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SAGAN: We are drifting in
a great ocean of space and time.
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00:00:59,207 --> 00:01:02,108
In that ocean, the events
that shape the future...
3
00:01:02,310 --> 00:01:04,710
...are working themselves out.
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00:01:06,915 --> 00:01:10,248
Each creature and every world,
to the remotest star...
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00:01:10,451 --> 00:01:12,351
...owe their existence to...
6
00:01:12,554 --> 00:01:15,387
...the great, coursing,
implacable forces of nature...
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00:01:15,590 --> 00:01:18,718
...but also, to minor happenstance.
8
00:01:21,262 --> 00:01:24,527
We are carried with our planet
around the sun.
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00:01:24,732 --> 00:01:28,168
The Earth has made more than
4 billion circuits of our star...
10
00:01:28,369 --> 00:01:30,200
...since its origin.
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00:01:32,240 --> 00:01:36,142
The sun itself travels about
the core of the Milky Way galaxy.
12
00:01:36,344 --> 00:01:38,972
Our galaxy is moving
among the other galaxies.
13
00:01:39,180 --> 00:01:42,547
We have always been space travelers.
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00:01:44,352 --> 00:01:49,221
These fine sand grains are all,
more or less, uniform in size.
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They're produced from bigger rocks
through ages of...
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00:01:53,561 --> 00:01:57,395
...jostling and rubbing,
abrasion and erosion.
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Driven in part by
the distant moon and sun.
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00:02:01,803 --> 00:02:05,364
So the roots of the present
lie buried in the past.
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00:02:05,573 --> 00:02:09,339
We are also travelers in time.
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00:02:13,815 --> 00:02:15,043
But trapped on Earth...
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00:02:15,249 --> 00:02:18,582
...we've had little to say about
where we go in time and space...
22
00:02:18,786 --> 00:02:20,151
...or how fast.
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00:02:20,355 --> 00:02:24,086
But now we're thinking
about true journeys in time...
24
00:02:24,292 --> 00:02:28,194
...and real voyages
to the distant stars.
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00:02:29,964 --> 00:02:34,560
A handful of sand contains
about 1 0,000 grains...
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00:02:34,769 --> 00:02:36,964
...more than all the stars
we can see...
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00:02:37,171 --> 00:02:39,503
...with the naked eye
on a clear night.
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00:02:39,707 --> 00:02:41,800
But the number of stars we can see...
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00:02:42,010 --> 00:02:45,605
...is only the tiniest fraction
of the number of stars that are.
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00:02:46,414 --> 00:02:49,440
What we see at night
is the merest smattering...
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...of the nearest stars...
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00:02:51,819 --> 00:02:56,085
...with a few more distant bright
stars thrown in for good measure.
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00:02:56,290 --> 00:03:00,158
Meanwhile, the cosmos
is rich beyond measure.
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00:03:00,361 --> 00:03:02,488
The number of stars
in the universe...
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00:03:02,697 --> 00:03:06,030
...is larger than all the grains
of sand on all the beaches...
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00:03:06,234 --> 00:03:07,826
...of the planet Earth.
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00:03:11,706 --> 00:03:16,507
Long ago, before we had figured out
that the stars are distant suns...
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00:03:16,711 --> 00:03:19,839
...they seemed to us
to make pictures in the sky.
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00:03:20,048 --> 00:03:22,846
Just follow the dots.
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00:03:24,152 --> 00:03:27,315
The Big Dipper constellation
today in North America...
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00:03:27,522 --> 00:03:29,683
...has had many other incarnations.
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00:03:29,891 --> 00:03:32,155
Every culture, ancient and modern...
43
00:03:32,427 --> 00:03:36,193
...has placed its totems
and concerns among the stars.
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00:03:36,397 --> 00:03:40,390
From a Chinese bureaucrat
to a German wagon.
45
00:03:44,439 --> 00:03:48,239
But very ancient cultures would have
seen different constellations...
46
00:03:48,443 --> 00:03:51,742
...because the stars move
with respect to one another.
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00:03:51,946 --> 00:03:56,713
We can give a computer the present
positions and motions of stars...
48
00:03:56,918 --> 00:04:00,615
...and then run the patterns
back into time.
49
00:04:02,623 --> 00:04:06,320
Every constellation is a single frame
in a cosmic movie...
50
00:04:06,527 --> 00:04:08,893
...but because our lives
are so short...
51
00:04:09,097 --> 00:04:11,088
...because star patterns
change slowly...
52
00:04:11,299 --> 00:04:14,234
...we tend not to notice
it's a movie.
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00:04:14,435 --> 00:04:18,235
A million years ago,
there was no Big Dipper.
54
00:04:18,639 --> 00:04:21,938
Our ancestors, looking up
and wondering about the stars...
55
00:04:22,143 --> 00:04:25,772
...saw some other pattern
in the northern skies.
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00:04:26,814 --> 00:04:31,183
We can also run a constellation,
Leo the Lion, say, forward in time...
57
00:04:31,385 --> 00:04:35,287
...and see what the patterns
in the stars will be in the future.
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00:04:37,058 --> 00:04:39,458
A million years from now,
Leo might be renamed...
59
00:04:39,660 --> 00:04:42,356
...the constellation
of the Radio Telescope.
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00:04:42,563 --> 00:04:45,862
Although I suspect radio telescopes
then will be as obsolete...
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00:04:46,067 --> 00:04:47,967
...as stone spears are now.
62
00:04:48,169 --> 00:04:52,196
Or, here's the constellation
of Cetus the Whale.
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00:05:01,215 --> 00:05:05,948
A million years ago, it may have
been called something else.
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00:05:06,154 --> 00:05:07,644
Perhaps the Spear.
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00:05:12,760 --> 00:05:17,288
Now, let's run fast-forward
through a billion nights.
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00:05:22,436 --> 00:05:24,063
Millions of years from now...
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00:05:24,272 --> 00:05:28,470
...some other very different image
will be featured in this cosmic movie.
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00:05:36,417 --> 00:05:39,443
In Orion the Hunter,
things are changing...
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00:05:39,654 --> 00:05:41,588
...not only because
the stars are moving...
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00:05:41,789 --> 00:05:44,553
...but also because
the stars are evolving.
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00:05:44,759 --> 00:05:48,320
Many of Orion's stars are
hot, young and short-lived.
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00:05:48,529 --> 00:05:52,590
They're born, live and die within
a span of only a few million years.
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00:05:52,800 --> 00:05:55,633
If we run Orion forward in time...
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00:05:55,837 --> 00:05:58,670
...we see the births
and explosive deaths...
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00:05:58,873 --> 00:06:00,397
...of dozens of stars...
76
00:06:00,608 --> 00:06:04,738
...flashing on and winking off
like fireflies in the night.
77
00:06:07,181 --> 00:06:10,708
If we wait long enough,
we see the constellations change.
78
00:06:10,918 --> 00:06:14,854
But if we go far enough,
we also see the star patterns alter.
79
00:06:15,056 --> 00:06:16,921
Two-dimensional constellations...
80
00:06:17,124 --> 00:06:21,117
...are only the appearance of stars
strewn through three dimensions.
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00:06:21,329 --> 00:06:25,629
Some are dim and near,
others are bright but farther away.
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00:06:26,868 --> 00:06:29,200
Could a space traveler
actually see...
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00:06:29,403 --> 00:06:32,099
...the patterns of
the constellations change?
84
00:06:32,306 --> 00:06:37,175
For that, you must travel roughly as
far as the constellation is from us.
85
00:06:37,645 --> 00:06:40,671
Here, we're traveling
hundreds of light-years...
86
00:06:40,882 --> 00:06:45,182
...circling all the way around
the stars of the Big Dipper.
87
00:06:48,723 --> 00:06:50,850
Inhabitants of planets
around other stars...
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00:06:51,058 --> 00:06:53,492
...will see different constellations
than us...
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00:06:53,694 --> 00:06:56,663
...because their vantage points
are different.
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00:07:04,739 --> 00:07:08,402
Here we are
in the constellation Andromeda...
91
00:07:08,609 --> 00:07:13,410
...or at least a model of it
next to the constellation Perseus.
92
00:07:13,614 --> 00:07:15,775
Andromeda, in the Greek myth...
93
00:07:15,983 --> 00:07:19,783
...was the maiden
who was saved by Perseus...
94
00:07:19,987 --> 00:07:21,921
...from a sea monster.
95
00:07:22,123 --> 00:07:27,026
This star just above me
is Beta Andromedae...
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00:07:27,228 --> 00:07:29,856
...the second brightest star
in the constellation...
97
00:07:30,064 --> 00:07:33,227
...75 light-years from the Earth.
98
00:07:33,434 --> 00:07:36,528
The light by which we see this star...
99
00:07:36,737 --> 00:07:40,867
...has spent 75 years
traversing interstellar space...
100
00:07:41,075 --> 00:07:43,066
...on its journey to the Earth.
101
00:07:43,277 --> 00:07:46,872
In the unlikely event
that Beta Andromedae...
102
00:07:47,081 --> 00:07:49,606
...blew itself up
a week ago Tuesday...
103
00:07:49,817 --> 00:07:52,581
...we will not know of it
for another 75 years...
104
00:07:52,787 --> 00:07:56,780
...as this interesting information,
traveling at the speed of light...
105
00:07:56,991 --> 00:08:00,620
...crosses the enormous
interstellar distances.
106
00:08:00,828 --> 00:08:03,319
When the light we see
from this star set out...
107
00:08:03,531 --> 00:08:06,261
...on its long
interstellar voyage...
108
00:08:06,467 --> 00:08:09,027
...the young Albert Einstein...
109
00:08:09,236 --> 00:08:12,103
...working as a Swiss patent clerk...
110
00:08:12,306 --> 00:08:16,208
...had just published his epochal
special theory of relativity...
111
00:08:16,410 --> 00:08:17,741
...here on Earth.
112
00:08:18,612 --> 00:08:19,840
We see...
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00:08:20,047 --> 00:08:24,177
...that space and time
are intertwined.
114
00:08:24,385 --> 00:08:27,252
We cannot look out into space...
115
00:08:27,455 --> 00:08:30,754
...without looking back into time.
116
00:08:31,025 --> 00:08:34,119
The speed of light is very fast...
117
00:08:34,328 --> 00:08:37,525
...but space is very empty...
118
00:08:37,732 --> 00:08:41,031
...and the stars are very far apart.
