What is Spin: Definition and 1000 Discussions

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  1. K

    A Rotations on the Bloch sphere

    Hello! I am curious about how different rotations on the Bloch sphere are done in practice. For example, assuming we start in the lower energy state of the z-axis (call it |0>), a resonant rotation on the Bloch sphere by ##\pi/2## around the x-axis will take you to ##\frac{|0>-i|1>}{\sqrt{2}}##...
  2. S

    I Why do all fermions have the same spin 1/2?

    We are taught that all fermions have spin ##\frac{1}{2}##, short for spin angular momentum ##\frac{\hbar}{2}##, which can be added to the orbital angular momentum. Considering spin is a kind of angular momentum, it must be dependent on the mass (or moment of inertia) of the particle. However...
  3. cianfa72

    I Representation of Spin 1/2 quantum state

    Hi, I'm aware of the wave function ##\Psi## of a quantum system represents basically the "continuous components" of a quantum state (a point/vector in the infinite-dimension Hilbert space) in a basis. If we take the ##\delta(x - \bar x)## eigenfunctions as basis on Hilbert space then the wave...
  4. U

    I Spin state, base transformation

    how do i write the z spin state using the spin state on x or on y?
  5. H

    A Does attraction vs repulsion depend on graviton spin?

    Hi Pfs, I read somewhere that if the graviton had a spin 1, then gravity would be repelling. Is there a formula showing that the attract-repel depends on the parity of the carrier's spin? thanks
  6. W

    B How are the spins of two entangled photons measured at light speed?

    and how is it known that the two photons are entangled in the first place? I mean before measuring how do you know that you have the correct two photons?
  7. Garlic

    A Quantitative description of successive Stern-Gerlach measurements

    I'm trying to understand how exactly we calculate the detection rate in this specific multiple Stern-Gerlach setup. As written on the image, an (unpolarized) atomic beam is sent through a three Stern-Gerlach apparatuses, and the detector supposedly clicks 25% of the time. When I try to...
  8. D

    A Position and Spin Commutation

    I know how position and momentum commute, but now I have the spin angular momentum operator involved as well as a dot product. Specifically, what would the commutation [x,S·p] be?
  9. B

    B Is Quantum Mechanics necessary for understanding electromagnetic phenomena?

    Is there a link between electron spin and magnetism? can a magnetic field also be thought of as a 'spin' field. If the eletron spin is linked to the magnetic field then can magnetic repulsion be caused be the pauli exclusion principle?
  10. Pixelex

    B Quantum Entanglement in Practice

    How is quantum entanglement done in practice for different particles with different properties eg. spin, polarization, etc.? Why is entanglement useful for quantum circuits? *I am curious about how they are actually entangled and used. I don't know how is entanglement applied in practice so I...
  11. Frigorifico9

    I In what chapter do Mehra and Rechenberg discuss Pauli matrices?

    I am very interested in how Pauli found the Pauli matrices, so I read his original paper, but it didn't give me the perspective I wanted, so I went to Mehra and Rechenberg, but here's the thing, after reading Volumes 1, 2 and most of volume 3, I can't find any mention of Pauli matrices anywhere...
  12. okaythanksbud

    I Question about Stern-Gerlach experiment

    I just learned about the Stern-Gerlach experiment and have some questions: 1: clearly there's no objective "up" or "down"--the directions are measured relative to the magnetic field, correct? And well always find just 2 spots of equal and opposite distance on the detector, implying the magnetic...
  13. Frigorifico9

    I Who was the first to make a Stern-Gerlach experiment with two magnets?

    I'm trying to fill a conceptual gap I have in the history of physics In 1922 Stern and Gerlach make their experiment, proving that electrons have intrinsic angular momentum, however it takes a while for people to understand this. At first they think this is somehow caused by quantization of...
  14. Frigorifico9

    I When was the formula for the Stern-Gerlach experiment found?

