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Today in my RSS feeds an APS paper summary came up called, "Heat Flows in a Circle Without Gradients. It was about this ring of spaced ring oscillators that supported both mechanical excitation modes and phonon modes but where phonon excitations flowed around forever. It's interesting but more interesting was the offhand comment in support of infinite currents saying,
Undriven flows do occur in nature, as evidenced by persistent electric currents in permanent magnets and superconductors.
At first I thought they might be talking analogy, like spins as currents or electron "orbits", or something, but no, the wikipedia article, https://en.wikipedia.org/wiki/Persistent_current#In_resistive_conductors talks about real nanoamp currents physically existing. And then that leads to any even weirder rabbit hole.
I feel like that I can normally ignore quantum physics when doing radio and electromagnetics work. I don't do a lot of polarization stuff and quantum effects normally average out in imprecise many-sample systems. But apparently at specific mesoscopic length scales the quantum stuff I don't understand actually starts coupling to electromagnetic excitation modes. But the gist is that there's a neutral AC oscillation normally and applying an external magnetic bias to these magnetic material rings creates an internal asymmetry causing an perpetual tiny electrical current to flow.
This type of persistent current is a mesoscopic low temperature effect: the magnitude of the current becomes appreciable when the size of the metallic system is reduced to the scale of the electron quantum phase coherence length and the thermal length. Persistent currents decrease with increasing temperature and will vanish exponentially above a temperature known as the Thouless temperature. This temperature scales as the inverse of the circuit diameter squared.
And it does so in regions where there should be both zero electric and magnetic field the https://en.wikipedia.org/wiki/Aharonov-Bohm_effect. It's apparently strong philosophical support for the mathematical formalisms of the electric and magnetic potentials (as opposed to fields) as *real* physical things and beyond that delocalized physical things.
The Aharonov-Bohm effect, sometimes called the Ehrenberg-Siday-Aharonov-Bohm effect, is a quantum mechanical phenomenon in which an electrically charged particle is affected by an electromagnetic potential, despite being confined to a region in which both the magnetic field B and electric field E are zero. The underlying mechanism is the coupling of the electromagnetic potential with the complex phase of a charged particle's wave function, and the Aharonov-Bohm effect is accordingly illustrated by interference experiments.
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