119
00:08:41,235 --> 00:08:44,363
The distances that we've been
talking about up to now...
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00:08:44,572 --> 00:08:48,372
...are very small
by the usual astronomical standards.
121
00:08:48,576 --> 00:08:51,204
In fact, the distance
from the Earth...
122
00:08:51,412 --> 00:08:53,403
...to the center
of the Milky Way galaxy...
123
00:08:53,614 --> 00:08:56,481
...is 30,000 light-years.
124
00:08:57,451 --> 00:09:02,388
From our galaxy to the nearest
spiral galaxy like our own...
125
00:09:02,723 --> 00:09:04,520
...called M31 ...
126
00:09:04,725 --> 00:09:07,626
...and which is also within,
that means behind...
127
00:09:07,828 --> 00:09:09,796
...the constellation Andromeda...
128
00:09:10,531 --> 00:09:14,262
...is 2 million light-years.
129
00:09:15,503 --> 00:09:18,700
When the light we see today
from M31 ...
130
00:09:18,906 --> 00:09:21,431
...left on its journey for Earth...
131
00:09:21,642 --> 00:09:23,576
...there were no human beings...
132
00:09:23,778 --> 00:09:27,077
...although our ancestors
were nicely evolving...
133
00:09:27,281 --> 00:09:30,341
...and very rapidly,
to our present form.
134
00:09:31,018 --> 00:09:33,509
There are much greater distances
in astronomy.
135
00:09:33,721 --> 00:09:37,316
The distance from the Earth
to the most distant quasars...
136
00:09:37,525 --> 00:09:40,961
...is 8 or 1 0 billion light-years.
137
00:09:41,162 --> 00:09:45,861
We see them as they were before
the Earth itself accumulated...
138
00:09:46,067 --> 00:09:49,628
...before the Milky Way galaxy
was formed.
139
00:09:49,837 --> 00:09:53,466
The fastest space vehicles ever
launched by the human species...
140
00:09:53,674 --> 00:09:55,608
...are the Voyager spacecraft.
141
00:09:55,810 --> 00:09:57,505
They are traveling so fast...
142
00:09:57,711 --> 00:10:01,306
...that it's only
1 0,000 times slower...
143
00:10:01,816 --> 00:10:03,113
...than the speed of light.
144
00:10:03,317 --> 00:10:05,945
The Voyager spacecraft
will take 40,000 years...
145
00:10:06,153 --> 00:10:08,212
...to go the distance
to the nearest stars...
146
00:10:08,422 --> 00:10:11,619
...and they're not even headed
towards the nearest stars.
147
00:10:11,826 --> 00:10:14,420
But is there a method
by which we could travel...
148
00:10:14,628 --> 00:10:18,120
...in a conveniently short time
to the stars?
149
00:10:18,332 --> 00:10:21,165
Can we travel close
to the speed of light?
150
00:10:21,368 --> 00:10:24,098
And what's magic
about the speed of light?
151
00:10:24,305 --> 00:10:26,899
Can't we travel faster than that?
152
00:10:28,909 --> 00:10:32,743
It turns out that
there is something very strange...
153
00:10:32,947 --> 00:10:34,414
...about the speed of light.
154
00:10:34,615 --> 00:10:36,674
Something that provides the key...
155
00:10:36,884 --> 00:10:40,285
...to our understanding
of time and space.
156
00:10:42,022 --> 00:10:43,717
The story of its discovery...
157
00:10:43,924 --> 00:10:47,519
...takes us to Tuscany
in northern Italy.
158
00:10:49,830 --> 00:10:52,264
There's something timeless
about this place.
159
00:10:52,466 --> 00:10:56,129
A century ago, it probably
looked very much the same.
160
00:11:08,249 --> 00:11:12,447
If you had traveled these roads
in the summer of 1895...
161
00:11:12,653 --> 00:11:16,783
...you might have come upon a
16-year-old German high-school dropout.
162
00:11:16,991 --> 00:11:20,154
His teacher told him that
he'd never amount to anything...
163
00:11:20,361 --> 00:11:23,819
...that his attitude destroyed
classroom discipline...
164
00:11:24,031 --> 00:11:25,760
...that he should drop out.
165
00:11:25,966 --> 00:11:27,957
So he left and came here...
166
00:11:28,169 --> 00:11:30,603
...where he enjoyed
wandering these roads...
167
00:11:30,804 --> 00:11:33,272
...and giving his mind
free rein to explore.
168
00:11:35,276 --> 00:11:37,744
One day, he began
to think about light...
169
00:11:37,945 --> 00:11:39,936
...about how fast it travels.
170
00:11:40,147 --> 00:11:43,207
We always measure
the speed of a moving object...
171
00:11:43,417 --> 00:11:45,385
...relative to something else.
172
00:11:45,586 --> 00:11:49,613
I'm moving at about 10 kilometers
an hour relative to the ground.
173
00:11:49,823 --> 00:11:51,450
But the ground isn't at rest.
174
00:11:51,659 --> 00:11:55,425
The Earth is turning at more
than 1600 kilometers an hour.
175
00:11:55,629 --> 00:11:58,097
The Earth itself is
in orbit around the sun.
176
00:11:58,299 --> 00:12:02,360
The sun is moving among
the drifting stars, and so on.
177
00:12:02,570 --> 00:12:05,971
It was hard for the young man
to imagine some absolute standard...
178
00:12:06,173 --> 00:12:09,040
...to measure all these
relative motions against.
179
00:12:18,852 --> 00:12:22,720
He knew that sound waves are
a vibration of the air...
180
00:12:22,923 --> 00:12:26,086
...and their speed is measured
relative to the air itself.
181
00:12:26,293 --> 00:12:29,490
But sunlight travels across
the vacuum of empty space.
182
00:12:29,697 --> 00:12:31,961
"Do light waves move
relative to something else?
183
00:12:32,166 --> 00:12:35,761
And if so," he wondered,
"relative to what?"
184
00:12:39,306 --> 00:12:43,436
That teenage dropout's name...
185
00:12:44,111 --> 00:12:45,772
...was Albert Einstein.
186
00:12:45,980 --> 00:12:48,915
And his ruminations changed the world.
187
00:12:54,255 --> 00:12:58,385
He had been fascinated
by Bernstein's 1 869...
188
00:12:59,260 --> 00:13:02,855
...People's Book of Natural Science.
189
00:13:03,063 --> 00:13:06,226
Here, on its very first page...
190
00:13:07,101 --> 00:13:11,231
...it describes the astonishing speed
of electricity through wires...
191
00:13:11,572 --> 00:13:13,506
...and light through space.
192
00:13:13,841 --> 00:13:17,777
Einstein wondered, perhaps for
the first time, in northern Italy...
193
00:13:18,545 --> 00:13:22,743
...what the world would look like if
you could travel on a wave of light.
194
00:13:23,317 --> 00:13:25,512
To travel at the speed of light.
195
00:13:25,719 --> 00:13:30,520
What an engaging and magical thought
for a teenage boy on the road...
196
00:13:30,724 --> 00:13:34,592
...where the countryside is dappled
and rippling in sunlight.
197
00:13:45,606 --> 00:13:50,009
You couldn't tell you were on a light
wave if you were traveling with it.
198
00:13:50,210 --> 00:13:52,838
If you started on a wave crest...
199
00:13:53,047 --> 00:13:57,848
...you would stay on the crest and
lose all notion of it being a wave.
200
00:13:58,052 --> 00:14:02,887
Something funny happens
at the speed of light.
201
00:14:33,487 --> 00:14:37,253
The more Einstein thought about it,
the more troubling it became.
202
00:14:37,458 --> 00:14:40,018
Paradoxes seemed to pop up all over...
203
00:14:40,227 --> 00:14:42,195
...if you could travel
at the speed of light.
204
00:14:42,396 --> 00:14:45,797
Certain ideas had been
accepted as true...
205
00:14:45,999 --> 00:14:48,467
...without sufficiently
careful thought.
206
00:14:50,604 --> 00:14:54,631
One of those ideas had to do
with the light from a moving object.
207
00:14:56,110 --> 00:14:59,341
The images by which we see the world
are made of light...
208
00:14:59,546 --> 00:15:01,673
...and are carried
at the speed of light...
209
00:15:01,882 --> 00:15:05,147
...300,000 kilometers a second.
210
00:15:05,753 --> 00:15:09,553
You might think that the image of me
should be moving out ahead of me...
211
00:15:09,757 --> 00:15:12,988
...at the speed of light
plus the speed of the bicycle.
212
00:15:13,193 --> 00:15:16,720
If I'm moving towards you
faster than a horse-and-cart...
213
00:15:16,930 --> 00:15:19,899
...then my image should be
approaching you that much faster.
214
00:15:20,100 --> 00:15:22,694
My image ought to arrive earlier.
215
00:15:24,838 --> 00:15:27,363
But in reality
you don't see any time delay.
216
00:15:27,808 --> 00:15:31,676
In a near collision, for example,
you see everything happen at once.
217
00:15:31,879 --> 00:15:35,542
Horse, cart, swerve, bicycle.
All simultaneous.
218
00:15:36,283 --> 00:15:40,242
But how would it look if
it were proper to add the velocities?
219
00:15:40,454 --> 00:15:43,890
Since I'm heading toward you, you'd
add my speed to the speed of light.
220
00:15:44,091 --> 00:15:48,858
So my image ought to arrive before
the image of the horse-and-cart.
221
00:15:49,763 --> 00:15:52,254
I'd be cycling towards you
quite normally.
222
00:15:52,466 --> 00:15:55,958
To me, a collision
would seem imminent.
223
00:15:56,170 --> 00:15:59,071
But you'd see me swerve
for no apparent reason...
224
00:15:59,273 --> 00:16:01,605
...and have a collision with nothing.
225
00:16:02,643 --> 00:16:05,441
Now, the horse-and-cart
aren't headed towards you.
226
00:16:05,646 --> 00:16:09,377
Their image would arrive only
at the speed of light.
227
00:16:10,117 --> 00:16:12,711
Could it seem to me that
I just missed colliding...
228
00:16:13,253 --> 00:16:15,813
...while to you it wasn't even close?
229
00:16:16,023 --> 00:16:18,184
In precise laboratory experiments...
230
00:16:18,392 --> 00:16:21,520
...scientists have never observed
any such thing.
231
00:16:22,129 --> 00:16:24,495
If the world is to be understood...
232
00:16:24,832 --> 00:16:29,735
...if we are to avoid logical paradoxes
when traveling at high speeds...
