Spin angular momentum of electron

[dev70]

hi pf, i got following questions in my mind while reading about the spin angular momentum of an electron.
1. From rotational dynamics we know that to have angular momentum a body must necessarily rotation or motion? so does an electron rotates as it has spin angular momentum or does it vibrates?

2. And if it does, won't it radiate ?

 

[jmcelve]

hi pf, i got following questions in my mind while reading about the spin angular momentum of an electron.
1. From rotational dynamics we know that to have angular momentum a body must necessarily rotation or motion? so does an electron rotates as it has spin angular momentum or does it vibrates?

2. And if it does, won't it radiate ?

Spin is a quantum mechanical phenomenon. It doesn't have a classical analogue, so you can't treat the electron as if it's rotating. In fact, if we naively treated the angular momentum classically, we'd find that the electron (if it were spherical) would have a tangential velocity many times the speed of light -- obviously absurd!

The name "spin" itself is therefore misleading. Spin is simply an intrinsic degree of freedom that we can't intuitively understand.

 

[dev70]

ok.i got it. so does that mean an electron is a rest always with no motion. if we have a look at Hisenberg's Uncertainty Principle then i think we can say that the electron is always in a motion?? could we?

 

[jmcelve]

ok.i got it. so does that mean an electron is a rest always with no motion.

No, it doesn't imply that.

if we have a look at Hisenberg's Uncertainty Principle then i think we can say that the electron is always in a motion?? could we?

Precisely. For the electron to be at rest, it would necessarily have a well-defined position (x=x_rest). But it must also necessarily have a well-defined momentum since it is at rest (v=0), and this violates the uncertainty relation.

 

[dev70]

Then a electron is always in a kind of motion. If we just take up a thought experiment where an electron is moving just in an empty space i.e., vacuum should it radiate?

 

[jmcelve]

Then a electron is always in a kind of motion. If we just take up a thought experiment where an electron is moving just in an empty space i.e., vacuum should it radiate?

The problem is that you're applying classical electromagnetism to a particle that doesn't behave classically. Hopefully someone who knows QED will step in at this point and maybe elaborate a little, but I can't really give a deeper reason than that at the moment.

 

[dev70]

Yaa. You are right. With Classical physics i am getting confused. I need quantum mechanism. I did a brief search and i think i have got it.