    Today we know that if you make successive Stern-Gerlach measurements the beam of atoms will split according to this formula: > cos^2 (theta/2) And this is something people back then could have figured out, they could have done many measurements, plotted the values, and realized it followed...
  15. spin_100

    A SU(2) and SU(3) representations to describe spin states

    Spin 1/2 particles are two states system in C^2 and so it is natural for the rotations to be described by SU(2), for three states systems like spin - 1 particle, Why do we still use SU(2) and not SU(3) to describe the rotations? Is it possible to derive them without resorting to the eigenvalue...
  16. JBlue

    I HUP for spin seems violated: 0 x sy > sz/2...?

    HUP for spins reads $$\langle\sigma_z^2\rangle\langle\sigma_x^2\rangle \ge \frac{1}{4}|\langle\sigma_y\rangle|^2$$ Right after measuring ##\sigma_z##, we know it exactly, and so ##\langle\sigma_z^2\rangle=0##. However, HUP then implies that ##\langle\sigma_y^2\rangle=\infty## Even if we say...
  17. I

    A Monte Carlo simulation for the classical isotropic 3D Heisenberg model

    here is my attempt to implement using python import numpy as np import matplotlib.pyplot as plt def initialize_spins(L): """Initialize a random spin configuration with unit magnitudes.""" spins = np.random.normal(size=(L, L, L, 3)) magnitudes = np.linalg.norm(spins, axis=-1...
  18. RobbyQ

    I Generating Entangled Electron Pairs

    I have read, what I believe, misleading articles about generating entangled electron pairs. Some suggesting the electron is split. But this isn't possible because it's an elementary particle with charge/mass and Spin properties. So how do we achieve generating entangled electrons with opposite...
  19. O

    I What do you need to establish that spin is conserved?

    Hi. Question as in the summary. Spin has no obvious classical interpretation but it is often a conserved quantity and considered as some sort of angular momentum. What do you need to establish that spin is a conserved quantity? I'm finding references to situations where spin is not a...
  20. 4

    I Spin operator and spin quantum number give different values, why?

    Assume spin 1/2 particle So the spin operator gives +/- hbar/2 eg. S |n+> = +/- hbar/2 |n+> But S= s(s+1) hbar = sqrt(3)/2 hbar So I'm off by a factor of sqrt(3). I suspect I am missing something fundamental about my understanding of spin. My apologies and thanks in advance.
  21. P

    Quantum spin superposition interpretation

    Here is my workings out: $$$$ If a particle's spin of magnitude ##\frac {\hbar}{2}## is prepared along direction ##\vec r_1## and subsequently its spin is measured along direction ##\vec r_2 ## at an angle ##\vec \theta ## to ##\vec r_1##, the probability of its being found "spin up" along is...
  22. thomas_shvekher

    Newton's First Law to explain Washing Machine Spin Cycle

    I truly am not sure. I assume it is that because everything has inertia, an a tendency to remain in a constant state of motion, when the clothes are quickly spun around they cannot remain in a constant state of motion (of either rest or constant velocity), but the water is "pushed"/spun out of...
  23. S

    I Potential energy of spin anti-alignment

    Hello everybody, I consider two electrons that have enough kinetic energy to reach their respective classical electron radius. This would be: 2.0514016772310431402e-13 J The corresponding speed is v = 287336682 m/s. The electric field is E = \frac{k_{e}}{R_e^2} = 1.8133774657059088443 ×...
  24. snoopies622

    B How to show that particle spin includes angular momentum?

    I understand how a massive, electrically charged spinning ball would have both angular momentum and a magnetic dipole, and i can see how the Stern–Gerlach experiment shows that the magnetic dipole of an electron is quantized. What kind of experiment demonstrates a connection between electron...
  25. DomDominate

    B What exactly is spin? Does the standard model work without spin?

    I did some research online and found that "When certain elementary particles move through a magnetic field, they are deflected in a manner that suggests they have the properties of little magnets." To explain this phenomenon, physicists invented the concept of spin. So far so good. What I...
  26. H

    Why a particle with spin=0 can't posses a magnetic dipole moment?

    Hi, I would like to know why a particle with spin=0 can't posses a magnetic dipole moment? Using Wigner-Eckart theorem for ##\langle j,1,m,0|j,m \rangle## I get ##\langle j'|| \vec{J}|| j \rangle = \hbar \sqrt{j(j+1)} \delta_{jj'}## It seems like the right hand side is the magnetic dipole...
  27. S

    B Magnetic Field & Particle Spin: Does It Matter?