233
00:16:29,937 --> 00:16:32,462
...then there are rules
which must be obeyed.
234
00:16:32,673 --> 00:16:37,406
Einstein called these rules
the special theory of relativity.
235
00:16:37,611 --> 00:16:40,136
Light from a moving object
travels at the same speed...
236
00:16:40,347 --> 00:16:43,976
...no matter whether the object
is at rest or in motion.
237
00:16:44,184 --> 00:16:48,848
"Thou shalt not add my speed
to the speed of light."
238
00:16:49,056 --> 00:16:53,891
Also, no material object can travel
at or beyond the speed of light.
239
00:16:54,094 --> 00:16:58,155
Nothing in physics prevents you from
traveling close to the speed of light.
240
00:16:58,365 --> 00:17:02,165
99.9 percent the speed of light
is just fine.
241
00:17:02,369 --> 00:17:04,735
But no matter how hard you try...
242
00:17:04,938 --> 00:17:07,702
...you can never gain
that last decimal point.
243
00:17:07,908 --> 00:17:10,172
For the world
to be logically consistent...
244
00:17:10,377 --> 00:17:13,505
...there must be a cosmic speed limit.
245
00:17:14,281 --> 00:17:16,977
The crack of a whip is,
due to its tip...
246
00:17:17,184 --> 00:17:19,175
...moving faster
than the speed of sound.
247
00:17:21,822 --> 00:17:23,050
It makes a shock wave...
248
00:17:23,257 --> 00:17:27,091
...a small sonic boom
in the Italian countryside.
249
00:17:27,294 --> 00:17:29,626
A thunderclap has a similar origin.
250
00:17:29,830 --> 00:17:33,163
So does the sound of
a supersonic airplane.
251
00:17:35,202 --> 00:17:39,764
So why is the speed of light a barrier
any more than the speed of sound?
252
00:17:39,973 --> 00:17:41,838
The answer is not just that...
253
00:17:42,042 --> 00:17:44,636
...light travels a million times
faster than sound.
254
00:17:44,845 --> 00:17:48,508
It's not merely an engineering problem
like the supersonic airplane.
255
00:17:49,082 --> 00:17:53,143
Instead, the light barrier is
a fundamental law of nature...
256
00:17:53,353 --> 00:17:55,253
...as basic as gravity.
257
00:17:55,455 --> 00:17:58,822
Einstein found his absolute framework
for the world:
258
00:17:59,026 --> 00:18:03,395
This sturdy pillar among all
the relative motions of the cosmos.
259
00:18:03,597 --> 00:18:07,658
Light travels just as fast,
no matter how its source is moving.
260
00:18:07,868 --> 00:18:11,895
The speed of light is constant,
relative to everything else.
261
00:18:12,105 --> 00:18:15,404
Nothing can ever catch up with light.
262
00:18:18,378 --> 00:18:21,905
Einstein's prohibition against
traveling faster than light...
263
00:18:22,115 --> 00:18:24,845
...seems to clash with
our common sense notions.
264
00:18:25,052 --> 00:18:27,850
But why should we expect
our common sense notions...
265
00:18:28,055 --> 00:18:30,956
...to have any reliability
in a matter of this sort?
266
00:18:31,158 --> 00:18:35,026
Why should our experience
at 1 0 kilometers an hour...
267
00:18:35,228 --> 00:18:37,423
...constrain the laws of nature...
268
00:18:37,631 --> 00:18:40,930
...at 300,000 kilometers a second?
269
00:18:43,170 --> 00:18:45,764
Relativity sets limits...
270
00:18:45,973 --> 00:18:49,033
...on what humans ultimately can do.
271
00:18:49,576 --> 00:18:52,204
The universe is not required...
272
00:18:52,412 --> 00:18:56,781
...to be in perfect harmony
with human ambition.
273
00:19:00,253 --> 00:19:03,154
Imagine a place
where the speed of light...
274
00:19:03,357 --> 00:19:06,724
...isn't its true value
of 300,000 kilometers a second...
275
00:19:06,927 --> 00:19:09,487
...but something a lot less.
276
00:19:09,696 --> 00:19:13,097
Let's say, 40 kilometers an hour...
277
00:19:13,300 --> 00:19:15,359
...and strictly enforced.
278
00:19:16,269 --> 00:19:20,171
Just as in the real world we can
never reach the speed of light...
279
00:19:20,374 --> 00:19:22,239
...the commandment here is still...
280
00:19:22,442 --> 00:19:25,775
..."Thou shalt not travel
faster than light."
281
00:19:25,979 --> 00:19:30,473
We can do thought experiments on
what happens near the speed of light...
282
00:19:30,684 --> 00:19:35,018
...here 40 kilometers per hour,
the speed of a motor scooter.
283
00:19:38,358 --> 00:19:42,351
You can't break the laws of nature.
There are no penalties for doing so.
284
00:19:42,729 --> 00:19:44,594
The real world and this one...
285
00:19:44,931 --> 00:19:49,061
...are merely so arranged
that transgressions can't happen.
286
00:19:49,269 --> 00:19:53,205
The job of physics is to find out
what those laws are.
287
00:19:55,542 --> 00:19:58,511
Before Einstein,
physicists thought that...
288
00:19:58,712 --> 00:20:01,112
...there were privileged frames
of reference...
289
00:20:01,314 --> 00:20:03,805
...some special places and times...
290
00:20:04,017 --> 00:20:06,577
...against which everything else
had to be measured.
291
00:20:06,787 --> 00:20:10,154
Einstein encountered
a similar notion in human affairs.
292
00:20:10,357 --> 00:20:13,053
The idea that the customs
of a particular nation...
293
00:20:13,260 --> 00:20:16,821
...his native Germany
or Italy or anywhere...
294
00:20:17,030 --> 00:20:20,966
...are the standard which all
other societies must be measured.
295
00:20:21,835 --> 00:20:25,271
But Einstein rejected the strident
nationalism of his time.
296
00:20:25,472 --> 00:20:28,566
He believed every culture
had its own validity.
297
00:20:28,775 --> 00:20:30,709
Also in physics,
he understood that...
298
00:20:30,911 --> 00:20:33,175
...there are no privileged
frames of reference.
299
00:20:33,380 --> 00:20:36,508
Every observer,
in any place, time or motion...
300
00:20:36,717 --> 00:20:39,345
...must deduce
the same laws of nature.
301
00:20:39,553 --> 00:20:41,453
(SPEAKING IN ITALIAN)
302
00:20:42,989 --> 00:20:46,823
A speed is simply how much space
you cover in a given time...
303
00:20:47,027 --> 00:20:49,689
...as any kid on
a motor scooter knows.
304
00:20:53,133 --> 00:20:54,998
Since near the velocity of light...
305
00:20:55,202 --> 00:20:57,693
...we cannot simply add speeds...
306
00:20:57,904 --> 00:21:01,533
...the familiar notions of
absolute space and absolute time...
307
00:21:01,742 --> 00:21:04,734
...independent of your
relative motion, must give way.
308
00:21:04,945 --> 00:21:07,505
That's why, as Einstein showed...
309
00:21:07,714 --> 00:21:11,673
...funny things have to happen
close to the speed of light.
310
00:21:12,419 --> 00:21:16,253
There, our conventional perspectives
of space and time...
311
00:21:16,456 --> 00:21:18,515
...strangely change.
312
00:21:20,961 --> 00:21:24,897
Your nose is just a little closer
to me than your ears.
313
00:21:25,098 --> 00:21:27,328
Light reflected off your nose
reaches me...
314
00:21:27,534 --> 00:21:29,764
...an instant in time
before your ears.
315
00:21:29,970 --> 00:21:33,201
But suppose I had a magic camera...
316
00:21:33,406 --> 00:21:36,398
...so that I could see
your nose and your ears...
317
00:21:36,610 --> 00:21:38,578
...at precisely the same instant?
318
00:21:38,779 --> 00:21:40,246
(SCOOTER STARTS UP)
319
00:21:40,447 --> 00:21:41,914
(SCOOTER HONKS)
320
00:21:42,115 --> 00:21:46,552
With such a camera you could take
some pretty interesting pictures.
321
00:21:47,921 --> 00:21:51,322
Paolo says goodbye to
his little brother, Vincenzo...
322
00:21:51,691 --> 00:21:53,784
-Ciao, Vincenzo.
-Ciao, Paolo.
323
00:21:54,594 --> 00:21:56,221
...and rides off.
324
00:21:56,429 --> 00:21:58,954
He's now going more than
half the speed of light.
325
00:21:59,166 --> 00:22:01,726
He is almost catching up
with his own light waves.
326
00:22:01,935 --> 00:22:04,495
This compresses the light waves
in front of him...
327
00:22:04,704 --> 00:22:06,433
...and his image becomes blue.
328
00:22:06,740 --> 00:22:10,437
The shorter wavelength is
what makes blue light waves blue.
329
00:22:11,011 --> 00:22:14,742
Also Paolo becomes skinny
in the direction of motion.
330
00:22:14,948 --> 00:22:17,007
This isn't just some optical illusion.
331
00:22:17,217 --> 00:22:20,448
It really happens when you travel
near the speed of light.
332
00:22:21,221 --> 00:22:25,555
As he roars away, he leaves his own
light waves stretched out behind him.
333
00:22:25,759 --> 00:22:27,124
Long light waves are red.
334
00:22:27,327 --> 00:22:30,888
We say that his receding image
is red-shifted.
335
00:22:32,465 --> 00:22:37,129
Now Paolo leaves for
a short tour of the countryside.
336
00:22:37,571 --> 00:22:41,098
He experiences something
even stranger.
337
00:22:43,710 --> 00:22:45,940
Everything he can see is squeezed...
338
00:22:46,146 --> 00:22:48,512
...into a moving window
just ahead of him...
339
00:22:48,715 --> 00:22:52,116
...blue-shifted at the center,
red-shifted at the edges.
340
00:22:52,319 --> 00:22:55,618
To a passerby, Paolo appears
blue-shifted when approaching...
341
00:22:55,822 --> 00:22:57,551
...red-shifted when receding.
342
00:22:57,757 --> 00:23:00,817
But to him, the entire world
is both coming and going...
343
00:23:01,027 --> 00:23:02,756
...at nearly the speed of light.
344
00:23:02,963 --> 00:23:06,592
Roadside houses and trees
that has already gone past...
