What is Spin 1/2: Definition and 107 Discussions

In quantum mechanics, spin is an intrinsic property of all elementary particles. All known fermions, the particles that constitute ordinary matter, have a spin of 1/2. The spin number describes how many symmetrical facets a particle has in one full rotation; a spin of 1/2 means that the particle must be rotated by two full turns (through 720°) before it has the same configuration as when it started.
Particles having net spin 1/2 include the proton, neutron, electron, neutrino, and quarks. The dynamics of spin-1/2 objects cannot be accurately described using classical physics; they are among the simplest systems which require quantum mechanics to describe them. As such, the study of the behavior of spin-1/2 systems forms a central part of quantum mechanics.

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  1. R

    Basic Probability question for a spin 1/2 Particle

    Homework Statement A beam of spin-1/2 particles is prepared in the state \left|\psi\right\rangle=\frac{2}{\sqrt{13}}\left|+\right\rangle_x+i \frac{2}{\sqrt{13}}\left|-\right\rangle_x a) What are the possible results of a measurement of the spin component Sz, and with what probabilities...
  2. S

    Spin 1/2 particle in |z+> state hits a magnetic field

    Homework Statement "A spin 1/2 electron in a hydrogen atom is placed in a time-independent magnetic field B=B0(k) The electron's spin starts in the |z+> state at t=0. Calculate probability o ffinding the electron in the |y-> state as a function of time for t>0. Homework Equations...
  3. N

    Statistical Physics number of particles with spin 1/2 in a B-field

    Homework Statement Homework Equations The Attempt at a Solution To be honest I'm clueless. I've missed a large amount of the course and just struggling to find any sources that explain this. I don't want the answer to the question, I want to figure that out for myself. What I really need is...
  4. P

    Why does a deuteron nucleus have spin 1? Hydrogen spin 1/2?

    I'm learning about NMR and it's been stated that deuteron has a spin 1 nucleus and so should exhibit different behavior. Why? I've heard of electrons having spin, and now protons, but I don't see why the spin of a neutron should matter since it's uncharged. I'm thinking a spin of 1 means...
  5. D

    N spin 1/2 particles in 3D harmonic oscillator potential

    Homework Statement The 3-dimensional harmonic oscillator potential holds N identical non-reacting spin 1/2 particles a)How many particles are needed to fill the low lying states through E=(3+3/2)\bar{h}ω b)What is the total energy of the system c)what is the fermi energyHomework Equations...
  6. H

    Understanding the Contradiction: Spin Rotation in Quantum Systems

    This has been a contradiction in my brain for some time. If I want to rotate one nuclei (spin 1/2), with say an applied magnetic field B and RF pulse (at the appropriate larmor frequency), how does the spin actually rotate? I thought it can only take on discrete values of 1/2 or -1/2...
  7. T

    Understanding Spin 1/2 and Other Spin Values Explained

    What's a pin 1/2? How can one spin 2 whole revolution to get to the same side? I don't get it. How about spin 0,1,or 2. What do those mean?
  8. S

    Obtaining the angular momentum state of two spin 1/2 particles for (S=0)

    Homework Statement consider the possible angular momentum states |s,m>, of a system of two spin-1/2 particles construct all possible states with total spin zero (S=0) Homework Equations The Attempt at a Solution if total S of system is zero, m must also equal zero. So the...
  9. C

    Origin of Spin 1/2: Exploring Quantum Field Theory

    Dear Forum, I would like to understand what the origin of spin 1/2 is. I read in Feynman's lectures that the origin is related to quantum field theory. I know nothing about quantum field theory. Is there an easy explanation? Thanks Cabrera
  10. P

    Identical spin 1/2 particles in a box.

    Homework Statement two identical particles of spin 1/2 that confined in a cubical box of side L. find the energy and wave function (non-interacting between particles) Homework Equations for a cubic boxby and reducing the Schrodinger equation: ψ(x,y,z)=√(8/L3 ) sin((nx πx)/L)sin((ny...
  11. B

    Why does Dirac Equation describe spin 1/2 particles?

