Central banks cannot encourage spending by lowering interest rates, because people would simply hold cash instead.
The belief that monetary policy under the ZLB was effective in promoting economy growth has been critiqued by Paul Krugman, Gauti Eggertsson, and Michael Woodford among others.
Milton Friedman, on the other hand, argued that a zero nominal interest rate presents no problem for monetary policy.
According to Friedman, a central bank can increase the monetary base even if the interest rate vanishes; it only needs to continue buying bonds.
Friedman used the example of a helicopter flying over a town dropping dollar bills from the sky, which households then gathered in perfectly equal shares.