345
00:23:06,800 --> 00:23:10,236
...still appear to him at the edge
of his forward field of view...
346
00:23:10,437 --> 00:23:13,133
...but distorted and red-shifted.
347
00:23:14,307 --> 00:23:18,038
When he slows down,
everything again looks normal.
348
00:23:19,579 --> 00:23:21,945
Only very close
to the speed of light...
349
00:23:22,148 --> 00:23:25,743
...does the visible world
get squeezed into a kind of tunnel.
350
00:23:26,653 --> 00:23:30,111
You'd see these distortions if you
traveled near the speed of light.
351
00:23:30,323 --> 00:23:33,019
Someday, perhaps,
interstellar navigators...
352
00:23:33,226 --> 00:23:35,751
...will take their bearings
on stars behind them...
353
00:23:35,962 --> 00:23:40,456
...whose images have all crowded
together on the forward view screen.
354
00:23:42,736 --> 00:23:45,864
The most bizarre aspect of traveling
near the speed of light...
355
00:23:46,072 --> 00:23:48,666
...is that time slows down.
356
00:23:49,509 --> 00:23:51,807
All clocks,
mechanical and biological...
357
00:23:52,012 --> 00:23:54,810
...tick more slowly
near the speed of light.
358
00:23:55,015 --> 00:23:58,348
But stationary clocks tick
at their usual rate.
359
00:23:58,551 --> 00:24:00,712
If we travel close to light speed...
360
00:24:00,921 --> 00:24:04,015
...we age more slowly
than those we left behind.
361
00:24:09,462 --> 00:24:12,295
Paolo's watch and his internal
sense of time show...
362
00:24:12,832 --> 00:24:16,359
...that he has been gone from
his friends for only a few minutes.
363
00:24:16,870 --> 00:24:21,136
But from their point of view,
he has been away for many decades.
364
00:24:21,341 --> 00:24:24,936
His friends have grown up,
moved on and died.
365
00:24:25,912 --> 00:24:27,539
And his younger brother has been...
366
00:24:27,747 --> 00:24:31,080
...patiently waiting
for him all this time.
367
00:24:33,219 --> 00:24:38,156
The two brothers experience
the paradox of time dilation.
368
00:24:38,358 --> 00:24:42,055
They've encountered
Einstein's special relativity.
369
00:24:43,096 --> 00:24:44,063
Vincenzo.
370
00:24:57,944 --> 00:25:00,378
This was just a thought experiment.
371
00:25:00,580 --> 00:25:03,743
But atomic particles traveling
near the speed of light...
372
00:25:03,950 --> 00:25:07,147
...do decay more slowly
than stationary particles.
373
00:25:07,354 --> 00:25:10,551
As strange and counterintuitive
as it seems...
374
00:25:10,757 --> 00:25:14,022
...time dilation is a law of nature.
375
00:25:15,895 --> 00:25:18,693
Traveling close
to the speed of light...
376
00:25:18,999 --> 00:25:21,900
...is a kind of elixir of life.
377
00:25:22,869 --> 00:25:25,963
Because time slows down
close to the speed of light...
378
00:25:26,172 --> 00:25:29,039
...special relativity provides us...
379
00:25:29,242 --> 00:25:31,938
...with a means of going to the stars.
380
00:25:32,979 --> 00:25:36,176
This region of northern Italy
is not only the caldron...
381
00:25:36,383 --> 00:25:39,682
...of some of the thinking
of the young Albert Einstein...
382
00:25:40,020 --> 00:25:43,353
...it is also the home
of another great genius...
383
00:25:43,556 --> 00:25:45,854
...who lived 400 years earlier.
384
00:25:46,059 --> 00:25:48,357
Leonardo da Vinci.
385
00:25:49,629 --> 00:25:53,998
Leonardo delighted
in climbing these hills...
386
00:25:54,200 --> 00:25:57,567
...and viewing the ground
from a great height...
387
00:25:57,771 --> 00:26:00,069
...as if he were soaring like a bird.
388
00:26:00,273 --> 00:26:02,867
He drew the first aerial views...
389
00:26:03,076 --> 00:26:06,773
...of landscapes, villages,
fortifications.
390
00:26:07,247 --> 00:26:11,183
I've been talking about Einstein
in and around this town of Vinci...
391
00:26:11,384 --> 00:26:13,545
...in which Leonardo grew up.
392
00:26:13,753 --> 00:26:16,813
Einstein greatly respected Leonardo...
393
00:26:17,023 --> 00:26:19,685
...and their spirits, in some sense...
394
00:26:19,893 --> 00:26:23,192
...inhabit this countryside still.
395
00:26:48,154 --> 00:26:51,214
Among Leonardo's
many accomplishments...
396
00:26:51,424 --> 00:26:54,791
...in painting, sculpture,
architecture, natural history...
397
00:26:54,994 --> 00:26:59,431
...anatomy, geology,
civil and military engineering...
398
00:26:59,933 --> 00:27:01,798
...he had a great passion.
399
00:27:02,035 --> 00:27:05,527
He wished to construct a machine...
400
00:27:05,738 --> 00:27:07,399
...which would fly.
401
00:27:07,941 --> 00:27:11,741
He made sketches of such machines,
built miniature models...
402
00:27:11,945 --> 00:27:16,075
...constructed great,
full-scale prototypes.
403
00:27:17,884 --> 00:27:21,547
And not a one of them ever worked.
404
00:27:22,755 --> 00:27:27,124
There were no machines of adequate
capacity available in his time.
405
00:27:27,327 --> 00:27:30,694
The technology was just not ready.
406
00:27:31,598 --> 00:27:34,829
The designs, however, were brilliant.
407
00:27:35,034 --> 00:27:38,197
For example, this bird-like machine...
408
00:27:38,404 --> 00:27:42,397
...here in the Leonardo Museum
in the town of Vinci.
409
00:27:43,576 --> 00:27:48,513
Leonardo's great designs encouraged
engineers in later epochs...
410
00:27:48,715 --> 00:27:53,084
...although Leonardo himself
was very depressed at these failures.
411
00:27:53,286 --> 00:27:55,117
But it's not his fault...
412
00:27:55,321 --> 00:27:58,347
...he was trapped in the 1 5th century.
413
00:27:59,192 --> 00:28:02,889
A somewhat similar case
occurred in 1 939...
414
00:28:03,096 --> 00:28:07,430
...when a group of engineers called
the British Interplanetary Society...
415
00:28:07,634 --> 00:28:09,898
...decided to design a ship...
416
00:28:10,103 --> 00:28:13,072
...which would carry people
to the moon.
417
00:28:13,273 --> 00:28:15,571
Now, it was by no means
the same design...
418
00:28:15,775 --> 00:28:20,439
...as the Apollo ship which actually
took people to the moon years later.
419
00:28:20,647 --> 00:28:23,207
But that design suggested that...
420
00:28:23,416 --> 00:28:25,213
...a mission to the moon
might one day...
421
00:28:25,418 --> 00:28:28,012
...be a practical
engineering possibility.
422
00:28:28,488 --> 00:28:29,682
Today...
423
00:28:30,690 --> 00:28:34,319
...we have preliminary
designs of ships...
424
00:28:34,527 --> 00:28:37,724
...which will take people
to the stars.
425
00:28:37,931 --> 00:28:42,630
They are constructed in Earth orbit
and from there...
426
00:28:42,835 --> 00:28:47,772
...they venture on their great
interstellar journeys.
427
00:28:48,141 --> 00:28:49,540
One of them...
428
00:28:50,009 --> 00:28:53,001
...is called Project Orion.
429
00:28:54,147 --> 00:28:56,081
It utilizes nuclear weapons...
430
00:28:56,282 --> 00:29:00,719
...hydrogen bombs
against an inertial plate.
431
00:29:00,920 --> 00:29:04,651
Each explosion providing
a kind of "putt-putt"...
432
00:29:04,857 --> 00:29:08,793
...a vast nuclear motorboat in space.
433
00:29:09,195 --> 00:29:12,631
Orion seems entirely practical...
434
00:29:12,832 --> 00:29:15,266
...and was under development
in the U.S...
435
00:29:15,468 --> 00:29:18,835
...until the signing
of the international treaty...
436
00:29:19,038 --> 00:29:22,235
...forbidding nuclear weapons
explosions in space.
437
00:29:22,842 --> 00:29:27,779
I think, the Orion starship
is the best use of nuclear weapons...
438
00:29:27,981 --> 00:29:31,815
...provided the ships don't depart
from very near the Earth.
439
00:29:41,394 --> 00:29:44,124
Project Daedalus is
a recent design...
440
00:29:44,330 --> 00:29:46,730
...of the British
Interplanetary Society.
441
00:29:46,933 --> 00:29:50,425
It assumes the existence
of a nuclear fusion reactor...
442
00:29:50,637 --> 00:29:52,935
...something much safer
and more efficient...
443
00:29:53,139 --> 00:29:56,939
...than the existing nuclear
fission power plants.
444
00:30:01,180 --> 00:30:03,444
We do not yet have fusion reactors.
445
00:30:03,650 --> 00:30:06,380
One day, quite soon, we may.
446
00:30:12,091 --> 00:30:15,492
Orion and Daedalus might go...
447
00:30:15,695 --> 00:30:18,323
...10 percent the speed of light.
448
00:30:19,465 --> 00:30:22,332
So a trip to Alpha Centauri,
4 1 l2 light-years away...
449
00:30:22,535 --> 00:30:26,369
...would take 45 years,
less than a human lifetime.
450
00:30:26,939 --> 00:30:30,773
Such ships could not travel
close enough to the speed of light...
451
00:30:30,977 --> 00:30:33,912
...for the time-slowing effects
of special relativity...
452
00:30:34,113 --> 00:30:35,774
...to become important.
453
00:30:36,349 --> 00:30:39,045
It does not seem likely
that such ships...
454
00:30:39,252 --> 00:30:41,846
...would be built before
the middle of the 21 st century...
455
00:30:42,055 --> 00:30:45,855
...although we could build
an Orion starship now.
456
00:30:46,292 --> 00:30:50,353
For voyages beyond the nearest stars,
something must be added.
457
00:30:50,563 --> 00:30:53,657
Perhaps they could be used
as multigeneration ships...
458
00:30:54,033 --> 00:30:57,025
...so those arriving would be
the remote descendants...
459
00:30:57,236 --> 00:31:00,967
...of those who had originally
set out centuries before.
