"Ordinary photons do have spin, they have a notion of helicity so they spin around their direction on motion"
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Penrose is doing what he does best: turning a seemingly innocent clarification into a quiet rebuke of sloppy intuition. “Ordinary photons” sounds casual, almost dismissive, but it’s a strategic move. He’s separating the everyday particle of light from the mystical, overburdened “photon” of pop science, then insisting on a technical truth: yes, light carries angular momentum. Just not in the cartoonish way people picture “spinning” as a tiny planet on an axis.
The key phrase is “notion of helicity.” Penrose reaches for the more precise word because “spin” is both indispensable and misleading. In quantum field theory, photon spin isn’t a little rotating billiard ball; it’s encoded in how the electromagnetic field transforms, and for a massless particle it locks to the direction of motion. That’s why he immediately pivots to “around their direction of motion”: he’s anchoring the concept in geometry, not mechanics. Helicity is spin projected along momentum, and for photons it’s basically all you get - two clean options (left- or right-handed polarization), no rest frame, no third “spin state” to rescue classical imagery.
The subtext is a lesson about language as much as physics. Penrose is reminding you that nature’s most familiar thing - light - still refuses to be narrated in purely everyday terms. The intent isn’t to impress; it’s to prevent a category error. If you insist on imagining photons as spinning objects, you’ll miss what’s actually spinning: the phase and orientation of a field, a handedness written into how light propagates.
The key phrase is “notion of helicity.” Penrose reaches for the more precise word because “spin” is both indispensable and misleading. In quantum field theory, photon spin isn’t a little rotating billiard ball; it’s encoded in how the electromagnetic field transforms, and for a massless particle it locks to the direction of motion. That’s why he immediately pivots to “around their direction of motion”: he’s anchoring the concept in geometry, not mechanics. Helicity is spin projected along momentum, and for photons it’s basically all you get - two clean options (left- or right-handed polarization), no rest frame, no third “spin state” to rescue classical imagery.
The subtext is a lesson about language as much as physics. Penrose is reminding you that nature’s most familiar thing - light - still refuses to be narrated in purely everyday terms. The intent isn’t to impress; it’s to prevent a category error. If you insist on imagining photons as spinning objects, you’ll miss what’s actually spinning: the phase and orientation of a field, a handedness written into how light propagates.
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| Topic | Science |
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