QR3.7.1 The Curious Case of Quantum Spin

Quantum spin is so strange that when Pauli first proposed it he was not believed:

… the spin of a fundamental particle has the curious feature that its magnitude always has the same value, although the direction of its spin axis can vary…(Penrose, 1994) p270

Figure 3.17. Classical spin

A classical object like the earth spins in a rotation plane around an axis of rotation (Figure 3.17), so its spin on any other axis is a fraction of its total spin. If the spin is unknown, measuring spin on any three orthogonal axes is needed to give the total spin. So that one can measure quantum spin on any axis to get the total spin makes no sense in classical terms.

Quantum realism explains why spin measured on any axis always gives the full spin. A photon gives all its spin to any axis measurement for the same reason that measuring a photon in either of Young’s slits always gives the full photon. A physical event is an all or nothing restart, so if it happens the result is the entire photon including all its spin. The spin result for a photon is, as expected, one quantum process which is Planck’s constant in radians

Imagine a coin spun on a table too fast to see its spin direction except that a quantum coin is also spinning at every point on the table. The only way to find out the spin is to stop it and that can’t be repeated unless the coin is re-spun in a new case that could be either direction again.