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We are drifting in
a great ocean of space and time.
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In that ocean, the events
that shape the future...
3
00:01:09,310 --> 00:01:11,710
...are working themselves out.
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00:01:13,915 --> 00:01:17,248
Each creature and every world,
to the remotest star...
5
00:01:17,451 --> 00:01:19,351
...owe their existence to...
6
00:01:19,554 --> 00:01:22,387
...the great, coursing,
implacable forces of nature...
7
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...but also, to minor happenstance.
8
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We are carried with our planet
around the sun.
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The Earth has made more than
4 billion circuits of our star...
10
00:01:35,369 --> 00:01:37,200
...since its origin.
11
00:01:39,240 --> 00:01:43,142
The sun itself travels about
the core of the Milky Way galaxy.
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00:01:43,344 --> 00:01:45,972
Our galaxy is moving
among the other galaxies.
13
00:01:46,180 --> 00:01:49,547
We have always been space travelers.
14
00:01:51,352 --> 00:01:56,221
These fine sand grains are all,
more or less, uniform in size.
15
00:01:57,124 --> 00:02:00,355
They're produced from bigger rocks
through ages of...
16
00:02:00,561 --> 00:02:04,395
...jostling and rubbing,
abrasion and erosion.
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00:02:04,599 --> 00:02:08,467
Driven in part by
the distant moon and sun.
18
00:02:08,803 --> 00:02:12,364
So the roots of the present
lie buried in the past.
19
00:02:12,573 --> 00:02:16,339
We are also travelers in time.
20
00:02:20,815 --> 00:02:22,043
But trapped on Earth...
21
00:02:22,249 --> 00:02:25,582
...we've had little to say about
where we go in time and space...
22
00:02:25,786 --> 00:02:27,151
...or how fast.
23
00:02:27,355 --> 00:02:31,086
But now we're thinking
about true journeys in time...
24
00:02:31,292 --> 00:02:35,194
...and real voyages
to the distant stars.
25
00:02:36,964 --> 00:02:41,560
A handful of sand contains
about 10,000 grains...
26
00:02:41,769 --> 00:02:43,964
...more than all the stars
we can see...
27
00:02:44,171 --> 00:02:46,503
...with the naked eye
on a clear night.
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00:02:46,707 --> 00:02:48,800
But the number of stars we can see...
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00:02:49,010 --> 00:02:52,605
...is only the tiniest fraction
of the number of stars that are.
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00:02:53,414 --> 00:02:56,440
What we see at night
is the merest smattering...
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00:02:56,651 --> 00:02:58,619
...of the nearest stars...
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00:02:58,819 --> 00:03:03,085
...with a few more distant bright
stars thrown in for good measure.
33
00:03:03,290 --> 00:03:07,158
Meanwhile, the cosmos
is rich beyond measure.
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00:03:07,361 --> 00:03:09,488
The number of stars
in the universe...
35
00:03:09,697 --> 00:03:13,030
...is larger than all the grains
of sand on all the beaches...
36
00:03:13,234 --> 00:03:14,826
...of the planet Earth.
37
00:03:18,706 --> 00:03:23,507
Long ago, before we had figured out
that the stars are distant suns...
38
00:03:23,711 --> 00:03:26,839
...they seemed to us
to make pictures in the sky.
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00:03:27,048 --> 00:03:29,846
Just follow the dots.
40
00:03:31,152 --> 00:03:34,315
The Big Dipper constellation
today in North America...
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00:03:34,522 --> 00:03:36,683
...has had many other incarnations.
42
00:03:36,891 --> 00:03:39,155
Every culture, ancient and modern...
43
00:03:39,427 --> 00:03:43,193
...has placed its totems
and concerns among the stars.
44
00:03:43,397 --> 00:03:47,390
From a Chinese bureaucrat
to a German wagon.
45
00:03:51,439 --> 00:03:55,239
But very ancient cultures would have
seen different constellations...
46
00:03:55,443 --> 00:03:58,742
...because the stars move
with respect to one another.
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00:03:58,946 --> 00:04:03,713
We can give a computer the present
positions and motions of stars...
48
00:04:03,918 --> 00:04:07,615
...and then run the patterns
back into time.
49
00:04:09,623 --> 00:04:13,320
Every constellation is a single frame
in a cosmic movie...
50
00:04:13,527 --> 00:04:15,893
...but because our lives
are so short...
51
00:04:16,097 --> 00:04:18,088
...because star patterns
change slowly...
52
00:04:18,299 --> 00:04:21,234
...we tend not to notice
it's a movie.
53
00:04:21,435 --> 00:04:25,235
A million years ago,
there was no Big Dipper.
54
00:04:25,639 --> 00:04:28,938
Our ancestors, looking up
and wondering about the stars...
55
00:04:29,143 --> 00:04:32,772
...saw some other pattern
in the northern skies.
56
00:04:33,814 --> 00:04:38,183
We can also run a constellation,
Leo the Lion, say, forward in time...
57
00:04:38,385 --> 00:04:42,287
...and see what the patterns
in the stars will be in the future.
58
00:04:44,058 --> 00:04:46,458
A million years from now,
Leo might be renamed...
59
00:04:46,660 --> 00:04:49,356
...the constellation
of the Radio Telescope.
60
00:04:49,563 --> 00:04:52,862
Although I suspect radio telescopes
then will be as obsolete...
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00:04:53,067 --> 00:04:54,967
...as stone spears are now.
62
00:04:55,169 --> 00:04:59,196
Or, here's the constellation
of Cetus the Whale.
63
00:05:08,215 --> 00:05:12,948
A million years ago, it may have
been called something else.
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00:05:13,154 --> 00:05:14,644
Perhaps the Spear.
65
00:05:19,760 --> 00:05:24,288
Now, let's run fast-forward
through a billion nights.
66
00:05:29,436 --> 00:05:31,063
Millions of years from now...
67
00:05:31,272 --> 00:05:35,470
...some other very different image
will be featured in this cosmic movie.
68
00:05:43,417 --> 00:05:46,443
In Orion the Hunter,
things are changing...
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00:05:46,654 --> 00:05:48,588
...not only because
the stars are moving...
70
00:05:48,789 --> 00:05:51,553
...but also because
the stars are evolving.
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00:05:51,759 --> 00:05:55,320
Many of Orion's stars are
hot, young and short-lived.
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00:05:55,529 --> 00:05:59,590
They're born, live and die within
a span of only a few million years.
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00:05:59,800 --> 00:06:02,633
If we run Orion forward in time...
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00:06:02,837 --> 00:06:05,670
...we see the births
and explosive deaths...
75
00:06:05,873 --> 00:06:07,397
...of dozens of stars...
76
00:06:07,608 --> 00:06:11,738
...flashing on and winking off
like fireflies in the night.
77
00:06:14,181 --> 00:06:17,708
If we wait long enough,
we see the constellations change.
78
00:06:17,918 --> 00:06:21,854
But if we go far enough,
we also see the star patterns alter.
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00:06:22,056 --> 00:06:23,921
Two-dimensional constellations...
80
00:06:24,124 --> 00:06:28,117
...are only the appearance of stars
strewn through three dimensions.
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00:06:28,329 --> 00:06:32,629
Some are dim and near,
others are bright but farther away.
82
00:06:33,868 --> 00:06:36,200
Could a space traveler
actually see...
83
00:06:36,403 --> 00:06:39,099
...the patterns of
the constellations change?
84
00:06:39,306 --> 00:06:44,175
For that, you must travel roughly as
far as the constellation is from us.
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00:06:44,645 --> 00:06:47,671
Here, we're traveling
hundreds of light-years...
86
00:06:47,882 --> 00:06:52,182
...circling all the way around
the stars of the Big Dipper.
87
00:06:55,723 --> 00:06:57,850
Inhabitants of planets
around other stars...
88
00:06:58,058 --> 00:07:00,492
...will see different constellations
than us...
89
00:07:00,694 --> 00:07:03,663
...because their vantage points
are different.
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00:07:11,739 --> 00:07:15,402
Here we are
in the constellation Andromeda...
91
00:07:15,609 --> 00:07:20,410
...or at least a model of it
next to the constellation Perseus.
92
00:07:20,614 --> 00:07:22,775
Andromeda, in the Greek myth...
93
00:07:22,983 --> 00:07:26,783
...was the maiden
who was saved by Perseus...
94
00:07:26,987 --> 00:07:28,921
...from a sea monster.
95
00:07:29,123 --> 00:07:34,026
This star just above me
is Beta Andromedae...
96
00:07:34,228 --> 00:07:36,856
...the second brightest star
in the constellation...
97
00:07:37,064 --> 00:07:40,227
...75 light-years from the Earth.
98
00:07:40,434 --> 00:07:43,528
The light by which we see this star...
99
00:07:43,737 --> 00:07:47,867
...has spent 75 years
traversing interstellar space...
100
00:07:48,075 --> 00:07:50,066
...on its journey to the Earth.
101
00:07:50,277 --> 00:07:53,872
In the unlikely event
that Beta Andromedae...
102
00:07:54,081 --> 00:07:56,606
...blew itself up
a week ago Tuesday...
103
00:07:56,817 --> 00:07:59,581
...we will not know of it
for another 75 years...
104
00:07:59,787 --> 00:08:03,780
...as this interesting information,
traveling at the speed of light...
105
00:08:03,991 --> 00:08:07,620
...crosses the enormous
interstellar distances.
106
00:08:07,828 --> 00:08:10,319
When the light we see
from this star set out...
107
00:08:10,531 --> 00:08:13,261
...on its long
interstellar voyage...
108
00:08:13,467 --> 00:08:16,027
...the young Albert Einstein...
109
00:08:16,236 --> 00:08:19,103
...working as a Swiss patent clerk...
110
00:08:19,306 --> 00:08:23,208
...had just published his epochal
special theory of relativity...
111
00:08:23,410 --> 00:08:24,741
...here on Earth.
112
00:08:25,612 --> 00:08:26,840
We see...
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00:08:27,047 --> 00:08:31,177
...that space and time
are intertwined.
114
00:08:31,385 --> 00:08:34,252
We cannot look out into space...
115
00:08:34,455 --> 00:08:37,754
...without looking back into time.
116
00:08:38,025 --> 00:08:41,119
The speed of light is very fast...
117
00:08:41,328 --> 00:08:44,525
...but space is very empty...
