# Questions tagged [quantum-spin]

Fundamental characteristic property of particles which together with orbital angular momentum acts as the generator of rotations and which doesn't have a classical equivalent but is sometimes compared to and contrasted with classical intrinsic angular momentum.

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### Why can't a spin 0 graviton couple to a photon? [closed]

I read that a spin 0 graviton can't couple to traceless energy, such as a photon, in the introduction in the book The Feynman Lectures On Gravitation. Why is this true? What does it mean for a ...
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### Free Will Theorem question

The Kochen-Specker Theorem says, if I understand it correctly, that the results of spin measurements cannot be predetermined independent of measurement. They get to this conclusion by describing 33 ...
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### Calculating lagrangian density from first principle

In most of the field theory text they will start with lagrangian density for spin 1 and spin 1/2 particles. But i could find any text where this lagrangian density is derived from first principle.
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### Is it possible to have a photon with -1 spin?

Wikipedia states that a photon has a spin of value 1. What I want to know is this: are there two possible spins for photons, i.e. +1 and -1 (like electrons, which can have +1/2 or -1/2)? If the spin ...
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### Demystifying the connection between magnetic and electric fields

Part (1): In the classical theory of electromagnesitm, as given by Maxwell, we know that by just looking at the four famous equations: An electric field has a source: there are charged particles (...
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### Tensor Product vs. Direct Product for three spin-1/2 particles

Let us consider three spin-1/2 particles and only focusing on their intrinsic spin $S$. The Hilbert space has then to be $\mathcal H = ℂ^2 ⊗ ℂ^2 ⊗ ℂ^2$. The spin can be described by $V ∈ \text{SU(2)}$ ...
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### There's only time, no space in Quantum Mechanics

In this lecture (44:23) Nathan Seiberg: Topics in 2+1 Dimensional Quantum Field Theories 2. Nathan Seiberg says there's no space in QM and therefore fermions have spin 0. This sounds pretty revolting ...
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### What is the difference between a charged rho meson and a charged pion?

They both seem to have the same quark content: $$\rho^{+} = u\bar{d} = \pi^{+}$$ and $$\rho^{-} = \bar{u}d = \pi^{-}$$ What is different about the two?
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### Can I feed enough spin up electrons to a black hole to affect its angular momentum?

I was reading classical spin vs quantum field spin. I know spin in quantum mechanics is just a quantum number. But what happens if I try to intentionally feed many electrons all in the same spin state ...
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### The symmetry group and representation of spin-$N$ particle

I am confused with the symmetry group and the representation of spin-$N$ particles, and will appreciate any help or suggestions of reference. There are $2N+1$ internal states for a (massive) spin-$N$ ...
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### What does it mean for a particle to have spin of 2? [duplicate]

When I first started to study quantum mechanics, my physics text book told that particles have spin of either 1/2 or -1/2. Then I recently read an article saying that gravitons are expected to be ...
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### Can the Pauli equation be used to derive the usual $\mathbf L\cdot \mathbf S$ spin-orbit coupling? If not, what's the simplest way?

Can the Pauli equation, $$\left[{\frac {1}{2m}}({\boldsymbol {\sigma }}\cdot (\mathbf {p} -q\mathbf {A} ))^{2}+q\phi \right]|\psi \rangle =i\hbar {\frac {\partial }{\partial t}}|\psi \rangle,$$ be ...
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### Problem counting spin states

I can't figure out how many different spin states I can create with a four-electron system. I think I can create a spin-zero state, three spin-one states, and five spin-two states. That gives me nine ...
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### What is the spin of the Kalb-Ramond field?

In bosonic string theory the massless states of the closed string are given by a rank 2 tensor, which is divided into its three irreducible spherical tensors: symmetric traceless, antisymmetric and ...
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### Magnetic moment of uncharged particles

As we know that particles, only having charge, can have magnetic moment, then how particle like neutrino (having mass) can have magnetic moment? Don't bother about neutron because it has charge ...
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### Doubts concerning Wigner's classification

Wigner classified particles in function of the eigenvalues of $P_\mu P^\mu$ and $W_\mu W^\mu$. Then, it can be proved that for massless particles spin values can be only $\pm s_{max}$. But for a ...
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### Spin-2 Lagrangian

I can't seem to find this anywhere. I would like to know what the Lagrangian density for a general spin-2 particle is. Note that I am not talking about gravitons. What I am asking is for something ...
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### Intuitively, why is the energy decrease of the singlet state three times larger than the energy increase of the triplet state (hyperfine correction)?

Following Griffith's Hyperfine Splitting section for the Hydrogen atom, he concludes that the energy decrease for the singlet configuration is 3 times larger than the energy increase for the triplet ...
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### Is quark and gluon orbital angular momentum predicted to have the rest of the proton's spin?

The Phys.org news item How did the proton get its spin? outlines various experiments that probe proton structure and are sensitive to spin, and the current news is that CEBAF's major energy upgrade ...
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### Spin 3/2 matrices in terms of Pauli matrices

Let $\sigma_i (\frac{3}{2})$ be the three generators of the irreducible spin 3/2 representation of $SU(2)$ (see http://easyspin.org/documentation/spinoperators.html for their explicit forms). ...
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### Question on Pauli's Exclusion Principle

Pauli's EP says that electrons in a subshell will have opposing spins. Now, lets say I prepare two electrons with spin up. If I took an alpha particle ($\mathrm{He^{2+}}$) and added one of these ...
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### Why do electrons in singly occupied states have parallel spin?

I've been reading about Hund's rule and I realized that the Pauli exclusion principle only says that the electrons singly occupy states first, and nothing about their spins needing to be parallel. Why ...
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### When an electron orbits in a magnetic field, how exactly does its spin precess?

In the case of a cyclotron, with a constant magnetic field $B$ in the vertical direction, a moving electron circles in a horizontal orbit. The cyclotron frequency is $\omega = eB/m$. At the same ...
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### Atomic physics, determining levels and terms

In atomic physics I understand there a configurations, terms and levels. I think levels for instance appear because of spin-orbit interactions, so that terms are split. But I'm confused about the ...
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### Is there a way to get the spin naturally in nonrelativistic theories?

We all know how spin is added in a rather ad-hoc way in quantum mechanics. In the other hand, in relativistic quantum field theories the spin structure arises quite naturally from the fields. Is ...
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### Which component shows spin squeezing under twisting Hamiltonian?

Given a many body spin system, a collection of N spin-1/2 particles, under the interaction of the twisting Hamiltonian: $$H_{int} = \sum_{i,j=1}^Na_{i,j}\sigma_{z,i}\sigma_{z,j}= A J_{z}^{2}$$ assume ...