How Time Travel Works

From millennium-skipping Victorians to phone booth-hopping teenagers, the termtime travel often summons our most fantastic visions of what it means to move through the fourth dimension. But of course you don't need a time machine or a fancy wormhole to jaunt through the years.

As you've probably noticed, we're all constantly engaged in the act of time travel. At its most basic level, time is the rate of change in the universe -- and like it or not, we are constantly undergoing change. We age, the planets move around the sun, and things fall apart.

We measure the passage of time in seconds, minutes, hours and years, but this doesn't mean time flows at a constant rate. Just as the water in a river rushes or slows depending on the size of the channel, time flows at different rates in different places. In other words, time is relative.

But what causes this fluctuation along our one-way trek from the cradle to the grave? It all comes down to the relationship between time and space. Human beings frolic about in the three spatial dimensions of length, width and depth. Time joins the party as that most crucial fourth dimension. Time can't exist without space, and space can't exist without time. The two exist as one: the space-time continuum. Any event that occurs in the universe has to involve both space and time.

In this article, we'll look at the real-life, everyday methods of time travel in our universe, as well as some of the more far-fetched methods of dancing through the fourth dimension.

Time Travel Into the Future

If you want to advance through the years a little faster than the next person, you'll need to exploit space-time. Global positioning satellites pull this off every day, accruing an extra third-of-a-billionth of a second daily. Time passes faster in orbit, because satellites are farther away from the mass of the Earth. Down here on the surface, the planet's mass drags on time and slows it down in small measures.

We call this effect gravitational time dilation. According to Einstein's theory of general relativity, gravity is a curve in space-time and astronomers regularly observe this phenomenon when they study light moving near a sufficiently massive object. Particularly large suns, for instance, can cause an otherwise straight beam of light to curve in what we call the gravitational lensing effect.

What does this have to do with time? Remember: Any event that occurs in the universe has to involve both space and time. Gravity doesn't just pull on space; it also pulls on time.  

You wouldn't be able to notice minute changes in the flow of time, but a sufficiently massive object would make a huge difference -- say, like the supermassive black hole Sagittarius A at the center of our galaxy. Here, the mass of 4 million suns exists as a single, infinitely dense point, known as a singularity [source:NASA]. Circle this black hole for a while (without falling in) and you'd experience time at half the Earth rate. In other words, you'd round out a five-year journey to discover an entire decade had passed on Earth [source: Davies].

Speed also plays a role in the rate at which we experience time. Time passes more slowly the closer you approach the unbreakable cosmic speed limit we call the speed of light. For instance, the hands of a clock in a speeding train move more slowly than those of a stationary clock. A human passenger wouldn't feel the difference, but at the end of the trip the speeding clock would be slowed by billionths of a second. If such a train could attain 99.999 percent of light speed, only one year would pass onboard for every 223 years back at the train station [source: Davies].

In effect, this hypothetical commuter would have traveled into the future. But what about the past? Could the fastest starship imaginable turn back the clock?

Black Holes and Kerr Rings

Circle a black hole long enough, and gravitational time dilation will take you into the future. But what would happen if you flew right into the maw of this cosmic titan? Most scientists agree the black hole would probably crush you, but one unique variety of black hole might not: theKerr black hole or Kerr ring.

In 1963, New Zealand mathematician Roy Kerr proposed the first realistic theory for a rotating black hole. The concept hinges on neutron stars, which are massive collapsed stars the size of Manhattan but with the mass of Earth's sun [source: Kaku]. Kerr postulated that if dying stars collapsed into a rotating ring of neutron stars, their centrifugal force would prevent them from turning into a singularity. Since the black hole wouldn't have a singularity, Kerr believed it would be safe to enter without fear of the infinite gravitational force at its center.

If Kerr black holes exist, scientists speculate that we might pass through them and exit through a white hole. Think of this as the exhaust end of a black hole. Instead of pulling everything into its gravitational force, the white hole would push everything out and away from it -- perhaps into another time or even another universe.

Kerr black holes are purely theoretical, but if they do exist they offer the adventurous time traveler a one-way trip into the past or future. And while a tremendously advanced civilization might develop a means of calibrating such a method of time travel, there's no telling where or when a "wild" Kerr black hole might leave you.

Wormholes

Theoretical Kerr black holes aren't the only possible cosmic shortcut to the past or future. As made popular by everything from "Star Trek: Deep Space Nine" to "Donnie Darko," there's also the equally theoretical Einstein-Rosen bridgeto consider. But of course you know this better as a wormhole.

Einstein's general theory of relativity allows for the existence of wormholes since it states that any mass curves space-time. To understand this curvature, think about two people holding a bedsheet up and stretching it tight. If one person were to place a baseball on the bedsheet, the weight of the baseball would roll to the middle of the sheet and cause the sheet to curve at that point. Now, if a marble were placed on the edge of the same bedsheet it would travel toward the baseball because of the curve.

In this simplified example, space is depicted as a two-dimensional plane rather than a four-dimensional one. Imagine that this sheet is folded over, leaving a space between the top and bottom. Placing the baseball on the top side will cause a curvature to form. If an equal mass were placed on the bottom part of the sheet at a point that corresponds with the location of the baseball on the top, the second mass would eventually meet with the baseball. This is similar to how wormholes might develop.

In space, masses that place pressure on different parts of the universe could combine eventually to create a kind of tunnel. This tunnel would, in theory, join two separate times and allow passage between them. Of course, it's also possible that some unforeseen physical or quantum property prevents such a wormhole from occurring. And even if they do exist, they may be incredibly unstable.

According to astrophysicist Stephen Hawking, wormholes may exist in quantum foam, the smallest environment in the universe. Here, tiny tunnels constantly blink in and out of existence, momentarily linking separate places and time like an ever-changing game of "Chutes and Ladders."

Wormholes such as these might prove too small and too brief for human time travel, but might we one day learn to capture, stabilize and enlarge them? Certainly, says Hawking, provided you're prepared for some feedback. If we were to artificially prolong the life of a tunnel through folded space-time, a radiation feedback loop might occur, destroying the time tunnel in the same way audio feedback can wreck a speaker.

Cosmic String

We've blown through black holes and wormholes, but there's yet another possible means of time traveling via theoretic cosmic phenomena. For this scheme, we turn to physicist J. Richard Gott, who introduced the idea of cosmic string back in 1991. As the name suggests, these are stringlike objects that some scientists believe were formed in the early universe.

These strings may weave throughout the entire universe, thinner than an atom and under immense pressure. Naturally, this means they'd pack quite a gravitational pull on anything that passes near them, enabling objects attached to a cosmic string to travel at incredible speeds and benefit from time dilation. By pulling two cosmic strings close together or stretching one string close to a black hole, it might be possible to warp space-time enough to create what's called a closed timelike curve.

Using the gravity produced by the two cosmic strings (or the string and black hole), a spaceship theoretically could propel itself into the past. To do this, it would loop around the cosmic strings.

Quantum strings are highly speculative, however. Gott himself said that in order to travel back in time even one year, it would take a loop of string that contained half the mass-energy of an entire galaxy. In other words, you'd have to split half the atoms in the galaxy to power your time machine. And, as with any time machine, you couldn't go back farther than the point at which the time machine was created.

Oh yes, and then there are the time paradoxes.