I just recalled an article I read in discovery magazine (IIRC) that listed 10 or 20 ways the world could end. I recall one scenario they explained where an exotic, and never before seen particle, synthesized in a particle accelerator could react negatively with normal particles and destroy the universe as we know it. The magazine gave this a fairly high likelihood of this happening compared to other scenarios. I find this extremely hard to believe. In the early universe (very, very, early) high energy particles were slamming into each other all of the time. Trillions upon trillions of times more high energy reactions between particles were occurring than occur during the operational lifetime of a particle accelerator. Furthermore, there were probably a richer diversity of high energy particle reactions then man has ever witnessed during the entire history of modern physics. Isn't it safe to say that every particle we are capable of producing in the next thousand years has already existed in the past with no adverse effect on the fabric of the universe?