    Does the magnetic field caused by moving particles depend on the particle spin value? Eg a stream of say protons spin 1/2 is creating a magnetic field. If the particles are (say) lithium nuclei spin 3/2 instead, does that create the same strength field ? (same conditions of course)
  28. H

    Probabilities of measuring ##\pm \hbar/2## along ##\hat{n}##?

    Hi, Given a spin in the state ##|z + \rangle##, i.e., pointing up along the z-axis what are the probabilities of measuring ##\pm \hbar/2## along ##\hat{n}##? My problem is that I'm not sure to understand the statement. It seems like I have to find the probabilities of measuring an eigenvalue...
  29. E

    Probability of Negative Value in Sz 1/2 Spin System w/ Lambda 1 & 2

    Will the probability to provide a negative value of a Sz 1/2 spin system always be 0? If lambda 1 = hbar/2 and lambda 2 = -h bar/2 ?
  30. Omega0

    B Is a Spin 2 Particle the Key to Understanding Gravitation in Quantum Mechanics?

    I have some questions regarding the expected exchange particles for gravitation. From my understanding the following was valid: We can linearize the equations of GTR for weak fields "Quantum mechanics" (Schrödinger, Dirac equations) are linear Those linear equations allow eigenstates and...
  31. Like Tony Stark

    Collision between two particles with different spin

    1) The Hilbert space for each particle and the system are: ##H_1={\ket{\frac{1}{2} \frac{1}{2}}; \ket{\frac{1}{2} -\frac{1}{2}}}## ##H_2={\ket{1 1}; \ket{1 0}; \ket{1 -1}}## ##H=H_1 \otimes H_2## 2) I'm not sure what "considering the total Hamiltonian" means, but I think that the two CSCO...
  32. uxioq99

    Exploring Solutions to Spin Evolution Equations

    By the statement of the question, a solution must take the form ##\begin{pmatrix} \Psi_1 \\ \Psi_2 \end{pmatrix}## and the energy operator will be as per usual ##\hat{E} = i\hbar \frac{\partial}{\partial t}##. I am confused by the fact that ##S_y = \frac{\hbar}{2} \begin{pmatrix} 0 & -i \\ i & 0...
  33. H

    B Spin And Angular Momentum of Large Objects

    I read that quantum spin is the measure of the angular momentum of a quantum object. Suppose you have a rotating Thing 1. Quantum objects bounce off of it then collide with Thing 2. Will this transfer angular momentum from Thing 1 to 2, causing it to rotate?
  34. E

    A Can four electrons form a completely antisymmetric joint spin WF?

    Can four (or more) electrons form a completely antisymmetric joint spin wave function?
  35. G

    I What Could Cause the Earth's Rotation to Dramatically Slow Down?

    Hi everyone, I’ve been thinking about this for a while and have done a bit of research but can’t seem to get a straight answer. Aside from collision with another object in space, is there anything that could cause the Earth’s rotation to dramatically slow down? If a sufficiently massive object...
  36. V

    A Black holes: merged remnants have similar spin - why?

    Hello everyone, I hope I'm asking in the correct section, if not please point me. I read a list of gravitational wave detection. I focused on black hole - black hole events and I noticed the resulting black hole spin is very similar about a=0.7. I didn't find any explanation for this. List of...
  37. H

    I The left-right symmetry and forbidden triplet spin state

    Suppose that we have a system including two single (and identical) orbitals as shown in the attached image. Then, consider the states that we have two different spins on the two orbitals. It seems that because of the left right symmetry of the system the special part of teh wave function can not...
  38. H

    I Spin networks: what exactly is a trivalent node?

