"It's the sun, stupid,' is probably how James Carville would summarize the message of Daniel G. Nocera, a chemist and professor of energy at the Massachusetts Institute of Technology, when he addressed a meeting on environmental issues that began in Aspen Wednesday night. The event is the first Aspen Environmental Forum, a conclave of scientists, policy experts, industry executives, environmental campaigners and communicators (I probably left out a category or two) organized by the Aspen Institute and National Geographic Magazine. A prime focus of Dr. Nocera's lab is unraveling photosynthesis to find ways to turn sunlight efficiently into chemical fuels (hydrogen, for example). "All scientists ultimately believe solar has to be the answer,' he said. On Thursday, he laid out his "big idea' as a formula: "If you take sunlight plus water, that equals oil plus coal plus methane.' The night before, he described what he said was an achievable energy future