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I would like to understand what the origin of spin 1/2 is. I read in Feynman's lectures that the origin is related to quantum field theory. I know nothing about quantum field theory. Is there an easy explanation?

Thanks Cabrera

- Thread starter cabrera
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- #1

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I would like to understand what the origin of spin 1/2 is. I read in Feynman's lectures that the origin is related to quantum field theory. I know nothing about quantum field theory. Is there an easy explanation?

Thanks Cabrera

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Could anybody suggest a link for an introduction in quantum field theory?

regards,

Cabrera

regards,

Cabrera

- #4

tom.stoer

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Can any one explain what spin 1, 1/2, -1/2 actually mean?

I'm not asking to know their mathematical origin, but what this spins

What

photon has spin 1/2

Z boson has spin 1

What physical property Z boson has but photon does not have?

I once read some where, the

Then where does spin angular momentum comes from?

Or is it the other way around? From spin (extra) angular momentum, which is measurable, physicists determine its spin number?

An inquiring mind wants to know.

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Before you continue with this line of questioning, ask yourself whether you can explain what an apple really means. What is it really?Can any one explain what spin 1, 1/2, -1/2 actually mean?

I'm not asking to know their mathematical origin, but what this spinsphysicallymean?

Whatphysical propertya particle has to have to be rewarded with spin 1 or spin 1/2.

photon has spin 1/2

Z boson has spin 1

What physical property Z boson has but photon does not have?

I once read some where, thespindoes not meanphysical spinof the particle.

Then where does spin angular momentum comes from?

Or is it the other way around? From spin (extra) angular momentum, which is measurable, physicists determine its spin number?

An inquiring mind wants to know.

If you think that you can only explain what an apple is via a list of properties and characteristics on what an apple is, then you have just discovered why what you are requesting makes no sense. To completely rule out the mathematical description of spin is to completely rule out using "fruits" as a category to explain what an apple is.

Zz.

- #7

tom.stoer

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[tex]\vec{L} = \vec{r} \times \vec{p}[/tex]

From this equation we can derive several properties of rbital angular momentum, e.g. that it "generates rotations", that two rotations with different axes of rotation "do not commute", that "orbital angular momentum is consvered" (given that certain symmetries hold) etc. I think in that sense

Now replace

[tex]\vec{L} \to \vec{S} [/tex]

[tex]\ldots = \vec{r} \times \vec{p}[/tex]

while keeping most of the other statements. Now you have to explain again

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Apart from that, spin is just a quantum property that has some of the characteristics we associate with spinning objects in the big world.

IMO it probably IS spin -it's just that there are so many fine states in a macroscopic object we don't see the quantum effects. Trying to argue it backwards from the macroscopic experience to 'explain' the fundamental behaviour is like trying to stuff smoke back in a cigarette.

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In my student years, when dinosaurs roamed the planet, I used to

Apart from that, spin is just a quantum property that has some of the characteristics we associate with spinning objects in the big world.

IMO it probably IS spin -it's just that there are so many fine states in a macroscopic object we don't see the quantum effects. Trying to argue it backwards from the macroscopic experience to 'explain' the fundamental behaviour is like trying to stuff smoke back in a cigarette.

Now I see there are many mathematical quantities (or variables) that have no physical counterparts.

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tom.stoer

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... no [STRIKE]physical[/STRIKE] visualizable counterparts; they are physicakl in the sense that they are measurable![/QUOTE]Now I see there are many mathematical quantities (or variables) that have no physical counterparts.

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Photon has spin 1, the same as Z boson. Photon is a boson, so it must have integer spin. Half integer spin is possible only for fermions.photon has spin 1/2

Z boson has spin 1

The number 0, 1/2, 1 or 2 tell you how much is the wave function of a particle rotated when you turn it by 360 degrees. Please note this is not about rotating some point like object, byt an actual wave in a field. The wave has a complex phase and this phase is changing if you rotate the wave.

If you rotate the wave by 360 degrees and the wave is rotated only by 50%, this particle has spin 1/2. If the wave is rotated by 100%, the spin is 1. If the wave is rotated by 200% (i.e. the phase changed fully twice during the 360 degree "revolution"), the spin is 2.

So the spin is associated with the structure of the wave and is closely connected to the features of the field. Spin 0 fields are scalar (they have a single value in each point of space), spin 1 fields are vector fields (they have a direction and magnitude in each point of space), spin 2 fields are tensor fields (they are more complicated in each point of space).

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So "spin" can be visualized as the rotation of the wave function of a particle? That's a good start for me but what "wave function" does something like a proton/neutron have exactly?Photon has spin 1, the same as Z boson. Photon is a boson, so it must have integer spin. Half integer spin is possible only for fermions.

The number 0, 1/2, 1 or 2 tell you how much is the wave function of a particle rotated when you turn it by 360 degrees. Please note this is not about rotating some point like object, byt an actual wave in a field. The wave has a complex phase and this phase is changing if you rotate the wave.

If you rotate the wave by 360 degrees and the wave is rotated only by 50%, this particle has spin 1/2. If the wave is rotated by 100%, the spin is 1. If the wave is rotated by 200% (i.e. the phase changed fully twice during the 360 degree "revolution"), the spin is 2.

So the spin is associated with the structure of the wave and is closely connected to the features of the field. Spin 0 fields are scalar (they have a single value in each point of space), spin 1 fields are vector fields (they have a direction and magnitude in each point of space), spin 2 fields are tensor fields (they are more complicated in each point of space).

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