460
00:31:01,607 --> 00:31:05,634
Or perhaps some safe means
of human hibernation might be found...
461
00:31:05,845 --> 00:31:09,713
...so that space travelers might be
frozen and then thawed out...
462
00:31:09,916 --> 00:31:13,818
...when they arrive at
the destination centuries later.
463
00:31:14,520 --> 00:31:18,820
But fast interstellar space flight
approaching the speed of light...
464
00:31:19,025 --> 00:31:20,890
...is much more difficult.
465
00:31:21,094 --> 00:31:23,824
That's an objective
not for a hundred years...
466
00:31:24,030 --> 00:31:27,158
...but for a thousand
or for 1 0 thousand...
467
00:31:27,367 --> 00:31:29,835
...but it also is possible.
468
00:31:32,205 --> 00:31:35,174
A kind of interstellar ramjet
has been proposed...
469
00:31:35,375 --> 00:31:37,809
...which scoops up
the hydrogen atoms...
470
00:31:38,010 --> 00:31:39,944
...which float between the stars...
471
00:31:40,146 --> 00:31:44,207
...accelerates them into an engine
and spits them out the back.
472
00:31:45,051 --> 00:31:47,451
But in deep space,
there is one atom...
473
00:31:47,653 --> 00:31:51,714
...for every 1 0 cubic centimeters
of space.
474
00:31:51,924 --> 00:31:53,391
For the ramjet to work...
475
00:31:53,893 --> 00:31:56,453
...it has to have a frontal scoop...
476
00:31:56,662 --> 00:31:59,529
...hundreds of kilometers across.
477
00:31:59,799 --> 00:32:03,997
Reaching relativistic velocities,
the hydrogen atoms will be moving...
478
00:32:04,203 --> 00:32:06,763
...with respect
to the interstellar spaceship...
479
00:32:06,973 --> 00:32:08,998
...at close to the speed of light.
480
00:32:09,208 --> 00:32:10,937
If precautions aren't taken...
481
00:32:11,144 --> 00:32:15,706
...the passengers will be fried
by these induced cosmic rays.
482
00:32:15,915 --> 00:32:17,780
There's a proposed solution:
483
00:32:17,984 --> 00:32:21,283
A laser is used to strip
electrons off the atoms...
484
00:32:21,487 --> 00:32:24,718
...and electrically charge them
while they're some distance away.
485
00:32:25,291 --> 00:32:27,919
And an extremely strong
magnetic field...
486
00:32:28,161 --> 00:32:31,426
...is used to deflect
the charged atoms into the scoop...
487
00:32:31,631 --> 00:32:33,258
...and away from the spacecraft.
488
00:32:33,466 --> 00:32:34,763
This is engineering...
489
00:32:34,967 --> 00:32:38,767
...on a scale so far
unprecedented on the Earth.
490
00:32:38,971 --> 00:32:43,567
We are talking of engines
the size of small worlds.
491
00:32:52,985 --> 00:32:57,888
Suppose that the spacecraft is
designed to accelerate at 1 g...
492
00:32:58,090 --> 00:33:00,388
...so we'd be comfortable aboard it.
493
00:33:00,593 --> 00:33:03,027
We'd go closer and closer
to the speed of light...
494
00:33:03,229 --> 00:33:05,493
...until the midpoint of the journey.
495
00:33:05,698 --> 00:33:08,258
Then the spacecraft is
turned around...
496
00:33:08,468 --> 00:33:12,029
...and we decelerate at 1 g
to the destination.
497
00:33:12,605 --> 00:33:16,735
For most of the trip, the velocity
would be close to the speed of light...
498
00:33:16,943 --> 00:33:20,106
...and time would
slow down enormously.
499
00:33:20,446 --> 00:33:21,970
By how much?
500
00:33:22,949 --> 00:33:26,385
Barnard's Star could be reached
by such a ship...
501
00:33:26,586 --> 00:33:29,214
...in eight years, ship time.
502
00:33:29,822 --> 00:33:33,781
The center of the Milky Way galaxy
in 21 years.
503
00:33:33,993 --> 00:33:37,861
The Andromeda galaxy in 28 years.
504
00:33:38,397 --> 00:33:40,331
Of course, the people
left behind on the Earth...
505
00:33:40,533 --> 00:33:42,831
...would see things
somewhat differently.
506
00:33:43,102 --> 00:33:45,070
Instead of 21 years to the galaxy...
507
00:33:45,271 --> 00:33:48,832
...they would measure it
as 30,000 years.
508
00:33:49,041 --> 00:33:50,406
When we got back...
509
00:33:50,610 --> 00:33:54,068
...very few of our friends
would be around to greet us.
510
00:33:54,914 --> 00:33:56,677
In principle, such a journey...
511
00:33:56,883 --> 00:34:01,286
...mounting the decimal points closer
and closer to the speed of light...
512
00:34:01,487 --> 00:34:05,253
...would even permit us to
circumnavigate the known universe...
513
00:34:05,458 --> 00:34:08,518
...in 56 years, ship time.
514
00:34:09,462 --> 00:34:13,558
We would return tens
of billions of years...
515
00:34:13,766 --> 00:34:15,757
...in the far future...
516
00:34:15,968 --> 00:34:19,028
...with the Earth a charred cinder...
517
00:34:19,238 --> 00:34:21,706
...and the sun dead.
518
00:34:22,341 --> 00:34:25,742
Relativistic space flight makes
the universe accessible...
519
00:34:25,945 --> 00:34:28,311
...to advanced civilizations...
520
00:34:28,514 --> 00:34:30,709
...but only to those
who go on the journey...
521
00:34:30,917 --> 00:34:33,283
...not to those who stay home.
522
00:34:34,120 --> 00:34:38,454
These designs are probably further...
523
00:34:38,658 --> 00:34:42,389
...from the actual interstellar
spacecraft of the future...
524
00:34:43,596 --> 00:34:46,394
...than Leonardo's models are...
525
00:34:46,599 --> 00:34:49,932
...from the supersonic transports
of the present.
526
00:34:50,503 --> 00:34:52,368
But if we do not destroy ourselves...
527
00:34:52,572 --> 00:34:57,475
...I believe that we will,
one day, venture to the stars.
528
00:34:58,144 --> 00:35:00,704
When our solar system
is all explored...
529
00:35:00,913 --> 00:35:04,041
...the planets of other stars
will beckon.
530
00:35:37,850 --> 00:35:42,048
Space travel and time travel
are connected.
531
00:35:43,189 --> 00:35:45,123
To travel fast into space...
532
00:35:45,324 --> 00:35:48,521
...is to travel fast into the future.
533
00:35:51,397 --> 00:35:55,561
We travel into the future,
although slowly, all the time.
534
00:35:55,768 --> 00:36:00,068
But what about the past?
Could we journey into yesterday?
535
00:36:00,272 --> 00:36:03,400
Many physicists think this is
fundamentally impossible...
536
00:36:03,609 --> 00:36:05,941
...that we could
not build a device...
537
00:36:06,145 --> 00:36:08,875
...which would carry us
backwards into time.
538
00:36:09,081 --> 00:36:12,482
Some say that even if we were
to build such a device...
539
00:36:12,685 --> 00:36:13,947
...it wouldn't do much good.
540
00:36:14,153 --> 00:36:16,678
We couldn't significantly
affect the past.
541
00:36:16,889 --> 00:36:20,552
For example, suppose you
traveled into the past...
542
00:36:20,760 --> 00:36:22,853
...and somehow or other prevented...
543
00:36:23,062 --> 00:36:25,860
...your own parents from meeting.
544
00:36:26,065 --> 00:36:29,626
Why, then you would probably
never have been born...
545
00:36:29,835 --> 00:36:31,860
...which is something
of a contradiction, isn't it...
546
00:36:32,071 --> 00:36:34,198
...since you are clearly there.
547
00:36:34,807 --> 00:36:36,138
Other people think that...
548
00:36:36,342 --> 00:36:39,470
...the two alternative histories
have equal validity...
549
00:36:39,679 --> 00:36:43,080
...that they're parallel threads,
skeins of time...
550
00:36:43,282 --> 00:36:45,842
...that they could exist side by side.
551
00:36:49,722 --> 00:36:52,247
The history in which
you were never born...
552
00:36:52,458 --> 00:36:55,427
...and the history that
you know all about.
553
00:36:55,661 --> 00:36:58,858
Perhaps time itself has
many potential dimensions...
554
00:36:59,065 --> 00:37:02,330
...despite the fact that
we are condemned to experience...
555
00:37:02,535 --> 00:37:04,730
...only one of those dimensions.
556
00:37:05,371 --> 00:37:08,636
Now, suppose you could go back
into the past...
557
00:37:08,841 --> 00:37:12,299
...and really change it by,
let's say something like...
558
00:37:12,511 --> 00:37:17,005
...persuading Queen Isabella not
to bankroll Christopher Columbus.
559
00:37:17,216 --> 00:37:19,480
Then you would have set into motion...
560
00:37:19,685 --> 00:37:22,711
...a different sequence
of historical events...
561
00:37:22,922 --> 00:37:25,982
...which those people
you left behind you in our time...
562
00:37:26,192 --> 00:37:28,353
...would never get to know about.
563
00:37:28,561 --> 00:37:31,291
If that kind of time travel
were possible...
564
00:37:31,497 --> 00:37:34,694
...then every imaginable sequence...
565
00:37:34,900 --> 00:37:36,868
...of alternative history...
566
00:37:37,069 --> 00:37:39,264
...might in some sense really exist.
567
00:37:40,239 --> 00:37:42,298
Would it be possible
for a time traveler...
568
00:37:42,508 --> 00:37:45,875
...to change the course of history
in a major way?
569
00:37:46,078 --> 00:37:48,308
Well, let's think about that.
570
00:37:51,884 --> 00:37:54,148
History consists for the most part...
571
00:37:54,353 --> 00:37:58,414
...of a complex multitude
of deeply interwoven threads...
572
00:37:58,624 --> 00:38:01,286
...biological, economic
and social forces...
573
00:38:01,494 --> 00:38:03,985
...that are not so easily unraveled.
574
00:38:05,264 --> 00:38:09,894
The ancient Greeks imagined the course
of human events to be a tapestry...
575
00:38:10,102 --> 00:38:13,868
...created by three goddesses:
the Fates.
576
00:38:15,541 --> 00:38:19,773
Random minor events generally
have no long-range consequences.
577
00:38:19,979 --> 00:38:23,176
But some which occur
at critical junctures...
