One of the most popular questions from our "ask a physicist" feature was, "What happens if you're driving at the speed of light and you turn on your headlights?" The simple answer: You can't. So quit trying.
Noted time-travel expert Dave Goldberg, a physics professor at Drexel University, just saw Hot Tub Time Machine and was - disturbed. Here he explains the "egregious number of inviolable paradoxes" in the film, in an open letter to its creators.