118
00:08:44,732 --> 00:08:48,031
...and the stars are very far apart.
119
00:08:48,235 --> 00:08:51,363
The distances that we've been
talking about up to now...
120
00:08:51,572 --> 00:08:55,372
...are very small
by the usual astronomical standards.
121
00:08:55,576 --> 00:08:58,204
In fact, the distance
from the Earth...
122
00:08:58,412 --> 00:09:00,403
...to the center
of the Milky Way galaxy...
123
00:09:00,614 --> 00:09:03,481
...is 30,000 light-years.
124
00:09:04,451 --> 00:09:09,388
From our galaxy to the nearest
spiral galaxy like our own...
125
00:09:09,723 --> 00:09:11,520
...called M31...
126
00:09:11,725 --> 00:09:14,626
...and which is also within,
that means behind...
127
00:09:14,828 --> 00:09:16,796
...the constellation Andromeda...
128
00:09:17,531 --> 00:09:21,262
...is 2 million light-years.
129
00:09:22,503 --> 00:09:25,700
When the light we see today
from M31...
130
00:09:25,906 --> 00:09:28,431
...left on its journey for Earth...
131
00:09:28,642 --> 00:09:30,576
...there were no human beings...
132
00:09:30,778 --> 00:09:34,077
...although our ancestors
were nicely evolving...
133
00:09:34,281 --> 00:09:37,341
...and very rapidly,
to our present form.
134
00:09:38,018 --> 00:09:40,509
There are much greater distances
in astronomy.
135
00:09:40,721 --> 00:09:44,316
The distance from the Earth
to the most distant quasars...
136
00:09:44,525 --> 00:09:47,961
...is 8 or 10 billion light-years.
137
00:09:48,162 --> 00:09:52,861
We see them as they were before
the Earth itself accumulated...
138
00:09:53,067 --> 00:09:56,628
...before the Milky Way galaxy
was formed.
139
00:09:56,837 --> 00:10:00,466
The fastest space vehicles ever
launched by the human species...
140
00:10:00,674 --> 00:10:02,608
...are the Voyager spacecraft.
141
00:10:02,810 --> 00:10:04,505
They are traveling so fast...
142
00:10:04,711 --> 00:10:08,306
...that it's only
10,000 times slower...
143
00:10:08,816 --> 00:10:10,113
...than the speed of light.
144
00:10:10,317 --> 00:10:12,945
The Voyager spacecraft
will take 40,000 years...
145
00:10:13,153 --> 00:10:15,212
...to go the distance
to the nearest stars...
146
00:10:15,422 --> 00:10:18,619
...and they're not even headed
towards the nearest stars.
147
00:10:18,826 --> 00:10:21,420
But is there a method
by which we could travel...
148
00:10:21,628 --> 00:10:25,120
...in a conveniently short time
to the stars?
149
00:10:25,332 --> 00:10:28,165
Can we travel close
to the speed of light?
150
00:10:28,368 --> 00:10:31,098
And what's magic
about the speed of light?
151
00:10:31,305 --> 00:10:33,899
Can't we travel faster than that?
152
00:10:35,909 --> 00:10:39,743
It turns out that
there is something very strange...
153
00:10:39,947 --> 00:10:41,414
...about the speed of light.
154
00:10:41,615 --> 00:10:43,674
Something that provides the key...
155
00:10:43,884 --> 00:10:47,285
...to our understanding
of time and space.
156
00:10:49,022 --> 00:10:50,717
The story of its discovery...
157
00:10:50,924 --> 00:10:54,519
...takes us to Tuscany
in northern Italy.
158
00:10:56,830 --> 00:10:59,264
There's something timeless
about this place.
159
00:10:59,466 --> 00:11:03,129
A century ago, it probably
looked very much the same.
160
00:11:15,249 --> 00:11:19,447
If you had traveled these roads
in the summer of 1895...
161
00:11:19,653 --> 00:11:23,783
...you might have come upon a
16-year-old German high-school dropout.
162
00:11:23,991 --> 00:11:27,154
His teacher told him that
he'd never amount to anything...
163
00:11:27,361 --> 00:11:30,819
...that his attitude destroyed
classroom discipline...
164
00:11:31,031 --> 00:11:32,760
...that he should drop out.
165
00:11:32,966 --> 00:11:34,957
So he left and came here...
166
00:11:35,169 --> 00:11:37,603
...where he enjoyed
wandering these roads...
167
00:11:37,804 --> 00:11:40,272
...and giving his mind
free rein to explore.
168
00:11:42,276 --> 00:11:44,744
One day, he began
to think about light...
169
00:11:44,945 --> 00:11:46,936
...about how fast it travels.
170
00:11:47,147 --> 00:11:50,207
We always measure
the speed of a moving object...
171
00:11:50,417 --> 00:11:52,385
...relative to something else.
172
00:11:52,586 --> 00:11:56,613
I'm moving at about 10 kilometers
an hour relative to the ground.
173
00:11:56,823 --> 00:11:58,450
But the ground isn't at rest.
174
00:11:58,659 --> 00:12:02,425
The Earth is turning at more
than 1600 kilometers an hour.
175
00:12:02,629 --> 00:12:05,097
The Earth itself is
in orbit around the sun.
176
00:12:05,299 --> 00:12:09,360
The sun is moving among
the drifting stars, and so on.
177
00:12:09,570 --> 00:12:12,971
It was hard for the young man
to imagine some absolute standard...
178
00:12:13,173 --> 00:12:16,040
...to measure all these
relative motions against.
179
00:12:25,852 --> 00:12:29,720
He knew that sound waves are
a vibration of the air...
180
00:12:29,923 --> 00:12:33,086
...and their speed is measured
relative to the air itself.
181
00:12:33,293 --> 00:12:36,490
But sunlight travels across
the vacuum of empty space.
182
00:12:36,697 --> 00:12:38,961
"Do light waves move
relative to something else?
183
00:12:39,166 --> 00:12:42,761
And if so," he wondered,
"relative to what?"
184
00:12:46,306 --> 00:12:50,436
That teenage dropout's name...
185
00:12:51,111 --> 00:12:52,772
...was Albert Einstein.
186
00:12:52,980 --> 00:12:55,915
And his ruminations changed the world.
187
00:13:01,255 --> 00:13:05,385
He had been fascinated
by Bernstein's 1869...
188
00:13:06,260 --> 00:13:09,855
...People's Book of Natural Science.
189
00:13:10,063 --> 00:13:13,226
Here, on its very first page...
190
00:13:14,101 --> 00:13:18,231
...it describes the astonishing speed
of electricity through wires...
191
00:13:18,572 --> 00:13:20,506
...and light through space.
192
00:13:20,841 --> 00:13:24,777
Einstein wondered, perhaps for
the first time, in northern Italy...
193
00:13:25,545 --> 00:13:29,743
...what the world would look like if
you could travel on a wave of light.
194
00:13:30,317 --> 00:13:32,512
To travel at the speed of light.
195
00:13:32,719 --> 00:13:37,520
What an engaging and magical thought
for a teenage boy on the road...
196
00:13:37,724 --> 00:13:41,592
...where the countryside is dappled
and rippling in sunlight.
197
00:13:52,606 --> 00:13:57,009
You couldn't tell you were on a light
wave if you were traveling with it.
198
00:13:57,210 --> 00:13:59,838
If you started on a wave crest...
199
00:14:00,047 --> 00:14:04,848
...you would stay on the crest and
lose all notion of it being a wave.
200
00:14:05,052 --> 00:14:09,887
Something funny happens
at the speed of light.
201
00:14:40,487 --> 00:14:44,253
The more Einstein thought about it,
the more troubling it became.
202
00:14:44,458 --> 00:14:47,018
Paradoxes seemed to pop up all over...
203
00:14:47,227 --> 00:14:49,195
...if you could travel
at the speed of light.
204
00:14:49,396 --> 00:14:52,797
Certain ideas had been
accepted as true...
205
00:14:52,999 --> 00:14:55,467
...without sufficiently
careful thought.
206
00:14:57,604 --> 00:15:01,631
One of those ideas had to do
with the light from a moving object.
207
00:15:03,110 --> 00:15:06,341
The images by which we see the world
are made of light...
208
00:15:06,546 --> 00:15:08,673
...and are carried
at the speed of light...
209
00:15:08,882 --> 00:15:12,147
...300,000 kilometers a second.
210
00:15:12,753 --> 00:15:16,553
You might think that the image of me
should be moving out ahead of me...
211
00:15:16,757 --> 00:15:19,988
...at the speed of light
plus the speed of the bicycle.
212
00:15:20,193 --> 00:15:23,720
If I'm moving towards you
faster than a horse-and-cart...
213
00:15:23,930 --> 00:15:26,899
...then my image should be
approaching you that much faster.
214
00:15:27,100 --> 00:15:29,694
My image ought to arrive earlier.
215
00:15:31,838 --> 00:15:34,363
But in reality
you don't see any time delay.
216
00:15:34,808 --> 00:15:38,676
In a near collision, for example,
you see everything happen at once.
217
00:15:38,879 --> 00:15:42,542
Horse, cart, swerve, bicycle.
All simultaneous.
218
00:15:43,283 --> 00:15:47,242
But how would it look if
it were proper to add the velocities?
219
00:15:47,454 --> 00:15:50,890
Since I'm heading toward you, you'd
add my speed to the speed of light.
220
00:15:51,091 --> 00:15:55,858
So my image ought to arrive before
the image of the horse-and-cart.
221
00:15:56,763 --> 00:15:59,254
I'd be cycling towards you
quite normally.
222
00:15:59,466 --> 00:16:02,958
To me, a collision
would seem imminent.
223
00:16:03,170 --> 00:16:06,071
But you'd see me swerve
for no apparent reason...
224
00:16:06,273 --> 00:16:08,605
...and have a collision with nothing.
225
00:16:09,643 --> 00:16:12,441
Now, the horse-and-cart
aren't headed towards you.
226
00:16:12,646 --> 00:16:16,377
Their image would arrive only
at the speed of light.
227
00:16:17,117 --> 00:16:19,711
Could it seem to me that
I just missed colliding...
228
00:16:20,253 --> 00:16:22,813
...while to you it wasn't even close?
229
00:16:23,023 --> 00:16:25,184
In precise laboratory experiments...
230
00:16:25,392 --> 00:16:28,520
...scientists have never observed
any such thing.
231
00:16:29,129 --> 00:16:31,495
If the world is to be understood...
