Tagged Questions
1
vote
1answer
72 views
Can 3 photons be combined to give a spin-0 projection?
Motivation: The neutral pion decays to 2 photons ($\pi^0\to\gamma\gamma$) most of the time. For the decay of the neutral to 3 photons ($\pi^0\to 3\gamma$) we have an upper limit on the branching ...
2
votes
1answer
46 views
Possible states for two electrons in the helium atom
Consider the helium atom with two electrons, but ignore coupling of angular momenta, relativistic effects, etc.
The spin state of the system is a combination of the triplet states and the singlet ...
2
votes
2answers
143 views
In quantum mechanics(QM), can we define a high-dimensional “spin” angular momentum other than the ordinary 3D one?
Inspired by my previous question Questions about angular momentum and 3-dimensional(3D) space? and another relevant question How to define angular momentum in other than three dimensions? , now I get ...
2
votes
2answers
64 views
Hamiltonian of Harmonic Oscillator with Spin Term
We have the usual Hamiltonian for the 1D Harmonic Oscillator:
$\hat{H_{0}}=\frac{\hat{P^2}}{2m} + \frac{1}{2}m \omega \hat{X^2}$
Now a new term has been added to the Hamiltonian, $\hat{H} = ...
1
vote
2answers
96 views
Is it only the spin of a particle that can be entangled with another particles spin?
Is it only the spin of a particle that can be entangled with another particles spin?
Also is there any good physical interpretation of the spin of a particle? because the rotational invariance of ...
1
vote
0answers
48 views
Helicity operator in Non relativistic limit
Helicity operator in Dirac equation is given by
$$H=\frac{\vec{S}\times \vec{P}}{P^{2}}$$
This operator commutes with dirac hamiltonian.We can also define a helicity(with same form) operator in case ...
10
votes
3answers
317 views
How to tackle 'dot' product for spin matrices
I read a textbook today on quantum mechanics regarding the Pauli spin matrices for two particles, it gives the Hamiltonian as
$$
H = \alpha[\sigma_z^1 + \sigma_z^2] + ...
3
votes
0answers
102 views
Meaning of spin
I'm pretty astounded that I did not hear about this sooner, but in my course on QFT our professor told us that the concept of spin can be used to mean three things:
Mechanical spin (apparently a ...
2
votes
0answers
137 views
How is parity relevant to determining angular momentum?
Question:
Particle A, whose spin $\mathbf{J}$ is less than 2, decays into two identical spin-1/2 particles of type B.
What are the allowed values of the orbital angular momentum $\mathbf{L}$, ...
1
vote
2answers
109 views
Spin of a particle and spin quantum number [duplicate]
what actually does the spin quantum number of a particle describe about? What it means when we say photon has spin 1, Higgs boson has spin 0, etc..?? What actually does that numerical value explain? I ...
2
votes
2answers
75 views
Reaction force in electron spin measurements
Consider the following (thought) experiment, where an electron is emitted, then deflected by a magnetic field, and then detected:
Because the momentum of the electron changes when it gets ...
3
votes
1answer
130 views
Spin about an arbitrary axis
This is based off question 4.30 from Griffith's Introduction to Quanum Mechanics. It asks for the matrix $\textbf{S}_r$ representing the component of spin angular momentum about an axis defined by: ...
3
votes
2answers
283 views
Pauli matrix rotations
When doing physics with two-level systems and introducing rotations, a term that appears quite often is the rotation of a pauli matrix by another one:
$e^{- i \sigma_i \theta/2} \sigma_k e^{i ...
0
votes
1answer
80 views
Quantum entanglement, quantum measurement, spin and position
By uncertainty principle, we know that determining particle's position at some location is limited. So we cannot determine the position of a particle at some exact point location as this would make ...
1
vote
0answers
96 views
How is multiplicity given by 2S+1?
Suppose there are two electrons in an atom with $s_1 = \frac{1}{2}, l_1 = 1$ and $s2 = \frac{1}{2}, l_2 = 1$. Hence the total $S$ (of the atom) may be +1 or 0. And total $L$ is either +2,+1 or 0.
Now ...
0
votes
0answers
43 views
What exactly is the spin of a particle? [duplicate]
Possible Duplicate:
What is spin as it relates to subatomic particles?
I'm having a hard time grasping the concept of spin, my textbook describes it very vaguely:
Stable matter contains ...
0
votes
4answers
359 views
Could one argue that h (Planck constant) and $\hbar$/2 (Dirac constant) are in fact independant constants?
My question is very naive and could sound strange but it seems to me natural in so far as the Planck constant is related to the first quantization (of newtonian particle mechanics/galilean relativity) ...
