# What happens to spin as a particle increases in velocity?

1. May 23, 2012

### idea2000

Hi,

If we think of an electron's spin as just being a classical kind of a spin, what happens as it speeds up close to the speed of light relative to a laboratory frame of reference? Does the lab frame see the spin of the particle "slow down"?

2. May 23, 2012

### Staff: Mentor

There's no reasonable answer to your question, because the electron's spin is not a classical spin - it's sort of like asking "If 2+2=5, then what happens when..."

However, you may be asking a different question that does have a reasonable answer: "If a very small (but not so small that quantum mechanics dominates) spinning ball is accelerated to close to the speed of light, what happens to the spin as observed in the lab frame?". If that's what you're asking, you may get a better answer in the relativity forum.

3. May 23, 2012

### Mark M

The spin of a particle is invariant, it does not change. As Nugatory said, spin is not a rotation about an axis. It is a quantized unit of angular momentum, that is given by $S = \frac {n} {2}.$ So, allowed values for spin are $\frac {1} {2}$, $1$, $\frac {3} {2}$, $2$, and so on. There is no classical analogue to spin - there is a form of angular momentum in quantum mechanics, known as orbital angular momentum, that is essentially the same concept as classical angular momentum. But, spin does not have any apparent classical meaning since particles such as electrons do not have internal structures.

4. May 24, 2012

### micky_gta

Spin does change and it remains static at the same time just like all the other things, gravity, time, mass etc... its all about perspective.

If you could somehow observe the particle spin from earth as it travels close to the speed of light around our solar system it will barely be spinning, it will slow down.

If you travel with the particle near the speed of light and observe you will notice no difference.

5. May 24, 2012

### idea2000

what happens to electron spin and magnetic moment at high velocity?

Hi,

I read that an electron's spin and magnetic moment are invariant. What does this mean exactly from the perspective of a laboratory observer? Would the laboratory frame see the magnetic moment of an electron moving at near the speed of light as being completely unchanged? And, would the effects of time dilation make the electron seem as if it were not spinning at all? Thanks in advance for any help anyone can provide...

6. May 24, 2012

### fzero

Re: what happens to electron spin and magnetic moment at high velocity?

The magnitude of a particle's spin is an invariant, but the direction is not. So the lab observer would continue to measure spin 1/2 for an electron, but as the velocity increases, he'd find that the spin vector starts to become aligned with the direction of motion. To really understand this, you need to know something about Lorentz transformations and how spinors transform under them http://en.wikipedia.org/wiki/Spin_(physics)#Spin_and_Lorentz_transformations

7. May 24, 2012

### jartsa

If we have small classical objects spinning at different rates, and we shoot one electron into each of the objects, then the objects experience various changes of spinning rates.

Some of the objects experience no change of spinning rate at all. We could perhaps say that these objects have the same spinning rate as an electron.

Now if we accelerate all of this experimental setup to great speed, we know that the spinning classical objects will spin more slowly, while we also know that the result of this experiment does not change. This suggest that electrons also spin more slowly.