It’s rather shocking that nearly a billion dollars can vanish like that, but such things happen when valuations are based, like Northvolt’s, on hazy guesses about how great things are going to be a few years from now. The U.S. stock market is packed with such optimistic assumptions. Excitement over the potential of artificial intelligence (AI) has driven valuations in the tech sector to staggering heights. Meanwhile, steady but unexciting performers such as Walmart Inc. WMT-N +0.67%increase and Home Depot Inc. HD-N +0.45%increase are trading at the type of lofty price-to-earnings multiples that used to be reserved for high-growth startups. Wall Street analysts don’t deny the frothiness. The Levkovitch Index of economic indicators compiled by Citigroup analysts