Aired: September 16, 2008 on BBC 2 ....... Horizon asks some of the biggest names in science to have a quiet word with the new president, be it Obama or McCain, in the Presidents Guide to Science. The United States president is quite simply the most powerful man on earth, but they often know little about science. That's a problem when the decisions they make will affect every one of us, from nuclear proliferation to climate change. To help the new president get to grips with this intimidating responsibility some of the world's leading scientists, from Dawkins to Watson, share some crucial words of advice.