $p_A=P(A)$ and $p_B=P(B)$, a proportion ...
2
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0answers
23 views

Making An Energy Momentum Plot For A Rashba Model (Using Discretization)

I want to make a plot of the Energy versus the Momentum of the Rashba model, using discrete matrices. First Ill show how I did this for the free particle. Subsequently I will show what goes wrong for ...
0
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2answers
91 views

Bound electrons don't move, right?

I've never been satisfied with the explanation that electrons have a “speed” and move close to the speed of light, thus incurring relativistic effects that make gold pretty and mercury wet. I learned ...
0
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0answers
13 views

Derive allowed transitions in IR spectroscopy [closed]

I have question which I didn't find in the forum yet and hope someone could help me. It's in the field of quantum mechanics, which I know a bit about, but am not a total expert in. I have an ...
0
votes
1answer
37 views

Pressure in Harmonic Oscillation

Classical Harmonic oscillator's energy depends on temperature as it equals $k_B$$T/2$. However, quantum harmonic oscillator energy is $(n+1/2)hf$. So, when T=0, quantum predicts motion. I have been ...
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1answer
50 views

Quantum Tunneling

Why is it, that quantum tunneling is only significant between states of nearly equal energy (as claimed here: 'Since tunnelling is significant only between states of nearly equal energy, tunnelling is ...
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0answers
77 views

The double-slit experiment with electrons illustrates the surprising idea that (multiple choice) [closed]

A. electrons sometimes behave like tiny particles B. electrons sometimes behave like waves C. Light is a wave
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0answers
47 views

Commutation relation of a operator with Hamiltonian [duplicate]

Given that the eigenvalues of a Hamiltonian operator $H$ are bounded below, will a Hermitian operator $T$ exist such that $[T, H] = i\hbar{\bf 1}$ identity operator?
0
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1answer
45 views

What is many-body bound state?

Bound state by definition is a state when particles are bounded together, so then "many-body bound state" would be bound state for a system of many bodies. Then I have several puzzles: 1. is the state ...
2
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2answers
41 views

Does an excited state wave function depend on state preparation?

Consider a quantum system with a ground state and many excited states (e.g. an atom). If the system is in an excited state, to what extent does its wave function depend on the method of state ...
6
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2answers
163 views

How can we differentiate between matter and antimatter? [duplicate]

For instance if there was a galaxy, assume it to be made up of antimatter (isolated from other "normal" galaxies), how would we, or rather, would we be able to distinguish if it was made up of matter ...
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0answers
35 views

Do anybody know about Monte Carlo Methods in simple logic? [closed]

Do anybody know about Monte Carlo Methods in simple logic ?
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1answer
48 views

Photon and Wave

There are some aspects of light that can be easily demonstrated by using the concept of wave. However I really want to know what it would be like in term of photon point of view. So I have some ...
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1answer
43 views

Koopman Von Neumann state vs Quantum state

Is it correct to think that a state in Hilbert space represents the "most we can know" about a system? Is therefore a state in KvN Hilbert space the same as a state in the usual quantum Hilbert space, ...
2
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1answer
38 views

Everyone calls Electromagnetic Induced Transparencyan interference phenomenon, but is it also an interference phenomenon in classical systems?

Electromagnetically induced transparency is a hot topic in physics. However I'm curious about its mechanics in physics. Physicists think that it's a phenomenon of interference from transition of two ...
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0answers
39 views

Wave function of N electrons in a superconductor

Assuming that the wave function consisting of $N$ electrons is $\Psi_{N}(\bf{r_1,r_2,\cdots r_N)}$ then in the presence of a magnetic field ($\bf{B}=\nabla \times A$), how do I show that the current ...
0
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0answers
14 views

Many body quantum rotors

I'm stuck on a particular problem about quantum rotors. Suppose we have $N$ such rotors and they are connected to a thermal reservoir of temperature $T$. Neglecting any center of mass motion, I'm ...
1
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2answers
107 views

Why is time evolution of wavefunctions non-trivial?

(Note: This post focuses on a single simple example, however I'm asking about the error in general in my logic). Consider the infinite potential well "particle in a box" system described by ...
1
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1answer
58 views

Scalable tool (engine) for simulating the universe

Does there exist a large scale scalable tool (engine) for simulating the universe that incorporates both quantum mechanics and cosmology, i.e. micro & macro scales? (It would be best if this tool ...
3
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2answers
90 views

What is the physical interpretation of a field operator

So far in our lecture we defined creation operators $a^{\dagger}_{n}$ in the following way, that we said: Somebody got you a antisymmetric or symmetric N- particle state and now $a^{\dagger}_{n}$ ...
1
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1answer
53 views

Energy as charge with respect to time translations in QM

Consider a non relativistic quantum mechanical system with Hamiltonian $\mathcal{H}$, and denote the states by $\psi \equiv \psi(t) \equiv | \psi(t) \rangle$. From the Schrödinger equation we know ...
5
votes
3answers
99 views

Ground state of Spherical symmetric potential always have $\ell=0$?

I was given a problem where I have a spherically symmetric potential (the exact form is not relevant to this question, I think - but anyway is it 0 for $r\in[a,b]$ and $\infty$ everywhere else) and I ...
1
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1answer
69 views

What is the smallest length scale ever measured?

