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 ...

learn more… | top users | synonyms (4)

1
vote
2answers
26 views

With respect to the Casimir effect, why can't the wavelengths of the virtual particles between two plates just “pass through” the plates themselves?

I've read over the years that the suppression of many of the possible wavelengths between the two plates in a Casimir experiment is what causes the phenomenon (top comment on this Askscience thread is ...
0
votes
0answers
24 views

Superposition of waves with different initial phase in Quantum Mechanics

In Quantum Mechanics, if a particle's state is a superposition of many states of definite momentum, then we say that it's position is well-defined (by the Heisenberg uncertainty principle, because ...
0
votes
1answer
55 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 ...
0
votes
0answers
5 views

What exactly is an integral kernel? [migrated]

I am not sure if I have seen integral transforms in the right way, but given a transform like fourier transform - its actually a basis transformation right ? $$ F(y) = \int K(x,y) f(x) \text{d}x $$ ...
3
votes
1answer
208 views

First order coherence through double slit

The state $$|\Psi \rangle = |0\rangle + \sum_j \int d\omega f_j(\omega)\hat{a}^\dagger_j (\omega) |0\rangle $$ is coming from a far field and incident on a double slit setup. Here j is the index of ...
0
votes
0answers
22 views

Periodicity of function as a result of superposition in Quantum Mechanics

Say we add infinitely many waves (states of definite momentum) so as to produce a function that gives a very well-defined position, does that addition(using Fourier series) make that function ...
-2
votes
1answer
95 views

Are black holes in a binary system with white holes, and are they both wormholes? [on hold]

What is a black hole? The general explanation is that a black hole is small region in space with such strong gravitational effects that nothing can escape and even light is trapped inside of the event ...
1
vote
1answer
22 views

Question about what a simultaneous measurement of entangled spins means

I was working through a problem I found online and ran into something that is confusing me. We have a system of three spin-1/2 particles, in the state $$ |\psi\rangle = ...
2
votes
1answer
5k views

What is the difference between Quantum Physics, Quantum Theory, Quantum Mechanics, and Quantum Field Theory?

What is the difference between Quantum Physics, Quantum Theory, Quantum Mechanics, and Quantum Field Theory? Are they the same subject? I believe that they are not the same subject! Maybe there is not ...
1
vote
1answer
59 views

Can someone explain why this QM FTL communication setup is wrong?

So I thought I understood the double-slit experiment and EPR paradox until this setup occured to me. It combines EPR entanglement with "which-way" double slit setups. I know it must be wrong, but I ...
0
votes
0answers
31 views

Addition of spins and projection

This is a calculational question but is not a homework question (rest assured I'm past that stage). Still, I don't quite know how to show the following. Say we have two $SU(2)$ spins $\vec{S}_1, ...
-1
votes
0answers
14 views

Quantum mechanics Hermitian problems [on hold]

how to determine its non zero Eigenvalues and eigenfunctions how to prove A is hermitian in problem 2
5
votes
2answers
93 views

Why do we not require higher derivatives to match at boundary when solving the Schrödinger equation in a given potential?

When solving the time independent Schrödinger equation for a given potential in 1D, the main part of the solving involves matching boundary conditions. Usually, we require the value and the first ...
1
vote
1answer
185 views

Are the authors saying that the observer effect plays no role in Bohr's thought experiment of the Heisenberg uncertainty principle?

Here is an excerpt from Eisberg & Resnick's Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles. Here is introducing Bohr's though experiment to establish a physical origin for the ...
0
votes
1answer
45 views

How to prove that sum converges to integral using density of states?

Essentially, I would like to prove $$ \sum_k f(k) \to \int f(k) \rho dE \tag{1}$$ where $$ \rho = \frac{dk}{dE} \tag{2}$$ is the density of states and $k \to \infty$. The model is that there is a ...
0
votes
0answers
16 views

Determining photon flux at a particular distance from a source, given frequency and transmitter wattage

I want to check my process and formulae for determining the observed number of photons per square meter per second, when the signal frequency is $f$, transmitter power is $P$ and the distance from the ...
-2
votes
0answers
25 views

gravity and inflation

Looking at the interaction between mass and the higgs field. Could gravity be a result of the topographical distortion of the fabric of space-time as the universe accelerates under inflationary ...
-1
votes
1answer
95 views
+50

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 ...
7
votes
3answers
1k views

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 ...
3
votes
1answer
311 views

Least Action Principle (Classical and Quantum Theory)

I) My first question would be "why should classical systems obey the principle of least action ?" When we find out the propagator in quantum physics, we find the amplitude to be equal to the sum over ...
2
votes
2answers
71 views

Can atoms with top / bottom quarks exist?

Do we have examples in nature or the lab of atoms constructed of 2 top and 1 bottom quark and a tau selectron? How would its properties differ from a hydrogen atom?
4
votes
1answer
23 views

Why exactly do sometimes universal covers, and sometimes central extensions feature in the application of a symmetry group to quantum physics?

