@quadrants I'm glad it's a compelling opening! I became fascinated by how life was able to flip electron spins (and hence symmetry) in such huge quantities!

Conservation of angular momentum, which forbids singlet-triplet reactions, is a consequence of Emmy Noether's theorem:

If there's a continuously running symmetry in some space—in this case rotational—and no special paths, then quantities are conserved.

Fundamental, but not one single universal symmetry.

The shielding is incidental.

@keiran_rowell I’m familiar with the work of Emmy Noether—she’s one of my favorite scientists, and I think we owe her an enormous amount.

I interpret your point as follows: different symmetries give rise to different conservation laws. At the same time, some symmetries can be broken (symmetry breaking), and at higher energy levels the rules can change.

That suggests that what is “forbidden” in one context may become possible in another—or at least far less strictly constrained.

@quadrants Oh yes precisely! Noether is amazing, and it's so elegant that conservation flows smoothly from symmetries.

Yes, "spin-forbidden" in particular is *exactly* a term that becomes less strictly constrained in some contexts.

Something is introduced that weakens the "law".

Spin-orbit coupling is the main one, and why the Mn-cluster works: the electric field of the metal atoms facilitates the spin-flip.

Here's an actual physics person talking about oxygen:

https://galileo-unbound.blog/tag/spin-orbit-coupling/

@quadrants A flame gets around this, not reacting triplet oxygen directly with singlet organic matter. Heat imparts enough energy to split a 2e- bond into two radicals.

Each radical now never formally violates a spin symmetry, so *those* can react pretty smoothly. So, yes the higher energy in the system introduced another way around a 'forbidden' process.

Without any external factor or high energy regime, then the rules would hold.

I know proper physicists look at this for other symmetries!

@keiran_rowell Wow, that’s really cool, Keiran! 🔥 Radical formation is such a clever way for nature to get around the “forbidden” spin rules.

I love how energy can open doors that are otherwise closed—and I’m curious which other symmetries physicists find the most striking!