# What do we mean by spin of a particle when we say it a point particle

nouveau_riche
what do we mean by spin of a particle when we say it a point particle?how do we measure spin experimentally and give it values like +1,+2 etc.
what does it mean by a spin 0 particle?

## Answers and Replies

shahbaznihal
The problem which occurs in understanding spin stems from our lack of visualization within the classical world in this regards. Spin of a particle cannot be understood as a spinning point particle. That idea was shown to be inconsistent as any such attempt will make the surface speed of the particle exceed the speed of light or make it larger than the current bounds (even if you want to think about it as a point particle, which itself is of limited value within the quantum world where the spinning property of particle belongs). However, the behavior of particle in an external influence such as magnetic field can only be explained if the particles have a intrinsic spin. Regardless of the lack of visualization, in the classical sense, the name "spin" for this property of particles has survived through the ages.

As far as the question of measurement and subsequent quantization of Spin is concerned I will agree with Simon that you can find tons of information on this in any undergraduate book on Quantum Mechanics.

Good luck!

mpv_plate
what do we mean by spin of a particle when we say it a point particle?how do we measure spin experimentally and give it values like +1,+2 etc.
what does it mean by a spin 0 particle?

If the wave function of a particle is unchanged when rotated by 2π/s radians, the particle has spin s. For example photon has spin 1, thus its wave function is unchanged when rotated by 2π (360 degrees, or one full revolution around an axis). Electron has spin 1/2, so you need to rotate its wave function twice around the axis (720 degrees) to get the same wave function. If a particle has spin 2, it is sufficient to rotate it by 180 degrees to get the same wave function.

Fields with spin 0 are scalar, fields with spin 1 are vector fields, fields with spin 2 are tensor fields.

nouveau_riche
If the wave function of a particle is unchanged when rotated by 2π/s radians, the particle has spin s. For example photon has spin 1, thus its wave function is unchanged when rotated by 2π (360 degrees, or one full revolution around an axis). Electron has spin 1/2, so you need to rotate its wave function twice around the axis (720 degrees) to get the same wave function. If a particle has spin 2, it is sufficient to rotate it by 180 degrees to get the same wave function.

Fields with spin 0 are scalar, fields with spin 1 are vector fields, fields with spin 2 are tensor fields.

i am not getting the reason to bring the concept of spin

nouveau_riche
Gold Member

i am not getting the reason to include the concept of spin

Well, without this additional degree of freedom, it would not be possible of accurately describe many things. How about the number of electrons in different atomic shells? Or behavior of particles with known spin?

The point being that spin needs to be included to properly explain various particle behavior.

Kholdstare
When we talk about "spin", we really don't mean spin. We talk about charged electrons doing "something" that has a similar effect as if they are spinning. You can name it any thing else, but that's another thing (we don't know what that electrons do, when they "spin").

Mentor
"Spin" in QM is shorthand for "intrinsic angular momentum." We know it's angular momentum because it contributes to an object's total macroscopic angular momentum. See the Einstein-DeHaas effect.

nouveau_riche
When we talk about "spin", we really don't mean spin. We talk about charged electrons doing "something" that has a similar effect as if they are spinning. You can name it any thing else, but that's another thing (we don't know what that electrons do, when they "spin").

what is that "something" you are talking about?

Kholdstare
what is that "something" you are talking about?

I told you already. Electrons obey quantum mechanics and the quantum mechanical description of spin can not be understood by drawing analogy between that and the classical concept of spin.

Mentor
what is that "something" you are talking about?

He already gave you the best current answer to this question:

(we don't know what that electrons do, when they "spin").

All we can do is calculate the effects of "spin" on things that we can actually measure.

Homework Helper

i am not getting the reason to include the concept of spin

It is a name for a property... like strangeness or charm for quarks. In this case chosen for the similarity in the math and its relationship to things like moment of inertia and magnetic moment. A name does not have to mean anything: it's just a handy label.

nouveau_riche
"Spin" in QM is shorthand for "intrinsic angular momentum." We know it's angular momentum because it contributes to an object's total macroscopic angular momentum. See the Einstein-DeHaas effect.

could you please show me what you said, with an example

DrewD
Imagine a ball that is spinning and think about how it would move. Now imagine its not a ball and not spinning. That what quantum particles are like, but they still have the interactions with other particles similar to those which the spinning ball would have.

If you bounce a spinning ball it will deflect from the floor (or whatever it hits) and move off in a different direction. There is no exact analogy, but electrons will have their paths changed in a way that makes sense if one assumes they have the same mathematical trait that a spinning ball has (angular momentum). But they are not balls and they are not spinning, so it is just a trait that they intrinsically have.

This may not be satisfying, but none of us can do anything about that.

Staff Emeritus