evidence: Though the theory may sound far-fetched to some, chemtrails have become a common conspiracy both in the U.S. and around the world, despite evidence to the contrary. [...] The chemtrails idea has been around since 1996, and is largely rooted in an Air Force research paper from the same year, "Weather as a Force Multiplier: Owning the weather in 2025." It outlines a "future weather modification system to achieve military objectives" using "aerospace forces," and "does not reflect current military policy, practice, or capability," the Environmental Protection Agency has stated. [...] Scientists have said there is no evidence for the existence of chemtrails. Even if there was a government conspiracy at work in aircraft contrails, such a large-scale program would be difficult to cover up given the number of people who would be necessary for operation, Harvard researchers noted.