    Hi, Everybody! Currently, I am reading the book "Lectures on Quantum Field Theory" (by Ashok Das) But I am a bit confusing. Why does Dirac Equation describe spin 1/2 particles? I have already known that Dirac Equation bears some angular momentum structure, but why it just describe spin...
  12. C

    Feynman diagrams for spin 1/2 particles

    I think I have a pretty good handle on how scalar field scattering works in QFT, so now I'm trying to wrap my head around spin 1/2 particles, and I'm having a bit of trouble with it. For instance, in N + \phi \rightarrow N + \phi scattering, an application of the Feynman rules leads to an...
  13. A

    Two spin 1/2 partcles and spin operators

    Dear forumers, I was thinking about how the Sz operator "couples" (has non zero matrix elements) states with the same expectation values for the projection of spin on the z-axis (duh! α and β are its eigenvectors), and how Sx and Sy couple different states (once again, duh!). I was also...
  14. G

    Spin 1/2 Correlation: Chance of Measuring Spin Down

    Hello all, Imagine two spin 1/2 particles that are entangled, going towards two stern-gerlach apparatuses, with some relative angle. Now imagine one stern-gerlach device measures the spin of one of the particles as up. What is the chance that the other stern-gerlach device measures the spin...
  15. G

    Spin 1/2 and 720 degree rotation

    Hello everybody, I was watching a lecture on quantum mechanics, and the lecturer was talking about the 720 degree rotation required for a spin 1/2 particle to return to an identical state. I'm aware of this, but I was more interested in the experiment to show that there is an actual...
  16. X

    Need help understanding this: Deriving expressions for spin 1/2 systems

    So this is my first quantum mechanics class and so far we have spent about a month (well 3 weeks) on just the mathematics. So in my book, the author explores the ideas presented by the stern gerlach experiment. It related the beam of silver atoms traveling through the separators to light...
  17. A

    How Many Quantum Numbers Define a Spin 1/2 Particle's State?

    this sounds like a question that should be answered somewhere, but i can't find it. how many quantum numbers does it take to specify the state of a spin 1/2? 1. if it points along z, then just 1: the eigenvalue of Sz will do (up to global phase) A. whether or not in a B field B. if...
  18. B

    Spin 1/2 Basis Change Homework Solution

    Homework Statement This is from my first-quarter graduate QM course. Part 4 of this problem asks me to compute the unitary operator U which transforms Sn into Sz, where Sn is the spin operator for spin 1/2 quantized along some arbitrary axis n = icos\phisinθ + jsin\phisinθ + zcosθ.Homework...
  19. O

    Explicit form for representing two spin 1/2 system

    explicit form for representing "two spin 1/2 system" Hello, Are there any explicit forms(functions) for representing a "two spin(1/2) particle system" whose states are "proton1 up / proton2 up", "proton1 up / proton2 down", "proton1 down / proton2 up", "proton1 down / proton2 down"? Can it...
  20. L

    Gauge symmetry for spin 1/2 fields

    Why is there no gauge symmetry for spin-1/2 fields? Has gauge symmetry to be related to spin-1 fields/ particles? thanks
  21. kreil

    How can the bound states of two spin 1/2 particles be split into two equations?

    Homework Statement Two spin 1/2 particles interact through the spin-dependent potential V(r)=V_1(r)+V_2(r) \sigma ^{(1)} \cdot \sigma^{(2)} Show that the equation determining the bound states can be split into two equations, one having the effective potential V_1(r)+V_2(2) and the other...
  22. T

    Quantum Mechanics - Unitary Operators and Spin 1/2

    Hi, I'm doing question 2/II/32D at the top of page 68 here (http://www.maths.cam.ac.uk/undergrad/pastpapers/2005/Part_2/list_II.pdf ). I have done everything except for the last sentence of the question. This is what I have attempted so far: |\chi\rangle=|\uparrow\rangle=\left(...
  23. T

    Quantum Mechanics - Two spin 1/2 particles

    How do we show that the state |\chi>=\frac{1}{\sqrt{2}}(|\uparrow>|\downarrow>-|\downarrow>|\uparrow>) has total spin zero? Does it involve acting some combination of the spin operators on it? I know that the total spin operator \underline{S^2}=S_x^2+S_y^2+S_z^2=\frac{3\hbar^2}{4}I where I...
  24. Shackleford

    How are (u,v) and alpha+ related in the Eigenstates of Spin 1/2?

