Underpinning much of modern technology, from smartphones to scanning tunneling microscopes to particle colliders, is Fermi’s Golden Rule. Named for 20th-century Italian American physicist Enrico Fermi (but actually discovered by British physicist Paul Dirac), the rule is a formula that connects what can be measured in an experiment—such as how fast atoms “jump” between energy states—to the microscopic properties of a quantum mechanical system. The formula is taught in every undergraduate quantum physics class.

Yet scientists sometimes misapply it. They either misjudge the conditions under which the formula works, or they miss the “window” for its use. A “user manual” for Fermi’s Golden Rule would be a boon to researchers, says Yale physicist Nir Navon—and now he and his lab partners have provided one.

“We put one of the most famous formulas in all of quantum mechanics to the test, and found where it works and where it fails, including ways that many physicists weren’t fully aware of,” said Navon, an associate professor of physics in Yale’s Faculty of Arts and Sciences and senior author of a new study published in the journal Nature Physics. “We’re telling everyone who uses it to take a breath first and check their ingredients.”