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.
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].
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 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.
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
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.
My brain hurts a little, but it was totally worth it.