One of the most replayed commercials on television right now is the DirecTV ad with Doc Brown from Back to the Future. Doc, we learn, has forgotten to tell Marty McFly to buy DirecTV in the future. Never mind that the 1955 version of Doc never traveled through time, and therefore wouldn't know about DirecTV. More importantly, how's that whole time machine thing coming? When can we rev up the DeLorean and, like Marty, go to our parent's high school dance with our mother?
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Never. But not never, never. Just never for us. First, back to the basics.
A physical time machine—a device available at Wal-Mart, as opposed to a natural wormhole somewhere in the cosmos—is possible. You begin with something square. Next, install mirrors at the corners and send a beam of light, perhaps from a laser, at one of the mirrors. The light will bounce to the second mirror, the third, the fourth and back through this cycle forever.
The force of this constantly circulating light will begin twisting the empty space in the middle. Einstein's theory of relativity dictates that everything happening to space must happen to time, so time begins twisting, too.
To fit a human inside this time machine we need to stack a bunch of these mirrors on top of each other, and add more light beams. Eventually, we'll have a cylinder of circulating light. Once we step inside, we're ready to fly through time.
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.
So, we can't travel back to our mother's high school prom. But, putting incest matters aside, it's conceivable for some future Marty McFly to do so. In this scenario, even if Marty interrupted the meeting of his parents, he would continue to exist—the picture of his family that fades in the movie would remain intact in our new world.
Instead, as soon as he stepped out of the light cylinder, a parallel universe would begin. Marty can alter this new universe if he'd like—perhaps even take credit for "Johnny B. Goode" and become a famous musician. He can even travel in time within this parallel world. But once the new universe changes, he's unable to return to the original one. (In which case, I call dibs on his girl Jennifer.)