    Hi Pfs Rovelli defines spin networks in this paper https://arxiv.org/abs/1004.1780 for a trivalent node Vn = 0 (the volume) nodes begin to "get" volume with the four valent case. take a cube or a tetrahedron, each vertex is linkes to 3 nodes so they would have a null volume. things are...
  39. sal1854

    I Direction of motion of particles with total spin under magnetic field

    According to Chapter 8 of Griffiths' book Introduction to Electrodynamics, the magnetization force that acts on a magnetic dipole is $$F_M=\nabla (m \cdot B)$$, where ##m## is the magnetic moment and ##B## is the magnetic field. For a paramagnetic or diamagnetic particle...
  40. Z

    Angular deceleration of a washing machine spin cycle

    6.0 rev - 1 s 1 rev - 1/6 5.0 rev - (1/6) x 5 = (5/6) s = 0.833 s I do not know how to calculate the angular acceleration
  41. earthling75

    Stuck calculating probability of measuring ##S_y## for spin 1 particle

    I know how to construct Sy for spin = 1 case from the raising and lowering operators. I get $$ S_y=\frac{i\hbar}{\sqrt{2}}\begin{pmatrix} 0 & -1 & 0 \\ 1 & 0 & -1 \\ 0 & 1 & 0 \\ \end{pmatrix} $$ From what I have seen, the eigenspinor for $\hbar$ is found by solving $$...
  42. BiGyElLoWhAt

    A How can you tell the spin of a particle by looking at the Lagrangian?

    I'm just starting to get into QFT as some self study. I've watched some lectures and videos, read some notes, and am trying to piece some things together. Take ##U(1)_{EM}: L = \bar{\psi}[i\gamma^{\mu}(\partial_{\mu} - ieA_{\mu}) - m]\psi - 1/4 F_{\mu\nu}F^{\mu\nu}## This allegedly governs spin...
  43. redtree

    I Pin & Spin Groups: Double Covers of Orthogonal & SO Groups

    Pin Groups are the double cover of the Orthogonal Group and Spin Groups are the double cover of the Special Orthogonal Group. Both sets of the double cover are considered to be groups, but it seems that only one of the sets of the double cover actually contains the identity element, which means...
  44. H

    Partition function for a spin i

    ##Z = \sum_{-i}^{i} = e^{-E_n \beta}## ##Z = \sum_{0}^j e^{nh\beta} + \sum_{0}^j e^{-nh\beta}## Those sums are 2 finites geometric series ##Z = \frac{1- e^{h\beta(i+1)}}{1-e^{h\beta}} + \frac{1-e^{-h\beta(i+1)}}{1-e^{-h\beta}}## I don't think this is ring since from that I can't get 2 sinh...
  45. D

    A dial can spin on a fixed rotational axis

    What I have done is on my Ipad that I cant upload or at least don't know how to... :/ With hope of help DJ
  46. SR_0301

    I Question about orbital and spin magnetic dipole

    Hello guys, i want to ask you a question about orbital and spin dipole, and how this is going to influence diamagnetic or paramagnetic substances. So my question is: we know in a atom there is orbital and spin motion by electrons so possibly two magnetic dipoles. Is it correct to say that in...
  47. H

    A Spin networks with different intertwiners

    Hi Pfs Spin networks are defined by the way their links and their nodes are equipped with SU(2) representations and intertwiners. Could you give an example of two different spin networks with the same number of nodes, links between them, the same coloring of the links (and their orientations)...
  48. BadgerBadger92

    B How do particles get their spin?

    https://physics.stackexchange.com/questions/418865/why-does-an-electron-have-spin I’ve been trying to do research on this and haven’t gotten a clear answer. It sounds like they get it through particle interactions, but I may be misreading it. Is there an answer to this? And if there is, how...
  49. Ahmed1029

    I Spin expectation value for one particle vs actual measurement

    When the expectation value of spin in the z direction for one particle is zero and I make measurements for an even number of particles in the same state, do I get exactly half to be spin up and half to be spin down along the z direction? More generally, what does spin expectation value for one...
  50. H

    A What do you think of this definition of Spin Networks?

    Hi Pfs i found a paper: https://arxiv.org/abs/math-ph/0306059 in which the author gives a definition of spin networks it is at the bottom of page 5 the words node or vertex does not appear. what do you think of it? Does the author think that all the information is in the hilbert spaces on which...
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