232
00:16:31,832 --> 00:16:36,735
...if we are to avoid logical paradoxes
when traveling at high speeds...
233
00:16:36,937 --> 00:16:39,462
...then there are rules
which must be obeyed.
234
00:16:39,673 --> 00:16:44,406
Einstein called these rules
the special theory of relativity.
235
00:16:44,611 --> 00:16:47,136
Light from a moving object
travels at the same speed...
236
00:16:47,347 --> 00:16:50,976
...no matter whether the object
is at rest or in motion.
237
00:16:51,184 --> 00:16:55,848
"Thou shalt not add my speed
to the speed of light."
238
00:16:56,056 --> 00:17:00,891
Also, no material object can travel
at or beyond the speed of light.
239
00:17:01,094 --> 00:17:05,155
Nothing in physics prevents you from
traveling close to the speed of light.
240
00:17:05,365 --> 00:17:09,165
99.9 percent the speed of light
is just fine.
241
00:17:09,369 --> 00:17:11,735
But no matter how hard you try...
242
00:17:11,938 --> 00:17:14,702
...you can never gain
that last decimal point.
243
00:17:14,908 --> 00:17:17,172
For the world
to be logically consistent...
244
00:17:17,377 --> 00:17:20,505
...there must be a cosmic speed limit.
245
00:17:21,281 --> 00:17:23,977
The crack of a whip is,
due to its tip...
246
00:17:24,184 --> 00:17:26,175
...moving faster
than the speed of sound.
247
00:17:28,822 --> 00:17:30,050
It makes a shock wave...
248
00:17:30,257 --> 00:17:34,091
...a small sonic boom
in the Italian countryside.
249
00:17:34,294 --> 00:17:36,626
A thunderclap has a similar origin.
250
00:17:36,830 --> 00:17:40,163
So does the sound of
a supersonic airplane.
251
00:17:42,202 --> 00:17:46,764
So why is the speed of light a barrier
any more than the speed of sound?
252
00:17:46,973 --> 00:17:48,838
The answer is not just that...
253
00:17:49,042 --> 00:17:51,636
...light travels a million times
faster than sound.
254
00:17:51,845 --> 00:17:55,508
It's not merely an engineering problem
like the supersonic airplane.
255
00:17:56,082 --> 00:18:00,143
Instead, the light barrier is
a fundamental law of nature...
256
00:18:00,353 --> 00:18:02,253
...as basic as gravity.
257
00:18:02,455 --> 00:18:05,822
Einstein found his absolute framework
for the world:
258
00:18:06,026 --> 00:18:10,395
This sturdy pillar among all
the relative motions of the cosmos.
259
00:18:10,597 --> 00:18:14,658
Light travels just as fast,
no matter how its source is moving.
260
00:18:14,868 --> 00:18:18,895
The speed of light is constant,
relative to everything else.
261
00:18:19,105 --> 00:18:22,404
Nothing can ever catch up with light.
262
00:18:25,378 --> 00:18:28,905
Einstein's prohibition against
traveling faster than light...
263
00:18:29,115 --> 00:18:31,845
...seems to clash with
our common sense notions.
264
00:18:32,052 --> 00:18:34,850
But why should we expect
our common sense notions...
265
00:18:35,055 --> 00:18:37,956
...to have any reliability
in a matter of this sort?
266
00:18:38,158 --> 00:18:42,026
Why should our experience
at 10 kilometers an hour...
267
00:18:42,228 --> 00:18:44,423
...constrain the laws of nature...
268
00:18:44,631 --> 00:18:47,930
...at 300,000 kilometers a second?
269
00:18:50,170 --> 00:18:52,764
Relativity sets limits...
270
00:18:52,973 --> 00:18:56,033
...on what humans ultimately can do.
271
00:18:56,576 --> 00:18:59,204
The universe is not required...
272
00:18:59,412 --> 00:19:03,781
...to be in perfect harmony
with human ambition.
273
00:19:07,253 --> 00:19:10,154
Imagine a place
where the speed of light...
274
00:19:10,357 --> 00:19:13,724
...isn't its true value
of 300,000 kilometers a second...
275
00:19:13,927 --> 00:19:16,487
...but something a lot less.
276
00:19:16,696 --> 00:19:20,097
Let's say, 40 kilometers an hour...
277
00:19:20,300 --> 00:19:22,359
...and strictly enforced.
278
00:19:23,269 --> 00:19:27,171
Just as in the real world we can
never reach the speed of light...
279
00:19:27,374 --> 00:19:29,239
...the commandment here is still...
280
00:19:29,442 --> 00:19:32,775
..."Thou shalt not travel
faster than light."
281
00:19:32,979 --> 00:19:37,473
We can do thought experiments on
what happens near the speed of light...
282
00:19:37,684 --> 00:19:42,018
...here 40 kilometers per hour,
the speed of a motor scooter.
283
00:19:45,358 --> 00:19:49,351
You can't break the laws of nature.
There are no penalties for doing so.
284
00:19:49,729 --> 00:19:51,594
The real world and this one...
285
00:19:51,931 --> 00:19:56,061
...are merely so arranged
that transgressions can't happen.
286
00:19:56,269 --> 00:20:00,205
The job of physics is to find out
what those laws are.
287
00:20:02,542 --> 00:20:05,511
Before Einstein,
physicists thought that...
288
00:20:05,712 --> 00:20:08,112
...there were privileged frames
of reference...
289
00:20:08,314 --> 00:20:10,805
...some special places and times...
290
00:20:11,017 --> 00:20:13,577
...against which everything else
had to be measured.
291
00:20:13,787 --> 00:20:17,154
Einstein encountered
a similar notion in human affairs.
292
00:20:17,357 --> 00:20:20,053
The idea that the customs
of a particular nation...
293
00:20:20,260 --> 00:20:23,821
...his native Germany
or Italy or anywhere...
294
00:20:24,030 --> 00:20:27,966
...are the standard which all
other societies must be measured.
295
00:20:28,835 --> 00:20:32,271
But Einstein rejected the strident
nationalism of his time.
296
00:20:32,472 --> 00:20:35,566
He believed every culture
had its own validity.
297
00:20:35,775 --> 00:20:37,709
Also in physics,
he understood that...
298
00:20:37,911 --> 00:20:40,175
...there are no privileged
frames of reference.
299
00:20:40,380 --> 00:20:43,508
Every observer,
in any place, time or motion...
300
00:20:43,717 --> 00:20:46,345
...must deduce
the same laws of nature.
301
00:20:49,989 --> 00:20:53,823
A speed is simply how much space
you cover in a given time...
302
00:20:54,027 --> 00:20:56,689
...as any kid on
a motor scooter knows.
303
00:21:00,133 --> 00:21:01,998
Since near the velocity of light...
304
00:21:02,202 --> 00:21:04,693
...we cannot simply add speeds...
305
00:21:04,904 --> 00:21:08,533
...the familiar notions of
absolute space and absolute time...
306
00:21:08,742 --> 00:21:11,734
...independent of your
relative motion, must give way.
307
00:21:11,945 --> 00:21:14,505
That's why, as Einstein showed...
308
00:21:14,714 --> 00:21:18,673
...funny things have to happen
close to the speed of light.
309
00:21:19,419 --> 00:21:23,253
There, our conventional perspectives
of space and time...
310
00:21:23,456 --> 00:21:25,515
...strangely change.
311
00:21:27,961 --> 00:21:31,897
Your nose is just a little closer
to me than your ears.
312
00:21:32,098 --> 00:21:34,328
Light reflected off your nose
reaches me...
313
00:21:34,534 --> 00:21:36,764
...an instant in time
before your ears.
314
00:21:36,970 --> 00:21:40,201
But suppose I had a magic camera...
315
00:21:40,406 --> 00:21:43,398
...so that I could see
your nose and your ears...
316
00:21:43,610 --> 00:21:45,578
...at precisely the same instant?
317
00:21:49,115 --> 00:21:53,552
With such a camera you could take
some pretty interesting pictures.
318
00:21:54,921 --> 00:21:58,322
Paolo says goodbye to
his little brother, Vincenzo...
319
00:21:58,691 --> 00:22:00,784
- Ciao, Vincenzo.
- Ciao, Paolo.
320
00:22:01,594 --> 00:22:03,221
...and rides off.
321
00:22:03,429 --> 00:22:05,954
He's now going more than
half the speed of light.
322
00:22:06,166 --> 00:22:08,726
He is almost catching up
with his own light waves.
323
00:22:08,935 --> 00:22:11,495
This compresses the light waves
in front of him...
324
00:22:11,704 --> 00:22:13,433
...and his image becomes blue.
325
00:22:13,740 --> 00:22:17,437
The shorter wavelength is
what makes blue light waves blue.
326
00:22:18,011 --> 00:22:21,742
Also Paolo becomes skinny
in the direction of motion.
327
00:22:21,948 --> 00:22:24,007
This isn't just some optical illusion.
328
00:22:24,217 --> 00:22:27,448
It really happens when you travel
near the speed of light.
329
00:22:28,221 --> 00:22:32,555
As he roars away, he leaves his own
light waves stretched out behind him.
330
00:22:32,759 --> 00:22:34,124
Long light waves are red.
331
00:22:34,327 --> 00:22:37,888
We say that his receding image
is red-shifted.
332
00:22:39,465 --> 00:22:44,129
Now Paolo leaves for
a short tour of the countryside.
333
00:22:44,571 --> 00:22:48,098
He experiences something
even stranger.
334
00:22:50,710 --> 00:22:52,940
Everything he can see is squeezed...
335
00:22:53,146 --> 00:22:55,512
...into a moving window
just ahead of him...
336
00:22:55,715 --> 00:22:59,116
...blue-shifted at the center,
red-shifted at the edges.
337
00:22:59,319 --> 00:23:02,618
To a passerby, Paolo appears
blue-shifted when approaching...
338
00:23:02,822 --> 00:23:04,551
...red-shifted when receding.
339
00:23:04,757 --> 00:23:07,817
But to him, the entire world
is both coming and going...
340
00:23:08,027 --> 00:23:09,756
...at nearly the speed of light.
341
00:23:09,963 --> 00:23:13,592
Roadside houses and trees
that has already gone past...
342
00:23:13,800 --> 00:23:17,236
...still appear to him at the edge
of his forward field of view...
343
00:23:17,437 --> 00:23:20,133
...but distorted and red-shifted.