There seem to be two different things one must consider when representing a symmetry group in quantum mechanics: The universal cover: For instance, when representing the rotation group ...
-3
votes
0answers
28 views

Can atoms exist [duplicate]

Can an atom be created from 2 top and one bottom quark together with a tau selectron? How would its properties differ from a hydrogen atom? What combinations of quarks and selectrons are possible to ...
0
votes
4answers
105 views

“FTL” Communication with Quantum Entanglement? [duplicate]

Can quantum entanglement make sending a message, whether audio, video, or even Morse code, instantaneous between two points (faster than it could travel normally at the speed of light)? Let me first ...
-2
votes
0answers
11 views

Bandlike vs. Excitonic Semiconductors

What is the different between 'bandlike' semiconductors and 'excitonic' semiconductors and how does this relate to whether they are organic or inorganic? I also wonder how such bandlike or excitonic ...
25
votes
7answers
4k views

What is Quantum Mechanics really 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 ...
1
vote
2answers
72 views

(Local) Conservation of Energy in Quantum Mechanics

Generally, we say that conservation of energy is a local law; the change in energy in some small region of space is equal to the energy flux out of that region. However, in quantum mechanics, we can ...
-3
votes
0answers
59 views

Why photon have zero rest mass?

If photon have zero rest mass then the term E=hf should also be zero because if rest mass is zero then relativistic mass is also zero and so on by Einstein mass energy relation energy shoud be zero ...
4
votes
5answers
680 views

Why does optical pumping of Rubidium require presence of magnetic field?

The optical pumping experiment of Rubidium requires the presence of magnetic field, but I don't understand why. The basic principle of pumping is that the selection rule forbids transition from ...
4
votes
2answers
248 views

Has this experiment really demonstrated wave-function collapse?

My question is: why did the following experiment claim that it had demonstrated the wave-function collapse? Experimental proof of nonlocal wavefunction collapse for a single particle using ...
0
votes
1answer
219 views

Difference: Fermi wave length vs. phase-breaking length?

I am reading a quantum transport book, where they often mention: phase breaking length and Fermi wavelength. I have looked up and found that: Phase breaking length= length over which electron remains ...
0
votes
0answers
38 views

Electron double slit, different probabilities of going through each slit [on hold]

The Problem: Consider a two slit experiment where the slits are not the same. Specifically, consider the case where the probability of an electron passing through one slit is different from that of ...
2
votes
0answers
61 views

Continuum analogue of $ \langle \psi | \psi \rangle = \sum _i a_i^* a_i$

I'm learning Dirac notation, and I found the following exercise: Verify that if $$|\psi \rangle=\sum_i a_i |i\rangle, \tag{1}$$ (where $a_i=\langle i|\psi \rangle$), with $$\langle i|j ...
3
votes
1answer
96 views

Separating the hamiltonian for a superlattice — is it this easy?

I've been banging my head against a wall trying to figure out what I'm sure is a very simple problem. I want to solve the Kronig Penney model for a superlattice, which is just a normal periodic 1D ...
0
votes
0answers
39 views

Quantum Mechanics, Free Particle with Gaussian Distribution [on hold]

Assume a free particle of mass $m$ is initially at rest, $v = 0$, which has a time dependent Gaussian distribution $V= \text{expected value of }v$. Write down the expression for the wave function: Is ...
4
votes
2answers
93 views

Is there a mathematical relationship between Legendre conjugates and Fourier conjugates?

In quantum mechanics, there is an uncertainty principle between conjugate variables, giving rise to complementary descriptions of a quantum system. But the variables are conjugates in two different ...
0
votes
1answer
50 views

Possible values for $L_x$

I've a physical system with $l=1$ and I have to calculate the values I can obtain if I measure $L_x$ and their probability. I know that: the values I can obtain are $\ m=0, \pm 1$ $\displaystyle ...
32
votes
16answers
55k views

What is a good introductory book on quantum mechanics?

I'm really interested in quantum theory and would like to learn all that I can about it. I've followed a few tutorials and read a few books but none satisfied me completely. I'm looking for ...
0
votes
0answers
54 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 unperturbed kets that are degenerated and find the matrix $V$, ...
3
votes
2answers
80 views

What all has intrinsic spin?

What does and does not have intrinsic spin? Wikipedia Spin (Physics) https://en.wikipedia.org/wiki/Spin_(physics) says: “In quantum mechanics and particle physics, spin is an intrinsic form of ...
0
votes
2answers
62 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 ...
2
votes
2answers
78 views

Superposition in Quantum Mechanics

First of all, let $V$ be a vector space over the field $\mathbb{F}$. It is possible then to show, by Zorn's Lemma that there is a basis for $V$. The main point is that although basis are quite ...
0
votes
1answer
112 views

Forward-scattering off a potential well

In his book The Physics of Quantum Mechanics, James Binney writes the following: The scattering cross-section. In the case that $V_0<0$, so the scattering potential forms a potential well, the ...
1
vote
0answers
24 views

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 ...
6
votes
2answers
126 views

Infinite square well that suddenly decreases in size

A well known exercise in basic quantum mechanics is the sudden (diabatic) increase of the length of an infinite square well. Now consider a particle in an eigenstate of an infinite well that is ...
-1
votes
0answers
25 views

Will water vapour rise in vacuum?

If I put water vapour in vacuum, will it behave normally like a gas? Will it rise up in the vacuum?
-3
votes
0answers
19 views

Best thermal (heat) radiation barrier (insulator)? [on hold]

Which material provides best insulation from thermal (heat) radiations? In other words, which material blocks most of heat radiation from escaping?
0
votes
0answers
27 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 ...
0
votes
1answer
26 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 ...
1
vote
2answers
257 views

Measurement of the energy of an atom using a cold substance

An atom was prepared in a superposition of ground state and excited states.I propose to measure the state by coupling the system to a cold enough substance. By cold enough I mean $$kT\ll E_1,$$ where ...