1. Hello.
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2. My name is Stephen Hawking...
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3. physicist, cosmologist...
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4. and something of a dreamer.
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5. Although I cannot move...
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6. and I have to
speak through a computer...
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7. in my mind, I am free.
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8. Free to explore the universe
and ask the big questions...
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9. such as, do aliens exist?
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10. If so,
where could they be found?
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11. What do they look like?
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12. What are they made of?
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13. Are they intelligent?
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14. And if we met them,
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15. what would it mean
for humankind?
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16. Check it out.
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17. Wherever I go in the world,
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18. people ask me,
"Do aliens exist?"
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19. It's a good question,
because it cuts to the heart
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20. of how we see
our place in the universe.
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21. Are we alone on our small,
round, blue ball?
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22. I think probably not,
because of one fact.
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23. The universe is big.
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24. Really big.
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25. Our planet is just one of eight
in orbit around our sun...
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26. which itself
is hardly special,
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27. being one of about 200 billion
stars in a vast spiral...
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28. our galaxy, the Milky Way.
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29. It's so big, some days
I find it hard to comprehend.
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30. But even the Milky Way
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31. is just a tiny drop
in the cosmic ocean.
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32. Just one of 100 billion galaxies
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33. formed into an enormous web
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34. stretching away
in all directions.
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35. At this scale,
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36. each point of light
is an entire galaxy,
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37. which not only puts
our little world in perspective,
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38. but also makes it difficult
to believe we really are alone.
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39. So to my mathematical brain...
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40. the numbers alone
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41. make thinking about aliens
perfectly rational.
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42. The real challenge
is to try and work out
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43. what aliens
might actually be like,
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44. living on some far-off world.
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45. The possibilities are infinite
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46. and infinitely intriguing.
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47. Alien life could range
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48. from simple, green slime
that doesn't do much but drip
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49. to more advanced animals.
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50. Something with a bit more bite.
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51. But, of course,
that's just the start
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52. of what could be out here.
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53. In such a massive universe,
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54. it's logical to wonder
if there are intelligent beings,
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55. perhaps even civilizations
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56. like those in science-fiction
TV shows and movies.
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57. "Star Wars" and "Star Trek,"
two of my personal favorites,
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58. may be closer to reality
than we think.
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59. Similar scenarios
are at least conceivable.
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60. But think about it more,
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61. and even this
is limiting the options.
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62. There could be life-forms
so strange,
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63. we wouldn't even recognize them
as life.
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64. Perhaps there are
really exotic creatures
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65. that live
at the center of stars.
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66. Or even huge communities
of microorganisms
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67. that look like
clouds of cosmic dust.
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68. Maybe aliens live and die
so fast
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69. that they come and go
in the blink of an eye.
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70. So in such a vast universe
with so many possibilities,
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71. how do we know what to look for
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72. or, for that matter,
where to look for it?
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73. The answer
is right back where we began.
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74. The information we need
is here at home,
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75. for the simple reason that home
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76. harbors the only known examples
of life.
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77. The laws of physics appear
to be the same everywhere,
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78. so it follows that the laws of
life should be universal, too,
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79. even if the detail
is different.
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80. We can use life on Earth
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81. as a kind
of alien-hunters' handbook,
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82. a field guide to what life
actually is and how it works,
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83. no matter where it occurs.
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84. Chapter 1,
in our particular case,
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85. takes us back 4.5 billion years
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86. to when the Earth
was really quite young.
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87. Exactly what triggered life here
is still a mystery,
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88. but there are several theories.
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89. The most common one is that
life began purely by accident
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90. in pools of primordial soup
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91. full of chemicals
called amino acids.
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92. These molecules
would have collided at random
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93. for millions of years
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94. until the perfect combination
just happened.
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95. The ultimate lucky break
that started the chain of life.
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96. It is extremely unlikely
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97. that life could
spontaneously create itself.
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98. But I don't think that's
a problem with this theory.
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99. It's like winning a lottery.
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100. Although the odds
are astronomical...
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101. most weeks,
someone hits the jackpot.
