HOW MUCH money should companies lavish on artificial intelligence? American tech giants think the answer is: a lot. Their capital spending on data centres may surpass $740bn this year, according to Bloomberg Intelligence, a research firm. Investors, who used to agree, are having second thoughts. When on July 26th it emerged that Nvidia was in talks to underwrite $250bn of a gargantuan $500bn-plus data centre, stuffed full of its chips and run by OpenAI, a leading model-maker, its share price tumbled. Days earlier investors blanched when Alphabet, Google’s parent company, said it would boost its AI spending to $205bn this…