0:00:06.865,0:00:09.437
Have you ever daydreamed[br]about traveling through time,

0:00:09.461,0:00:13.119
perhaps fast forward in the centuries[br]and seeing the distant future?

0:00:13.143,0:00:15.185
Well, time travel is possible,

0:00:15.209,0:00:17.639
and what's more, it's already been done.

0:00:17.663,0:00:18.934
Meet Sergei Krikalev,

0:00:18.958,0:00:21.726
the greatest time traveler[br]in human history.

0:00:21.750,0:00:23.762
This Russian cosmonaut holds the record

0:00:23.786,0:00:26.349
for the most amount of time[br]spent orbiting our planet,

0:00:26.373,0:00:30.061
a total of 803 days,[br]9 hours, and 39 minutes.

0:00:31.140,0:00:32.457
During his stay in space,

0:00:32.481,0:00:34.357
he time traveled into his own future

0:00:34.381,0:00:36.520
by 0.02 seconds.

0:00:37.925,0:00:40.435
Traveling at 17,500 miles an hour,

0:00:40.459,0:00:43.456
he experienced an effect[br]known as time dilation,

0:00:43.480,0:00:45.086
and one day the same effect

0:00:45.110,0:00:48.387
might make significant time travel[br]to the future commonplace.

0:00:49.575,0:00:52.853
To see why moving faster through space[br]affects passage of time,

0:00:52.877,0:00:55.008
we need to go back to the 1880s,

0:00:55.032,0:00:56.413
when two American scientists,

0:00:56.437,0:00:58.342
Albert Michelson and Edward Morley,

0:00:58.366,0:01:01.778
were trying to measure the effect[br]of the Earth's movement around the Sun

0:01:01.802,0:01:03.096
on the speed of light.

0:01:03.120,0:01:06.267
When a beam of light was moving[br]in the same direction as the Earth,

0:01:06.291,0:01:08.263
they expected the light to travel faster.

0:01:08.287,0:01:10.920
And when the Earth was moving[br]in the opposite direction,

0:01:10.944,0:01:12.871
they expected it to go slower.

0:01:12.895,0:01:14.856
But they found something very curious.

0:01:14.880,0:01:18.381
The speed of light remained the same[br]no matter what the Earth was doing.

0:01:18.405,0:01:20.644
Two decades later,[br]Albert Einstein was thinking

0:01:20.668,0:01:23.959
about the consequences[br]of that never-changing speed of light.

0:01:23.983,0:01:25.321
And it was his conclusions,

0:01:25.345,0:01:27.922
formulated in the theory[br]of special relativity,

0:01:27.946,0:01:30.764
that opened the door[br]into the world of time travel.

0:01:30.788,0:01:32.515
Imagine a man named Jack,

0:01:32.539,0:01:34.563
standing in the middle[br]of a train carriage,

0:01:34.587,0:01:36.232
traveling at a steady speed.

0:01:36.256,0:01:38.963
Jack's bored and starts bouncing[br]a ball up and down.

0:01:38.987,0:01:42.035
What would Jill, standing on the platform,[br]see through the window

0:01:42.059,0:01:43.605
as the train whistles through?

0:01:43.629,0:01:46.570
Well, between Jack dropping the ball[br]and catching it again,

0:01:46.594,0:01:49.653
Jill would have seen him move[br]slightly further down the track,

0:01:49.676,0:01:52.495
resulting in her seeing the ball[br]follow a triangular path.

0:01:53.407,0:01:56.184
This means Jill sees the ball[br]travel further than Jack does

0:01:56.208,0:01:58.079
in the same time period.

0:01:58.103,0:02:00.294
And because speed[br]is distance divided by time,

0:02:00.318,0:02:02.840
Jill actually sees the ball move faster.

0:02:03.830,0:02:06.712
But what if Jack's bouncing ball[br]is replaced with two mirrors

0:02:06.736,0:02:08.828
which bounce a beam of light between them?

0:02:08.852,0:02:11.085
Jack still sees the beam dropping down

0:02:11.109,0:02:14.286
and Jill still sees the light beam[br]travel a longer distance,

0:02:14.310,0:02:17.425
except this time Jack and Jill[br]cannot disagree on the speed

0:02:17.449,0:02:20.599
because the speed of light[br]remains the same no matter what.

0:02:21.535,0:02:24.464
And if the speed is the same[br]while the distance is different,

0:02:24.488,0:02:26.998
this means the time taken[br]will be different as well.

