The Physics of Time Travel

Scientists tell us it's technically possible. Here's a how-to guide for the ambitious tinkerer.

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.

If Einstein and Rosen are the architects of the space-time shortcut, then Kip Thorne of Caltech is its structural engineer. Starting from the rough sketch that Einstein and Rosen left behind, Thorne created an algorithm that describes in strict mathematical terms the physics of a working time machine. Of course, actually building Thorne's time portal would require a technological prowess that is at least many centuries away. But his work proves that time travel is possible-at least in theory.

Thorne's problem was finding a way to hold open the wormhole's channel, or throat, long enough for an explorer to pass through. Ordinary matter won't do: No matter how strong it is, any scaffolding made of matter cannot brace against the crush of space-time. Thorne needed a substance that could counteract the squeeze of a black hole. Thorne needed antigravity.

Instead of contracting the space around it, as ordinary matter does, antigravity-or negative energy, as it is sometimes called-pushes it apart. In theory, antigravity would be placed inside a wormhole's throat, opening it wide enough for an astronaut, or possibly even a spaceship, to pass through.
Antigravity does the trick; the problem is finding it. Einstein first postulated the existence of antigravity on cosmic scales in 1915, a conjecture proven correct eight decades later. But Einstein's antigravity is wispy and dilute, a spoonful of sugar dissolved in the Pacific Ocean. Opening a wormhole requires a regular torrent of antigravity.

The best current candidate for creating concentrated antigravity is called the Casimir effect. Because of the quirks of quantum mechanics, two flat metal plates held a hair's width apart generate a small amount of negative energy. That energy, multiplied many times over, could in principle be used to create a traversable wormhole. The widening, meanwhile, would dilute the strength of nearby gravity, preventing the traveler from being torn apart.

Once the antigravity scaffolding is holding open the portal, the traveler passing through would emerge in a distant place. But time travelers, of course, want to journey not just geographically but temporally. So Thorne's next step was to desynchronize the two regions on either side of the wormhole.

To do this, he applied an old trick of Einstein's. A major consequence of Einstein's Special Theory of Relativity is that time slows for objects that move quickly. Thorne applied this principle to one of the two black holes that make up a wormhole. Imagine lassoing one of the black holes-perhaps by trapping it inside a cage of negative energy-and towing it around the universe at close to the speed of light. That black hole, and therefore that end of the wormhole, would age more slowly than the stationary end of the wormhole. Over time, the black holes would become desynchronized, two objects connected through the wormhole but existing in different eras. An explorer who entered the stationary end of the wormhole would exit the moving end, many years earlier than when he departed, making the wormhole a true time portal.

Or Try It on a Shoestring

The most recent development in the physics of time travel came in 1991, when Princeton astrophysicist J. Richard Gott III suggested that hypothetical objects called cosmic strings might enable an astronaut to travel backward in time. Cosmic strings are long, thin objects that some cosmologists believe coalesced out of the universe's very earliest days. They are infinitely long, no wider than a single atom, and so dense that a few miles of a single cosmic string would outweigh Earth itself.

Gott's proposal relies on idealized versions of cosmic strings. In order to be em-ployed in the service of a time traveler, two cosmic strings, perfectly parallel and traveling at nearly the speed of light, must whiz past one another like two cars traveling in opposite directions on a highway. As the strings pass each other, space-time would become profoundly distorted by the influence of these fast-moving filaments. A savvy time traveler, waiting in a nearby spaceship, could exploit those distortions by flying around the coupled strings. If he timed it just right, the twists in space-time would enable him to return to his starting point before he began-making the voyage a one-way trip back in time. Which means that, according to the laws of physics, journeys through time are conceivable, if rather difficult to arrange. It may be only a matter of time.