Quantum spin is so strange that when Pauli first proposed it, he wasn’t 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.
A physical object like the earth spins in a rotation plane around an axis of rotation (Figure 3.17), so this axis must be known to measure its spin. It turns out that measuring its spin on another axis, not the rotation axis, reveals less than its total spin. If the spin axis is unknown, one must measure spin on three orthogonal axes to get its total spin. In contrast, the total spin of a quantum entity can be measured from any axis, and is always the same, so quantum spin doesn’t work like physical spin.
However, detecting a photon’s spin on any axis could give all its spin for the same reason that detecting a photon anywhere gives all its energy. In both cases, measurement is a physical event, an all or nothing restart of the entire photon that includes all its energy or spin. This property of quantum spin is then expected.
In addition, quantum spin is measured in angular radians defined by Planck’s constant (Note 1). If Planck’s constant represents the transfer rate of a Planck process, as argued earlier, it will also represent its spin rate. Planck’s constant in radians is then expected to define quantum spin, as it does.
Finally, quantum spin is said to occur in both directions at once, and when measured, can give either direction randomly. This again recalls the earlier finding that a photon can go through two slits at the same time, not just one, and when measured, can also be in either slit randomly. The principle then is the quantum law of all action (that quantum reality tries every option) applied to rotational movement as well as linear movement. It makes sense that if photon instances can travel through two slits at once, they can also rotate in two directions at once. The result of measuring either case is then random because it depends on which instance accesses an unobservable server first. This property of quantum spin is then also expected by a quantum processing model.
In conclusion, spin is a fundamental property of every quantum entity because quantum processing spreads not only in every linear direction possible, but also in every angular direction possible.
Note 1. Quantum spin is defined as Plank’s reduced constant ħ = h/2p (in angular radians).