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102. But there is another intriguing
idea called panspermia,
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103. which says that life could have
originated somewhere else
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104. and have been spread
from planet to planet...
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105. by asteroids.
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106. It seems possible
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107. that lumps of rock could carry
frozen organisms inside them,
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108. organisms able to withstand
extremes of temperature
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109. and the vacuum of space.
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110. If so,
asteroids could even now
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111. be transporting life
to other worlds.
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112. Regardless
of which theory is right,
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113. once life begins,
the next chapter starts.
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114. And that's all about survival.
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115. Survival links you, me,
and E.T.,
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116. and it generates rules
all of its own.
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117. Survival demands
a source of energy...
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118. what we call food... or else
it would grind to a halt.
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119. Once nourished,
life can then copy itself
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120. to protect against the death
of any one individual.
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121. Ultimately,
that leads to evolution...
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122. evolution that would happen
even on alien worlds,
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123. producing, in some instances,
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124. animals that I think we would
recognize as being alive,
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125. even if they look
a bit strange.
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126. So, the next step
on our alien hunt
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127. is to find a place, or places,
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128. where organisms might find food
and replicate and evolve.
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129. And as far as we know,
that requires one thing.
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130. Like most people,
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131. I find water
both beautiful and fascinating.
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132. But it's also the key
to all known forms of life,
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133. from bacteria to blue whales.
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134. Find water elsewhere,
and aliens could exist nearby.
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135. The good news is that water
is very common, indeed...
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136. out in space.
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137. Frozen water
litters the universe,
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138. from tiny, single crystals
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139. to icy comets
the size of mountains.
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140. But to find liquid water,
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141. we need somewhere
at the right temperature.
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142. Around every star is a region
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143. where it's not too hot
or too cold, but just right,
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144. like the porridge in the story
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145. of Goldilocks
and the three bears.
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146. Around our sun,
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147. there are two planets that lie
in this Goldilocks Zone...
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148. the Earth and Mars.
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149. Which is why one day
I'm sure we'll pay it a visit.
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150. Robots have been exploring Mars
since the '70s,
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151. but they have yet to find life.
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152. I don't think
we should give up.
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153. Beneath the martian surface,
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154. NASA's Spirit rover discovered
these white salts,
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155. which are formed
in contact with liquid water.
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156. Satellite images reveal
drainage patterns and erosion
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157. of the kinds
caused by rivers and oceans.
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158. There may well still be moisture
under Mars' surface...
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159. moisture that perhaps
could support life.
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160. I hope one day
we will find the money
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161. to send men and women to Mars.
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162. It would capture
the public's imagination
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163. just as the Apollo
moon missions captured mine
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164. back in the '60s.
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165. If they found even a few
martian microbes, in my opinion,
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166. it would be one of the most
exciting discoveries ever made.
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167. But even if Mars is barren,
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168. there are other places
to look for liquid water.
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169. One of them lies a mere
30 million miles from Mars
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170. on a small, mysterious moon
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171. that orbits
the giant planet Jupiter.
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172. This is Europa.
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173. Europa is tiny, just under
2,000 miles in diameter,
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174. and it's very cold,
minus 260 degrees.
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175. The entire moon is covered
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176. in a layer of ice
perhaps 15 miles thick.
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177. But Europa may have
a hidden heat source
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178. beneath the surface.
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179. Europa orbits Jupiter
once every 3.6 days,
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180. in an egg-shaped path.
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181. The gravitational pull from
Jupiter changes constantly,
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182. stretching,
then compressing Europa.
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183. This process is like
kneading a piece of clay
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184. to make it warm and soft.
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185. And the heat produced
may be enough
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186. to melt the underside
of the ice sheet...
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187. creating a hidden ocean
of liquid water
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188. protected from the vacuum
of space by the solid ice above.
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189. If so, there could be aliens
living here...
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190. creatures that have evolved
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191. to exploit this dark
and ancient water world.
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192. I think it's even reasonable
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193. to guess at some
of their physical features.
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194. Aliens here would probably swim
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195. in a similar way
to our own ocean life,
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196. since liquid water
is the same stuff everywhere.
