There is a strong case for an energy future that moves beyond highly polluting coal and extremely volatile nuclear power, and towards a marriage of efficiency/smart grid, renewables, and natural gas. But in a world of low-cost natural gas, currently at less than $2.50 per million BTU in the U.S. (it's closer to six times that amount, or $15, in Europe and Asia), how can solar, wind, and other clean energy sources compete? Does low-cost natural gas, which could last for years or decades in the U.S., mean the end of clean tech as we know it?