344
00:23:21,307 --> 00:23:25,038
When he slows down,
everything again looks normal.
345
00:23:26,579 --> 00:23:28,945
Only very close
to the speed of light...
346
00:23:29,148 --> 00:23:32,743
...does the visible world
get squeezed into a kind of tunnel.
347
00:23:33,653 --> 00:23:37,111
You'd see these distortions if you
traveled near the speed of light.
348
00:23:37,323 --> 00:23:40,019
Someday, perhaps,
interstellar navigators...
349
00:23:40,226 --> 00:23:42,751
...will take their bearings
on stars behind them...
350
00:23:42,962 --> 00:23:47,456
...whose images have all crowded
together on the forward view screen.
351
00:23:49,736 --> 00:23:52,864
The most bizarre aspect of traveling
near the speed of light...
352
00:23:53,072 --> 00:23:55,666
...is that time slows down.
353
00:23:56,509 --> 00:23:58,807
All clocks,
mechanical and biological...
354
00:23:59,012 --> 00:24:01,810
...tick more slowly
near the speed of light.
355
00:24:02,015 --> 00:24:05,348
But stationary clocks tick
at their usual rate.
356
00:24:05,551 --> 00:24:07,712
If we travel close to light speed...
357
00:24:07,921 --> 00:24:11,015
...we age more slowly
than those we left behind.
358
00:24:16,462 --> 00:24:19,295
Paolo's watch and his internal
sense of time show...
359
00:24:19,832 --> 00:24:23,359
...that he has been gone from
his friends for only a few minutes.
360
00:24:23,870 --> 00:24:28,136
But from their point of view,
he has been away for many decades.
361
00:24:28,341 --> 00:24:31,936
His friends have grown up,
moved on and died.
362
00:24:32,912 --> 00:24:34,539
And his younger brother has been...
363
00:24:34,747 --> 00:24:38,080
...patiently waiting
for him all this time.
364
00:24:40,219 --> 00:24:45,156
The two brothers experience
the paradox of time dilation.
365
00:24:45,358 --> 00:24:49,055
They've encountered
Einstein's special relativity.
366
00:24:50,096 --> 00:24:51,063
Vincenzo.
367
00:25:04,944 --> 00:25:07,378
This was just a thought experiment.
368
00:25:07,580 --> 00:25:10,743
But atomic particles traveling
near the speed of light...
369
00:25:10,950 --> 00:25:14,147
...do decay more slowly
than stationary particles.
370
00:25:14,354 --> 00:25:17,551
As strange and counterintuitive
as it seems...
371
00:25:17,757 --> 00:25:21,022
...time dilation is a law of nature.
372
00:25:22,895 --> 00:25:25,693
Traveling close
to the speed of light...
373
00:25:25,999 --> 00:25:28,900
...is a kind of elixir of life.
374
00:25:29,869 --> 00:25:32,963
Because time slows down
close to the speed of light...
375
00:25:33,172 --> 00:25:36,039
...special relativity provides us...
376
00:25:36,242 --> 00:25:38,938
...with a means of going to the stars.
377
00:25:39,979 --> 00:25:43,176
This region of northern Italy
is not only the caldron...
378
00:25:43,383 --> 00:25:46,682
...of some of the thinking
of the young Albert Einstein...
379
00:25:47,020 --> 00:25:50,353
...it is also the home
of another great genius...
380
00:25:50,556 --> 00:25:52,854
...who lived 400 years earlier.
381
00:25:53,059 --> 00:25:55,357
Leonardo da Vinci.
382
00:25:56,629 --> 00:26:00,998
Leonardo delighted
in climbing these hills...
383
00:26:01,200 --> 00:26:04,567
...and viewing the ground
from a great height...
384
00:26:04,771 --> 00:26:07,069
...as if he were soaring like a bird.
385
00:26:07,273 --> 00:26:09,867
He drew the first aerial views...
386
00:26:10,076 --> 00:26:13,773
...of landscapes, villages,
fortifications.
387
00:26:14,247 --> 00:26:18,183
I've been talking about Einstein
in and around this town of Vinci...
388
00:26:18,384 --> 00:26:20,545
...in which Leonardo grew up.
389
00:26:20,753 --> 00:26:23,813
Einstein greatly respected Leonardo...
390
00:26:24,023 --> 00:26:26,685
...and their spirits, in some sense...
391
00:26:26,893 --> 00:26:30,192
...inhabit this countryside still.
392
00:26:55,154 --> 00:26:58,214
Among Leonardo's
many accomplishments...
393
00:26:58,424 --> 00:27:01,791
...in painting, sculpture,
architecture, natural history...
394
00:27:01,994 --> 00:27:06,431
...anatomy, geology,
civil and military engineering...
395
00:27:06,933 --> 00:27:08,798
...he had a great passion.
396
00:27:09,035 --> 00:27:12,527
He wished to construct a machine...
397
00:27:12,738 --> 00:27:14,399
...which would fly.
398
00:27:14,941 --> 00:27:18,741
He made sketches of such machines,
built miniature models...
399
00:27:18,945 --> 00:27:23,075
...constructed great,
full-scale prototypes.
400
00:27:24,884 --> 00:27:28,547
And not a one of them ever worked.
401
00:27:29,755 --> 00:27:34,124
There were no machines of adequate
capacity available in his time.
402
00:27:34,327 --> 00:27:37,694
The technology was just not ready.
403
00:27:38,598 --> 00:27:41,829
The designs, however, were brilliant.
404
00:27:42,034 --> 00:27:45,197
For example, this bird-like machine...
405
00:27:45,404 --> 00:27:49,397
...here in the Leonardo Museum
in the town of Vinci.
406
00:27:50,576 --> 00:27:55,513
Leonardo's great designs encouraged
engineers in later epochs...
407
00:27:55,715 --> 00:28:00,084
...although Leonardo himself
was very depressed at these failures.
408
00:28:00,286 --> 00:28:02,117
But it's not his fault...
409
00:28:02,321 --> 00:28:05,347
...he was trapped in the 15th century.
410
00:28:06,192 --> 00:28:09,889
A somewhat similar case
occurred in 1939...
411
00:28:10,096 --> 00:28:14,430
...when a group of engineers called
the British Interplanetary Society...
412
00:28:14,634 --> 00:28:16,898
...decided to design a ship...
413
00:28:17,103 --> 00:28:20,072
...which would carry people
to the moon.
414
00:28:20,273 --> 00:28:22,571
Now, it was by no means
the same design...
415
00:28:22,775 --> 00:28:27,439
...as the Apollo ship which actually
took people to the moon years later.
416
00:28:27,647 --> 00:28:30,207
But that design suggested that...
417
00:28:30,416 --> 00:28:32,213
...a mission to the moon
might one day...
418
00:28:32,418 --> 00:28:35,012
...be a practical
engineering possibility.
419
00:28:35,488 --> 00:28:36,682
Today...
420
00:28:37,690 --> 00:28:41,319
...we have preliminary
designs of ships...
421
00:28:41,527 --> 00:28:44,724
...which will take people
to the stars.
422
00:28:44,931 --> 00:28:49,630
They are constructed in Earth orbit
and from there...
423
00:28:49,835 --> 00:28:54,772
...they venture on their great
interstellar journeys.
424
00:28:55,141 --> 00:28:56,540
One of them...
425
00:28:57,009 --> 00:29:00,001
...is called Project Orion.
426
00:29:01,147 --> 00:29:03,081
It utilizes nuclear weapons...
427
00:29:03,282 --> 00:29:07,719
...hydrogen bombs
against an inertial plate.
428
00:29:07,920 --> 00:29:11,651
Each explosion providing
a kind of "putt-putt"...
429
00:29:11,857 --> 00:29:15,793
...a vast nuclear motorboat in space.
430
00:29:16,195 --> 00:29:19,631
Orion seems entirely practical...
431
00:29:19,832 --> 00:29:22,266
...and was under development
in the U.S...
432
00:29:22,468 --> 00:29:25,835
...until the signing
of the international treaty...
433
00:29:26,038 --> 00:29:29,235
...forbidding nuclear weapons
explosions in space.
434
00:29:29,842 --> 00:29:34,779
I think, the Orion starship
is the best use of nuclear weapons...
435
00:29:34,981 --> 00:29:38,815
...provided the ships don't depart
from very near the Earth.
436
00:29:48,394 --> 00:29:51,124
Project Daedalus is
a recent design...
437
00:29:51,330 --> 00:29:53,730
...of the British
Interplanetary Society.
438
00:29:53,933 --> 00:29:57,425
It assumes the existence
of a nuclear fusion reactor...
439
00:29:57,637 --> 00:29:59,935
...something much safer
and more efficient...
440
00:30:00,139 --> 00:30:03,939
...than the existing nuclear
fission power plants.
441
00:30:08,180 --> 00:30:10,444
We do not yet have fusion reactors.
442
00:30:10,650 --> 00:30:13,380
One day, quite soon, we may.
443
00:30:19,091 --> 00:30:22,492
Orion and Daedalus might go...
444
00:30:22,695 --> 00:30:25,323
...10 percent the speed of light.
445
00:30:26,465 --> 00:30:29,332
So a trip to Alpha Centauri,
41 l2 light-years away...
446
00:30:29,535 --> 00:30:33,369
...would take 45 years,
less than a human lifetime.
447
00:30:33,939 --> 00:30:37,773
Such ships could not travel
close enough to the speed of light...
448
00:30:37,977 --> 00:30:40,912
...for the time-slowing effects
of special relativity...
449
00:30:41,113 --> 00:30:42,774
...to become important.
450
00:30:43,349 --> 00:30:46,045
It does not seem likely
that such ships...
451
00:30:46,252 --> 00:30:48,846
...would be built before
the middle of the 21 st century...
452
00:30:49,055 --> 00:30:52,855
...although we could build
an Orion starship now.
453
00:30:53,292 --> 00:30:57,353
For voyages beyond the nearest stars,
something must be added.
454
00:30:57,563 --> 00:31:00,657
Perhaps they could be used
as multigeneration ships...
455
00:31:01,033 --> 00:31:04,025
...so those arriving would be
the remote descendants...
456
00:31:04,236 --> 00:31:07,967
...of those who had originally
set out centuries before.
457
00:31:08,607 --> 00:31:12,634
Or perhaps some safe means
of human hibernation might be found...
