0:00:07.038,0:00:08.008
Have you ever daydreamed

0:00:08.008,0:00:09.460
about traveling through time,

0:00:09.460,0:00:11.128
perhaps fast forward in the centuries

0:00:11.128,0:00:13.143
and seen the distant future?

0:00:13.143,0:00:15.209
Well, time travel is possible,

0:00:15.209,0:00:15.834
and what's more,

0:00:15.834,0:00:17.663
it's already been done.

0:00:17.663,0:00:18.958
Meet Serge Krikalev,

0:00:18.958,0:00:21.750
the greatest time traveler in human history.

0:00:21.750,0:00:23.547
This Russian cosmonaut holds the record

0:00:23.547,0:00:24.753
for the most amount of time

0:00:24.753,0:00:26.373
he's been orbiting our planet,

0:00:26.373,0:00:30.978
a total of 803 days, 9 hours, and 39 minutes.

0:00:30.978,0:00:32.133
During his stay in space,

0:00:32.133,0:00:34.381
he time traveled into his own future

0:00:34.381,0:00:37.504
by 0.02 seconds.

0:00:37.504,0:00:40.459
Travelling at 17,500 miles an hour,

0:00:40.459,0:00:41.713
he experienced an effect

0:00:41.713,0:00:43.480
known as time dilation,

0:00:43.480,0:00:44.963
and one day the same effect

0:00:44.963,0:00:46.568
might make significant time travel

0:00:46.568,0:00:48.596
to the future commonplace.

0:00:49.397,0:00:51.242
To see why moving faster through space

0:00:51.242,0:00:52.877
affects passage of time,

0:00:52.877,0:00:55.032
we need to go back to the 1880s,

0:00:55.032,0:00:56.324
when two American scientists,

0:00:56.324,0:00:58.461
Albert Michelson and Edward Morley,

0:00:58.461,0:00:59.672
were trying to measure the effect

0:00:59.672,0:01:01.376
of the Earth's movement around the Sun

0:01:01.376,0:01:03.165
on the speed of light.

0:01:03.165,0:01:04.294
When a beam of light was moving

0:01:04.294,0:01:05.830
in the same direction as the Earth,

0:01:05.830,0:01:08.253
they expected the light to travel faster.

0:01:08.253,0:01:10.590
And when the Earth was moving in the opposite direction,

0:01:10.590,0:01:12.895
they expected it to go slower.

0:01:12.895,0:01:14.880
But they found something very curious.

0:01:14.880,0:01:16.200
The speed of light remained the same

0:01:16.200,0:01:17.943
no matter what the Earth was doing.

0:01:18.405,0:01:20.542
Two decades later, Albert Einstein was thinking

0:01:20.542,0:01:21.784
about the consequences

0:01:21.784,0:01:23.983
of that never-changing speed of light.

0:01:23.983,0:01:25.345
And it was his conclusions,

0:01:25.345,0:01:27.946
formulated in the Theory of Special Relativity,

0:01:27.946,0:01:28.867
that opened the door

0:01:28.867,0:01:30.788
into the world of time travel.

0:01:30.788,0:01:32.539
Imagine a man named Jack,

0:01:32.539,0:01:34.090
standing in the middle of a train carriage,

0:01:34.090,0:01:35.758
traveling at a steady speed.

0:01:35.758,0:01:37.012
Jack's bored

0:01:37.012,0:01:38.987
and starts bouncing a ball up and down.

0:01:38.987,0:01:40.836
What would Jill, standing on the platform,

0:01:40.836,0:01:41.845
see through the window

0:01:41.845,0:01:43.629
as the train rustles through.

0:01:43.629,0:01:45.340
Well, between Jack dropping the ball

0:01:45.340,0:01:46.594
and catching it again,

0:01:46.594,0:01:47.762
Jill would have seen him move

0:01:47.762,0:01:49.677
slightly further down the track,

0:01:49.677,0:01:50.845
resulting in her seeing the ball

0:01:50.845,0:01:53.034
follow a triangular path.

0:01:53.034,0:01:54.543
This means Jill sees the ball

0:01:54.543,0:01:55.904
travel further than Jack does

0:01:55.904,0:01:58.103
in the same time period.

0:01:58.103,0:01:59.974
And because speed is distance divided by time,

0:01:59.974,0:02:02.677
Jill actually sees the ball move faster.

0:02:03.830,0:02:05.302
But what if Jack's bouncing ball

0:02:05.302,0:02:06.684
is replaced with two mirrors

0:02:06.684,0:02:08.852
which bounce a beam of light between them?

0:02:08.852,0:02:11.109
Jack still sees the beam dropping down

0:02:11.109,0:02:12.753
and Jill still sees the light beam

0:02:12.753,0:02:14.310
travel a longer distance,

0:02:14.310,0:02:15.991
except this time Jack and Jill

0:02:15.991,0:02:17.449
cannot disagree on the speed

0:02:17.449,0:02:18.366
because the speed of light

0:02:18.366,0:02:20.757
remains the same no matter what.