578
00:38:23,382 --> 00:38:25,907
...may alter the weave of history.
579
00:38:26,118 --> 00:38:29,110
There may be cases where
profound changes can be made...
580
00:38:29,321 --> 00:38:31,812
...by relatively trivial adjustments.
581
00:38:32,024 --> 00:38:36,791
The further in the past such an event
is, the more powerful its influence.
582
00:38:37,296 --> 00:38:40,754
What if our time traveler had
persuaded Queen Isabella that...
583
00:38:40,966 --> 00:38:42,866
...Columbus' geography was wrong?
584
00:38:43,068 --> 00:38:47,266
Almost certainly, some other European
would have sailed to the New World.
585
00:38:47,473 --> 00:38:49,134
There were many inducements:
586
00:38:49,341 --> 00:38:52,401
The lure of the spice trade,
improvements in navigation...
587
00:38:52,611 --> 00:38:55,045
...competition among
rival European powers.
588
00:38:55,247 --> 00:38:59,149
The discovery of America
around 1 500 was inevitable.
589
00:38:59,351 --> 00:39:02,752
Of course, there wouldn't be any
postage stamps showing Columbus...
590
00:39:02,955 --> 00:39:05,890
...and the Republic of Colombia
would have another name.
591
00:39:06,091 --> 00:39:10,084
But the big picture would have
turned out more or less the same.
592
00:39:14,700 --> 00:39:17,897
In order to affect
the future profoundly...
593
00:39:18,103 --> 00:39:20,663
...a time traveler
has to pick and choose.
594
00:39:20,873 --> 00:39:24,331
He'd probably have to intervene
in a number of events...
595
00:39:24,543 --> 00:39:27,239
...which are
very carefully selected...
596
00:39:27,446 --> 00:39:32,281
...so he could change
the weave of history.
597
00:39:32,685 --> 00:39:35,210
It's a lovely fantasy...
598
00:39:35,421 --> 00:39:39,357
...to explore those other worlds
that never were.
599
00:39:41,861 --> 00:39:45,456
If you had H.G. Wells' time machine...
600
00:39:45,664 --> 00:39:48,690
...maybe you could understand
how history really works.
601
00:39:48,901 --> 00:39:51,768
If an apparently pivotal person
had never lived...
602
00:39:51,971 --> 00:39:56,135
...Paul the Apostle or Peter the Great
or Pythagoras...
603
00:39:56,342 --> 00:39:59,038
...how different would
the world really be?
604
00:39:59,845 --> 00:40:01,938
What if the scientific tradition...
605
00:40:02,147 --> 00:40:04,911
...of the ancient Ionian Greeks...
606
00:40:05,117 --> 00:40:07,779
...had prospered and flourished?
607
00:40:07,987 --> 00:40:10,854
It would have required
many social factors at the time...
608
00:40:11,056 --> 00:40:12,785
...to have been different...
609
00:40:12,992 --> 00:40:15,426
...including the common feeling...
610
00:40:15,628 --> 00:40:18,324
...that slavery was right and natural.
611
00:40:18,530 --> 00:40:22,091
But what if that light
that had dawned...
612
00:40:22,301 --> 00:40:25,668
...on the eastern Mediterranean
some 2500 years ago...
613
00:40:25,871 --> 00:40:27,839
...had not flickered out?
614
00:40:28,040 --> 00:40:31,339
What if scientific method
and experiment...
615
00:40:31,543 --> 00:40:33,534
...had been vigorously pursued...
616
00:40:33,746 --> 00:40:36,010
...2000 years before
the industrial revolution...
617
00:40:36,215 --> 00:40:38,183
...our industrial revolution?
618
00:40:38,384 --> 00:40:42,445
What if the power of this new mode
of thought, the scientific method...
619
00:40:42,655 --> 00:40:44,953
...had been generally appreciated?
620
00:40:45,324 --> 00:40:49,055
I think we might have saved
1 0 or 20 centuries.
621
00:40:49,261 --> 00:40:51,957
Perhaps the contributions
that Leonardo made...
622
00:40:52,164 --> 00:40:54,997
...would have been made
1 000 years earlier...
623
00:40:55,200 --> 00:40:59,000
...and the contributions
of Einstein 500 years ago.
624
00:40:59,204 --> 00:41:01,331
Not that it would have
been those people...
625
00:41:01,540 --> 00:41:03,974
...who would've made
those contributions...
626
00:41:04,176 --> 00:41:07,168
...because they lived only
in our timeline.
627
00:41:07,746 --> 00:41:10,681
If the Ionians had won...
628
00:41:10,883 --> 00:41:14,876
...we might by now, I think,
be going to the stars.
629
00:41:15,087 --> 00:41:19,820
We might at this moment have
the first survey ships...
630
00:41:20,025 --> 00:41:24,689
...returning with astonishing results
from Alpha Centauri...
631
00:41:24,897 --> 00:41:29,493
...and Barnard's Star,
Sirius and Tau Ceti.
632
00:41:29,702 --> 00:41:33,001
There would now be great fleets...
633
00:41:33,205 --> 00:41:35,173
...of interstellar transports...
634
00:41:35,374 --> 00:41:37,865
...being constructed in Earth orbit...
635
00:41:38,077 --> 00:41:41,069
...small, unmanned survey ships...
636
00:41:41,280 --> 00:41:44,738
...liners for immigrants, perhaps...
637
00:41:44,950 --> 00:41:46,349
...great trading ships...
638
00:41:46,552 --> 00:41:50,010
...to ply the spaces
between the stars.
639
00:41:50,489 --> 00:41:53,686
On all these ships
there would be symbols...
640
00:41:53,892 --> 00:41:56,622
...and inscriptions on the sides.
641
00:41:56,829 --> 00:41:59,093
The inscriptions,
if we looked closely...
642
00:41:59,298 --> 00:42:01,926
...would be written in Greek.
643
00:42:02,434 --> 00:42:03,765
The symbol...
644
00:42:03,969 --> 00:42:07,427
...perhaps, would be the dodecahedron.
645
00:42:07,639 --> 00:42:12,269
And the inscription on the sides
of the ships to the stars...
646
00:42:12,478 --> 00:42:13,945
...something like:
647
00:42:14,146 --> 00:42:18,879
"Starship Theodorus
of the Planet Earth."
648
00:42:21,887 --> 00:42:24,685
If you were a really
ambitious time traveler...
649
00:42:27,893 --> 00:42:30,657
...you might not dally
with human history...
650
00:42:30,863 --> 00:42:33,491
...or even pause to examine
the evolution on Earth.
651
00:42:33,699 --> 00:42:35,929
Instead, you would journey back...
652
00:42:36,135 --> 00:42:37,534
...to witness the origin
of our solar system...
653
00:42:38,003 --> 00:42:42,531
...from the gas and dust
between the stars.
654
00:42:43,709 --> 00:42:45,142
Five billion years ago...
655
00:42:45,344 --> 00:42:48,939
...an interstellar cloud was
collapsing to form our solar system.
656
00:42:49,148 --> 00:42:52,379
Most clumps of matter
gravitated towards the center...
657
00:42:52,584 --> 00:42:55,144
...and were destined
to form the sun.
658
00:42:55,354 --> 00:42:59,586
Smaller peripheral clumps
would become the planets.
659
00:42:59,792 --> 00:43:04,195
Long ago, there was a kind of
natural selection among the worlds.
660
00:43:04,396 --> 00:43:08,594
Those on highly elliptical orbits
tended to collide and be destroyed...
661
00:43:08,801 --> 00:43:12,396
...but planets in circular orbits
tended to survive.
662
00:43:12,604 --> 00:43:14,868
But if events had been
a little different...
663
00:43:15,074 --> 00:43:16,735
...the Earth would never have formed...
664
00:43:16,942 --> 00:43:20,901
...and another planet at another
distance from the sun would be around.
665
00:43:21,113 --> 00:43:23,377
We owe the existence of our world...
666
00:43:23,582 --> 00:43:27,416
...to random collisions
in a long-vanished cloud.
667
00:43:30,255 --> 00:43:33,315
Soon, the central mass
became very hot.
668
00:43:33,525 --> 00:43:37,359
Thermonuclear reactions were initiated
and the sun turned on...
669
00:43:37,563 --> 00:43:40,430
...flooding the solar system
with light.
670
00:43:42,801 --> 00:43:44,769
But the growing smaller lumps...
671
00:43:44,970 --> 00:43:47,200
...would never achieve
such high temperatures...
672
00:43:47,406 --> 00:43:50,239
...and would never generate
thermonuclear reactions.
673
00:43:50,442 --> 00:43:54,173
They would become
the Earth and the other planets...
674
00:43:54,379 --> 00:43:58,611
...heated not from within,
but mainly by the distant sun.
675
00:44:03,288 --> 00:44:05,085
The accretion continued until...
676
00:44:05,290 --> 00:44:08,555
...almost all the gas and dust
and small worldlets...
677
00:44:08,760 --> 00:44:12,161
...were swept up
by the surviving planets.
678
00:44:14,533 --> 00:44:16,831
Our time traveler would witness...
679
00:44:17,035 --> 00:44:19,970
...the collisions
that made the worlds.
680
00:44:26,145 --> 00:44:28,170
Except for the comets and asteroids...
681
00:44:28,380 --> 00:44:31,008
...the chaos of the early
solar system was reduced...
682
00:44:31,216 --> 00:44:33,548
...to a remarkable simplicity:
683
00:44:33,752 --> 00:44:37,586
Nine or so principal planets
in almost circular orbits...
684
00:44:37,789 --> 00:44:39,848
...and a few dozen moons.
685
00:44:44,329 --> 00:44:47,264
Now, let's take a different look.
686
00:44:48,901 --> 00:44:51,369
If we view the solar system edge on...
687
00:44:51,570 --> 00:44:53,834
...and move the sun
off-screen to the left...
688
00:44:54,039 --> 00:44:56,701
...we see that
the small terrestrial planets...
689
00:44:56,909 --> 00:45:00,538
...the ones about as massive as Earth,
tend to be close to the sun.
690
00:45:00,746 --> 00:45:04,580
The big Jupiter-like planets tend
to be much further from the sun.
691
00:45:04,783 --> 00:45:07,650
But is that the way it has to be?
692
00:45:08,854 --> 00:45:10,515
Computer studies suggest...
693
00:45:10,722 --> 00:45:13,486
...that there may be many
similar systems about stars...