458
00:31:12,845 --> 00:31:16,713
...so that space travelers might be
frozen and then thawed out...
459
00:31:16,916 --> 00:31:20,818
...when they arrive at
the destination centuries later.
460
00:31:21,520 --> 00:31:25,820
But fast interstellar space flight
approaching the speed of light...
461
00:31:26,025 --> 00:31:27,890
...is much more difficult.
462
00:31:28,094 --> 00:31:30,824
That's an objective
not for a hundred years...
463
00:31:31,030 --> 00:31:34,158
...but for a thousand
or for 10 thousand...
464
00:31:34,367 --> 00:31:36,835
...but it also is possible.
465
00:31:39,205 --> 00:31:42,174
A kind of interstellar ramjet
has been proposed...
466
00:31:42,375 --> 00:31:44,809
...which scoops up
the hydrogen atoms...
467
00:31:45,010 --> 00:31:46,944
...which float between the stars...
468
00:31:47,146 --> 00:31:51,207
...accelerates them into an engine
and spits them out the back.
469
00:31:52,051 --> 00:31:54,451
But in deep space,
there is one atom...
470
00:31:54,653 --> 00:31:58,714
...for every 10 cubic centimeters
of space.
471
00:31:58,924 --> 00:32:00,391
For the ramjet to work...
472
00:32:00,893 --> 00:32:03,453
...it has to have a frontal scoop...
473
00:32:03,662 --> 00:32:06,529
...hundreds of kilometers across.
474
00:32:06,799 --> 00:32:10,997
Reaching relativistic velocities,
the hydrogen atoms will be moving...
475
00:32:11,203 --> 00:32:13,763
...with respect
to the interstellar spaceship...
476
00:32:13,973 --> 00:32:15,998
...at close to the speed of light.
477
00:32:16,208 --> 00:32:17,937
If precautions aren't taken...
478
00:32:18,144 --> 00:32:22,706
...the passengers will be fried
by these induced cosmic rays.
479
00:32:22,915 --> 00:32:24,780
There's a proposed solution:
480
00:32:24,984 --> 00:32:28,283
A laser is used to strip
electrons off the atoms...
481
00:32:28,487 --> 00:32:31,718
...and electrically charge them
while they're some distance away.
482
00:32:32,291 --> 00:32:34,919
And an extremely strong
magnetic field...
483
00:32:35,161 --> 00:32:38,426
...is used to deflect
the charged atoms into the scoop...
484
00:32:38,631 --> 00:32:40,258
...and away from the spacecraft.
485
00:32:40,466 --> 00:32:41,763
This is engineering...
486
00:32:41,967 --> 00:32:45,767
...on a scale so far
unprecedented on the Earth.
487
00:32:45,971 --> 00:32:50,567
We are talking of engines
the size of small worlds.
488
00:32:59,985 --> 00:33:04,888
Suppose that the spacecraft is
designed to accelerate at 1 g...
489
00:33:05,090 --> 00:33:07,388
...so we'd be comfortable aboard it.
490
00:33:07,593 --> 00:33:10,027
We'd go closer and closer
to the speed of light...
491
00:33:10,229 --> 00:33:12,493
...until the midpoint of the journey.
492
00:33:12,698 --> 00:33:15,258
Then the spacecraft is
turned around...
493
00:33:15,468 --> 00:33:19,029
...and we decelerate at 1 g
to the destination.
494
00:33:19,605 --> 00:33:23,735
For most of the trip, the velocity
would be close to the speed of light...
495
00:33:23,943 --> 00:33:27,106
...and time would
slow down enormously.
496
00:33:27,446 --> 00:33:28,970
By how much?
497
00:33:29,949 --> 00:33:33,385
Barnard's Star could be reached
by such a ship...
498
00:33:33,586 --> 00:33:36,214
...in eight years, ship time.
499
00:33:36,822 --> 00:33:40,781
The center of the Milky Way galaxy
in 21 years.
500
00:33:40,993 --> 00:33:44,861
The Andromeda galaxy in 28 years.
501
00:33:45,397 --> 00:33:47,331
Of course, the people
left behind on the Earth...
502
00:33:47,533 --> 00:33:49,831
...would see things
somewhat differently.
503
00:33:50,102 --> 00:33:52,070
Instead of 21 years to the galaxy...
504
00:33:52,271 --> 00:33:55,832
...they would measure it
as 30,000 years.
505
00:33:56,041 --> 00:33:57,406
When we got back...
506
00:33:57,610 --> 00:34:01,068
...very few of our friends
would be around to greet us.
507
00:34:01,914 --> 00:34:03,677
In principle, such a journey...
508
00:34:03,883 --> 00:34:08,286
...mounting the decimal points closer
and closer to the speed of light...
509
00:34:08,487 --> 00:34:12,253
...would even permit us to
circumnavigate the known universe...
510
00:34:12,458 --> 00:34:15,518
...in 56 years, ship time.
511
00:34:16,462 --> 00:34:20,558
We would return tens
of billions of years...
512
00:34:20,766 --> 00:34:22,757
...in the far future...
513
00:34:22,968 --> 00:34:26,028
...with the Earth a charred cinder...
514
00:34:26,238 --> 00:34:28,706
...and the sun dead.
515
00:34:29,341 --> 00:34:32,742
Relativistic space flight makes
the universe accessible...
516
00:34:32,945 --> 00:34:35,311
...to advanced civilizations...
517
00:34:35,514 --> 00:34:37,709
...but only to those
who go on the journey...
518
00:34:37,917 --> 00:34:40,283
...not to those who stay home.
519
00:34:41,120 --> 00:34:45,454
These designs are probably further...
520
00:34:45,658 --> 00:34:49,389
...from the actual interstellar
spacecraft of the future...
521
00:34:50,596 --> 00:34:53,394
...than Leonardo's models are...
522
00:34:53,599 --> 00:34:56,932
...from the supersonic transports
of the present.
523
00:34:57,503 --> 00:34:59,368
But if we do not destroy ourselves...
524
00:34:59,572 --> 00:35:04,475
...I believe that we will,
one day, venture to the stars.
525
00:35:05,144 --> 00:35:07,704
When our solar system
is all explored...
526
00:35:07,913 --> 00:35:11,041
...the planets of other stars
will beckon.
527
00:35:44,850 --> 00:35:49,048
Space travel and time travel
are connected.
528
00:35:50,189 --> 00:35:52,123
To travel fast into space...
529
00:35:52,324 --> 00:35:55,521
...is to travel fast into the future.
530
00:35:58,397 --> 00:36:02,561
We travel into the future,
although slowly, all the time.
531
00:36:02,768 --> 00:36:07,068
But what about the past?
Could we journey into yesterday?
532
00:36:07,272 --> 00:36:10,400
Many physicists think this is
fundamentally impossible...
533
00:36:10,609 --> 00:36:12,941
...that we could
not build a device...
534
00:36:13,145 --> 00:36:15,875
...which would carry us
backwards into time.
535
00:36:16,081 --> 00:36:19,482
Some say that even if we were
to build such a device...
536
00:36:19,685 --> 00:36:20,947
...it wouldn't do much good.
537
00:36:21,153 --> 00:36:23,678
We couldn't significantly
affect the past.
538
00:36:23,889 --> 00:36:27,552
For example, suppose you
traveled into the past...
539
00:36:27,760 --> 00:36:29,853
...and somehow or other prevented...
540
00:36:30,062 --> 00:36:32,860
...your own parents from meeting.
541
00:36:33,065 --> 00:36:36,626
Why, then you would probably
never have been born...
542
00:36:36,835 --> 00:36:38,860
...which is something
of a contradiction, isn't it...
543
00:36:39,071 --> 00:36:41,198
...since you are clearly there.
544
00:36:41,807 --> 00:36:43,138
Other people think that...
545
00:36:43,342 --> 00:36:46,470
...the two alternative histories
have equal validity...
546
00:36:46,679 --> 00:36:50,080
...that they're parallel threads,
skeins of time...
547
00:36:50,282 --> 00:36:52,842
...that they could exist side by side.
548
00:36:56,722 --> 00:36:59,247
The history in which
you were never born...
549
00:36:59,458 --> 00:37:02,427
...and the history that
you know all about.
550
00:37:02,661 --> 00:37:05,858
Perhaps time itself has
many potential dimensions...
551
00:37:06,065 --> 00:37:09,330
...despite the fact that
we are condemned to experience...
552
00:37:09,535 --> 00:37:11,730
...only one of those dimensions.
553
00:37:12,371 --> 00:37:15,636
Now, suppose you could go back
into the past...
554
00:37:15,841 --> 00:37:19,299
...and really change it by,
let's say something like...
555
00:37:19,511 --> 00:37:24,005
...persuading Queen Isabella not
to bankroll Christopher Columbus.
556
00:37:24,216 --> 00:37:26,480
Then you would have set into motion...
557
00:37:26,685 --> 00:37:29,711
...a different sequence
of historical events...
558
00:37:29,922 --> 00:37:32,982
...which those people
you left behind you in our time...
559
00:37:33,192 --> 00:37:35,353
...would never get to know about.
560
00:37:35,561 --> 00:37:38,291
If that kind of time travel
were possible...
561
00:37:38,497 --> 00:37:41,694
...then every imaginable sequence...
562
00:37:41,900 --> 00:37:43,868
...of alternative history...
563
00:37:44,069 --> 00:37:46,264
...might in some sense really exist.
564
00:37:47,239 --> 00:37:49,298
Would it be possible
for a time traveler...
565
00:37:49,508 --> 00:37:52,875
...to change the course of history
in a major way?
566
00:37:53,078 --> 00:37:55,308
Well, let's think about that.
567
00:37:58,884 --> 00:38:01,148
History consists for the most part...
568
00:38:01,353 --> 00:38:05,414
...of a complex multitude
of deeply interwoven threads...
569
00:38:05,624 --> 00:38:08,286
...biological, economic
and social forces...
570
00:38:08,494 --> 00:38:10,985
...that are not so easily unraveled.
571
00:38:12,264 --> 00:38:16,894
The ancient Greeks imagined the course
of human events to be a tapestry...
572
00:38:17,102 --> 00:38:20,868
...created by three goddesses:
the Fates.
573
00:38:22,541 --> 00:38:26,773
Random minor events generally
have no long-range consequences.
574
00:38:26,979 --> 00:38:30,176
But some which occur
at critical junctures...
