1
00:00:06,865 --> 00:00:09,437
Have you ever daydreamed
about traveling through time,

2
00:00:09,461 --> 00:00:13,119
perhaps fast forward in the centuries
and seeing the distant future?

3
00:00:13,143 --> 00:00:15,185
Well, time travel is possible,

4
00:00:15,209 --> 00:00:17,639
and what's more, it's already been done.

5
00:00:17,663 --> 00:00:18,934
Meet Sergei Krikalev,

6
00:00:18,958 --> 00:00:21,726
the greatest time traveler
in human history.

7
00:00:21,750 --> 00:00:23,762
This Russian cosmonaut holds the record

8
00:00:23,786 --> 00:00:26,349
for the most amount of time
spent orbiting our planet,

9
00:00:26,373 --> 00:00:30,061
a total of 803 days,
9 hours, and 39 minutes.

10
00:00:31,140 --> 00:00:32,457
During his stay in space,

11
00:00:32,481 --> 00:00:34,357
he time traveled into his own future

12
00:00:34,381 --> 00:00:36,520
by 0.02 seconds.

13
00:00:37,925 --> 00:00:40,435
Traveling at 17,500 miles an hour,

14
00:00:40,459 --> 00:00:43,456
he experienced an effect
known as time dilation,

15
00:00:43,480 --> 00:00:45,086
and one day the same effect

16
00:00:45,110 --> 00:00:48,387
might make significant time travel
to the future commonplace.

17
00:00:49,575 --> 00:00:52,853
To see why moving faster through space
affects passage of time,

18
00:00:52,877 --> 00:00:55,008
we need to go back to the 1880s,

19
00:00:55,032 --> 00:00:56,413
when two American scientists,

20
00:00:56,437 --> 00:00:58,342
Albert Michelson and Edward Morley,

21
00:00:58,366 --> 00:01:01,778
were trying to measure the effect
of the Earth's movement around the Sun

22
00:01:01,802 --> 00:01:03,096
on the speed of light.

23
00:01:03,120 --> 00:01:06,267
When a beam of light was moving
in the same direction as the Earth,

24
00:01:06,291 --> 00:01:08,263
they expected the light to travel faster.

25
00:01:08,287 --> 00:01:10,920
And when the Earth was moving
in the opposite direction,

26
00:01:10,944 --> 00:01:12,871
they expected it to go slower.

27
00:01:12,895 --> 00:01:14,856
But they found something very curious.

28
00:01:14,880 --> 00:01:18,381
The speed of light remained the same
no matter what the Earth was doing.

29
00:01:18,405 --> 00:01:20,644
Two decades later,
Albert Einstein was thinking

30
00:01:20,668 --> 00:01:23,959
about the consequences
of that never-changing speed of light.

31
00:01:23,983 --> 00:01:25,321
And it was his conclusions,

32
00:01:25,345 --> 00:01:27,922
formulated in the theory
of special relativity,

33
00:01:27,946 --> 00:01:30,764
that opened the door
into the world of time travel.

34
00:01:30,788 --> 00:01:32,515
Imagine a man named Jack,

35
00:01:32,539 --> 00:01:34,563
standing in the middle
of a train carriage,

36
00:01:34,587 --> 00:01:36,232
traveling at a steady speed.

37
00:01:36,256 --> 00:01:38,963
Jack's bored and starts bouncing
a ball up and down.

38
00:01:38,987 --> 00:01:42,035
What would Jill, standing on the platform,
see through the window

39
00:01:42,059 --> 00:01:43,605
as the train whistles through?

40
00:01:43,629 --> 00:01:46,570
Well, between Jack dropping the ball
and catching it again,

41
00:01:46,594 --> 00:01:49,653
Jill would have seen him move
slightly further down the track,

42
00:01:49,676 --> 00:01:52,495
resulting in her seeing the ball
follow a triangular path.

43
00:01:53,407 --> 00:01:56,184
This means Jill sees the ball
travel further than Jack does

44
00:01:56,208 --> 00:01:58,079
in the same time period.

45
00:01:58,103 --> 00:02:00,294
And because speed
is distance divided by time,

46
00:02:00,318 --> 00:02:02,840
Jill actually sees the ball move faster.

47
00:02:03,830 --> 00:02:06,712
But what if Jack's bouncing ball
is replaced with two mirrors

48
00:02:06,736 --> 00:02:08,828
which bounce a beam of light between them?

49
00:02:08,852 --> 00:02:11,085
Jack still sees the beam dropping down

50
00:02:11,109 --> 00:02:14,286
and Jill still sees the light beam
travel a longer distance,

51
00:02:14,310 --> 00:02:17,425
except this time Jack and Jill
cannot disagree on the speed

52
00:02:17,449 --> 00:02:20,599
because the speed of light
remains the same no matter what.

53
00:02:21,535 --> 00:02:24,464
And if the speed is the same
while the distance is different,

54
00:02:24,488 --> 00:02:26,998
this means the time taken
will be different as well.

