Physics professor writes open letter to Hot Tub Time Machine.

Great letter from a real professor. Click here to read the whole thing.

Dear Sirs:

As a noted time-travel expert, I looked forward to your new film, "Hot Tub Time Machine" with great anticipation. Of course, one expects a fair amount of artistic license in movies. For example, even in 1985, the most energy efficient flux capacitors could be powered at far less than 1.21 Gigawatts of electricity. It is also wholly understandable that you have ignored (or are perhaps ignorant of) the vast literature on time machine design and have therefore built yours around a hot tub, which has been shown to be unstable, rather than around the more conventional wormhole. Further, you exhibit an admirable attention to detail on many particulars. Following on the Terminator model, you correctly realize that time travel may only be undertaken while in the nude.

However, I cannot stand idly by as you subject your characters to a number of inviolable paradoxes. Considered from least to most egregious:

1. Predestination Paradoxes. In models of time travel with a single timeline, a traveler who has already experienced the past has no choice but to repeat his/her actions. It is troubling to me that you address predestination in matters of only the most trifling detail, such as whether John Cusack is destined to get a fork in the eye or the fate of Crispin Glover's arm, all while allowing gross details of history to be changed with impunity. Tinkering with history isn't a matter of "close enough." Once things are changed, however slightly, they're changed for good.

Your characters, showing better temporal intuition than you, yourselves, comment on this point, and warn of the "Butterfly Effect." A couple of observations are in order. First, the movie of that name was unspeakably terribly, and you do yourself and your characters a disservice to speak of it approvingly. Secondly, the changes wrought in the past would have so changed the future that any foreknowledge would quickly become worthless. If you don't make your fortune in the first couple of days, it's already too late. The alternate future you will have created will likely have everyone running around in goatees and building doomsday devices.