575
00:38:30,382 --> 00:38:32,907
...may alter the weave of history.
576
00:38:33,118 --> 00:38:36,110
There may be cases where
profound changes can be made...
577
00:38:36,321 --> 00:38:38,812
...by relatively trivial adjustments.
578
00:38:39,024 --> 00:38:43,791
The further in the past such an event
is, the more powerful its influence.
579
00:38:44,296 --> 00:38:47,754
What if our time traveler had
persuaded Queen Isabella that...
580
00:38:47,966 --> 00:38:49,866
...Columbus' geography was wrong?
581
00:38:50,068 --> 00:38:54,266
Almost certainly, some other European
would have sailed to the New World.
582
00:38:54,473 --> 00:38:56,134
There were many inducements:
583
00:38:56,341 --> 00:38:59,401
The lure of the spice trade,
improvements in navigation...
584
00:38:59,611 --> 00:39:02,045
...competition among
rival European powers.
585
00:39:02,247 --> 00:39:06,149
The discovery of America
around 1500 was inevitable.
586
00:39:06,351 --> 00:39:09,752
Of course, there wouldn't be any
postage stamps showing Columbus...
587
00:39:09,955 --> 00:39:12,890
...and the Republic of Colombia
would have another name.
588
00:39:13,091 --> 00:39:17,084
But the big picture would have
turned out more or less the same.
589
00:39:21,700 --> 00:39:24,897
In order to affect
the future profoundly...
590
00:39:25,103 --> 00:39:27,663
...a time traveler
has to pick and choose.
591
00:39:27,873 --> 00:39:31,331
He'd probably have to intervene
in a number of events...
592
00:39:31,543 --> 00:39:34,239
...which are
very carefully selected...
593
00:39:34,446 --> 00:39:39,281
...so he could change
the weave of history.
594
00:39:39,685 --> 00:39:42,210
It's a lovely fantasy...
595
00:39:42,421 --> 00:39:46,357
...to explore those other worlds
that never were.
596
00:39:48,861 --> 00:39:52,456
If you had H.G. Wells' time machine...
597
00:39:52,664 --> 00:39:55,690
...maybe you could understand
how history really works.
598
00:39:55,901 --> 00:39:58,768
If an apparently pivotal person
had never lived...
599
00:39:58,971 --> 00:40:03,135
...Paul the Apostle or Peter the Great
or Pythagoras...
600
00:40:03,342 --> 00:40:06,038
...how different would
the world really be?
601
00:40:06,845 --> 00:40:08,938
What if the scientific tradition...
602
00:40:09,147 --> 00:40:11,911
...of the ancient lonian Greeks...
603
00:40:12,117 --> 00:40:14,779
...had prospered and flourished?
604
00:40:14,987 --> 00:40:17,854
It would have required
many social factors at the time...
605
00:40:18,056 --> 00:40:19,785
...to have been different...
606
00:40:19,992 --> 00:40:22,426
...including the common feeling...
607
00:40:22,628 --> 00:40:25,324
...that slavery was right and natural.
608
00:40:25,530 --> 00:40:29,091
But what if that light
that had dawned...
609
00:40:29,301 --> 00:40:32,668
...on the eastern Mediterranean
some 2500 years ago...
610
00:40:32,871 --> 00:40:34,839
...had not flickered out?
611
00:40:35,040 --> 00:40:38,339
What if scientific method
and experiment...
612
00:40:38,543 --> 00:40:40,534
...had been vigorously pursued...
613
00:40:40,746 --> 00:40:43,010
...2000 years before
the industrial revolution...
614
00:40:43,215 --> 00:40:45,183
...our industrial revolution?
615
00:40:45,384 --> 00:40:49,445
What if the power of this new mode
of thought, the scientific method...
616
00:40:49,655 --> 00:40:51,953
...had been generally appreciated?
617
00:40:52,324 --> 00:40:56,055
I think we might have saved
10 or 20 centuries.
618
00:40:56,261 --> 00:40:58,957
Perhaps the contributions
that Leonardo made...
619
00:40:59,164 --> 00:41:01,997
...would have been made
1000 years earlier...
620
00:41:02,200 --> 00:41:06,000
...and the contributions
of Einstein 500 years ago.
621
00:41:06,204 --> 00:41:08,331
Not that it would have
been those people...
622
00:41:08,540 --> 00:41:10,974
...who would've made
those contributions...
623
00:41:11,176 --> 00:41:14,168
...because they lived only
in our timeline.
624
00:41:14,746 --> 00:41:17,681
If the lonians had won...
625
00:41:17,883 --> 00:41:21,876
...we might by now, I think,
be going to the stars.
626
00:41:22,087 --> 00:41:26,820
We might at this moment have
the first survey ships...
627
00:41:27,025 --> 00:41:31,689
...returning with astonishing results
from Alpha Centauri...
628
00:41:31,897 --> 00:41:36,493
...and Barnard's Star,
Sirius and Tau Ceti.
629
00:41:36,702 --> 00:41:40,001
There would now be great fleets...
630
00:41:40,205 --> 00:41:42,173
...of interstellar transports...
631
00:41:42,374 --> 00:41:44,865
...being constructed in Earth orbit...
632
00:41:45,077 --> 00:41:48,069
...small, unmanned survey ships...
633
00:41:48,280 --> 00:41:51,738
...liners for immigrants, perhaps...
634
00:41:51,950 --> 00:41:53,349
...great trading ships...
635
00:41:53,552 --> 00:41:57,010
...to ply the spaces
between the stars.
636
00:41:57,489 --> 00:42:00,686
On all these ships
there would be symbols...
637
00:42:00,892 --> 00:42:03,622
...and inscriptions on the sides.
638
00:42:03,829 --> 00:42:06,093
The inscriptions,
if we looked closely...
639
00:42:06,298 --> 00:42:08,926
...would be written in Greek.
640
00:42:09,434 --> 00:42:10,765
The symbol...
641
00:42:10,969 --> 00:42:14,427
...perhaps, would be the dodecahedron.
642
00:42:14,639 --> 00:42:19,269
And the inscription on the sides
of the ships to the stars...
643
00:42:19,478 --> 00:42:20,945
...something like:
644
00:42:21,146 --> 00:42:25,879
"Starship Theodorus
of the Planet Earth."
645
00:42:28,887 --> 00:42:31,685
If you were a really
ambitious time traveler...
646
00:42:34,893 --> 00:42:37,657
...you might not dally
with human history...
647
00:42:37,863 --> 00:42:40,491
...or even pause to examine
the evolution on Earth.
648
00:42:40,699 --> 00:42:42,929
Instead, you would journey back...
649
00:42:43,135 --> 00:42:44,534
...to witness the origin
of our solar system...
650
00:42:45,003 --> 00:42:49,531
...from the gas and dust
between the stars.
651
00:42:50,709 --> 00:42:52,142
Five billion years ago...
652
00:42:52,344 --> 00:42:55,939
...an interstellar cloud was
collapsing to form our solar system.
653
00:42:56,148 --> 00:42:59,379
Most clumps of matter
gravitated towards the center...
654
00:42:59,584 --> 00:43:02,144
...and were destined
to form the sun.
655
00:43:02,354 --> 00:43:06,586
Smaller peripheral clumps
would become the planets.
656
00:43:06,792 --> 00:43:11,195
Long ago, there was a kind of
natural selection among the worlds.
657
00:43:11,396 --> 00:43:15,594
Those on highly elliptical orbits
tended to collide and be destroyed...
658
00:43:15,801 --> 00:43:19,396
...but planets in circular orbits
tended to survive.
659
00:43:19,604 --> 00:43:21,868
But if events had been
a little different...
660
00:43:22,074 --> 00:43:23,735
...the Earth would never have formed...
661
00:43:23,942 --> 00:43:27,901
...and another planet at another
distance from the sun would be around.
662
00:43:28,113 --> 00:43:30,377
We owe the existence of our world...
663
00:43:30,582 --> 00:43:34,416
...to random collisions
in a long-vanished cloud.
664
00:43:37,255 --> 00:43:40,315
Soon, the central mass
became very hot.
665
00:43:40,525 --> 00:43:44,359
Thermonuclear reactions were initiated
and the sun turned on...
666
00:43:44,563 --> 00:43:47,430
...flooding the solar system
with light.
667
00:43:49,801 --> 00:43:51,769
But the growing smaller lumps...
668
00:43:51,970 --> 00:43:54,200
...would never achieve
such high temperatures...
669
00:43:54,406 --> 00:43:57,239
...and would never generate
thermonuclear reactions.
670
00:43:57,442 --> 00:44:01,173
They would become
the Earth and the other planets...
671
00:44:01,379 --> 00:44:05,611
...heated not from within,
but mainly by the distant sun.
672
00:44:10,288 --> 00:44:12,085
The accretion continued until...
673
00:44:12,290 --> 00:44:15,555
...almost all the gas and dust
and small worldlets...
674
00:44:15,760 --> 00:44:19,161
...were swept up
by the surviving planets.
675
00:44:21,533 --> 00:44:23,831
Our time traveler would witness...
676
00:44:24,035 --> 00:44:26,970
...the collisions
that made the worlds.
677
00:44:33,145 --> 00:44:35,170
Except for the comets and asteroids...
678
00:44:35,380 --> 00:44:38,008
...the chaos of the early
solar system was reduced...
679
00:44:38,216 --> 00:44:40,548
...to a remarkable simplicity:
680
00:44:40,752 --> 00:44:44,586
Nine or so principal planets
in almost circular orbits...
681
00:44:44,789 --> 00:44:46,848
...and a few dozen moons.
682
00:44:51,329 --> 00:44:54,264
Now, let's take a different look.
683
00:44:55,901 --> 00:44:58,369
If we view the solar system edge on...
684
00:44:58,570 --> 00:45:00,834
...and move the sun
off-screen to the left...
685
00:45:01,039 --> 00:45:03,701
...we see that
the small terrestrial planets...
686
00:45:03,909 --> 00:45:07,538
...the ones about as massive as Earth,
tend to be close to the sun.
687
00:45:07,746 --> 00:45:11,580
The big Jupiter-like planets tend
to be much further from the sun.
688
00:45:11,783 --> 00:45:14,650
But is that the way it has to be?
689
00:45:15,854 --> 00:45:17,515
Computer studies suggest...
690
00:45:17,722 --> 00:45:20,486
...that there may be many
similar systems about stars...
