Federal Reserve Chairman Alan Greenspan said it was hard to say why long-term interest rates were so low, but that even if they moved below short rates it need not signal a weakening economy. Speaking to a bankers' conference in Beijing via satellite from Washington, Greenspan said "new forces" in international markets were likely behind the unusually low level of long-term rates around the globe. "Their nature and their behavior is not something we are going to fully understand, if ever; certainly except in retrospect," he said. Although the Fed has raised overnight borrowing costs by 2 percentage points since June 2004, taking the benchmark federal funds rate to 3 percent, long-term rates have fallen -- confounding policymakers.