In his newly built palace near Tokyo, lined by stone statues of Roman emperors and surrounded by an 18-hole golf course, Masayoshi Son was stewing. After declaring for years the imminent arrival of the artificial-intelligence revolution,

Seattle-area company Helion Energy gets $425 million from investors including Sam Altman and Softbank to pursue clean energy's "Holy Grail."

Helion’s nuclear fusion promises have garnered the attention of Silicon Valley—and raised concerns amongst scientists wary of its aggressive timeline.