    How did they get (u,v) in the second line and then alpha+? http://i111.photobucket.com/albums/n149/camarolt4z28/2010-11-20113911.jpg?t=1290274889
  25. L

    Lagragians for spin 1/2 fields

    Hi, Why would having \partial\psi\partial\psi lead to a Hamiltonian that is unbounded below? Srednicki states that in order to have a bounded Hamiltonian one must include \psi^{\dag} in the combination too. Also why exactly do we require or Lagrangian to be Hermitian, is this somehow to...
  26. M

    Spin 1/2 planced in a magnetic field at x-direction

    spin 1/2 placed in a magnetic field at x-direction Hello guys I have been a regular reader of this forum. This is my first thread. Homework Statement A spin-1/2 is place in a magnetic field which points in the x-direction. The Hamiltonian is \hat{H} = - B Sx where Sx = h/2 * pauli...
  27. R

    Wave function of spin 1/2 under parity

    Homework Statement How does the wave function of spin 1/2 change under parity? Homework Equations The Attempt at a Solution The behavior of the eigenfunctions of orbital angular momentum L is easily seen from their explicit form, namely spherical function Yml is multiplied by...
  28. U

    States of a spin 1/2 particle in a magnetic field

    Hi, for my quantum mechanics course I'm confronted with the following question: Homework Statement A spin 1/2 particle has the following eigenstates of Sz: |+> = (1 0) and |-> = (0 1). A magnetic field is pointing in the z direction, B = (0,0,B). The Hamiltonian is H = -B * n, with n =...
  29. J

    Hamiltonian of spin 1/2 in tangential magnetic field

    Hi, I have this article in which I saw that for a spin 1/2 particle confined to move along a ring positiond in a magnetic field with a z and \varphi The Hamiltonian is given by: (in second attacment) What I do not understand is how do you get the last term in the Hamiltonian. Any help...
  30. M

    Solving Spin 1/2 Interactions with Hilbert Space Dimensions and J$^2$

    Homework Statement three distiguishable spin 1/2 particles interact via H = \lamda ( S_1 \cdot S_2 + S_2 \cdot S_3 + S_3 \cdot S_1 ) a) What is the demension of the hilbert space? b) Express H in terms of J^2 where J = S_1 + S_2 + S_3 c) I then need to find the energy and...
  31. C

    Spin 1/2 rotation and informetry

    Here is the problem: How can relate the magnetic field and the rotations and then the length of the area? I only know that 2pi rotation give the initial state a pi phase but that's all. where the hell I get the length from? thanks
  32. S

    Spin states of the addition of 2 spin 1/2 particles

    Hi everyone, A QM question that I am stuck on: Consider the addition of the spin of 2 electrons, s1=s2=½. Show that there are one ms=1; two ms=0 and one ms=-1 states. Show the 4 spin states are given by: a1a2 b1b2 (1/(sqrt(2)))(a1b2+b1a2) (1/(sqrt(2)))(a1b2-b1a2) I've used a...
  33. S

    Temperature in a nuclear spin 1/2 system?

    Temperature in a nuclear spin 1/2 system?? Homework Statement A solid at temperature T contains 10^20 protons which have a spin I = 1/2 and a nuclear g-factor of 5.59. Calculate the temperature such that 75% of the protons have their magnetic dipole moment aligned parallel to the applied...
  34. H

    Spin 1/2 particles and magnetic fields

    I was told that if you put a spin 1/2 particle in a magnetic field, it will align with or against the field. But some places it also says that it will precess around the field. Which one is right?
  35. L

    Particles with Spin 1/2: Unraveling the Mystery

    Why there can exist some particles having spin = 1/2? I understand, the postive numbers but what do the negative number mean? Also, I would like to ask why some photons can be detected when it comes to light, but photon is not detected when it comes to charge replusion? What is the difference...
  36. C

    Connection between the SU(2) group for the spin 1/2

    I wanted to ask if there is any connection between the SU(2) group for the spin 1/2 and the gauge group of weak interactions. I feel there isn't much of a connection other than the fact that they share the same group properties, but I am not sure. Thanks to anyone in advance that can clarify...
  37. H

    Uuu in Spin 1/2 Config - PhysicsForums.com

    First, this is my 1st post on physicsforums.com, so I'm not too sure I'm posting this in the right place, but guts tell me this is a good place to start. Second I'm sure you've all heard this question a million times. I've seen it answered here a couple of times, but what I've seen is of no...
  38. X

    Why spin 1/2 nucleus has no quadrupole?

