Answer: What is Atom Spin: Physical or Property?

[Mukilab]

Hi,

Just a quick question:
When people talk about an atom's spin do they actually mean it is physically spinning or is it a word for some different process or property of the atom - if so, what property is this?

Thank you for your time.

 

[jedishrfu]

wikipedia has a good description here where it talks about spin having the same units as angular momentum:

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

 

[Mukilab]

wikipedia has a good description here where it talks about spin having the same units as angular momentum:

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

I am still confused whether spin is actually spin as we perceive it.

Does this
"no analogue in classical mechanics"
mean that spin in terms of particles in not physically spinning but some other property? And what does this property denote? I.e. Weight (/mass/eV) denotes how heavy a particle is

 

[Dead Boss]

Spin represents intrinsic angular momentum of a particle, but the particle itself isn't spinning in classical sense like Earth around axis. You probably expect to hear: "spin is like x", but unfortunately there is no such x as far as I know.

 

[jtbell]

And what does this property denote?

It contributes to the total macroscopic angular momentum of an object. See the Einstein-deHaas effect, in which you can make a magnetized object start to rotate by reversing the direction of its magnetization (which flips the spins of the electrons that produce the magnetization). This is analogous to the classroom demonstration in which you sit on a rotatable stool holding a spinning bicycle wheel, flip the wheel over and you start rotating yourself.

 

[truesearch]

In other areas of physics you will find strange termsused to describe many properties. When dealing with nuclear structure you will meet Quarks that come in a range of 'flavours' and a range of 'colours'.
These are clearly invented names to make describing different properties easier.

 

[kloptok]

As pointed out, spin behaves like angular momentum and contributes to the total angular momentum of a particle, in the same way that the spin precession of the Earth contributes to its total angular momentum. This is probably why it is called spin. Remember though that the spin of the Earth actually comes from the rotation of all the stuff that makes up the Earth around the rotation axis. It is like the angular momentum from rotation around the sun but instead comes from rotations of the Earth around itself.

However, we consider elementary particles to be point-like, i.e. with NO internal structure and NO spatial extent. Yet they can have a spin value. So it is clear that we can't think of spin as the "stuff that makes up the electron" rotating around the electron center of mass, since as far as we know there is no "stuff" that makes up the electron. Thus the classical description fails.

 

[Mukilab]

In other areas of physics you will find strange termsused to describe many properties. When dealing with nuclear structure you will meet Quarks that come in a range of 'flavours' and a range of 'colours'.
These are clearly invented names to make describing different properties easier.

I never did quite understood why they called them 'charm' and 'strange' quarks, it seems a bit too wishy-washy for physics especially such new discoveries. Wikipedia cites it's etymology as "We called our construct the 'charmed quark', for we were fascinated and pleased by the symmetry it brought to the subnuclear world." Which doesn't make it any less trivial.

It contributes to the total macroscopic angular momentum of an object. See the Einstein-deHaas effect, in which you can make a magnetized object start to rotate by reversing the direction of its magnetization (which flips the spins of the electrons that produce the magnetization). This is analogous to the classroom demonstration in which you sit on a rotatable stool holding a spinning bicycle wheel, flip the wheel over and you start rotating yourself.

As pointed out, spin behaves like angular momentum and contributes to the total angular momentum of a particle, in the same way that the spin precession of the Earth contributes to its total angular momentum. This is probably why it is called spin. Remember though that the spin of the Earth actually comes from the rotation of all the stuff that makes up the Earth around the rotation axis. It is like the angular momentum from rotation around the sun but instead comes from rotations of the Earth around itself.

However, we consider elementary particles to be point-like, i.e. with NO internal structure and NO spatial extent. Yet they can have a spin value. So it is clear that we can't think of spin as the "stuff that makes up the electron" rotating around the electron center of mass, since as far as we know there is no "stuff" that makes up the electron. Thus the classical description fails.

Thank you for your explanations, especially the citing of the Einstein-de Haas experiment as I had never heard about it before and it helped me greatly to understand spin and also kloptok for your detailed analysis.

A big thanks to everyone answered this thread, I think I understand what spin means now, your help and time is greatly appreciated.