The physics of 9/11 — including how fast and symmetrically one of the World Trade Center buildings fell — prove that official explanations of the collapses are wrong, says a Brigham Young University physics professor. In fact, it's likely that there were"pre-positioned explosives" in all three buildings at ground zero, says Steven E. Jones. In a paper posted online Tuesday and accepted for peer-reviewed publication next year, Jones adds his voice to those of previous skeptics, including the authors of the Web site www.wtc7.net, whose research Jones quotes. Jones' article can be found at www.physics.byu.edu/.