The Physics of Time Travel

by Courtesy of Dreamworks Dreamworks' remake of The Time Machine is the latest expression of our fascination with time travel. Courtesy of Dreamworks

Start with a Black Hole ...

The physical possibility of time traveL is something of a catch-22. Any object that's surrounded by the twisted space-time that time travel requires must by its very nature be fantastically perilous, a maelstrom that would inevitably tear apart the foolhardy traveler. So physicists have labored to create a theoretically acceptable time machine that's free from nasty side effects like certain death. Their starting point: black holes.

Black holes are famous for sucking in everything around them-including light-and never letting go. But black holes have other characteristics, namely the way they bend nearby space-time. A black hole is infinitely dense, which means that it pulls the fabric of space-time to the breaking point-creating a deep pockmark, complete with a tiny rip at the bottom.

Many have wondered what lies on the other side of this rip. In 1935, Einstein and his colleague Nathan Rosen developed a scenario in which the tiny rip in a black hole could be connected to another tiny rip in another black hole, joining two disparate parts of space-time via a narrow channel, or throat. The Einstein-Rosen bridge, as the notion was then called, looks like a black hole attached to a mirror image of itself.




This bridge-a sort of back door leading from the interior of one black hole into another-is today known as a wormhole. Such a portal could in theory create a shortcut through space-time-just the thing a time traveler would need if he wanted to cheat Father Time out of a few million years.




Next, Modify the Wormhole ...




The problem with wormholes is that the channel
created between two black holes is minuscule, smaller than the center of a single atom, and remains open for only a fraction of a second. Even light, the fastest entity in the universe, would not have enough time to pass through. And no matter how sturdy his spacecraft, our traveler would inevitably be ripped apart by the black hole's immense gravitational forces. Because of these and other problems, the Einstein-Rosen bridge was for many years thought of as a geometric curiosity, a theoretical quirk that could never be of use to even a fictional time traveler. Einstein's equations might allow for wormholes, but the universe certainly did not. All that changed in the 1980s, however, when a physicist at the California Institute of Technology devised a better way to use wormholes as time machines.