about traveling through time,

perhaps fast forward in the centuries

and see the distant future?

Well, time travel is possible,

and what's more,

it's already been done.

Meet Sergei Krikalev,

the greatest time traveler in human history.

This Russian cosmonaut holds the record

for the most amount of time

spent 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 0.02 seconds.

Traveling at 17,500 miles an hour,

he experienced an effect

known as time dilation,

and one day the same effect

might make significant time travel

to the future commonplace.

To see why moving faster through space

affects 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 effect

of the Earth's movement around the Sun

on the speed of light.

When a beam of light was moving

in the same direction as the Earth,

they expected the light to travel faster.

And when the Earth was moving in the opposite direction,

they expected it to go slower.

But they found something very curious.

The speed of light remained the same

no matter what the Earth was doing.

Two decades later, Albert Einstein was thinking

about the consequences

of that never-changing speed of light.

And it was his conclusions,

formulated in the theory of special relativity,

that opened the door

into the world of time travel.

Imagine a man named Jack,

standing in the middle of a train carriage,

traveling at a steady speed.

Jack's bored

and starts bouncing a 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 ball

and catching it again,

Jill would have seen him move

slightly further down the track,

resulting in her seeing the ball

follow a triangular path.

This means Jill sees the ball

travel further than Jack does

in the same time period.

And because speed is distance divided by time,

Jill actually sees the ball move faster.

But what if Jack's bouncing ball

is 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 beam

travel a longer distance,

except this time Jack and Jill

cannot disagree on the speed

because the speed of light

remains the same no matter what.

And if the speed is the same

while the distance is different,

this means the time taken will be different as well.

Thus, time must tick at different rates

for people moving relative to each other.

Imagine that Jack and Jill have highly accurate watches

that they synchronize before Jack boarded the train.

During the experiment, Jack and Jill would each see

their own watch ticking normally.

But if they meet up again later

to compare watches,

less time would have elapsed on Jack's watch,

balancing the fact

that Jill saw the 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 synchronized

with some others left on the ground.

After the plane had flown around the world,

the clocks on board showed a different time

from those left behind.

Of course, at the speed of trains and planes,

the effect is minuscule.

But the faster you go,

the more time dilates.

For astronauts orbiting the Earth for 800 days,

it starts to add up.

But what affects humans also affects machines.

Satellites of the global positioning system

are also hurdling around the Earth

at thousands of miles an hour.

So, time dilation kicks in here, too.

In fact, their speed causes

the 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 have to 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 see when you returned?!?

Humanity merged with machines,

extinct due to climate change

or 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 requires

unimaginable amounts of energy.

It already takes enormous particle accelerators

like the Large Hadron Collider

to accelerate tiny subatomic particles

to close to light speed.

But one day, if we can develop the tools

to accelerate ourselves to similar speeds,

then we may regularly send time travelers

into the future,

bringing with them tales of a long, forgotten past.