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In this question, I'm not talking about particle spin.

I guess, when an object rotates, its atoms also rotate. When an atom rotates, its particles must move in space.

I wonder that if the particles have a direction.

Can they rotate or do they just move position around the axis (middle) of a proton so we consider that the proton rotates?

Let's think about single particles like electron instead of composite particles like hadrons.

Can electrons rotate ?

Edit : I think this is a simple and good question but I couldn't get a sufficient answer yet .

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  • $\begingroup$ If I was designing a universe simulation, would I implement the rotation info for the smallest building block of the simulation. This is how I came up with my question. $\endgroup$
    – Xtro
    Feb 20, 2014 at 21:34
  • $\begingroup$ Maybe I would implement the rotation info for the smallest building block. How else they would travel in different directions without an inner rotation info? $\endgroup$
    – Xtro
    Feb 20, 2014 at 21:35
  • $\begingroup$ If they had rotation info, what would cause them to rotate? The forces we know today can only pull or repel. Right? $\endgroup$
    – Xtro
    Feb 20, 2014 at 21:38
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    $\begingroup$ Well, you have to talk about particle spin to correctly answer this question. It has indeed no meaning to say that a point rotates. However, we can somehow make sense of the idea of a "point which rotates" as a representation of the Poincaré group. This particle "rotates" (i.e. has nonzero total angular momentum) if it has a spin. If you consider spinless particles, well, they do not rotate. $\endgroup$
    – Georg Sievelson
    Feb 20, 2014 at 22:13
  • $\begingroup$ If you want to forget about spin, then the rotation of a composite object can be described equivalently as the translations of the components. The law relating the rate of change of angular momentum and the torque applied to the system is a consequence of Newton's second law. $\endgroup$
    – Georg Sievelson
    Feb 20, 2014 at 22:18

3 Answers 3

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A particle as a point mass does not have rotation defined. So the question does not apply to point masses. In fact, rotations are used to describe the position of a point mass riding on a moving coordinate frame.

I see rotation as a property of the frame of reference, and not necessarily of the masses tracked.

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Electrons are leptons and are not composite particles and are therefore treated as point particles, aside from current experiments that are looking for electron dipole moments. When quantum mechanical spin was discovered various models tried to explain it via our standard notions of orbital angular momentum by having the electron rotate but it doesn't work.

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  • $\begingroup$ I didn't mentioned that electrons being composite. I wanted to simplify the question so I suggested to think about electrons instead of hadrons. I should say leptons, you are right. $\endgroup$
    – Xtro
    Feb 20, 2014 at 21:32
  • $\begingroup$ as far as I know, spin is completely different phenomenon. That's why I stated "I'm not talking about particle spin" at the beginning of my question. $\endgroup$
    – Xtro
    Feb 20, 2014 at 21:41
  • $\begingroup$ I know I was just providing some physics history. Either way, electrons and other point particles do not rotate because rotation would imply some finite size, i.e. composition. $\endgroup$
    – Elvex
    Feb 20, 2014 at 21:48
  • $\begingroup$ Many people think that finite size does not imply composition. I think this is the String Theory view. $\endgroup$
    – jinawee
    Feb 21, 2014 at 18:35
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Any particle with spin has a defined direction, even if it is a point particle. Now you have to distinguish two parts of the rotation of the system:

  • Any rotation around an axis perpendicular to the particle spin, will cause a change in the direction of the spin. So if for example you have an electron with spin up defined in the z-direction, and you rotate by 90 degrees in the z-x plane, then the electron will now have spin up defined in the x-direction.

  • Any rotation around an axis parallel to the spin, adds a phase to the particle, whose size is exp(iS*theta) for an angle theta and spin S (in h-bar untis).

So yes, the particles within the nucleus/atom will also change not only in their positions.

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