# Tagged Questions

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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### Is there a way of measuring the spin along an arbitrary direction of a spin 1 particle?

I am familiar with the expression for spin 1/2 but haven't seen one for spin 1.
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### Possible spin states?

Given a system of two particles with spin up and down, I have troubles to understand the possible states of this system. I would have normally thought, that the possible states are the tensor ...
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### How do you measure proton's spin?

I've probably read it somewhere in Sakurai but I cannot recall it at the moment. So how does one really measure the proton's spin? I mean the proton's spin and not its constituents. Do you measure ...
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### In which field theories with fermions do string- and fivebrane structures not come up?

A year ago, username @Greg Graviton asked in a thread here about the Spin group as covering of the spatial rotations. A subquestion was: What other groups, even larger than SU(2) are there that ...
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### Which of these two different forms of spin-orbit interaction is correct?

I am seeing the spin-orbit interaction in two different ways: $\lambda [\mathbf{p} \times \nabla V]\cdot \sigma$ $\lambda [\nabla V \times \mathbf{p}]\cdot \sigma$ I don't see how these two ...
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### Is the symmetry group of two spin 1/2 particles $SU(2) \times SU(2)$ or $SU(4)$?

This is a simple question. Please forgive me, as I am a lowly experimentalist. Suppose we have two free spin 1/2 particles, i.e. a 4-fold degenerate system. What is the set of symmetry operations ...
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### If space and time are equivalent, what's Spin in time dimension

This troubles me: We are talking about time and space being equivalent, but still only consider Spin in the $x$, $y$ or $z$-direction. What's Spin in time dimension? Is it distinction between ...
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### What is the spin state of a spin-1/2 particle when it comes out of a Stern-Gerlach apparatus?

Having a particle entering the apparatus with spin state $|+\rangle$, for which $\hat S_x|+\rangle=+\frac\hbar 2|+\rangle$, I have a question about how to express the spin state when it comes out. I ...
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### What spin-statistics is a magnetic monopole expected to obey?

What statistics (or spin) is a magnetic monopole expected to have? Does it depend on the theory used?
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### Does classical axial current conservation prevent the spontaneous emission of a photon by an electron?

I understand the axial current conservation (in the massless limit) in classial field theory to mean that the difference between right handed and left handed electrons is conserved in QED. j^{\mu ...
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### Is the third spin vector of a photon always suppressed?

I like to tell people interested in light polarization that the photon is a vector boson for which the third spin axis, the one in the direction of travel, is suppressed due to photons being massless ...
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### Adiabatic evolution for initial Hamiltonian on Hadamard basis and problem Hamiltonian as diagonal

This is spawned from a comment at the answer to one of my previous questions. Someone suggested to me that claiming the following statement might be NP-hard. Could anyone please help me to figure out ...
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### Confusion about a lemma on the time constraint of an adiabatic evolution (arXiv:quant-ph/0604077)

I am going through the paper Quantum adiabatic evolutions that can't be used to design efficient algorithms by Zhaohui Wei and Mingsheng Ying. On the second page they prove a lemma. The statement goes ...
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### Photons angular momentum / spin

I have a textbook that says that photons have a spin of absolute value $\hbar$ and at some other point, they say that it has angular momentum of absolute value $\hbar$. Now, since they are different ...
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### Simple QM question about Sy matrix

Given a spin 1/2 particle in state $|\alpha\rangle=\begin{bmatrix}a \\b\end{bmatrix}$, what is the probability of it being measured in the $S_{y+}$ state. Is this equivalent to, if $S_y$ is measured ...
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### What is the significance of electron spin quantum number?

Somewhere I read that spin quantum number is a particularly interesting theory of quantum mechanics as what it really implies is that particles like electrons do not come back to the initial state of ...
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### Demo of Classical Example of a Spin 1/2 System

I was wondering if there are demos of spin 1/2 systems that anyone knows of. Are there any classical systems that are symmetric under a rotation of $4\pi$ and not $2\pi$?
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### Trajectories of particles with spin in Einstein-Cartan theory

The Einstein-Cartan theory is a generalisation of General Relativity insofar as the condition that the metric affine connection is torsion-free is dropped. In other words, the space time is a ...
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### Has the spin and parity of the Higgs boson been experimentally confirmed? [closed]

I read in a newspaper that the Higgs boson might be the new boson but that this was not confirmed, because we don't know its properties, e.g. its spin or parity. Now I see it confirmed that it is the ...