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197. They might use chemicals
in their skin
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198. to generate their own light,
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199. as many deep-sea creatures do
back home.
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200. They might even swim
in school-like colonies,
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201. just as aquatic animals do
on Earth.
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202. But even if advanced animals
do live inside Europa,
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203. I think they're unlikely
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204. to be trying to make contact
with us anytime soon.
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205. They'd exist cocooned
in an icy shell 15 miles thick,
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206. so they'd be blissfully unaware
of the universe beyond.
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207. To find them, we'd need
to send a mission here,
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208. which would be even more risky
and expensive
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209. than visiting Mars.
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210. I hope one day we will discover
Europa's secrets.
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211. But before then, it's worth
continuing our journey
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212. to search for aliens
with a wider outlook.
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213. I think we need to leave
our solar system
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214. and voyage into the vastness
that lies beyond.
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215. Stars surround us
in the universe,
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216. but until recently,
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217. no one knew how many had
planets in orbit around them,
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218. let alone
if any of those planets
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219. could support alien life.
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220. Finding out is tough,
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221. because stars are big
and blindingly bright.
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222. Planets are tiny and dark.
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223. Spotting them requires
technology on an enormous scale.
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224. The binocular Keck telescope
in Hawaii,
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225. with its twin 30-foot mirrors,
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226. is one of the most powerful
land-based telescopes
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227. ever built.
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228. But even this vast machine
can't see distant planets.
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229. Instead, it looks
for stars that wobble...
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230. the telltale sign
of an unseen planet in orbit.
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231. A hammer thrower
demonstrates the principle.
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232. As he spins,
the hammer pulls on his body,
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233. and he wobbles
from side to side.
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234. The same thing happens as
a planet swings 'round its star.
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235. Planets also reveal themselves
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236. if they pass
between their star and us.
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237. The planet causes
regular dimming,
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238. and from the timing,
we can even determine
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239. if it's inside
the star's Goldilocks Zone.
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240. The first distant planet
was discovered in 1995.
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241. Since then,
hundreds more have been found.
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242. This, I think,
is a pretty exciting discovery.
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243. We could be on the verge
of a major breakthrough...
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244. one that will both
redefine our view
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245. of life in the universe
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246. and be a real triumph
for science itself.
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247. Somewhere out there,
perhaps not so far away,
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248. is a rocky planet
a bit like Earth.
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249. A planet with liquid water,
where life has begun.
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250. Due to the power of evolution,
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251. aliens here
might be surprisingly familiar,
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252. even if, at first,
they seem anything but.
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253. Aliens that eat, for example,
need an input orifice,
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254. or, as most people say, a mouth.
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255. Likewise, if they live
on a solid surface,
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256. they'll probably have legs.
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257. The detail might be different,
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258. but legs are good things
to have on land,
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259. especially if the animal is
clinging to the side of a cliff.
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260. If the planet is well-lit,
eyes are almost guaranteed.
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261. They let a creature accurately
sense its environment.
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262. Even the position of the eyes
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263. will follow the same rules
as on Earth.
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264. Prey animals tend to have eyes
on either side of their head,
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265. allowing them
to look out for predators.
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266. Predators, even alien ones,
need forward-facing eyes
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267. to accurately judge distance,
an essential skill when hunting.
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268. Alien struggles
of life and death
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269. are probably happening
right now,
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270. thanks to the universal power
of evolution.
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271. But in my opinion,
evolution is so remarkable,
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272. we can't really be sure
of its ultimate limits.
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273. Life-forms on earthlike planets
or in hidden oceans
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274. are not the only options.
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275. We can go even further
into the depths of the universe
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276. in search
of other kinds of aliens...
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277. extraterrestrials
that are totally unlike us.
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278. Life, but not as we know it.
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279. I like to think of myself
as an optimist,
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280. and so in our vast,
ancient universe,
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281. with its countless galaxies,
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282. almost any life-form
that is physically possible...
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283. is likely
to exist somewhere.
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284. So there could be...
perhaps should be...
