For more than 70 years, fusion scientists have leaned on a single equation to tell them whether a plasma is hot enough, dense enough, and stable enough to ignite. It is called the Lawson criterion, and it has guided labs from Oxfordshire to Hefei. But there was always a catch: the equation could confirm when ignition was possible, not how to get there using the least energy. That gap has puzzled researchers since the 1950s.

Now a team at the Princeton Plasma Physics Laboratory thinks they have found a way around it. Luis Delgado-Aparicio likens the challenge to crossing a mountain range. Most fusion strategies try to go straight over the peak: compress the plasma first, then pour in a punishing amount of heat to force ignition. Delgado-Aparicio, working with colleagues Masayuki Ono and Jonathan Menard, built a more detailed version of the Lawson criterion, adding four overlooked variables.

Their calculations suggest a longer but lower route around the mountain: heat the plasma first, compress it second. The order, it turns out, matters enormously. Plasma, the charged, superheated fourth state of matter, is the medium every fusion reactor depends on. Inside it, atomic nuclei are pushed to fuse, releasing bursts of energy mimicking the sun's own furnace.