How Time Travel Works

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, due to time dilation 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. The human body 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?

We've established that time travel into the future happens all the time. Scientists have proven it in experiments, and the idea is a fundamental aspect of Einstein's theory of relativity. You'll make it to the future; it's just a question of how fast the trip will be. But what about travel into the past? A glance into the night sky should supply an answer.

The Milky Way galaxy is roughly 100,000 light-years wide, so light from its more distant stars can take thousands upon thousands of years to reach Earth. Glimpse that light, and you're essentially looking back in time. When astronomers measure the cosmic microwave background radiation, they stare back more than 10 billion years into a primordial cosmic age. But can we do better than this?

There's nothing in Einstein's theory that precludes time travel into the past, but the very premise of pushing a button and going back to yesterday violates the law of causality, or cause and effect. One event happens in our universe, and it leads to yet another in an endless one-way string of events. In every instance, the cause occurs before the effect. Just try to imagine a different reality, say, in which a murder victim dies of his or her gunshot wound before being shot. It violates reality as we know it; thus, many scientists dismiss time travel into the past as an impossibility.

Some scientists like Einstein's theory of special relativity, have proposed the idea of using faster-than-light travel to journey back in time. After all, if time slows as an object approaches the speed of light, then might exceeding that speed cause time to flow backward? Of course, as an object nears the speed of light, its relativistic mass increases until, at the speed of light, it becomes infinite. Accelerating an infinite mass any faster than that is impossible. Warp speed technology could theoretically cheat the universal speed limit by propelling a bubble of space-time across the universe, but even this would come with colossal, far-future energy costs.

But what if time travel into the past and future depends less on speculative space propulsion technology and more on existing cosmic phenomena? Set a course for the black hole.

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: the Kerr 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.

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 bridge to 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 travelers, 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.

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 time machines. 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.

As we mentioned before, the concept of traveling into the past becomes a bit murky the second causality rears its head. Cause comes before effect, at least in this universe, which manages to muck up even the best-laid time traveling plans.

For starters, if you traveled back in time 200 years, you'd emerge in a time before you were born. Think about that for a second. In the flow of time, the effect (you) would exist before the cause (your birth).

To better understand what we're dealing with here, consider the famous grandfather paradox. You're a time-traveling assassin, and your target just happens to be your own grandfather. So you pop through the nearest wormhole and walk up to a spry 18-year-old version of your father's father. You raise your laser blaster, but just what happens when you pull the trigger?

Think about it. You haven't been born yet. Neither has your father. If you kill your own grandfather in the past, he'll never have a son. That son will never have you, and you'll never happen to take that job as a time-traveling assassin. You wouldn't exist to pull the trigger, thus negating the entire string of events. We call this an inconsistent causal loop.

On the other hand, we have to consider the idea of a consistent causal loop. While equally thought-provoking, this theoretical model of time travel is paradox free. According to physicist Paul Davies, such a loop might play out like this: A math professor travels into the future and steals a groundbreaking math theorem. The professor then gives the theorem to a promising student. Then, that promising student grows up to be the very person from whom the professor stole the theorem to begin with.

Then there's the post-selected model of time travel, which involves distorted probability close to any paradoxical situation [source: Sanders]. What does this mean? Well, put yourself in the shoes of the time-traveling assassin again. This time travel model would make your grandfather virtually death proof. You can pull the trigger, but the laser will malfunction. Perhaps a bird will poop at just the right moment, but some quantum fluctuation will occur to prevent a paradoxical situation from taking place.

But then there's another possibility: The quantum theory that the future or past you travel into might just be a parallel universe. Think of it as a separate sandbox: You can build or destroy all the castles you want in it, but it doesn't affect your home sandbox in the slightest. So if the past you travel into exists in a separate timeline, killing your grandfather in cold blood is no big whoop. Of course, this might mean that every time jaunt would land you in a new parallel universe and you might never return to your original sandbox.

Confused yet? Welcome to the world of time travel.

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