After a raucous year of supposed breakthroughs in superconducting physics, 2023 is looking like it’s going to end with a whimper. This summer, LK-99 was the room-temperature superconductor du jour.

On the surface it looks like a dullish gray rock, something you might scrape off your boot after a day of hiking. But the rock, known as LK-99, may hold the keys to a future barely imaginable today: ...