3
votes
3answers
200 views
Quantum mechanical angular momentum and spin formalism/notation
I am currently stuck on the following notation:
$\frac{1}{2}\otimes\frac{1}{2} = 0 \text{ (antisym) } \oplus 1 \text{ (sym) }$
No matter what I tried, I couldn't derive the identity. I am sure that ...
4
votes
2answers
174 views
When you apply the spin operator, what exactly is does it tell you?
The example I'm trying to understand is:
$ \hat{S}_{x} \begin{pmatrix}
\frac{1}{\sqrt{2}}\\
\frac{1}{\sqrt{2}}
\end{pmatrix} = 1/2 \begin{pmatrix}
\frac{1}{\sqrt{2}}\\
\frac{1}{\sqrt{2}}
...
2
votes
2answers
213 views
Why for a spin half particle, possible outcomes of measuring spin projection along any direction are the same?
If one measures the projection of spin of a spin half particle along the x axis one will always get plus or minus half $\hbar$
Measuring it along the y axis one will always get plus or minus half ...
7
votes
1answer
266 views
Classical vs. Quantum use of the spin 4-vector
I have a few basic questions about the Pauli-Lubanski spin 4-vector S.
I've used it in quantum mechanical calculations as an operator, that is to say each of the components of S is a matrix operator ...
3
votes
1answer
100 views
Decay of a particle
Would someone please explain the following found on P. 125 of these notes?
On the other hand, two $π^0$’s cannot be in an $l = 1$ state. The reason for this is that pions are bosons and so the ...
3
votes
1answer
123 views
Spin-orbit coupling constant for rubidium
I have come across the following question in my course notes:
The $5s\to 5p$ transition in rubidium is split into two components with wavelengths of 780nm and 795nm respectively. For the $5p$ state, ...
3
votes
1answer
105 views
Is there record of a bosonic Stern-Gerlach measurement?
I cannot seem to find any peer-reviewed (or other) reference to an integer-spin Stern-Gerlach experiment. It shouldn't be too hard to do: just find you friendly neighbourhood Deuterium ion and shoot ...
2
votes
0answers
122 views
Angular momentum confusion
Could somebody please explain what is going on here?
We have a system of two indistinguishable spin-1 bosons. We shall adopt the center of mass frame.
Let
$S$ = total spin
$L$ = relative orbital ...
1
vote
1answer
306 views
Commutation relation with Hamiltonian
How do we get $[\beta , L] = 0$ , where $L$= orbital angular momentum and $\beta$= matrix from Dirac equation?
6
votes
1answer
130 views
Can closed loops evade the spin-statistic theorem in 3 dimensions?
The famous spin-statistics result asserts that there are only bosons and fermions, and that they have integer and integer-and-a-half spin respectively. In two-dimensional condensed matter systems, ...
2
votes
1answer
317 views
What is the spin rotation operator for spin > 1/2?
For spin $\frac{1}{2}$, the spin rotation operator $R_\alpha(\textbf{n})=\exp(-i\frac{\alpha}{2}\vec{\sigma}\cdot\textbf{n})$ has a simple form:
...
2
votes
1answer
140 views
How quantum field transforms in case of some particular spin
Except when a particle is spin-0, field of all particles transforms when frame of reference is changed, and this defines what spin is. The question is, specifically how does the quantum field ...
1
vote
2answers
187 views
How to write Schrodinger equation when a particle with some spin quantity and orbital angular momentum
Quantum mechanics: Suppose that there is a particle with orbital angular momentum $|L|$. But the particle also has spin quantity $|S|$. The question is, how do I reflect this into Schrodinger ...
1
vote
1answer
163 views
An equation that describes massless spin-1 particle
Proca action/equation describes massive spin-1 particle, but I was unable to find an equation that describes massless spin-1 particle.
Can anyone tell me what the name of this equation is?
2
votes
4answers
472 views
Why would Klein-Gordon describe spin-0 scalar field while Dirac describe spin-1/2?
The derivation of both Klein-Gordon equation and Dirac equation is due the need of quantum mechanics (or to say more correctly, quantum field theory) to adhere to special relativity. However, excpet ...
2
votes
1answer
113 views
Normalization of a spin-like quantity in matrix mechanics
Suppose that there is a quantity in Heisenberg picture as the following:
$A=u_1\Sigma_1 + u_2\Sigma_2 +u_3\Sigma_3$
I am not sure why $u_1,u_2,u_3$ is normalized to be ${u_1}^2 + {u_2}^2 + {u_3}^2 ...