694
00:45:13,692 --> 00:45:18,061
...with the terrestrials in close
and the Jovian planets further away.
695
00:45:21,934 --> 00:45:25,665
But some systems might have Jovians
and terrestrials mixed together.
696
00:45:25,871 --> 00:45:30,399
There may be great worlds
like Jupiter looming in other skies.
697
00:45:31,710 --> 00:45:35,669
Rarely, the Jovian planets
may form close to the star...
698
00:45:35,881 --> 00:45:40,215
...the terrestrials trailing away
towards interstellar space.
699
00:45:41,553 --> 00:45:43,544
Our familiar arrangement of planets...
700
00:45:43,755 --> 00:45:46,519
...is only one,
perhaps typical, case...
701
00:45:46,725 --> 00:45:50,786
...in the vast expanse of systems.
702
00:45:50,996 --> 00:45:55,626
Often, one fledgling planet
accumulates so much gas and dust...
703
00:45:55,834 --> 00:45:58,029
...that thermonuclear reactions
do occur.
704
00:45:58,237 --> 00:46:00,330
It becomes a second sun.
705
00:46:00,539 --> 00:46:03,372
A binary star system has formed.
706
00:46:07,879 --> 00:46:11,781
From most of these worlds,
the vistas will be dazzling.
707
00:46:11,984 --> 00:46:14,350
Not one of them will be
identical to the Earth.
708
00:46:14,553 --> 00:46:18,922
A few will be hospitable.
Many will appear hostile.
709
00:46:20,325 --> 00:46:22,350
Where there are two suns in the sky...
710
00:46:22,561 --> 00:46:26,258
...every object will cast two shadows.
711
00:46:30,435 --> 00:46:33,063
What wonders are waiting for us...
712
00:46:33,272 --> 00:46:35,638
...on the planets of the nearby stars?
713
00:46:35,841 --> 00:46:38,708
Are there radically
different kinds of worlds...
714
00:46:38,910 --> 00:46:41,970
...unimaginably exotic forms of life?
715
00:46:45,250 --> 00:46:48,151
Perhaps in another century or two...
716
00:46:48,353 --> 00:46:50,344
...when our solar system
is all explored...
717
00:46:50,555 --> 00:46:53,786
...we will also have put
our own planet in order.
718
00:46:53,992 --> 00:46:57,450
Then we will set sail for the stars...
719
00:46:57,663 --> 00:47:00,325
...and the beckoning worlds
around them.
720
00:47:03,769 --> 00:47:06,897
In that day, our machines
and our descendants...
721
00:47:07,105 --> 00:47:10,802
...approaching the speed of light,
will skim the light-years...
722
00:47:11,009 --> 00:47:15,639
...leaping ahead through time,
seeking new worlds.
723
00:47:15,847 --> 00:47:19,374
Einstein has shown us
that it's possible.
724
00:47:20,619 --> 00:47:22,712
We will journey simultaneously...
725
00:47:22,921 --> 00:47:26,322
...to distant planets
and to the far future.
726
00:47:27,125 --> 00:47:28,820
Some worlds, like this one...
727
00:47:29,027 --> 00:47:32,292
...will look out onto
a vast gaseous nebula...
728
00:47:32,497 --> 00:47:34,226
...the remains of a star...
729
00:47:34,433 --> 00:47:37,561
...that once was and is no longer.
730
00:47:40,072 --> 00:47:42,597
In all those skies,
rich and distant...
731
00:47:42,808 --> 00:47:45,333
...and exotic constellations...
732
00:47:45,544 --> 00:47:49,503
...there may be a faint yellow star...
733
00:47:49,715 --> 00:47:52,616
...perhaps barely visible
to the naked eye...
734
00:47:52,818 --> 00:47:55,651
...perhaps seen only
through the telescope.
735
00:47:55,854 --> 00:47:59,722
The home star of a fleet
of interstellar transports...
736
00:47:59,925 --> 00:48:02,086
...exploring this tiny region...
737
00:48:02,294 --> 00:48:05,388
...of the great Milky Way galaxy.
738
00:48:05,831 --> 00:48:10,097
The themes of space and time
are intertwined.
739
00:48:10,302 --> 00:48:13,294
Worlds and stars, like people...
740
00:48:13,505 --> 00:48:17,305
...are born, live and die.
741
00:48:17,509 --> 00:48:20,239
The lifetime of a human being
is measured in decades.
742
00:48:20,445 --> 00:48:22,675
But the lifetime of the sun...
743
00:48:22,881 --> 00:48:25,714
...is a hundred million times longer.
744
00:48:27,886 --> 00:48:30,446
Matter is much older than life.
745
00:48:30,655 --> 00:48:33,886
Billions of years before
the sun and Earth even formed...
746
00:48:34,092 --> 00:48:37,459
...atoms were being synthesized
in the insides of hot stars...
747
00:48:37,662 --> 00:48:42,031
...and then returned to space
when the stars blew themselves up.
748
00:48:42,234 --> 00:48:45,567
Newly formed planets were
made of this stellar debris.
749
00:48:45,771 --> 00:48:50,071
The Earth and every living thing
are made of star stuff.
750
00:48:54,913 --> 00:48:58,747
But how slowly, in our human
perspective, life evolved...
751
00:48:58,950 --> 00:49:03,387
...from the molecules of the early
oceans to the first bacteria.
752
00:49:07,159 --> 00:49:10,094
Evolution is not immediately
obvious to everybody...
753
00:49:10,295 --> 00:49:13,594
...because it moves
so slowly and takes so long.
754
00:49:13,799 --> 00:49:16,927
How can creatures who
live for only 70 years...
755
00:49:17,135 --> 00:49:20,866
...detect events that
take 70 million years to unfold?
756
00:49:21,072 --> 00:49:22,869
Or 4 billion?
757
00:49:27,913 --> 00:49:30,279
By the time
one-celled animals had evolved...
758
00:49:30,482 --> 00:49:33,713
...the history of life
on Earth was half over.
759
00:49:38,156 --> 00:49:40,920
Not very far along to us,
you might think...
760
00:49:41,126 --> 00:49:43,788
...but by now almost all
the basic chemistry of life...
761
00:49:43,995 --> 00:49:46,020
...had been established.
762
00:49:47,265 --> 00:49:49,233
Forget our human time perspective.
763
00:49:49,434 --> 00:49:51,425
From the point of view of a star...
764
00:49:51,636 --> 00:49:54,901
...evolution was weaving
intricate new patterns...
765
00:49:55,106 --> 00:49:59,065
...from the star stuff on
the planet Earth, and very rapidly.
766
00:50:01,780 --> 00:50:04,476
Most evolutionary lines
became extinct.
767
00:50:04,683 --> 00:50:06,776
Many lines became stagnant.
768
00:50:06,985 --> 00:50:08,976
If things had gone
a bit differently...
769
00:50:09,187 --> 00:50:11,280
...a small change of climate,
say, or...
770
00:50:11,490 --> 00:50:12,548
...a new mutation...
771
00:50:12,757 --> 00:50:16,158
...or the accidental death
of a different humble organism...
772
00:50:16,361 --> 00:50:20,559
...the entire future history of life
might have been very different.
773
00:50:23,235 --> 00:50:26,136
Maybe the line to an intelligent
technological species...
774
00:50:26,338 --> 00:50:29,000
...would have passed through worms.
775
00:50:31,843 --> 00:50:34,004
Maybe the present masters
of the planet...
776
00:50:34,212 --> 00:50:37,875
...would have had ancestors
who were tunicates.
777
00:50:40,352 --> 00:50:41,876
We might not have evolved.
778
00:50:42,087 --> 00:50:45,113
Someone else,
someone very different...
779
00:50:45,323 --> 00:50:50,158
...would be here now in our stead,
maybe pondering their origins.
780
00:50:52,264 --> 00:50:54,357
But that's not what happened.
781
00:50:54,566 --> 00:50:57,626
There's a particular sequence
of environmental accidents...
782
00:50:57,836 --> 00:51:00,964
...and random mutations
in the hereditary material.
783
00:51:01,172 --> 00:51:04,869
One particular timeline
for life on Earth...
784
00:51:05,076 --> 00:51:06,907
...in this universe.
785
00:51:10,882 --> 00:51:14,716
As a result, the dominant organisms
on the planet today...
786
00:51:14,920 --> 00:51:16,854
...come from fish.
787
00:51:18,690 --> 00:51:22,387
Along the way, many more species
became extinct than now exist.
788
00:51:22,594 --> 00:51:25,825
If history had
a slightly different weave...
789
00:51:26,031 --> 00:51:30,491
...some of those extinct organisms
might have survived and prospered.
790
00:51:31,303 --> 00:51:34,466
But occasionally, a creature
thought to have become extinct...
791
00:51:34,673 --> 00:51:36,573
...hundreds of millions
of years ago...
792
00:51:36,775 --> 00:51:39,642
...turns out to be alive and well.
793
00:51:39,844 --> 00:51:42,506
The coelacanth, for example.
794
00:51:44,616 --> 00:51:49,451
For 3 1/2 billion years, life had
lived exclusively in the water.
795
00:51:49,654 --> 00:51:52,088
But now, in a great
breathtaking adventure...
796
00:51:52,290 --> 00:51:53,518
...it took to the land.
797
00:51:53,725 --> 00:51:55,852
But if things had gone
a little differently...
798
00:51:56,061 --> 00:51:58,791
...the dominant species might
still be in the ocean...
799
00:51:58,997 --> 00:52:03,366
...or developed spaceships to
carry them off the planet altogether.
800
00:52:09,007 --> 00:52:11,066
From our ancestors, the reptiles...
801
00:52:11,276 --> 00:52:13,642
...there developed
many successful lines...
802
00:52:13,845 --> 00:52:16,405
...including the dinosaurs.
803
00:52:16,715 --> 00:52:19,684
Some were fast, dexterous
and intelligent.
804
00:52:19,884 --> 00:52:21,852
A visitor from
another world or time...
805
00:52:22,053 --> 00:52:24,817
...might have thought them
the wave of the future.
806
00:52:25,023 --> 00:52:29,585
But after nearly 200 million years,
they were suddenly all wiped out.
807
00:52:29,794 --> 00:52:32,558
Perhaps it was a great meteorite
colliding with the Earth...
808
00:52:32,764 --> 00:52:35,494
...spewing debris into the air,
blotting out the sun...
