REUTERS/Andrew Kelly
It might sound like science fiction, but time travel is completely possible.
Well, at least time travel in one direction.
Back in the early 1900s, Einstein completely changed our understanding of the universe by proposing that we treat space and time as one thing: spacetime. Time just becomes an extra fourth dimension, and one we can move through.
So everything in the universe exists at both a place and a time. But it's impossible to pin down the exact position and time of an object in spacetime because everything in the universe is constantly in motion. We can only measure something from one frame of reference at a moment.
For example, if you're walking 3 miles per hour on a boat, you would clock your speed at just 3 miles per hour. But someone standing on the shore would clock your speed at 3 miles per hour plus whatever speed the boat is going.
We already know how to travel to the future
Time works the same way. The amount of time that passes between two events depends on your point of reference.
The closer something moves to the speed of light, the slower time passes for it. It's a phenomenon called time dilation, and that's why the clock on the International Space Station hurtling around Earth at 17,150 mph, ticks a little slower than clocks on Earth.
If you launched yourself off the Earth in an incredibly high-speed rocket, pulled a U-turn, and came back, you'd be returning to a future Earth. Time will have passed faster for people on Earth than it did for you in your rocket.
Like speed, gravity also messes with the passage of time (another type of time dilation). A huge gravitational field - from something like a black hole, for instance - is so powerful that it bends the fabric of spacetime around it.
If you were able to hang out in that warped spacetime near a black hole, and then return to your spaceship, time will have passed much faster for those on board the ship than it did for you. You're returning to the ship's future.
If you've seen the movie "Interstellar," that exact scenario unfolds. The characters return to the main ship after spending less than a day on a planet near a black hole - but back at the ship, years have passed:
So traveling forward in time is completely compatible with the laws of physics. And it's already technically possible, even though it's only on a fraction of a second scale.
So why can't we travel back in time?
Traveling to the past is a lot more complicated and would require a lot more energy than we're capable of harnessing.
Some physicists have proposed one could travel to the past through a spacetime tunnel called a wormhole.
Remember how massive objects can curve spacetime around them? It's theoretically possible to fold one of those curves back on itself, creating what's called a closed time-like curve (CTC). Time is on the y-axis and position is on the x-axis:
Tippet, Tsang
If you could somehow connect a bunch of those loops together, you could create a tunnel through spacetime that would allow you travel forward and backward in time. Some physicists have proposed this is how Doctor Who's time-traveling TARDIS works.
But in order to create a CTC, it appears that we'd need half of all the matter and energy in the universe, according to a paper published in the Physical Review Letters.
And even if we created a whole tunnel of CTCs, physicists say we'd need a ship that can travel faster than the speed of light to navigate it.
Still, scientists haven't given up hope. Some have devised crazy experiments to search for time travelers, including advertising for and then hosting a "time traveling convention" and hoping that some showed up. (None did.)
So for now, for those interested in revisiting the past, it looks like we're out of luck.