0
votes
0answers
102 views
Can experiment distinguish the basis in which a singlet state is represented?
Let $\left(|\uparrow\rangle,|\downarrow\rangle\right)$ and $\left(|\nearrow\rangle,|\swarrow\rangle\right)$ be two bases of the $2$-dimensional Hilbert space $H$.
Can an experiment distinguish ...
3
votes
0answers
138 views
Spin polarization of decay products
A relativistic moving particle, e.g. muon $\mu^+$, described by its four-momentum vector $p_\mu$, charge $e$ and with a given spin polarization, ${\bf S}=(S_x,S_y,S_z)$, decays into three particles, ...
1
vote
3answers
139 views
In solving the hydrogen atom, how to see intuitively in advance that the spin effects to the energy spectrum can be ignored?
When the hydrogen atom is solved in QM books spin is usually ignored because its effect is to add tiny piece to the energy. My question is, is there a way to see this in advance, to see that if we ...
3
votes
1answer
112 views
Determining the spin of wavefunction
We all know that by uncertainty principle, location of a wave-particle is perfectly determined when uncertainty of momentum becomes infinite. (I also heard that in reality, it is almost impossible to ...
1
vote
3answers
279 views
The Stern Gerlach Experiment Revisited
Is it possible to provide an explanation to the observations of the Stern Gerlach Experiment using the classical theories?
Some Considerations:
We consider the standard set-up for the ...
1
vote
3answers
279 views
Can the Klein-Gordon Equation represent Particles with non-zero spin?
Every Solution of the Dirac Equation is also a solution of the Klein-Gordon equation.
So the K-G equation does not necessarily represent particles with non-zero spin.
Would it be incorrect to ...
3
votes
1answer
309 views
Why and how is nondegenerate perturbation theory used for time evolution under $\vec{L}.\vec{S}$ coupling?
Let us say that we start with an electron which is in a spin up state and has a spatial wave-function of the form $xf(r)$. Then one turns on a perturbation of the form ...
4
votes
2answers
719 views
Why Silver atoms were used in Stern-Gerlach experiment?
For the Stern-Gerlach experiment done in 1922:
1-why silver atoms were used?
2-Silver atom contains many electrons in different orbits (different $l$'s). Wouldn't the inner -shell electrons be ...
3
votes
2answers
287 views
How is angular momentum measured in experiments/in practice? [duplicate]
Possible Duplicate:
How does one experimentally determine chirality, helicity and spin?
How do you find spin of a particle from experimental data?
We read about and study angular momentum ...
2
votes
1answer
31 views
Will an entangled idler electron induce a current in a conductor if the signal elctron's spin is measured?
I'm assuming a hypothetical setup as follows: Two labs (Alice and Bob) exist. Each has one electron of an entangled pair. At Alice, the electron travels through free space towards a magnetic field of ...
1
vote
2answers
275 views
what does it mean for a particle with no size to have angular momenta?
I recently was reading about higgs boson and particle spin recently and I stubble upon an question that contains an answer to what a spin is.
It explains that electrons etc. have no size yet they ...
2
votes
1answer
382 views
What does it really mean that particle has a spin of up/down? And how is spin actually meassured?
I been reading some physics articles (related to the recent discovery of the particle that could be a Higgs boson) posted online and it was talking about electron spin and how it can only have values ...
2
votes
1answer
212 views
Magnetic moment derivation from Dirac equation
I am reading a text book where they show the electron has spin 1/2 using Dirac's equation. At one point in the derivation they define $\pi=P-qA/c$ where $P$ is the momentum operator and A is the ...
4
votes
1answer
362 views
Why does photon have only two possible eigenvalues of helicity
Photon is a spin-1 particle. Were it massive, its spin projected along some direction would be either 1, -1, or 0. But photons can only be in an eigenstate of $S_z$ with eigenvalue $\pm 1$ (z as the ...
5
votes
3answers
394 views
Why does spin have a discrete spectrum?
Why is it that unlike other quantum properties such as momentum and velocity, which usually are given through (probabilistic) continuous values, spin has a (probabilistic) discrete spectrum?
10
votes
3answers
817 views
Adding 3 electron spins
I've learned how to add two 1/2-spins, which you can do with C-G-coefficients. There are 4 states (one singlet, three triplet states). States are symmetric or antisymmetric and the quantum numbers ...
0
votes
1answer
161 views
Quantum Entanglement - Measuring Twice
In the answer here and on the wiki article and many other articles it is mentioned that if one of 2 entangled particles is measured their state collapses according to the Copenhagen interpretation.
...
