# Rotation of a point particle?

1. May 8, 2010

### Darkmisc

Does it make sense to think of a point particle's rotation? Or does the particle need to be more than one point in dimension so that parts of it can exist either side of an axis of rotation?

2. May 8, 2010

### LukeD

If we have a universe with nothing but a point particle, and we rotate our universe, nothing changes, so we can't talk about the angular velocity of a point particle, and if it's rotating, we can't sensibly talk about how much of an angle its rotated by.

However, lets say we have 2 particles attracting each other by gravity so that they rotate with some angular momentum L. If we allow the 2 particles to collapse to a single point, because angular momentum is conserved, the single point still has angular momentum = L. (But the angular velocity goes to infinity! A point particle with angular momentum seems to spin infinitely fast)
If the single point would then split into multiple particles, the group would still have angular momentum = L, and we could again sensibly talk about angles & angular velocities.

So no, we can't talk about the speed or angle of rotation of a single point particle, but we can talk about its angular momentum.

It may not surprise you to hear that electrons are point particles that always have $$\hbar/2$$ of angular momentum about some axis.