North Korea has just tested a nuclear bomb. But it has tested five nuclear bombs before. It has demonstrated that it is an emerging nuclear-weapons power, but then again we already knew that. It has tested America’s largely untested president with the stuff of nightmares, but Donald Trump has already confronted several North Korean ballistic-missile tests—including one over Japanese territory and two involving missiles thought capable of reaching the United States. So why is North Korea’s latest nuclear test significant?
A nuclear test isn’t only an advertisement of military advances. It isn’t only a means of intimidating enemies and impressing allies. It is also—and primarily—a test in the scientific sense of the term: You don’t waste limited and valuable fissile material on a mere political provocation.
The North Koreans are “not just testing to say something—there’s a reason for testing,” Joel Wit, an expert on North Korea’s nuclear program, told me. “Any country that builds nuclear weapons proceeds down predictable technological paths.” With every trial of a weapon, “they’re trying to learn something new.”
In this case, North Korea may be experimenting with how to increase “yields,” or the amount of energy released when a nuclear weapon is detonated. Kim Jong Un’s government said Sunday’s test involved a hydrogen bomb, a two-stage device that combines nuclear fission and fusion reactions to generate much higher yields than atomic weapons, which rely only on nuclear fission. (The Kim regime has claimed to possess a hydrogen bomb since late 2015, though these claims are difficult to verify and had previously been met with skepticism. Hydrogen bombs have never been used in warfare.)