There's an old joke about a physicist, a chemist and an economist stranded on a desert island. A can of soup washes up on the beach. The physicist says, "Use a rock to smash it open." The chemist says, "Put it on the fire and let the heat burst it open." The economist says, "Assume a can opener."
In its forecasts of budget deficits, the Congressional Budget Office is assuming a can opener: Starting in 2015, it says, three straight years of 4 percent growth in real gross domestic product will get the U.S. out of its fiscal troubles (see chart).
