Quantum calculations amount to sophisticated estimates. But in 1931, Hans Bethe intuited precisely how a chain of particles would behave — an insight that had far-reaching consequences.

By 1928, quantum physicists seemed poised to unravel matter’s final secrets. The German researcher Walter Gordon had applied the emerging theory of quantum mechanics to the hydrogen atom, the universe’s simplest atom, and worked out exactly how it behaved. A mastery of all atoms seemed sure to follow.

It did not. When quantum particles influence each other, their possibilities intertwine in such a way that it overwhelms physicists’ ability to predict their future. In the search for sharp answers, the lone electron of the hydrogen atom marked the start and end of the road; even the two electrons of the helium atom doomed exact approaches like Gordon’s. It’s a limitation that physicists still grapple with today. Almost every quantum prediction is a little rough.

However, three years after Gordon’s triumph, his countryman Hans Bethe (pronounced “BAY-tah”) had found a striking way around this problem. Bethe’s “ansatz,” German for “starting point,” would turn out to perfectly capture the behavior of any number of quantum particles, from a single electron to the countless electrons in a sheet of ice. This extraordinary power comes with its own limitation, though, which would take decades to understand.

Bethe’s ansatz has captivated generations of researchers. Richard Feynman, the legendary theoretical physicist, was studying it when he died in the 1980s. Now few areas of physics remain untouched by Bethe’s nearly century-old insight.

“Its importance has continued growing to this day,” said Charlotte Kristjansen(opens a new tab), a professor at the Niels Bohr Institute in Copenhagen.

...read more at quantamagazine.org