# Thread: How are Spin and Linear Motion related?

1. ## How are Spin and Linear Motion related?

I could have simply asked "spin and motion" instead of linear motion.

Can anyone recommend some good reading on how these two phenomena are related?

2. As far as I am aware, they are not related. Is there a reason that you assume that they are?

3. Originally Posted by skepticalbip
As far as I am aware, they are not related. Is there a reason that you assume that they are?
I did a bit of googling and answered my own question. Right, they are not related. But thank-you!

4. As I understands it spin is a way of defining quantum states of a particle.

5. Originally Posted by T.G.G. Moogly
I could have simply asked "spin and motion" instead of linear motion.

Can anyone recommend some good reading on how these two phenomena are related?
If by "spin" you mean "angular momentum", Feynman's "The Character of Physical Law" discusses its relationship to linear momentum. (IIRC it doesn't cover quantum spin. But whichever you're looking for it's some good reading. )

6. Originally Posted by steve_bank
As I understands it spin is a way of defining quantum states of a particle.
Yes, but it was called "spin" because the particle was behaving as if it was spinning about an internal axis.

If Moogly means ordinary spinning objects like soccer balls, that has to do with spin and forward motion generating an aerodynamic force on the ball so that it curves clockwise as well. Since angular momentum is conserved, that means that the ball will lose spin to curving.

Spin in some fashion is used in many ball-based sports: baseball, tennis, ping pong. Even the sport that doesn't use a ball at all - American Football, rotates the prolate spheroid about its axis to stabilize it in flight.

Note that since it's an aerodynamic effect, curving would not work in a vacuum. But using spin to affect how a ball bounces would as that is interaction with the playing surface.

7. The Magnus effect can be quite impressive, if you have enough space to play with.

8. Originally Posted by Derec
Originally Posted by steve_bank
As I understands it spin is a way of defining quantum states of a particle.
Yes, but it was called "spin" because the particle was behaving as if it was spinning about an internal axis.

If Moogly means ordinary spinning objects like soccer balls, that has to do with spin and forward motion generating an aerodynamic force on the ball so that it curves clockwise as well. Since angular momentum is conserved, that means that the ball will lose spin to curving.

Spin in some fashion is used in many ball-based sports: baseball, tennis, ping pong. Even the sport that doesn't use a ball at all - American Football, rotates the prolate spheroid about its axis to stabilize it in flight.

Note that since it's an aerodynamic effect, curving would not work in a vacuum. But using spin to affect how a ball bounces would as that is interaction with the playing surface.
I mistakenly thought that spin such as on a soccer ball was the same spin that is associated with a particle, but that spin is not the same thing. The spin on a particle is just a way to designate a property of the particle, not a statement that the particle is spinning.

The particles may in fact be spinning the same way a soccer ball spins and curves for all we presently know but that is not how physicists presently use the word.

9. I only have a general knowledge of particle physics. How can one detect spin experimentally, or is it a model that interprets data?

Is there angular momentum?

10. Originally Posted by steve_bank
I only have a general knowledge of particle physics. How can one detect spin experimentally, or is it a model that interprets data?

Is there angular momentum?
No - The defining characteristic of quantum theory is that properties are quantised, so the classical concept of momentum (angular or linear) isn't really applicable when discussing single particles. Spin is conserved, but like charge, it comes in fractions. In much the same way that integer charge applies to protons and electrons, but quarks have charges in multiples of one third of that (an up quark has a charge of 2/3e, and a down has a charge of 1/3e), we find that Bosons have integer spin, while fermions have half integer spin.

You can't transfer some of the spin from one particle to another, in the way that you can transfer some angular momentum from one object to another in classical mechanics; But conservation of spin is important in QM nevertheless - the electron neutrino appears to be just a spin of 0.5, and was used as a theoretical device to balance the books in beta decay. It took a long time to demonstrate its existence experimentally, as it has no charge, and little or no rest mass. It's nothing but a spin of 0.5 - like having the spin without the football.

Clearly quantum spin is not completely analogous to classical spin - if you have a boson (with integer spin), you can pack as many as you like into the same quantum state. Two fermions cannot occupy the same quantum state, so for example the s orbital of an atom can contain a maximum of two electrons (one with spin +0.5, and the other with spin -0.5).

If this were repeated with regards to classical mechanics, a soccer ball could occupy the same space as another identical ball, as long as they were spinning in opposite directions.

Quantum mechanics is just fucking weird like that.

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