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285. really bizarre aliens
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286. that have followed a different
evolutionary path.
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287. Aliens that don't
depend on water,
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288. but on other chemicals instead.
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289. Nitrogen is one possibility.
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290. It's a gas on Earth,
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291. but it can exist as a liquid
when it's very, very cold...
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292. minus 320 degrees Fahrenheit.
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293. So, is there a world
of nitrogen oceans
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294. lapping frozen shores...
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295. where aliens have evolved
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296. in temperatures that would
kill a human instantly?
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297. Life here would need chemistry
very different than our own.
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298. A cold-weather remix
of the ingredients that make us.
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299. Ingredient number one,
of course, is water.
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300. The average male
holds 8 gallons.
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301. So, let's swap water
for liquid nitrogen.
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302. There's also
about 2 pounds of phosphorus,
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303. half a pound of salt,
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304. enough iron to make a nail,
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305. 3 pounds of lime,
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306. 15 trace elements,
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307. which might also work
in alien biology,
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308. and then this.
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309. Carbon, 45 pounds of it.
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310. But what if carbon was switched
with something else?
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311. Silicon, perhaps.
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312. Silicon has
slightly different properties,
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313. but it could do the same job.
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314. With the right ingredients,
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315. ultra-low-temperature life
might be possible.
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316. If so, I think
energy would be scarce,
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317. so things 'round here
would move very slowly.
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318. Other possibilities
are even stranger.
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319. The astronomers searching
for far-off planets
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320. have discovered that many
seem to be giant gas planets...
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321. like our own Jupiter and Saturn.
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322. Perhaps there are aliens
made of gas.
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323. Aliens living here would need
to consume something.
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324. I imagine they could use
the power of lightning storms
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325. that constantly rage
on planets like these.
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326. If such extreme life-forms
are possible,
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327. then life
elsewhere in the universe
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328. could be very common, indeed.
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329. There are certainly
many more planets
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330. that fall outside
the Goldilocks regions of stars
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331. than fall inside them.
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332. It suddenly seems
like there could be life
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333. nearly everywhere you look.
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334. But, ultimately, I think
it doesn't really matter
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335. what aliens are made of.
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336. To me, it's what they can do
that counts.
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337. Are they thinking
about the cosmos, too...
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338. trying to unlock its secrets
just as we are?
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339. In short, has alien life
evolved as we have
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340. and developed intelligence?
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341. If the universe
is full of intelligent,
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342. spacefaring aliens,
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343. I think at least some of them
might be interested in us...
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344. if only as a curiosity.
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345. Of course, many people believe
they are already here.
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346. Tales of alien abduction
have been common
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347. ever since I was
an undergraduate in the 1950s.
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348. And I watched
all those "B" movies, too.
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349. The story always goes the same.
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350. A lone individual
on a quiet road at night
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351. happens to take an unscheduled
detour and finds himself lost.
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352. I'm always a bit suspicious
when I hear these tales.
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353. Look at it
from the aliens' point of view.
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354. What's the point of crossing
vast tracts of the universe
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355. in a high-tech ship...
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356. just to abduct
some lone earthling?
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357. In my opinion,
if aliens are here,
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358. I suspect the newspapers
would be full of the story.
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359. And if governments are involved
in their cover-up,
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360. they're doing
a much better job at it
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361. than they seem to do
at anything else.
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362. So, the lack of alien contact
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363. raises a serious
scientific problem.
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364. Where is everybody?
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365. We've been listening to space
for over 40 years.
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366. And in all that time,
we've picked up nothing.
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367. Well, except
for one mysterious occasion.
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368. On August 16, 1977,
a radio telescope in Ohio
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369. picked up a signal
that became famous.
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370. The telescope listened to space
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371. by scanning the skies
as the Earth rotated.
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372. And just once,
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373. it recorded a signal
that got everyone excited...
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374. the "Wow!" signal,
as it became known.
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375. The signal was a steady source
of radio waves,
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376. just the kind
an alien race might send,
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377. because it stands out
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378. from the radio static
that fills the universe.
