Worries are rising that the White House and other authorities around the world can’t or won’t do what’s needed to help soon. The S&P 500 dropped about 7% within the first few minutes of Thursday’s trading, steep enough to trigger an automatic halt to trading on the New York Stock Exchange. European stocks tumbled 8%, even after the European Central Bank pledged to buy more bonds and offer more help for the economy. In Thailand and the Philippines, stocks fell so fast that trading was temporarily halted. Treasury yields, which were one of the first markets to sound the alarm on the economic risks of the virus, fell in morning trading.