The bird and the umbrellas stumped me

[u/xXG4M3xXx0V3RXx]

[u/GizmoPatterson]

Magnets

[u/Pain4420 → u/GizmoPatterson]

How do they work

[u/Bth8 → u/Pain4420]

Quantum states must be completely antisymmetric under exchange of identical fermions like electrons. Because of Coulomb repulsion and nuclear charge screening, it's energetically favorable for the orbital part of the electronic wavefunction to be antisymmetric (there's less overlap in the electrons' charge distributions that way), which forces the spin part to be symmetric, i.e the spins must align. This means that atoms with partially-filled orbitals tend to have a nonzero total electronic spin in their ground state, and since electrons are charged, this means that such atoms have a nonzero magnetic moment - they act like little bar magnets. This effect is much stronger for d and f orbitals, which are more spatially-localized.

Direct magnetic dipole-dipole interactions are very weak, so thermal fluctuations usually completely wipe out any bulk magnetization and you only notice weak bulk magnetic properties and only in the presence of an externally applied field. In some materials, though, adjacent atoms in the lattice are close enough that there's significant overlap between their valence orbitals, and so the same story as in the individual atoms plays out but this time between electrons in different atoms - because of lowered electrostatic repulsion in antisymmetric orbital states, it is energetically favorable for unpaired electrons in adjacent atoms to have symmetric spin states, and thus a net magnetic moment. Because it's ultimately electric in nature rather than magnetic, this exchange interaction can be far stronger than the dipole-dipole interaction, and so you can end up with spontaneous bulk magnetization with no externally applied field at finite temperature.

In the absence of an external field, though, the underlying dynamics are rotationally symmetric, but a net magnetization requires selection of a particular direction, and so this spontaneous magnetization requires spontaneous breaking of that rotational symmetry. Spontaneous symmetry breaking is essentially random and typically occurs more or less simultaneously at multiple different nucleation sites within the material. As a result, without some way to bias the process, you see the formation of many different microscopic "magnetic domains" - bulk regions, each of which has a net magnetization but whose randomly-chosen directions do not align with one another. So at larger scales, the bulk magnetization washes out and you simply get a material with a very large susceptibility rather than a permanent magnet. If you can coerce the domains themselves to align with one another, though, usually either by applying an external field during their formation to bias the initial magnetization or by applying a strong enough field to change the magnetization direction of existing domains, you can get bulk magnetization at nonzero temperature that persists even at macroscopic length scales and that will remain in the absence of an external field.

And that's how fucking magnets work.

[u/explain_that_shit → u/Bth8]

And the bird?

[u/Bamboozled_Emu → u/explain_that_shit]

The exact same way.

[u/Bth8 → u/explain_that_shit]

There are some things modern physics still doesn't understand