Electron Spin

[CarbonCopy]

What do people mean when they say that an electron has an INTRINSIC spin. Is it real ?

And how does this create a dipole moment ? I thought only moving charges create dipole moments not a stationary spinning electron ?

[ajb]

Electrons, for example, behave as if they have some "extra angular momentum" quite independent of any orbital angular momentum. This seems rather unavoidable and has consequences that we observe. This intrinsic angular momentum is what we call spin.

 

Classically a charged body rotating about its centre of mass has "classical spin angular momentum" and so is a moving charge. The interesting thing is that point-particles have no structure and so cannot be spinning in the classical sense. More than this, the measured spin magnetic moment of the electron is twice what one would expect classically.

[swansont]

The very small predicted electron EDM doesn't arise from the spin.

 

http://en.wikipedia.org/wiki/Electron_electric_dipole_moment

[MigL]

Well if it is a real spin, its a very strange kind of spin.

If you were to take the axis of spin and label the 'up' direction you would find that a rotation of 360 deg. would not bring the labelled axis to the 'up' direction again. You actually need to go 720 deg. to return to the original 'up' direction. Someone once described it to me 'as if the electron probability wave's peaks and troughs are out of sync at 360 deg. and interfere, while at 720 deg they are in sync again'.

[ajb]

Well if it is a real spin, its a very strange kind of spin.

Spin is real in the sense that we need it to explain the phenomenology of quantum mechanics we observe. However, this spin is not to be thought of as the electron spinning about form axis. The insistence of keeping this classical picture is the root of a lot of misunderstanding.

 

To get at a classical understanding of spin one needs to study the Lorentz group carefully. In special relativity spin does appear in a way similar to classical angular momentum. It has been said that spin is like a relativistic contribution to angular momentum, though I would say that this is not so straight forward as it might sound.

[MigL]

Sorry AJB, I should have said 'if it is a real classical spin, it would be very strange'.

[Mike Smith Cosmos]

Electrons, for example, behave as if they have some "extra angular momentum" quite independent of any orbital angular momentum. This seems rather unavoidable and has consequences that we observe. This intrinsic angular momentum is what we call spin.

 

Classically a charged body rotating about its centre of mass has "classical spin angular momentum" and so is a moving charge. The interesting thing is that point-particles have no structure and so cannot be spinning in the classical sense. More than this, the measured spin magnetic moment of the electron is twice what one would expect classically.

.

.."" Is there a case for the spin being complex. As the radius reduces toward a minimum ( Zero ? ), and the angular velocity goes sky high, might the 'motion' take on a complex style of motion ( spin ) such as to absorb a second value of angular momentum ? ""

 

(these descriptions "" xxxxxxxxxxxxx "" are in no ways correct descriptions, merely a possible concept )

Edited January 2, 2013 by Mike Smith Cosmos