Have you ever daydreamedabout traveling through time, perhaps fast forward in the centuriesand seeing the distant future? Well, time travel is possible, and what's more, it's already been done.
Meet Sergei Krikalev, the greatest time travelerin human history.
This Russian cosmonaut holds the record for the most amount of timespent orbiting our planet, a total of 803 days, 9 hours, and 39 minutes.
During his stay in space, he time traveled into his own future by 0.
Traveling at 17, 500 miles an hour, he experienced an effectknown as time dilation, and one day the same effect might make significant time travelto the future commonplace.
To see why moving faster through spaceaffects passage of time, we need to go back to the 1880s, when two American scientists, Albert Michelson and Edward Morley, were trying to measure the effectof the Earth's movement around the Sun on the speed of light.
When a beam of light was movingin the same direction as the Earth, they expected the light to travel faster.
And when the Earth was movingin the opposite direction, they expected it to go slower.
But they found something very curious.
The speed of light remained the sameno matter what the Earth was doing.
Two decades later, Albert Einstein was thinking about the consequencesof that never-changing speed of light.
And it was his conclusions, formulated in the theoryof special relativity, that opened the doorinto the world of time travel.
Imagine a man named Jack, standing in the middleof a train carriage, traveling at a steady speed.
Jack's bored and starts bouncinga ball up and down.
What would Jill, standing on the platform, see through the window as the train whistles through? Well, between Jack dropping the balland catching it again, Jill would have seen him moveslightly further down the track, resulting in her seeing the ballfollow a triangular path.
This means Jill sees the balltravel further than Jack does in the same time period.
And because speedis distance divided by time, Jill actually sees the ball move faster.
But what if Jack's bouncing ballis replaced with two mirrors which bounce a beam of light between them? Jack still sees the beam dropping down and Jill still sees the light beamtravel a longer distance, except this time Jack and Jillcannot disagree on the speed because the speed of lightremains the same no matter what.
And if the speed is the samewhile the distance is different, this means the time takenwill be different as well.
Thus, time must tick at different ratesfor people moving relative to each other.
Imagine that Jack and Jillhave highly accurate watches that they synchronizebefore Jack boards the train.
During the experiment, Jack and Jill would each see their own watch ticking normally.
But if they meet up again laterto compare watches, less time would haveelapsed on Jack's watch, balancing the fact that Jill sawthe light move further.
This idea may sound crazy, but like any good scientific theory, it can be tested.
In the 1970s, scientists boarded a plane with some super-accurate atomic clocks that were synchronizedwith some others left on the ground.
After the plane hadflown around the world, the clocks on boardshowed a different time from those left behind.
Of course, at the speedof trains and planes, the effect is minuscule.
But the faster you go, the more time dilates.
For astronauts orbitingthe Earth for 800 days, it starts to add up.
But what affects humansalso affects machines.
Satellites of the globalpositioning system are also hurdling around the Earth at thousands of miles an hour.
So, time dilation kicks in here, too.
In fact, their speed causesthe atomic clocks on board to disagree with clocks on the ground by seven millionths of a second daily.
Left uncorrected, this would cause GPS to lose accuracy by a few kilometers each day.
So, what does all this haveto do with time travel to the far, distant future? Well, the faster you go, the greater the effect of time dilation.
If you could travel really close to the speed of light, say 99.
9999%, on a round-trip through space for what seemed to you like ten years, you'd actually return to Earth around the year 9000.
Who knows what you'd seewhen you returned?! Humanity merged with machines, extinct due to climate changeor asteroid impact, or inhabiting a permanent colony on Mars.
But the trouble is, getting heavy things like people, not to mention space ships, up to such speeds requiresunimaginable amounts of energy.
It already takes enormousparticle accelerators like the Large Hadron Collider to accelerate tiny subatomic particlesto close to light speed.
But one day, if we can develop the toolsto accelerate ourselves to similar speeds, then we may regularly send time travelers into the future, bringing with them talesof a long, forgotten past.