TIME TRAVEL

From millennium-skipping Victorians to phone booth-hopping teenagers, the term time 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 holes 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. 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.

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

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?
Time Travel Into the Past

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 travels 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 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.