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How Time Travel Works

Time is malleable.
Time is malleable.
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Time Travel Into the Future

Real time travel in action
Real time travel in action
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Time Travel Into the Past

The stars above Flagstaff, Ariz., provide a backward view through time.
Dan and Cindy Duriscoe/FDSC

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 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.

Black Holes and Kerr Rings

What's on the other side of a black hole?
What's on the other side of a black hole?
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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.


Imagine space as a curved two-dimensional plane. Wormholes like this could form when two masses apply enough force on space-time to create a tunnel connecting distant points.

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.


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






Oh yes, and then there are the time paradoxes.

Time Travel Paradoxes

Bad news, time-traveling assassin: Grandpa is off-limits.
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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 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.

Explore the links on the next page for even more mind-blowing cosmology.

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More Great Links


  • Cleland, Andrew. Personal interview. April 2010.
  • Davies, Paul. "How to Build a Time Machine." Penguin. March 25, 2003.
  • Davies, Paul. Personal interview. April 2010.
  • Franknoi, Andrew. "Light as a Cosmic Time Machine." PBS: Seeing in the Dark. March 2008. (March 1, 2011)
  • Hawking, Stephen. "How to build a time machine." Mail Online. May 3, 2010. (March 1, 2011)
  • "Into the Universe with Stephen Hawking." Discovery Channel.
  • Kaku, Michio. "Parallel Worlds: A Journey Through Creation, Higher Dimensions, and the Future of the Cosmos." Anchor. Feb. 14, 2006.
  • "Kerr Black Holes and time travel." NASA. Dec. 8, 2008. (March 1, 2011)
  • Sanders, Laura. "Physicists Tame Time Travel by Forbidding You to Kill Your Grandfather." WIRED. July 20, 2010. (Mach 1, 2011)