conservation of angular momentum

A spinning figure skater pulls in their arms and spins faster; a planet circling the Sun keeps sweeping out area at a steady rate. Both are showing off the conservation of angular momentum — the 'amount of spinning' in a system. Angular momentum measures rotational motion, both orbiting around a point and spinning in place, and it has a direction (the axis of the turn). Left alone, the total amount of it never changes.

Particles carry angular momentum in two ways. There is orbital angular momentum — the motion of one particle going around another, like a planet around a star. And there is spin, an intrinsic angular momentum that every particle carries as a built-in property, as if it were always turning even when sitting still (though it is not literally a tiny spinning ball). In the quantum world both kinds come only in fixed-size steps, set by Planck's constant. When a particle decays or particles collide, the total angular momentum — orbital plus spin, added up — must be the same before and after.

This rule decides which decays and reactions are even possible and shapes their geometry. A spin-1 particle, for instance, cannot simply turn into two particles whose spins and orbital motion cannot be combined to total spin-1; such a decay is forbidden no matter how much energy is available. Conservation of angular momentum also tells experimenters how decay products tend to fly out — the angular pattern of the debris encodes the spin of the parent, which is exactly how physicists measured that the Higgs boson has zero spin. By Noether's theorem, angular momentum is conserved because the laws of physics look the same in every direction — space has no preferred orientation.