55
00:02:27,911 --> 00:02:32,163
Thus, time must tick at different rates
for people moving relative to each other.

56
00:02:32,187 --> 00:02:34,806
Imagine that Jack and Jill
have highly accurate watches

57
00:02:34,830 --> 00:02:38,021
that they synchronize
before Jack boards the train.

58
00:02:38,045 --> 00:02:40,426
During the experiment,
Jack and Jill would each see

59
00:02:40,450 --> 00:02:42,070
their own watch ticking normally.

60
00:02:43,053 --> 00:02:46,137
But if they meet up again later
to compare watches,

61
00:02:46,161 --> 00:02:48,327
less time would have
elapsed on Jack's watch,

62
00:02:48,351 --> 00:02:51,557
balancing the fact that Jill saw
the light move further.

63
00:02:52,394 --> 00:02:54,051
This idea may sound crazy,

64
00:02:54,075 --> 00:02:56,104
but like any good scientific theory,

65
00:02:56,128 --> 00:02:57,128
it can be tested.

66
00:02:58,051 --> 00:03:00,436
In the 1970s, scientists boarded a plane

67
00:03:00,460 --> 00:03:02,521
with some super-accurate atomic clocks

68
00:03:02,545 --> 00:03:05,316
that were synchronized
with some others left on the ground.

69
00:03:06,188 --> 00:03:08,235
After the plane had
flown around the world,

70
00:03:08,259 --> 00:03:10,260
the clocks on board
showed a different time

71
00:03:10,284 --> 00:03:11,578
from those left behind.

72
00:03:12,522 --> 00:03:14,727
Of course, at the speed
of trains and planes,

73
00:03:14,751 --> 00:03:16,201
the effect is minuscule.

74
00:03:16,225 --> 00:03:18,558
But the faster you go,
the more time dilates.

75
00:03:18,582 --> 00:03:21,152
For astronauts orbiting
the Earth for 800 days,

76
00:03:21,176 --> 00:03:22,736
it starts to add up.

77
00:03:22,760 --> 00:03:25,543
But what affects humans
also affects machines.

78
00:03:25,567 --> 00:03:27,718
Satellites of the global
positioning system

79
00:03:27,742 --> 00:03:29,385
are also hurdling around the Earth

80
00:03:29,409 --> 00:03:30,892
at thousands of miles an hour.

81
00:03:30,916 --> 00:03:33,239
So, time dilation kicks in here, too.

82
00:03:34,291 --> 00:03:36,979
In fact, their speed causes
the atomic clocks on board

83
00:03:37,003 --> 00:03:38,765
to disagree with clocks on the ground

84
00:03:38,789 --> 00:03:40,807
by seven millionths of a second daily.

85
00:03:40,831 --> 00:03:42,266
Left uncorrected,

86
00:03:42,290 --> 00:03:44,120
this would cause GPS to lose accuracy

87
00:03:44,144 --> 00:03:45,991
by a few kilometers each day.

88
00:03:47,589 --> 00:03:49,970
So, what does all this have
to do with time travel

89
00:03:49,994 --> 00:03:51,761
to the far, distant future?

90
00:03:51,785 --> 00:03:55,138
Well, the faster you go,
the greater the effect of time dilation.

91
00:03:55,162 --> 00:03:56,686
If you could travel really close

92
00:03:56,710 --> 00:04:00,769
to the speed of light, say 99.9999%,

93
00:04:00,793 --> 00:04:02,174
on a round-trip through space

94
00:04:02,198 --> 00:04:04,699
for what seemed to you like ten years,

95
00:04:04,723 --> 00:04:06,152
you'd actually return to Earth

96
00:04:06,176 --> 00:04:08,404
around the year 9000.

97
00:04:08,428 --> 00:04:10,551
Who knows what you'd see
when you returned?!

98
00:04:10,575 --> 00:04:12,222
Humanity merged with machines,

99
00:04:12,246 --> 00:04:15,309
extinct due to climate change
or asteroid impact,

100
00:04:15,333 --> 00:04:17,721
or inhabiting a permanent colony on Mars.

101
00:04:18,952 --> 00:04:19,988
But the trouble is,

102
00:04:20,012 --> 00:04:22,892
getting heavy things like people,
not to mention space ships,

103
00:04:22,916 --> 00:04:26,604
up to such speeds requires
unimaginable amounts of energy.

104
00:04:26,628 --> 00:04:29,346
It already takes enormous
particle accelerators

105
00:04:29,370 --> 00:04:30,893
like the Large Hadron Collider

106
00:04:30,917 --> 00:04:34,575
to accelerate tiny subatomic particles
to close to light speed.

107
00:04:35,483 --> 00:04:39,489
But one day, if we can develop the tools
to accelerate ourselves to similar speeds,

108
00:04:39,513 --> 00:04:41,466
then we may regularly send time travelers

109
00:04:41,490 --> 00:04:42,529
into the future,

110
00:04:42,553 --> 00:04:45,081
bringing with them tales
of a long, forgotten past.