# The Naked Scientists Forum

### Author Topic: Is particle spin conserved? If so, in what way?  (Read 5551 times)

#### Supercryptid

• Hero Member
• Posts: 606
##### Is particle spin conserved? If so, in what way?
« on: 19/09/2008 12:53:18 »
When it comes to conserving certain properties of subatomic particles before and after interactions, spin confuses me. Must the total spin of a system after an interaction be the same as it was before the interaction? Take neutron decay for example:

Neutron → Proton + Electron + Antineutrino

The spin of all of these particles is 1/2. However, I've also read that spin can be +1/2 or -1/2. In this case, one could imagine that the original neutron had a spin of +1/2, and that the spins of the proton and the electron are also +1/2, with the antineutrino having a spin of -1/2. This negative spin cancels out one of the positive spins and the net result is +1/2 spin of the products (equal to the original neutron).

However, some things seem not to do this. Take the absorption of a photon by a hydrogen atom. The photon has a spin of 1, and the electron has a spin of 1/2. The electron absorbs the photon and is raised into a higher energy state. However, all electrons have a spin of 1/2 and this cannot change (as far as I know). So what happened to the photon's spin of 1 during the electron's excitement? Did it just disappear? Does the nuclear spin play a role here?

Is there any way to predict the spin of the products of an interaction based on the spin of the particles entering the interaction?

#### common_sense_seeker

• Sr. Member
• Posts: 213
##### Is particle spin conserved? If so, in what way?
« Reply #1 on: 19/09/2008 13:28:57 »
Good question.

#### socratus

• Sr. Member
• Posts: 329
##### Is particle spin conserved? If so, in what way?
« Reply #2 on: 19/09/2008 13:57:26 »
The spin of particle is relative quantity.
For example.

An electron have spin  +1/2.
1.
Then electron leaves atom the spherical electro field changes
into ellipsoid field. It is possible to think that something
happened with electron and its spin.
2.
Then after interaction with vacuum all parameters of electron
became infinite. Where is spin of electron now?
============================

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### Is particle spin conserved? If so, in what way?
« Reply #3 on: 19/09/2008 17:50:53 »
When it comes to conserving certain properties of subatomic particles before and after interactions, spin confuses me. Must the total spin of a system after an interaction be the same as it was before the interaction? Take neutron decay for example:

Neutron → Proton + Electron + Antineutrino

The spin of all of these particles is 1/2. However, I've also read that spin can be +1/2 or -1/2. In this case, one could imagine that the original neutron had a spin of +1/2, and that the spins of the proton and the electron are also +1/2, with the antineutrino having a spin of -1/2. This negative spin cancels out one of the positive spins and the net result is +1/2 spin of the products (equal to the original neutron).

However, some things seem not to do this. Take the absorption of a photon by a hydrogen atom. The photon has a spin of 1, and the electron has a spin of 1/2. The electron absorbs the photon and is raised into a higher energy state. However, all electrons have a spin of 1/2 and this cannot change (as far as I know). So what happened to the photon's spin of 1 during the electron's excitement? Did it just disappear? Does the nuclear spin play a role here?

Is there any way to predict the spin of the products of an interaction based on the spin of the particles entering the interaction?
Spin is angular momentum and the associated magnetic momentum; in an atom those quantities are not only given by nuclear or (intrinsic) electronic spin, but from electronic orbital motion too.

#### socratus

• Sr. Member
• Posts: 329
##### Is particle spin conserved? If so, in what way?
« Reply #4 on: 19/09/2008 19:10:57 »
Lightarrow :
Spin is angular momentum and the associated magnetic momentum;
in an atom those quantities are not only given by nuclear or
(intrinsic) electronic spin, but from electronic orbital motion too.

Dodeca Dave :
Spin is conserved in the same way that angular momentum is conserved.

Socratus :
Is angular momentum of particle conserved ?

#### Soul Surfer

• Neilep Level Member
• Posts: 3345
• keep banging the rocks together
##### Is particle spin conserved? If so, in what way?
« Reply #5 on: 26/09/2008 23:43:55 »
In absorbing the photon the electron in the atom has beeen raised into a different orbital this orbital change represents a higher energy and angular momentum state and so energy and angular momentum are conserved.

#### DoctorBeaver

• Naked Science Forum GOD!
• Posts: 12656
• Thanked: 3 times
• A stitch in time would have confused Einstein.
##### Is particle spin conserved? If so, in what way?
« Reply #6 on: 27/09/2008 08:19:46 »
Does quantum spin involve angular momentum? I understood that it wasn't a physical spin at all, just that the particle acted as if it was.

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### Is particle spin conserved? If so, in what way?
« Reply #7 on: 27/09/2008 12:51:33 »
Does quantum spin involve angular momentum?
Yes.

#### DoctorBeaver

• Naked Science Forum GOD!
• Posts: 12656
• Thanked: 3 times
• A stitch in time would have confused Einstein.
##### Is particle spin conserved? If so, in what way?
« Reply #8 on: 27/09/2008 17:31:07 »
Does quantum spin involve angular momentum?
Yes.

Short & succinct! Thank you.

#### The Naked Scientists Forum

##### Is particle spin conserved? If so, in what way?
« Reply #8 on: 27/09/2008 17:31:07 »