    Hi: I am confused that we talk in general about only nuclear spin larger than 1/2 which has electric quadrupole. For spin half, there is no quadrupole because of its spherical shape. Anyone can comment on why they are sphere? what is physical significance behind? I read some research articles...
  39. N

    What is the explanation for spin 1/2 in the x-direction?

    Homework Statement Hi all. I'm reading about spin 1/2, and I am having a hard time understanding it. I hope you guys can fill the gaps. Here is where I stand at the moment: The basis vectors are "spin up" and "spin down" in the direction of the z-axis. Thus we have for spin 1/2 in the...
  40. A

    Calculating Spin Operators for Spin 1/2 Systems

    1. 1) Consider a spin 1/2 system... a) write expressions for the operators Sx Sy Sz in the basis composed of eigenkets of Sz b) Write eigenvalues of Sx Sy Sz c) Write eigenvectors of Sx and Sy in this basis 2) Write a matric corresponding to the operator S_ in the basis composed of the...
  41. S

    Identical, interacting spin 1/2 particles

    Homework Statement Given two identical, interacting spin 1/2 particles in 1D the Hamiltonian is: H = (p1)^2/2m + (p2)^2/2m + (mw/2)*(x1-x2)^2(a) Determine the energy spectrum. (b) Determine the corresponding eigenfunctions including spin state. The Attempt at a Solution I feel like I've...
  42. O

    2 spin 1/2 particles in magnetic field

    Homework Statement See pdf file: http://www.2shared.com/file/3469465/52e02653/q3_online.html Its the third problem I'm having trouble with. I'm pretty sure I have the two first questions correct. Homework Equations...
  43. C

    Natural Eigenbasis composite 2 X Spin 1/2 system

    The way I am being taught in my course is that instead of !s_1= 1/2, m_1 =1/2 ; s_2= 1/2, m_2 =1/2 > = !+ + > !s_1= 1/2, m_1 =1/2 ; s_2= 1/2, m_2 =-1/2 > = !+ - > !s_1= 1/2, m_1 =-1/2 ; s_2= 1/2, m_2 =1/2 > = !- + > !s_1= 1/2, m_1 =-1/2 ; s_2= 1/2, m_2 =-1/2 > = !- - > We have...
  44. N

    Rabi oscillations and spin 1/2 systems.

    Hi all, Can anybody please explain to me the connection between Rabi oscillations and spin-1/2 systems? I believe the connection lies in the bloch sphere and the ability to represent the spin-1/2 system by a superposition of Pauli matrices but I'm just not getting it. Thanks
  45. A

    Spin 1/2 vs Spin 2: Does 360 Degrees Change?

    As mentionned in the "A question about nonlocality" thread, the electron (a spin 1/2 particle) must turn twice to be back in its original position (so that its complex wave function is identically restore). Now my question is this: If a spin 1/2 particle must turn 2*360 for it's wave...
  46. E

    Finding Probabilities of Spin 1/2 System Measurement

    [SOLVED] Addition of Spin 1/2 Homework Statement A system is composed of two spin 1/2. The system's hamiltonian is H=w_1 S_1z + w_2 S_2z. The initial state of the system is, at t=0, | \psi (0) > = \frac{1}{\sqrt(2)}[|+ -> + |-+>] . a) At time t, we measure S^2 = (S1+S2)^2 What...
  47. U

    What is the Hamiltonian operator for a spin 1/2 particle in a magnetic field?

    Homework Statement Consider a spin 1/2 particle placed in a magnetic field \vec{B_0} with components: B_x = \frac{1}{\sqrt{2}} B_0 B_y = 0 B_z = \frac{1}{\sqrt{2}} B_0 a) Calculate the matrix representing, in the {| + >, | - >} basis, the operator H, the Hamiltonian of the...
  48. Q

    Understanding Spin 1/2 Particles - Writing Expressions

    Hello, I need help understanding spin 1/2 particles and writing down expressions for the amplitidues of spin up/down states as a function of time. Any help is appreciated or links to other information. Thanks!
  49. C

    Scalar field and spin 1/2 field

    are bosons represented by a scalar field and fermions represented by a spin 1/2 field or how does it work?
  50. V

    Spin 1/2 in B-field polarization direction?

    spin 1/2 in B-field polarization direction?? Homework Statement A spin 1/2 particle with magnetic momnet mu=mnot*sigma is placed in uniform B-field, where the magnetic field is pointing in (theta,phi) direction in spherical coordinates At t=0, the particle is in the (1 0) state...
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