0:02:21.234,0:02:22.656
And if the speed is the same

0:02:22.656,0:02:24.205
while the distance is different,

0:02:24.205,0:02:27.694
this means the time taken will be different as well.

0:02:27.694,0:02:29.578
Thus, time must tick at different rates

0:02:29.578,0:02:31.633
for people moving relative to each other.

0:02:32.187,0:02:34.499
Imagine that Jack and Jill have highly accurate watches

0:02:34.499,0:02:37.888
that they synchronize before Jack boards the train.

0:02:37.888,0:02:40.112
During the experiment, Jack and Jill would each see

0:02:40.112,0:02:42.011
their own watch ticking normally.

0:02:42.811,0:02:44.774
But if they meet up again later

0:02:44.774,0:02:46.161
to compare watches,

0:02:46.161,0:02:48.351
less time would have elapsed on Jack's watch,

0:02:48.351,0:02:49.311
balancing the fact

0:02:49.311,0:02:51.779
that Jill saw the light move further.

0:02:52.394,0:02:54.075
This idea may sound crazy,

0:02:54.075,0:02:56.128
but like any good scientific theory,

0:02:56.128,0:02:57.614
it can be tested.

0:02:57.614,0:03:00.460
In the 1970s, scientists boarded a plane

0:03:00.460,0:03:02.545
with some super-accurate atomic clocks

0:03:02.545,0:03:03.499
that were synchronized

0:03:03.499,0:03:05.255
with some others left on the ground.

0:03:05.947,0:03:07.750
After the plane had flown around the world,

0:03:07.750,0:03:09.976
the clocks on board showed a different time

0:03:09.976,0:03:12.066
from those left behind.

0:03:12.066,0:03:14.046
Of course, at the speed of trains and planes,

0:03:14.046,0:03:16.055
the effect is minuscule.

0:03:16.055,0:03:16.990
But the faster you go,

0:03:16.990,0:03:18.582
the more time dilates.

0:03:18.582,0:03:21.176
For astronauts orbiting the Earth for 800 days,

0:03:21.176,0:03:22.760
it starts to add up.

0:03:22.760,0:03:25.567
But what affects humans also affects machines.

0:03:25.567,0:03:27.742
Satellites of the global positioning system

0:03:27.742,0:03:29.409
are also hurdling around the Earth

0:03:29.409,0:03:30.916
at thousands of miles an hour.

0:03:30.916,0:03:34.121
So, time dilation kicks in here too.

0:03:34.121,0:03:35.562
In fact, their speed causes

0:03:35.562,0:03:37.003
the atomic clocks on board

0:03:37.003,0:03:38.445
to disagree with clocks on the ground

0:03:38.445,0:03:40.831
by seven millionths of a second daily.

0:03:40.831,0:03:42.290
Left uncorrected,

0:03:42.290,0:03:44.144
this would cause GPS to lose accuracy

0:03:44.144,0:03:46.575
by a few kilometers each day.

0:03:47.589,0:03:49.690
So what does all this have to do with time travel

0:03:49.690,0:03:51.496
to the far, distant future?

0:03:51.496,0:03:52.993
Well, the faster you go,

0:03:52.993,0:03:55.162
the greater the effect of time dilation.

0:03:55.162,0:03:56.551
If you could travel really close

0:03:56.551,0:04:00.793
to the speed of light, say 99.9999%,

0:04:00.793,0:04:02.161
on a round-trip through space

0:04:02.161,0:04:04.723
for what seemed to you like ten years,

0:04:04.723,0:04:06.149
you'd actually return to Earth

0:04:06.149,0:04:08.428
around the year 9000.

0:04:08.428,0:04:10.575
Who knows what you'd see when you returned?!?

0:04:10.575,0:04:12.246
Humanity merged with machines,

0:04:12.246,0:04:13.978
extinct due to climate change

0:04:13.978,0:04:15.333
or asteroid impact,

0:04:15.333,0:04:18.170
or inhabiting a permanent colony on Mars.

0:04:18.831,0:04:19.430
But the trouble is,

0:04:19.430,0:04:20.953
getting heavy things like people,

0:04:20.953,0:04:22.407
not to mention space ships,

0:04:22.407,0:04:24.042
up to such speeds requires

0:04:24.042,0:04:26.783
unimaginable amounts of energy.

0:04:26.783,0:04:29.037
It already takes enormous particle accelerators

0:04:29.037,0:04:30.917
at the Large Hadron Collider

0:04:30.917,0:04:33.246
to accelerate tiny subatomic particles

0:04:33.246,0:04:35.161
to close to light speed.

0:04:35.161,0:04:37.299
But one day, if we can develop the tools

0:04:37.299,0:04:39.513
to accelerate ourselves to similar speeds,

0:04:39.513,0:04:41.348
then we may regularly send time travelers

0:04:41.348,0:04:42.351
into the future,

0:04:42.351,0:04:45.397
bringing with them tales of a long, forgotten past.