691
00:45:20,692 --> 00:45:25,061
...with the terrestrials in close
and the Jovian planets further away.
692
00:45:28,934 --> 00:45:32,665
But some systems might have Jovians
and terrestrials mixed together.
693
00:45:32,871 --> 00:45:37,399
There may be great worlds
like Jupiter looming in other skies.
694
00:45:38,710 --> 00:45:42,669
Rarely, the Jovian planets
may form close to the star...
695
00:45:42,881 --> 00:45:47,215
...the terrestrials trailing away
towards interstellar space.
696
00:45:48,553 --> 00:45:50,544
Our familiar arrangement of planets...
697
00:45:50,755 --> 00:45:53,519
...is only one,
perhaps typical, case...
698
00:45:53,725 --> 00:45:57,786
...in the vast expanse of systems.
699
00:45:57,996 --> 00:46:02,626
Often, one fledgling planet
accumulates so much gas and dust...
700
00:46:02,834 --> 00:46:05,029
...that thermonuclear reactions
do occur.
701
00:46:05,237 --> 00:46:07,330
It becomes a second sun.
702
00:46:07,539 --> 00:46:10,372
A binary star system has formed.
703
00:46:14,879 --> 00:46:18,781
From most of these worlds,
the vistas will be dazzling.
704
00:46:18,984 --> 00:46:21,350
Not one of them will be
identical to the Earth.
705
00:46:21,553 --> 00:46:25,922
A few will be hospitable.
Many will appear hostile.
706
00:46:27,325 --> 00:46:29,350
Where there are two suns in the sky...
707
00:46:29,561 --> 00:46:33,258
...every object will cast two shadows.
708
00:46:37,435 --> 00:46:40,063
What wonders are waiting for us...
709
00:46:40,272 --> 00:46:42,638
...on the planets of the nearby stars?
710
00:46:42,841 --> 00:46:45,708
Are there radically
different kinds of worlds...
711
00:46:45,910 --> 00:46:48,970
...unimaginably exotic forms of life?
712
00:46:52,250 --> 00:46:55,151
Perhaps in another century or two...
713
00:46:55,353 --> 00:46:57,344
...when our solar system
is all explored...
714
00:46:57,555 --> 00:47:00,786
...we will also have put
our own planet in order.
715
00:47:00,992 --> 00:47:04,450
Then we will set sail for the stars...
716
00:47:04,663 --> 00:47:07,325
...and the beckoning worlds
around them.
717
00:47:10,769 --> 00:47:13,897
In that day, our machines
and our descendants...
718
00:47:14,105 --> 00:47:17,802
...approaching the speed of light,
will skim the light-years...
719
00:47:18,009 --> 00:47:22,639
...leaping ahead through time,
seeking new worlds.
720
00:47:22,847 --> 00:47:26,374
Einstein has shown us
that it's possible.
721
00:47:27,619 --> 00:47:29,712
We will journey simultaneously...
722
00:47:29,921 --> 00:47:33,322
...to distant planets
and to the far future.
723
00:47:34,125 --> 00:47:35,820
Some worlds, like this one...
724
00:47:36,027 --> 00:47:39,292
...will look out onto
a vast gaseous nebula...
725
00:47:39,497 --> 00:47:41,226
...the remains of a star...
726
00:47:41,433 --> 00:47:44,561
...that once was and is no longer.
727
00:47:47,072 --> 00:47:49,597
In all those skies,
rich and distant...
728
00:47:49,808 --> 00:47:52,333
...and exotic constellations...
729
00:47:52,544 --> 00:47:56,503
...there may be a faint yellow star...
730
00:47:56,715 --> 00:47:59,616
...perhaps barely visible
to the naked eye...
731
00:47:59,818 --> 00:48:02,651
...perhaps seen only
through the telescope.
732
00:48:02,854 --> 00:48:06,722
The home star of a fleet
of interstellar transports...
733
00:48:06,925 --> 00:48:09,086
...exploring this tiny region...
734
00:48:09,294 --> 00:48:12,388
...of the great Milky Way galaxy.
735
00:48:12,831 --> 00:48:17,097
The themes of space and time
are intertwined.
736
00:48:17,302 --> 00:48:20,294
Worlds and stars, like people...
737
00:48:20,505 --> 00:48:24,305
...are born, live and die.
738
00:48:24,509 --> 00:48:27,239
The lifetime of a human being
is measured in decades.
739
00:48:27,445 --> 00:48:29,675
But the lifetime of the sun...
740
00:48:29,881 --> 00:48:32,714
...is a hundred million times longer.
741
00:48:34,886 --> 00:48:37,446
Matter is much older than life.
742
00:48:37,655 --> 00:48:40,886
Billions of years before
the sun and Earth even formed...
743
00:48:41,092 --> 00:48:44,459
...atoms were being synthesized
in the insides of hot stars...
744
00:48:44,662 --> 00:48:49,031
...and then returned to space
when the stars blew themselves up.
745
00:48:49,234 --> 00:48:52,567
Newly formed planets were
made of this stellar debris.
746
00:48:52,771 --> 00:48:57,071
The Earth and every living thing
are made of star stuff.
747
00:49:01,913 --> 00:49:05,747
But how slowly, in our human
perspective, life evolved...
748
00:49:05,950 --> 00:49:10,387
...from the molecules of the early
oceans to the first bacteria.
749
00:49:14,159 --> 00:49:17,094
Evolution is not immediately
obvious to everybody...
750
00:49:17,295 --> 00:49:20,594
...because it moves
so slowly and takes so long.
751
00:49:20,799 --> 00:49:23,927
How can creatures who
live for only 70 years...
752
00:49:24,135 --> 00:49:27,866
...detect events that
take 70 million years to unfold?
753
00:49:28,072 --> 00:49:29,869
Or 4 billion?
754
00:49:34,913 --> 00:49:37,279
By the time
one-celled animals had evolved...
755
00:49:37,482 --> 00:49:40,713
...the history of life
on Earth was half over.
756
00:49:45,156 --> 00:49:47,920
Not very far along to us,
you might think...
757
00:49:48,126 --> 00:49:50,788
...but by now almost all
the basic chemistry of life...
758
00:49:50,995 --> 00:49:53,020
...had been established.
759
00:49:54,265 --> 00:49:56,233
Forget our human time perspective.
760
00:49:56,434 --> 00:49:58,425
From the point of view of a star...
761
00:49:58,636 --> 00:50:01,901
...evolution was weaving
intricate new patterns...
762
00:50:02,106 --> 00:50:06,065
...from the star stuff on
the planet Earth, and very rapidly.
763
00:50:08,780 --> 00:50:11,476
Most evolutionary lines
became extinct.
764
00:50:11,683 --> 00:50:13,776
Many lines became stagnant.
765
00:50:13,985 --> 00:50:15,976
If things had gone
a bit differently...
766
00:50:16,187 --> 00:50:18,280
...a small change of climate,
say, or...
767
00:50:18,490 --> 00:50:19,548
...a new mutation...
768
00:50:19,757 --> 00:50:23,158
...or the accidental death
of a different humble organism...
769
00:50:23,361 --> 00:50:27,559
...the entire future history of life
might have been very different.
770
00:50:30,235 --> 00:50:33,136
Maybe the line to an intelligent
technological species...
771
00:50:33,338 --> 00:50:36,000
...would have passed through worms.
772
00:50:38,843 --> 00:50:41,004
Maybe the present masters
of the planet...
773
00:50:41,212 --> 00:50:44,875
...would have had ancestors
who were tunicates.
774
00:50:47,352 --> 00:50:48,876
We might not have evolved.
775
00:50:49,087 --> 00:50:52,113
Someone else,
someone very different...
776
00:50:52,323 --> 00:50:57,158
...would be here now in our stead,
maybe pondering their origins.
777
00:50:59,264 --> 00:51:01,357
But that's not what happened.
778
00:51:01,566 --> 00:51:04,626
There's a particular sequence
of environmental accidents...
779
00:51:04,836 --> 00:51:07,964
...and random mutations
in the hereditary material.
780
00:51:08,172 --> 00:51:11,869
One particular timeline
for life on Earth...
781
00:51:12,076 --> 00:51:13,907
...in this universe.
782
00:51:17,882 --> 00:51:21,716
As a result, the dominant organisms
on the planet today...
783
00:51:21,920 --> 00:51:23,854
...come from fish.
784
00:51:25,690 --> 00:51:29,387
Along the way, many more species
became extinct than now exist.
785
00:51:29,594 --> 00:51:32,825
If history had
a slightly different weave...
786
00:51:33,031 --> 00:51:37,491
...some of those extinct organisms
might have survived and prospered.
787
00:51:38,303 --> 00:51:41,466
But occasionally, a creature
thought to have become extinct...
788
00:51:41,673 --> 00:51:43,573
...hundreds of millions
of years ago...
789
00:51:43,775 --> 00:51:46,642
...turns out to be alive and well.
790
00:51:46,844 --> 00:51:49,506
The coelacanth, for example.
791
00:51:51,616 --> 00:51:56,451
For 3 1/2 billion years, life had
lived exclusively in the water.
792
00:51:56,654 --> 00:51:59,088
But now, in a great
breathtaking adventure...
793
00:51:59,290 --> 00:52:00,518
...it took to the land.
794
00:52:00,725 --> 00:52:02,852
But if things had gone
a little differently...
795
00:52:03,061 --> 00:52:05,791
...the dominant species might
still be in the ocean...
796
00:52:05,997 --> 00:52:10,366
...or developed spaceships to
carry them off the planet altogether.
797
00:52:16,007 --> 00:52:18,066
From our ancestors, the reptiles...
798
00:52:18,276 --> 00:52:20,642
...there developed
many successful lines...
799
00:52:20,845 --> 00:52:23,405
...including the dinosaurs.
800
00:52:23,715 --> 00:52:26,684
Some were fast, dexterous
and intelligent.
801
00:52:26,884 --> 00:52:28,852
A visitor from
another world or time...
802
00:52:29,053 --> 00:52:31,817
...might have thought them
the wave of the future.
803
00:52:32,023 --> 00:52:36,585
But after nearly 200 million years,
they were suddenly all wiped out.
804
00:52:36,794 --> 00:52:39,558
Perhaps it was a great meteorite
colliding with the Earth...
805
00:52:39,764 --> 00:52:42,494
...spewing debris into the air,
blotting out the sun...
