In recent years, some academic economists renowned for their work on the implications of rational expectations have argued that open market operations are irrelevant. These include Robert Lucas Jr., Thomas Sargent, Neil Wallace, Finn E. Kydland, Edward C. Prescott and Scott Freeman. Keynesian economists point to the ineffectiveness of open market operations in 2008 in the United States, when short-term interest rates went as low as they could go in nominal terms, so that no more monetary stimulus could occur. This zero bound problem has been called the liquidity trap or "pushing on a string" (the pusher being the central bank and the string being the real economy).