Magnetic moment of the electron and spin

[Thoros]

Quote from wikipedia about the electron's spin

Electrons are spin-1⁄2 particles. These have only two possible spin angular momentum values measured along any axis, +ħ/2 or −ħ/2. If this value arises as a result of the particles rotating the way a planet rotates, then the individual particles would have to be spinning impossibly fast. Even if the electron radius were as large as 2.8 fm (the classical electron radius), its surface would have to be rotating at 2.3×10¹¹ m/s

But in optics and other fields we learned that speeds exceeding c are possible, if they do not propagate information. So is the concept of a classical electron with definite radius still physically correct in the sense that it's surface is actually allowed to move faster than c?

 

[dextercioby]

[...]But in optics and other fields we learned that speeds exceeding c are possible, if they do not propagate information. [...]

Can you give a textbook reference for that ?

 

[Vagn]

Can you give a textbook reference for that ?

I'd guess the OP is referring to greater than c group velocities.
A Systemized View of Superluminal Wave Propagation
http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=5535097&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D5535097

 

[Matterwave]

I'd guess the OP is referring to greater than c group velocities.
A Systemized View of Superluminal Wave Propagation
http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=5535097&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D5535097

I'm guessing you mean phase velocity here.

 

[Thoros]

Can you give a textbook reference for that ?

The effect is probably best known in the case of anomalous dispersion in which even the group velocity may exceed the speed of light in vacuo. Although the speed of information doesn't.

 

[Bill_K]

So is the concept of a classical electron with definite radius still physically correct in the sense that it's surface is actually allowed to move faster than c?

No, not at all. The electron does not have a surface. It's a pointlike particle, which means that, along with all other particles considered elementary, its size is smaller than anything we have been able to probe.

Secondly, despite having angular momentum, an electron does not rotate.

 

[andrien]

I recognize that statement.It is given on wikipedia which is interesting.It was uhlenbeck who came up to lorentz with the idea of electron having a spin .But lorentz showed him that if this would be the case then the surface would have to rotating that much fast,which was impossible on classical grounds.But nevertheless his idea was well received (along with goudsmith) by Heisenberg who said that this idea will remove all the problems of pauli theory.Sorry,I don't remember the reference of it.If I will find ,i will post it.

 

[PhilDSP]

Uhlenbeck and Goudsmith's idea was eventually considered by Pauli. One of Pauli's main objections was that the electron (as an extended particle) would necessarily rotate faster than what was allowed by special relativity.

http://www.lorentz.leidenuniv.nl/history/spin/spin.html
http://www.lorentz.leidenuniv.nl/history/spin/goudsmit.html