An old joke about nuclear fusion—that it is 30 years away and always will be—is so well-known that The Economist’s science editor forbids correspondents from repeating it. No one doubts sustained fusion is possible in principle. It powers every star in the universe. Making it work on Earth, though, has proved harder. Engineers have tried since the 1950s, so far without success. The latest and largest attempt—iter, a multinational test reactor in southern France—has been under construction for 11 years and is tens of billions of dollars over its initial, $6bn budget.
One day, one day….