809
00:52:35,700 --> 00:52:38,225
...and killing the plants
that the dinosaurs ate.
810
00:52:38,436 --> 00:52:42,930
I wonder when they first sensed
that something was wrong.
811
00:52:45,043 --> 00:52:48,809
The successors of the dinosaurs
came from the same reptilian stock...
812
00:52:49,014 --> 00:52:53,451
...but they survived the catastrophe
that destroyed their cousins.
813
00:52:56,054 --> 00:52:58,750
Again, there were many branches
which became extinct.
814
00:52:58,957 --> 00:53:01,255
And had events been
a little different...
815
00:53:01,459 --> 00:53:04,917
...those branches might have led
to the dominant form today.
816
00:53:07,799 --> 00:53:10,859
For 40 million years, a visitor
would not have been impressed...
817
00:53:11,069 --> 00:53:13,196
...by these timid little creatures...
818
00:53:13,405 --> 00:53:17,034
...but they led to all
the familiar mammals of today.
819
00:53:19,544 --> 00:53:22,513
And that includes the primates.
820
00:53:23,415 --> 00:53:26,782
About 20 million years ago,
a space time traveler...
821
00:53:26,985 --> 00:53:29,613
...might have recognized
these guys as promising...
822
00:53:29,821 --> 00:53:33,416
...bright, quick, agile,
sociable, curious.
823
00:53:33,658 --> 00:53:36,718
Their ancestors were once
atoms made in stars...
824
00:53:36,928 --> 00:53:39,761
...then simple molecules,
single cells...
825
00:53:39,964 --> 00:53:42,228
...polyps stuck to the ocean floor...
826
00:53:42,434 --> 00:53:45,835
...fish, amphibians, reptiles, shrews.
827
00:53:46,237 --> 00:53:50,640
But then they came down
from the trees and stood upright.
828
00:53:50,842 --> 00:53:53,208
They grew an enormous brain...
829
00:53:53,411 --> 00:53:56,539
...they developed culture,
invented tools...
830
00:53:56,748 --> 00:53:58,807
...domesticated fire.
831
00:54:02,353 --> 00:54:05,015
They discovered language and writing.
832
00:54:05,223 --> 00:54:07,214
They developed agriculture.
833
00:54:07,425 --> 00:54:11,020
They built cities and forged metal.
834
00:54:12,797 --> 00:54:16,631
And ultimately,
they set out for the stars...
835
00:54:16,835 --> 00:54:21,169
...from which they had come
5 billion years earlier.
836
00:54:23,575 --> 00:54:25,099
We are star stuff...
837
00:54:25,310 --> 00:54:28,711
...which has taken its destiny
into its own hands.
838
00:54:31,516 --> 00:54:33,643
The loom of time and space...
839
00:54:33,852 --> 00:54:37,481
...works the most astonishing
transformations of matter.
840
00:54:38,656 --> 00:54:41,648
Our own planet is only a tiny part...
841
00:54:41,860 --> 00:54:43,987
...of the vast cosmic tapestry...
842
00:54:44,195 --> 00:54:48,928
...a starry fabric
of worlds yet untold.
843
00:54:55,807 --> 00:54:59,834
Those worlds in space
are as countless...
844
00:55:00,044 --> 00:55:03,741
...as all the grains of sand
on all the beaches of the Earth.
845
00:55:04,415 --> 00:55:07,350
Each of those worlds
is as real as ours.
846
00:55:07,552 --> 00:55:10,043
In every one of them,
there's a succession of...
847
00:55:10,255 --> 00:55:14,885
...incidents, events, occurrences
which influence its future.
848
00:55:15,093 --> 00:55:18,688
Countless worlds,
numberless moments...
849
00:55:18,897 --> 00:55:22,355
...an immensity of space and time.
850
00:55:22,567 --> 00:55:25,365
And our small planet,
at this moment...
851
00:55:25,570 --> 00:55:30,007
...here, we face
a critical branchpoint in history.
852
00:55:30,208 --> 00:55:33,109
What we do with our world right now...
853
00:55:33,311 --> 00:55:35,609
...will propagate down
through the centuries...
854
00:55:35,814 --> 00:55:39,215
...and powerfully affect
the destiny of our descendants.
855
00:55:39,417 --> 00:55:43,217
It is well within our power
to destroy our civilization...
856
00:55:43,421 --> 00:55:46,254
...and perhaps our species as well.
857
00:55:46,457 --> 00:55:48,857
If we capitulate to superstition...
858
00:55:49,060 --> 00:55:51,426
...or greed or stupidity...
859
00:55:51,629 --> 00:55:55,565
...we can plunge our world into
a darkness deeper than the time...
860
00:55:55,767 --> 00:56:00,101
...between the collapse of classical
civilization and Italian Renaissance.
861
00:56:00,305 --> 00:56:02,569
But we are also capable...
862
00:56:02,774 --> 00:56:05,265
...of using our compassion
and our intelligence...
863
00:56:05,476 --> 00:56:08,070
...our technology and our wealth...
864
00:56:08,279 --> 00:56:10,747
...to make an abundant
and meaningful life...
865
00:56:10,949 --> 00:56:13,247
...for every inhabitant
of this planet...
866
00:56:13,451 --> 00:56:17,945
...to enhance enormously
our understanding of the universe...
867
00:56:18,356 --> 00:56:21,416
...and to carry us to the stars.
868
00:56:37,275 --> 00:56:38,902
In our motorbike sequence...
869
00:56:39,110 --> 00:56:41,601
...we showed how
the landscape might look...
870
00:56:41,813 --> 00:56:44,543
...if we barreled through it
at close to light speed.
871
00:56:44,749 --> 00:56:47,684
Since then,
inspired by this sequence...
872
00:56:47,886 --> 00:56:51,481
...Ping-Kang Hsiung
at Carnegie Mellon University...
873
00:56:51,689 --> 00:56:53,623
...produced an exact
computer animation.
874
00:56:54,125 --> 00:56:57,356
This is what you'd see if you
traveled at ordinary speeds...
875
00:56:57,562 --> 00:56:59,792
...through this red and white lattice.
876
00:56:59,998 --> 00:57:01,829
But this is how it would appear...
877
00:57:02,033 --> 00:57:05,969
...if you were traveling
at close to the speed of light.
878
00:57:06,838 --> 00:57:10,865
We're probably many centuries away
from traveling close to light speed...
879
00:57:11,075 --> 00:57:13,942
...and experiencing time dilation.
880
00:57:14,145 --> 00:57:17,012
But even then,
it might not be fast enough...
881
00:57:17,215 --> 00:57:20,616
...if we wanted to travel
to some distant place in the galaxy...
882
00:57:20,818 --> 00:57:23,548
...and then come back to Earth
in our own epoch.
883
00:57:24,055 --> 00:57:27,149
Some years after completing Cosmos...
884
00:57:27,358 --> 00:57:31,886
...I took time out from
my scientific work to write a novel.
885
00:57:32,397 --> 00:57:33,989
A novel about travel...
886
00:57:34,198 --> 00:57:37,395
...to the center
of the Milky Way galaxy.
887
00:57:37,669 --> 00:57:41,002
I was willing to imagine
beings and civilizations...
888
00:57:41,205 --> 00:57:43,230
...far more advanced than we...
889
00:57:43,441 --> 00:57:46,740
...but I wasn't willing
to ignore the laws of physics.
890
00:57:46,978 --> 00:57:51,540
Was there, even in principle,
a way to get very quickly...
891
00:57:51,749 --> 00:57:54,718
...to 30,000 light-years from Earth?
892
00:57:54,919 --> 00:57:56,784
So I asked my friend...
893
00:57:56,988 --> 00:58:00,219
...Kip Thorne of the California
Institute of Technology.
894
00:58:00,425 --> 00:58:03,451
He's a leading expert
on the nature of space and time.
895
00:58:03,661 --> 00:58:06,129
Kip thought about it for a while...
896
00:58:06,331 --> 00:58:09,425
...and then answered with
about 50 lines of equations...
897
00:58:09,634 --> 00:58:12,364
...which showed that
a really advanced civilization...
898
00:58:12,570 --> 00:58:16,506
...might establish
and hold open wormholes...
899
00:58:18,710 --> 00:58:22,407
...which we might think of as tubes
through the fourth dimension...
900
00:58:22,613 --> 00:58:25,446
...which connect the Earth
with another place...
901
00:58:25,650 --> 00:58:29,108
...without having to traverse
the intervening distance.
902
00:58:29,354 --> 00:58:33,222
Something like crawling
through a wormhole in an apple.
903
00:58:33,658 --> 00:58:35,523
I was happy with this result...
904
00:58:35,727 --> 00:58:39,219
...and used it as
a key plot device in Contact.
905
00:58:39,697 --> 00:58:41,722
But such wormholes through space...
906
00:58:41,933 --> 00:58:44,868
...would also be time machines,
it seemed to me.
907
00:58:45,069 --> 00:58:47,936
And I used that notion
in my novel Contact as well.
908
00:58:48,406 --> 00:58:52,502
Kip Thorne and his colleagues
later proved, or so it seemed...
909
00:58:52,710 --> 00:58:55,178
...that time travel
of this sort was possible.
910
00:58:55,380 --> 00:58:57,712
Here, look at this.
911
00:58:58,916 --> 00:59:01,544
The key question being explored now...
912
00:59:01,753 --> 00:59:05,211
...is whether such time travel
can be done consistently...
913
00:59:05,423 --> 00:59:09,723
...with causes preceding effects, say,
rather than following them.
914
00:59:09,927 --> 00:59:11,394
Does nature contrive it...
915
00:59:11,596 --> 00:59:14,827
...so that even with a time machine,
you can't intervene...
916
00:59:15,033 --> 00:59:18,127
...to prevent your own conception,
for example?
917
00:59:18,336 --> 00:59:21,362
Even if time travel of this sort
is really possible...
918
00:59:21,572 --> 00:59:24,439
...it's far in
our technological future.
919
00:59:24,642 --> 00:59:28,601
But maybe other beings
much more advanced than we...
920
00:59:28,813 --> 00:59:31,748
...are voyaging to the far future
and the remote past...
921
00:59:31,949 --> 00:59:34,941
...not a measly 40 years ago
on Earth...
922
00:59:35,153 --> 00:59:37,644
...but to witness
the death of the sun, say...
923
00:59:37,855 --> 00:59:40,050
...or the origin of the cosmos.
9999
00:00:0,500 --> 00:00:2,00
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