Understanding Spin of Particles & Atoms: FAQs

[Nim]

I don't quite understand what a "spin" is. My dictionary has two definitions.

11. PHYSICS angular momentum: the intrinsic angular momentum of an elementary particle or system of such particles independent of its motion.

10. PHYSICS quantum property of angular momentum: the quantum property or number of an elementary particle that is a measure of its intrinsic angular momentum and magnetic moment.

Is there a difference between the spin of a quantum particle and the spin of an atom, or combined atoms in a He3 BEC, or combined electrons in a superconducter? I would think an atom couldn't fall into the same category as a quantum particle or a photon at that. Quantum bosons are said to be force-carries, are other bosons force-carries too?

Also, what can or cannot have a spin? I thought only quantum particles had a spin at first. And then I found out that atoms can too. And then I found out that two atoms together can have a spin. But I guess that is only at really cold temperatures, why can't atoms combined in a solid have a spin? Why doesn't a solid itself have a spin? What is that largest thing that has had a spin before, just two atoms connected together?

 

[Ambitwistor]

"Spin" is just another word for "angular momentum". So anything can have "spin". Sometimes the word is used implicitly to refer to intrinsic spin, which is the angular momentum of an elementary (non-composite) particle.

By the way, what is a non-quantum boson?

 

[futz]

The spin of a composite particle depends on the spin of the elementary particles that make it up. For example, the combined electrons in superconductors that you mentioned are called Cooper pairs. They are made of two electrons, each with spin 1/2. When they join together, one is spin "up" and the other is spin "down", so they net spin of the Cooper pair is 0.

 

[jcsd]

Originally posted by Ambitwistor
"Spin" is just another word for "angular momentum". So anything can have "spin". Sometimes the word is used implicitly to refer to intrinsic spin, which is the angular momentum of an elementary (non-composite) particle.

By the way, what is a non-quantum boson?

Tell me if I'm wrong, but I always thought that spin (in the general sense, as obviously spin angular momentum is different to orbital angular momentum in quantum terms though of course they both contribute to the angualr momentum of a particle) was different to orbital angular momentum.

 

[Ambitwistor]

Originally posted by jcsd
Tell me if I'm wrong, but I always thought that spin (in the general sense, as obviously spin angular momentum is different to orbital angular momentum in quantum terms though of course they both contribute to the angualr momentum of a particle) was different to orbital angular momentum.

I've heard "spin" used to refer to the total angular momentum of a composite system. However, now that I think about it some more, this is may not be common usage.

 

[futz]

When the word spin is used, it usually refers to the intrinsic angular momentum S of a particle, and not the orbital angular momentum L.

 

[jcsd]

Originally posted by futz
When the word spin is used, it usually refers to the intrinsic angular momentum S of a particle, and not the orbital angular momentum L.

I meant in general terms, for example someone might talk about the spin of a neutron star in a binary system.

 

[Nim]

A boson that isn't a quantum particle would be an alpha particle. Fermions have a 1/2 spin. Bosons have a 0 or intregral spin. The two protons and the two neutrons in an alpha particle combine to make a boson because 1/2 + 1/2 = 1.

Spin means how much a particle must turn to look the same again.

Spin 1/2: 720 degrees (two turns)
Spin 1: 360 degrees (one turn)
Spin 2: 180 degrees (half turn)
Spin 3: 90 degress (1/3 a turn)

The higher the spin, the less it has to turn to look the same again. I don't really know how they turn them though, especially so precisely.

 

[futz]

Originally posted by jcsd
I meant in general terms, for example someone might talk about the spin of a neutron star in a binary system.

Oops, now I see what you meant. I was just referring to QM

 

[futz]

Originally posted by Nim
A boson that isn't a quantum particle would be an alpha particle. Fermions have a 1/2 spin. Bosons have a 0 or intregral spin. The two protons and the two neutrons in an alpha particle combine to make a boson because 1/2 + 1/2 = 1.

Spin means how much a particle must turn to look the same again.

Spin 1/2: 720 degrees (two turns)
Spin 1: 360 degrees (one turn)
Spin 2: 180 degrees (half turn)
Spin 3: 90 degress (1/3 a turn)

The higher the spin, the less it has to turn to look the same again. I don't really know how they turn them though, especially so precisely.

This is true, but it can be dangerous to think about particles "spinning" about some well-defined axis.

 

[Ambitwistor]

Originally posted by Nim
A boson that isn't a quantum particle would be an alpha particle.

I don't think that I would say that an alpha particle is not quantum mechanical. Maybe you just mean that it's not elementary (non-composite).

I don't really know how they turn them though, especially so precisely.

If you want to determine a particle's spin, there are various ways you can measure its angular momentum, but you don't literally "turn" a particle.

 

[quantumdude]

Originally posted by Nim
A boson that isn't a quantum particle would be an alpha particle. Fermions have a 1/2 spin. Bosons have a 0 or intregral spin. The two protons and the two neutrons in an alpha particle combine to make a boson because 1/2 + 1/2 = 1.

This really doesn't answer Ambitwistor's question.

Why do you think a boson is "nonquantum"?

The reason one would ask is that neither Bose-Einstein nor Fermi-Dirac statistics describe ensembles of classical particles. They are both "quantum".

 

[Nim]

I thought a quantum particle was an elementary particle. Isn't quantum the smallest amount of something? Am using the terminology wrong?

 

[Ambitwistor]

Originally posted by Nim
I thought a quantum particle was an elementary particle. Isn't quantum the smallest amount of something? Am using the terminology wrong?

Well, in the physics literature nobody really uses the term "quantum particle". The word "quantum" can simply mean "obeying the laws of quantum mechanics". All particles are "quantum" in this sense.

If you want to say that a "quantum particle" is an elementary particle, you can, but a physicist would be confused. For instance, is a proton a "quantum particle"? (It's not "elementary" in the sense that it's made up of smaller particles.) What about a Bose-Einstein condensate of atoms? The spin statistics of a particle (whether it is a boson or a fermion) is fundamentally a quantum mechanical property; classical particles can't be described in those terms.

 

[futz]

Originally posted by Nim
I thought a quantum particle was an elementary particle. Isn't quantum the smallest amount of something? Am using the terminology wrong?

You might me thinking of quanta. A quanta is a small piece of something. It is commonly used to describe something that is not continuous but quantized (hence the name). For example, the quanta of the electromagnetic field is a photon. Similarily, phonons are the quanta of vibrations inside crystal lattices.

 

[krab]

Originally posted by futz
You might me thinking of quanta. A quanta is a small piece of something. It is commonly used to describe something that is not continuous but quantized (hence the name). For example, the quanta of the electromagnetic field is a photon. Similarily, phonons are the quanta of vibrations inside crystal lattices.

No... Quanta is simply plural for Quantum.

 

[futz]

You say tomato, I say tomato... I usually just use quanta for everything, singular or otherwise. I guess I should pay more attention

BTW krab, I noticed in your profile that you work at TRIUMF. Very cool. My thesis research is currently based on μSR experiments conducted there.

 

[Nim]

So is a BEC the largest thing with a spin, that is, something that is either a fermion or a boson?