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379. A computer recorded the signal
as six letters and numbers.
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380. Astronomer Jerry Ehman
saw the data
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381. and wrote one word
in the margin.
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382. Ehman and others
subsequently searched
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383. the same patch of sky
many times...
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384. but found nothing.
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385. The "Wow!" signal had vanished.
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386. The whole mysterious episode
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387. reveals that making contact
with aliens via radio
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388. is always going to be difficult.
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389. In such a vast universe,
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390. messages take a long time
to reach their destination.
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391. The "Wow!" signal appeared
to come from a star system
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392. 200 light-years away.
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393. So it took at least 200 years
to reach us.
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394. If we sent a reply,
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395. it would take another 200 years
to reach them,
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396. by which time they might have
forgotten they sent anything
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397. and stopped listening
for a reply.
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398. Worse, they might well
have destroyed themselves
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399. in the meantime.
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400. The human race
very quickly discovered
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401. the power of the atom bomb.
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402. If the same holds
for intelligent aliens,
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403. then they might not last long.
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404. Perhaps they all
blow themselves up
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405. soon after they discover
that E equals MC squared.
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406. If civilizations take
billions of years to evolve
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407. only to vanish
virtually overnight,
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408. then, sadly, we've next to
no chance of hearing from them.
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409. They are simply too far away
in space and time to reach.
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410. But there is
one last possibility.
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411. That aliens who have avoided
destroying themselves
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412. are already colonizing
the universe.
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413. The human race
has only two options
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414. when it comes to looking
for advanced aliens.
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415. We can listen,
or we can be more active
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416. and broadcast
our willingness to talk.
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417. We'd have to think
very carefully
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418. about what we might say.
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419. I think this might be
just a little too risky.
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420. We only have to
look at ourselves
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421. to see how intelligent life
might develop
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422. into something
we wouldn't want to meet.
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423. We humans are already capable
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424. of manipulating the course
of our own evolution.
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425. Exactly the same, presumably,
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426. would be true
of advanced extraterrestrials.
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427. Ultimately,
they could halt aging
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428. and become virtually immortal.
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429. What's more, they might have
reached that point
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430. millions of years ago.
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431. It might sound unlikely, but if
you think about it logically,
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432. alien technology should be
as extraordinary to us
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433. as a rocket ship to a caveman.
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434. I imagine they might exist
in massive ships like these,
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435. having used up all the resources
from the home planet below.
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436. Such advanced aliens
would perhaps become nomads
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437. looking to conquer and colonize
whatever planets they can reach.
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438. If so, it makes sense for them
to exploit each new planet
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439. for materials
to build more spaceships
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440. so they could move on.
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441. Who knows
what the limits would be?
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442. Perhaps their capabilities
would only be limited
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443. by how much power
they could harness and control.
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444. And that could be far more
than we might first imagine.
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445. For example,
it might be possible
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446. to collect the energy
from an entire star.
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447. To do that, they could deploy
millions of mirrors in space,
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448. encircling the whole sun
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449. and feeding the power
to one single collection point.
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450. Such power
might make it possible
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451. to warp the very fabric of space
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452. and create a portal
called a wormhole.
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453. This portal would act
like a shortcut,
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454. allowing them to travel
huge distances
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455. in the blink of an eye.
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456. Like us, they would
probably have evolved
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457. from a species used to
exploiting whatever it can.
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458. So, if aliens ever visit us,
I think the outcome would be
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459. much as when
Christopher Columbus
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460. first landed in America.
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461. Which didn't turn out very well
for the Native Americans.
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462. So the journey that started
with the search for water
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463. has led us to far-off worlds
which I think could exist...
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464. worlds where the conditions
and chemistry
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465. allow life to begin...
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466. and then change into
many different forms.
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467. Even so, little green men
are probably pretty rare.
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468. But in such a massive place
as the cosmos,
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469. we only have to look
at ourselves
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470. for proof
that extremely unlikely things
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471. can and do happen all the time.
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472. Let's just hope
that if aliens do find us,
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473. they'll come in peace.
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