DeLorean Tremens

An interesting idiot's guide to time travel at the Smithsonian:

Rubbish, you say? Well, unlike Doc Brown's second-generation DeLorean, which ran on garbage, the model for our time machine is actually testable. Place subatomic particles—pion or muons—on one side of the light cylinder, and a particle detector on the other side. Then send the particles across. Because these particles all live for the same amount of time—about a millionth of a second—they should all reach the detector at the same instant. Unless, of course, a time loop exists inside our light cylinder. As soon as this time machine is built, time travel will commence, and continue to exist until someone turns off the machine. Here's the catch: The time machine only allows someone to travel as far back as when the machine was first activated. Since no time traveler has shown up yet—check-out aisle tabloids notwithstanding—no such machine has yet been invented.

These are the boundaries of time travel. If the machine is left on forever, you can travel forward forever, but you can't go back before the machine was built.

It continues by claiming that anyone who stepped into such a machine would step out in an alternative universe of the present or past, which they could then alter. I'm not really sure the explanation's very good: it seems clear to me that time travel is different than alternate universe travel, and time travel presumes that you can only be a part of the past, not change it, because the past already takes into account your interferences. Still, I always find these laymen's introductions to the problem fascinating.

DeLorean Tremens [Smithsonian]