The Federal Reserve in the United States has been significantly more aggressive since December 2008, when the Fed reduced its benchmark short-term interest rate nearly to zero. Ever since, it has pursued a pair of experiments aimed at dragging other interest rates closer to zero, too.

The Fed has tried to bolster confidence that rates will stay low by talking more about the future. In December, it said it intended to keep short-term rates near zero at least as long as the unemployment rate remained above 6.5 percent, the first time it had tied policy to a specific target. That, and buying almost $3 trillion in Treasury and mortgage-backed securities, has helped to cut borrowing costs for businesses and consumers.

While the share of Americans with jobs has barely budged and other economic indicators remain weak at best, the Standard & Poor’s 500-stock index has doubled since the Fed announced its first round of bond purchases in November 2008. Interest rates on mortgages and car loans are near the lowest levels on record. Average yields on junk bonds fell below 5 percent for the first time. Corporations with strong credit ratings, like Apple, also are

borrowing vast sums at little cost.

Still, for all the daring, some critics argue that the Fed is not trying hard enough. “It’s as if we went to the biggest fire we’ve ever seen and we poured more water on it than we’ve ever poured, and the fire isn’t completely out,” said Joseph E. Gagnon, a former Fed economist now at the Peterson Institute for International Economics. “Well, we should try more water.”

Officials in Britain, too, are debating its central bank’s ability to do more.

Last month, the departing governor of the Bank of England, Mervyn King, gave a speech at the International Monetary Fund in which he said — a bit acidly — that there was a limit to what monetary policy could do to spur recovery in a country like Britain, where a small number of stingy banks dominate the economy and the government is tightening its spending.

Like other central banks around the world, the Bank of England, by far the oldest of them all, has done its part to ward off a depression. It has bought, to date, the equivalent of $569 billion worth of government bonds — a bold use of the printing press for an institution known for its hidebound ways.

This shock treatment, the professorial Mr. King pointed out, equaled 20 percent of the British economy, outpacing the central bank interventions of the European Central Bank, the Bank of Japan and the Federal Reserve.