<chirp, chirp> “Moire here.”

“Hi, Sy, it’s Vinnie again. Hey, I just heard something on NPR I wanted to check with you on.”

“What’s that?”

“They said that even with the vaccine and all, it’s gonna take years for us to get back to normal ’cause the economy’s hysterical. Does that mean it’s cryin’‑funny or just cryin’? Neither one seems to fit.”

“You’re right about the no‑fit. Hmm… Ah! Could the word have been ‘hysteresis‘?”

“Somethin’ like that. What’s it about?”

“It’s an old Physics word that’s been picked up by other fields. Not misused as badly as ‘quantum,’ thank goodness, but still. The word itself gives you a clue. Do you hear the ‘history‘ in there?”

“Hysteresis, history … cute. So it’s about history?”

“Yup. The classic case is magnetism. Take an iron nail, for instance. The nail might already be magnetized strongly enough to pick up a paper clip. If it can, you can erase the magnetism by heating the nail white‑hot. If the nail’s not magnetic you may be able to magnetize it by giving it a few hammer‑whacks while it’s pointed north‑south, parallel to Earth’s magnetic field. Things get more interesting if we get quantitative. A strong‑enough magnetic field will induce magnetism in that nail no matter what direction it’s pointed. Reverse that field’s direction and the nail stays magnetized, only less so. It takes a stronger reverse field to demagnetize the nail than it took to magnetize it in the first place. See how the history makes a difference?”

“Yeah, for some things.”

“And that’s the point. Some of a system’s properties are as fixed as the nail’s weight or chemical composition. However, it may have other properties we can’t understand without knowing the history. Usually we can’t even predict them without looking at deeper structures. Hysteresis highlights two more gaps in Newton’s Physics. As usual he’s got a good excuse because many history‑dependent phenomena couldn’t even be detected with 17th‑Century technology. We couldn’t produce controllable magnetic fields until the 19th Century, when Oersted and Ampere studied magnetism and electricity. We didn’t understand magnetic hysteresis until the 20th Century.”

“Haw! You’re talking history of history. Anyway, to me it looks like what’s going on is that the strong field gets the magnetic atoms in there to all point the same way and heat undoes that by shaking them up to point random‑like.”

“What about the reversing field?”

“Maybe it points some of the atoms in the other direction and that makes the nail less and less magnetic until the field is strong enough to point everything backwards.”

“Close enough. The real story is that the atoms, iron in this case, are organized in groups called domains. The direction‑switching happens at the domain level — battalions of magnetically aligned atoms — but we had no way to know that until 20th‑Century microscopy came along.”

“So it takes ’em a while to get rearranged, huh?”

“Mmm, that’d be rate-dependent hysteresis, where the difference between forward and backward virtually disappears if you go slow enough. Think about putting your hand slowly into a tub of water versus splashing in there. Slow in, slow out reverses pretty well, but if you splash the water’s in turmoil for quite a long time. Magnetic hysteresis, though, doesn’t care about speed except in the extreme case. It’s purely controlled by the strength of the applied field.”

“I’m thinking about that poor frog.”

‘You would go there, wouldn’t you? Yeah, the legendary frog in slowly heating water would be another history dependency but it’s a different kind. The nail’s magnetism only depends on atoms standing in alignment. A frog is a highly organized system, lots of subsystems that all have to work together. Warming water adds energy that will speed up some subsystems more than others. If Froggy exits the pot before things desynchronize too far then it can recover its original lively state. If it’s trapped in there you’ve got frog soup. By the way, it’s a myth that the frog won’t try to hop out if you warm the water slowly. Frogs move to someplace cool if they get hotter than their personal threshold temperature.”

“Frogs are smarter than legends, huh?”

~~ Rich Olcott