Particle spin

[jason.p]

I've been trying to get my head round entaglement, locality etc and Bell and Aspect's experiments where particle spin was used as an indicator. If particles are in fact vibrating strings (if you accept this theory) how do they spin? It's easy to visualise a football like particle spinning around various axes but not a vibrating string. I'm sure it's not as simple as this and maybe "spin" is just a way of describing a mathematicall function, but I'm having to try and assimilate this in "quantum packets" and being able to visualise it helps. Thanks.

[ajb]

If you are talking about a particle's intrinsic spin, then this cannot simply be understood in terms of rotation about some axis. The origin of spin needs a good understanding of special relativity to be explained.

 

In non-relativistic quantum mechanics spin angular momentum is really just "bolted on" to the theory. It is at that level an observed fact that particles have spin: it is an intrinsic property of particles and composites.

 

Anyway, you should not think of spin as anything to do with spinning in any classical sense.

[jason.p]

Thanks AJB. So "spin" is not inconsistent with string theory. I must try and get out of need to visualise everything. I can accept that spin is a real function of a particle that I don't fully understand at the moment and move on from there.

[ajb]

Thanks AJB. So "spin" is not inconsistent with string theory.

 

 

Particles come in two types, bosons and fermions. Bosons have integer spin and fermions half integer spin. Bosons have the property that an arbitrary many can be placed in a quantum states. For fermions only one can be placed in each quantum state.

 

If you want to include fermionic particles in the string spectra then you need to consider the superstring.

[questionposter]

Couldn't spin just be the pattern in which a wave vibrates? I mean if you just imagine a sphere of water and you drop a wave through it, the waves will vibrate in a specific direction and merge on the other side, only with real matter, matter isn't just a wave so it doesn't exactly go both directions and it doesn't merge on the other side, I suppose different locations of an electrons can repel each other in superposition?

[ajb]

Couldn't spin just be the pattern in which a wave vibrates?

 

Sort of, you need to look at the quantum excitations of the string and identify these with elementary particles, including their intrinsic spin. However, you do need the superstring, rather than just the bosonic string to include both bosons and fermions in the spectra.

[questionposter]

Sort of, you need to look at the quantum excitations of the string and identify these with elementary particles, including their intrinsic spin. However, you do need the superstring, rather than just the bosonic string to include both bosons and fermions in the spectra.

 

Is superstring actually science? How would you even make 3 and 4+ dimensional movements from 1 dimensional object oscillating? That's like saying that playing a guitar sends me back in time. I don't even think "dimension" is the proper term, it should be more like "a different way to exist", because dimension implies you can measure the units of the "dimension" of an object, and I don't know how you measure 10 dimensions of something.

Edited January 7, 2012 by questionposter

[ajb]

Is superstring actually science?

 

People argue about string theory being science or maths.

 

Right now it would be fair to say that string theory is more maths than physics at the moment. Superstring theory offers a framework that may be the correct one for a unification scheme. Work in progress is what one should call it.

 

You maybe interested in my last blog entry.

[immijimmi]

Anyway, you should not think of spin as anything to do with spinning in any classical sense.

 

But the wiki for spin in particle physics says:

 

'Spin is a type of angular momentum, where angular momentum is defined in the modern way as the "generator of rotations"

I'm confused... >.<

[ajb]

But the wiki for spin in particle physics says:

 

'Spin is a type of angular momentum, where angular momentum is defined in the modern way as the "generator of rotations"

I'm confused... >.<

 

 

You can understand intrinsic spin in a way very similar to classical angular momentum, but you have to think 4d and in terms of the Lorentz group. Details I think we should omit at the moment. So spin is really a special relativistic thing.

 

In non-relativistic quantum mechanics spin appears rather unnatural and has no clear explanation. However, we can treat it just like an orbital angular momentum. In fact it is often useful to combine the orbital angular momentum with the spin and consider total angular momentum. This total angular momentum can be described in terms of generators that satisfy the relations of orbital angular momentum.

 

But it would be wrong to think of particles as actually rotating about some axis in 3d.

 

Spin I think is a rather poor choice of nomenclature.

Edited January 10, 2012 by ajb

[jason.p]

//Spin I think is a rather poor choice of nomenclature.//

 

I think this is where beginners like myself have problems, trying to visualise such things in "everyday literal terms". I suppose it's got to be called something .