0:02:27.911,0:02:32.163
Thus, time must tick at different rates[br]for people moving relative to each other.

0:02:32.187,0:02:34.806
Imagine that Jack and Jill[br]have highly accurate watches

0:02:34.830,0:02:38.021
that they synchronize[br]before Jack boards the train.

0:02:38.045,0:02:40.426
During the experiment,[br]Jack and Jill would each see

0:02:40.450,0:02:42.070
their own watch ticking normally.

0:02:43.053,0:02:46.137
But if they meet up again later[br]to compare watches,

0:02:46.161,0:02:48.327
less time would have[br]elapsed on Jack's watch,

0:02:48.351,0:02:51.557
balancing the fact that Jill saw[br]the light move further.

0:02:52.394,0:02:54.051
This idea may sound crazy,

0:02:54.075,0:02:56.104
but like any good scientific theory,

0:02:56.128,0:02:57.128
it can be tested.

0:02:58.051,0:03:00.436
In the 1970s, scientists boarded a plane

0:03:00.460,0:03:02.521
with some super-accurate atomic clocks

0:03:02.545,0:03:05.316
that were synchronized[br]with some others left on the ground.

0:03:06.188,0:03:08.235
After the plane had[br]flown around the world,

0:03:08.259,0:03:10.260
the clocks on board[br]showed a different time

0:03:10.284,0:03:11.578
from those left behind.

0:03:12.522,0:03:14.727
Of course, at the speed[br]of trains and planes,

0:03:14.751,0:03:16.201
the effect is minuscule.

0:03:16.225,0:03:18.558
But the faster you go,[br]the more time dilates.

0:03:18.582,0:03:21.152
For astronauts orbiting[br]the Earth for 800 days,

0:03:21.176,0:03:22.736
it starts to add up.

0:03:22.760,0:03:25.543
But what affects humans[br]also affects machines.

0:03:25.567,0:03:27.718
Satellites of the global[br]positioning system

0:03:27.742,0:03:29.385
are also hurdling around the Earth

0:03:29.409,0:03:30.892
at thousands of miles an hour.

0:03:30.916,0:03:33.239
So, time dilation kicks in here, too.

0:03:34.291,0:03:36.979
In fact, their speed causes[br]the atomic clocks on board

0:03:37.003,0:03:38.765
to disagree with clocks on the ground

0:03:38.789,0:03:40.807
by seven millionths of a second daily.

0:03:40.831,0:03:42.266
Left uncorrected,

0:03:42.290,0:03:44.120
this would cause GPS to lose accuracy

0:03:44.144,0:03:45.991
by a few kilometers each day.

0:03:47.589,0:03:49.970
So, what does all this have[br]to do with time travel

0:03:49.994,0:03:51.761
to the far, distant future?

0:03:51.785,0:03:55.138
Well, the faster you go,[br]the greater the effect of time dilation.

0:03:55.162,0:03:56.686
If you could travel really close

0:03:56.710,0:04:00.769
to the speed of light, say 99.9999%,

0:04:00.793,0:04:02.174
on a round-trip through space

0:04:02.198,0:04:04.699
for what seemed to you like ten years,

0:04:04.723,0:04:06.152
you'd actually return to Earth

0:04:06.176,0:04:08.404
around the year 9000.

0:04:08.428,0:04:10.551
Who knows what you'd see[br]when you returned?!

0:04:10.575,0:04:12.222
Humanity merged with machines,

0:04:12.246,0:04:15.309
extinct due to climate change[br]or asteroid impact,

0:04:15.333,0:04:17.721
or inhabiting a permanent colony on Mars.

0:04:18.952,0:04:19.988
But the trouble is,

0:04:20.012,0:04:22.892
getting heavy things like people,[br]not to mention space ships,

0:04:22.916,0:04:26.604
up to such speeds requires[br]unimaginable amounts of energy.

0:04:26.628,0:04:29.346
It already takes enormous[br]particle accelerators

0:04:29.370,0:04:30.893
like the Large Hadron Collider

0:04:30.917,0:04:34.575
to accelerate tiny subatomic particles[br]to close to light speed.

0:04:35.483,0:04:39.489
But one day, if we can develop the tools[br]to accelerate ourselves to similar speeds,

0:04:39.513,0:04:41.466
then we may regularly send time travelers

0:04:41.490,0:04:42.529
into the future,

0:04:42.553,0:04:45.081
bringing with them tales[br]of a long, forgotten past.