And, by the way, what is, or are, the measured values?
1
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1answer
30 views

How do discretize a finite crystal?

I am trying to find a general method to discretize a finite crystal system. How I have been discretizing systems so far (using Wannier functions): When you have an infinite crystal, you may apply ...
0
votes
1answer
46 views

Where did I go wrong in ket vector in quantum mechanics? [closed]

Consider a system of total angular momentum j = 1. The operator jx is given by: What are the possible values when measuring jx? My attempt: The eigenvalues after calculation are -1, 0, 1. Now I ...
1
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0answers
43 views

Phase and group velocity of a soliton? [closed]

How do I find the phase velocity and group velocity of a soliton with a $\operatorname{sech}$ (hyperbolic secant) envelope?
1
vote
1answer
20 views

Filled molecular orbitals: Avoidance vs Mixing

I am trying to figure out if the mixing of two filled orbitals requires some additional activation energy and, if it does, if there is some orbital avoidance area where the filled orbitals will not ...
0
votes
0answers
12 views

Crystal in Magnetic Field

When you have a 2D crystal in a uniform perpendicular magnetic field, you can use the Peierls substitution to convert one tight-binding before-magnetic-field-band into a $q$-bands under the influence ...
2
votes
3answers
90 views

Fock space and occupation number

I have troubles to understand the concept of a Fock space. We defined it as a direct sum of the 0-particle, single particle, two particle etc. Hilbert space. Unfortunately, I am not sure if I ...
0
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0answers
22 views

Hamiltonian symmetry Lie algebra

What is the connection between complete set of commuting observables and generators of the Lie group? I have a Hamiltonian written down in second quantized formalism and I also checked that it ...
2
votes
2answers
64 views

Why does dot product equal to one? (Pauli spin matrices)

I was reading these lecture notes (NB: PDF): For spin-1/2, the rotation operator $$ R_\alpha^{(s)}(\mathbf n)=\exp\left(-i\frac{\alpha}{2}\vec\sigma\cdot\mathbf{\hat n}\right) $$ can be ...
4
votes
2answers
68 views

How is the scattering length in 2d defined?

Scattering length is 3d is well-defined. In the literature, one can also see scattering length in 2d. How is it defined? Can we even generalize it to 1d?
2
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0answers
24 views

What symmetry operation mixes states with different $\ell$ in hydrogen atom? [duplicate]

We can mix states with different $m$ in hydrogen atom by rotating it around some axis (not coinciding with $z$). Thus rotation is the symmetry operation which mixes states with different $m$. As ...
2
votes
1answer
62 views

Why no Top Physicists Work on Bohmian Mechanics? [duplicate]

I'm curious to hear some opinions from serious physicists on this site as to why no top physicists have ever worked on Bohmian Mechanics. Except Bohm and Bell, the theory has received virtually no ...
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2answers
37 views

Are relative phases observable for identical particles but not for non-identical ones?

In quantum mechanics, amplitudes are represented by complex numbers $e^{i\phi}$, which have phase angles $\phi$. These phase angles are clearly not observable in absolute terms. If I have two ...
0
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1answer
21 views

Commutation of field and atomic ladder operators in the Dicke model

Consider the Dicke model, whose Hamiltonian is (in the rotating wave approximation) \begin{equation} \hat H=\omega_c \hat a^\dagger \hat a+\omega_0 \hat \sigma_z + g(\hat a^\dagger \hat \sigma + \hat ...
16
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3answers
2k views

How large can an atom get? What's the farthest an electron can be from its nucleus?

For example, would it be possible to excite a hydrogen atom so that it's the size of a tennis ball? I'm thinking the electron would break free at some point, or it just gets practically harder to keep ...
0
votes
2answers
65 views

A simple quantum mechanics question [closed]

Here is the problem: Find the energy of neutron, electron and electromagnetic waves of wavelength 0.1nm. English is my second language, so I am kind of confused about the meaning of problem ...
0
votes
4answers
49 views

Question about interpreting probabilities in QM [duplicate]

For the example of an infinite square well, $\psi(x)=0$ for $x$ outside the well/interval, and we are to interpret this as the particle cannot be found outside the well because ...
1
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1answer
74 views

Quantization from density of states

The density of states in quantum mechanics is obtained via the rather not so complicated relation below: $$\rho(E)=\delta (E-E_n)$$ Which means that if we know the energy quantization condition for a ...
0
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1answer
72 views

Do physicists use agent based models?

I am hoping that this is a simple and specific question. I just wanted to know whether physicists from any branch of physics use agent based models as a tool in their research? If so, then in which ...
7
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1answer
171 views
+50

Why isn't the path integral defined for non homotopic paths?

Context In the Aharonov Bohm effect, there is a solenoid which creates a magnetic field. Since the electron cannot be inside the solenoid, the configuration space is not simply connected. Question ...
0
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0answers
36 views

Addition in linear vector spaces [migrated]

In the definition of linear vector spaces, one of the axioms is that the addition must be commutative and associative. The addition of scalars and matrices are both commutative and associate. Can ...
0
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2answers
38 views

Can the matrix element of the momentum operator be found in the momentum basis?

In Shankar's Quantum Mechanics example 7.3.4, the problem is to find $\langle n'\rvert P\lvert n\rangle$ for the harmonic oscillator. The answer contains imaginary parts; you can derive such an answer ...