806
00:52:42,700 --> 00:52:45,225
...and killing the plants
that the dinosaurs ate.
807
00:52:45,436 --> 00:52:49,930
I wonder when they first sensed
that something was wrong.
808
00:52:52,043 --> 00:52:55,809
The successors of the dinosaurs
came from the same reptilian stock...
809
00:52:56,014 --> 00:53:00,451
...but they survived the catastrophe
that destroyed their cousins.
810
00:53:03,054 --> 00:53:05,750
Again, there were many branches
which became extinct.
811
00:53:05,957 --> 00:53:08,255
And had events been
a little different...
812
00:53:08,459 --> 00:53:11,917
...those branches might have led
to the dominant form today.
813
00:53:14,799 --> 00:53:17,859
For 40 million years, a visitor
would not have been impressed...
814
00:53:18,069 --> 00:53:20,196
...by these timid little creatures...
815
00:53:20,405 --> 00:53:24,034
...but they led to all
the familiar mammals of today.
816
00:53:26,544 --> 00:53:29,513
And that includes the primates.
817
00:53:30,415 --> 00:53:33,782
About 20 million years ago,
a space time traveler...
818
00:53:33,985 --> 00:53:36,613
...might have recognized
these guys as promising...
819
00:53:36,821 --> 00:53:40,416
...bright, quick, agile,
sociable, curious.
820
00:53:40,658 --> 00:53:43,718
Their ancestors were once
atoms made in stars...
821
00:53:43,928 --> 00:53:46,761
...then simple molecules,
single cells...
822
00:53:46,964 --> 00:53:49,228
...polyps stuck to the ocean floor...
823
00:53:49,434 --> 00:53:52,835
...fish, amphibians, reptiles, shrews.
824
00:53:53,237 --> 00:53:57,640
But then they came down
from the trees and stood upright.
825
00:53:57,842 --> 00:54:00,208
They grew an enormous brain...
826
00:54:00,411 --> 00:54:03,539
...they developed culture,
invented tools...
827
00:54:03,748 --> 00:54:05,807
...domesticated fire.
828
00:54:09,353 --> 00:54:12,015
They discovered language and writing.
829
00:54:12,223 --> 00:54:14,214
They developed agriculture.
830
00:54:14,425 --> 00:54:18,020
They built cities and forged metal.
831
00:54:19,797 --> 00:54:23,631
And ultimately,
they set out for the stars...
832
00:54:23,835 --> 00:54:28,169
...from which they had come
5 billion years earlier.
833
00:54:30,575 --> 00:54:32,099
We are star stuff...
834
00:54:32,310 --> 00:54:35,711
...which has taken its destiny
into its own hands.
835
00:54:38,516 --> 00:54:40,643
The loom of time and space...
836
00:54:40,852 --> 00:54:44,481
...works the most astonishing
transformations of matter.
837
00:54:45,656 --> 00:54:48,648
Our own planet is only a tiny part...
838
00:54:48,860 --> 00:54:50,987
...of the vast cosmic tapestry...
839
00:54:51,195 --> 00:54:55,928
...a starry fabric
of worlds yet untold.
840
00:55:02,807 --> 00:55:06,834
Those worlds in space
are as countless...
841
00:55:07,044 --> 00:55:10,741
...as all the grains of sand
on all the beaches of the Earth.
842
00:55:11,415 --> 00:55:14,350
Each of those worlds
is as real as ours.
843
00:55:14,552 --> 00:55:17,043
In every one of them,
there's a succession of...
844
00:55:17,255 --> 00:55:21,885
...incidents, events, occurrences
which influence its future.
845
00:55:22,093 --> 00:55:25,688
Countless worlds,
numberless moments...
846
00:55:25,897 --> 00:55:29,355
...an immensity of space and time.
847
00:55:29,567 --> 00:55:32,365
And our small planet,
at this moment...
848
00:55:32,570 --> 00:55:37,007
...here, we face
a critical branchpoint in history.
849
00:55:37,208 --> 00:55:40,109
What we do with our world right now...
850
00:55:40,311 --> 00:55:42,609
...will propagate down
through the centuries...
851
00:55:42,814 --> 00:55:46,215
...and powerfully affect
the destiny of our descendants.
852
00:55:46,417 --> 00:55:50,217
It is well within our power
to destroy our civilization...
853
00:55:50,421 --> 00:55:53,254
...and perhaps our species as well.
854
00:55:53,457 --> 00:55:55,857
If we capitulate to superstition...
855
00:55:56,060 --> 00:55:58,426
...or greed or stupidity...
856
00:55:58,629 --> 00:56:02,565
...we can plunge our world into
a darkness deeper than the time...
857
00:56:02,767 --> 00:56:07,101
...between the collapse of classical
civilization and Italian Renaissance.
858
00:56:07,305 --> 00:56:09,569
But we are also capable...
859
00:56:09,774 --> 00:56:12,265
...of using our compassion
and our intelligence...
860
00:56:12,476 --> 00:56:15,070
...our technology and our wealth...
861
00:56:15,279 --> 00:56:17,747
...to make an abundant
and meaningful life...
862
00:56:17,949 --> 00:56:20,247
...for every inhabitant
of this planet...
863
00:56:20,451 --> 00:56:24,945
...to enhance enormously
our understanding of the universe...
864
00:56:25,356 --> 00:56:28,416
...and to carry us to the stars.
865
00:56:44,275 --> 00:56:45,902
In our motorbike sequence...
866
00:56:46,110 --> 00:56:48,601
...we showed how
the landscape might look...
867
00:56:48,813 --> 00:56:51,543
...if we barreled through it
at close to light speed.
868
00:56:51,749 --> 00:56:54,684
Since then,
inspired by this sequence...
869
00:56:54,886 --> 00:56:58,481
...Ping-Kang Hsiung
at Carnegie Mellon University...
870
00:56:58,689 --> 00:57:00,623
...produced an exact
computer animation.
871
00:57:01,125 --> 00:57:04,356
This is what you'd see if you
traveled at ordinary speeds...
872
00:57:04,562 --> 00:57:06,792
...through this red and white lattice.
873
00:57:06,998 --> 00:57:08,829
But this is how it would appear...
874
00:57:09,033 --> 00:57:12,969
...if you were traveling
at close to the speed of light.
875
00:57:13,838 --> 00:57:17,865
We're probably many centuries away
from traveling close to light speed...
876
00:57:18,075 --> 00:57:20,942
...and experiencing time dilation.
877
00:57:21,145 --> 00:57:24,012
But even then,
it might not be fast enough...
878
00:57:24,215 --> 00:57:27,616
...if we wanted to travel
to some distant place in the galaxy...
879
00:57:27,818 --> 00:57:30,548
...and then come back to Earth
in our own epoch.
880
00:57:31,055 --> 00:57:34,149
Some years after completing Cosmos...
881
00:57:34,358 --> 00:57:38,886
...I took time out from
my scientific work to write a novel.
882
00:57:39,397 --> 00:57:40,989
A novel about travel...
883
00:57:41,198 --> 00:57:44,395
...to the center
of the Milky Way galaxy.
884
00:57:44,669 --> 00:57:48,002
I was willing to imagine
beings and civilizations...
885
00:57:48,205 --> 00:57:50,230
...far more advanced than we...
886
00:57:50,441 --> 00:57:53,740
...but I wasn't willing
to ignore the laws of physics.
887
00:57:53,978 --> 00:57:58,540
Was there, even in principle,
a way to get very quickly...
888
00:57:58,749 --> 00:58:01,718
...to 30,000 light-years from Earth?
889
00:58:01,919 --> 00:58:03,784
So I asked my friend...
890
00:58:03,988 --> 00:58:07,219
...Kip Thorne of the California
Institute of Technology.
891
00:58:07,425 --> 00:58:10,451
He's a leading expert
on the nature of space and time.
892
00:58:10,661 --> 00:58:13,129
Kip thought about it for a while...
893
00:58:13,331 --> 00:58:16,425
...and then answered with
about 50 lines of equations...
894
00:58:16,634 --> 00:58:19,364
...which showed that
a really advanced civilization...
895
00:58:19,570 --> 00:58:23,506
...might establish
and hold open wormholes...
896
00:58:25,710 --> 00:58:29,407
...which we might think of as tubes
through the fourth dimension...
897
00:58:29,613 --> 00:58:32,446
...which connect the Earth
with another place...
898
00:58:32,650 --> 00:58:36,108
...without having to traverse
the intervening distance.
899
00:58:36,354 --> 00:58:40,222
Something like crawling
through a wormhole in an apple.
900
00:58:40,658 --> 00:58:42,523
I was happy with this result...
901
00:58:42,727 --> 00:58:46,219
...and used it as
a key plot device in Contact.
902
00:58:46,697 --> 00:58:48,722
But such wormholes through space...
903
00:58:48,933 --> 00:58:51,868
...would also be time machines,
it seemed to me.
904
00:58:52,069 --> 00:58:54,936
And I used that notion
in my novel Contact as well.
905
00:58:55,406 --> 00:58:59,502
Kip Thorne and his colleagues
later proved, or so it seemed...
906
00:58:59,710 --> 00:59:02,178
...that time travel
of this sort was possible.
907
00:59:02,380 --> 00:59:04,712
Here, look at this.
908
00:59:05,916 --> 00:59:08,544
The key question being explored now...
909
00:59:08,753 --> 00:59:12,211
...is whether such time travel
can be done consistently...
910
00:59:12,423 --> 00:59:16,723
...with causes preceding effects, say,
rather than following them.
911
00:59:16,927 --> 00:59:18,394
Does nature contrive it...
912
00:59:18,596 --> 00:59:21,827
...so that even with a time machine,
you can't intervene...
913
00:59:22,033 --> 00:59:25,127
...to prevent your own conception,
for example?
914
00:59:25,336 --> 00:59:28,362
Even if time travel of this sort
is really possible...
915
00:59:28,572 --> 00:59:31,439
...it's far in
our technological future.
916
00:59:31,642 --> 00:59:35,601
But maybe other beings
much more advanced than we...
917
00:59:35,813 --> 00:59:38,748
...are voyaging to the far future
and the remote past...
918
00:59:38,949 --> 00:59:41,941
...not a measly 40 years ago
on Earth...
919
00:59:42,153 --> 00:59:44,644
...but to witness
the death of the sun, say...
920
00:59:44,855 --> 00:59:47,050
...or the origin of the cosmos.
78921
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