[07:21.34]11 Furthermore,if a star is very small and dim,
[07:25.62]a planet must be very close to it to get enough light and heat to support life as we know it.
[07:32.26]But at that close distance,tidal effects would cause the planet to face only one side to the sun,
[07:40.36]so that half the planet would be too hot and half too cold.
[07:45.35]12 In other words,we need stars about the size of our sun.
[07:50.20]13 Then again,such stars cannot be part of close binaries or in other regions
[07:57.02]where there would be too much energetic radiation from surrounding stars.
[08:02.72]Suppose we decide that only one out of three hundred stars has a chance of possessing a planet
[08:10.16]that would be hospitable to our kind of life,and only one out of three hundred of such stars
[08:18.42]has a planet of the right size,chemical composition,and temperature to actually support life.
[08:26.10]That might still mean the existence of millions of life-bearing planets scattered among the stars.
[08:33.65]14 However,what are the chances that on one of these planets intelligent life has developed,
[08:40.86]capable of developing a technology like ours?
[08:45.48]15 There are no optimistic answers to that question.After all,
[08:51.28]Earth had to exist for 4.6 billion years before a life form appeared that was capable of developing technology.
[09:00.76]16Even if the chances of its happening are small,it might still be that thousands of technologies have developed among the stars,
[09:09.46]but then there's a still more difficult question:How long would such technologies endure?
[09:16.64]17 Intelligent beings,as they learn to dispose of great sources of energy,might use them for self-destructive purposes.
[09:25.11]Certainly,now that mankind has developed advanced technologies,we have begun to use them in ruinous wars
[09:33.26]and are in the process of destroying our environment with them.If this is typical,
[09:39.79]then the universe might be full of life-bearing planets that have not yet achieved a technology,
[09:46.84]and equally full of others that have already achieved an advanced technology and have destroyed themselves.
[09:55.04]There would be only a very,very few besides ourselves
[09:59.75]who had achieved the technology and had not yet had time to destroy themselves.
[10:05.83]18 In about 1950,the Italian-American physicist Enrico Fermi asked the question:Where are they?What he meant was,
[10:17.43]if the stars are rich in technologies,why hasn't some alien life form reached us?
[10:23.88](4)(We can't count wild tales of flying saucers and ancient astronauts,because the evidence in their favor is extremely weak.)
[10:33.26]19 Perhaps aliens have not appeared because the distances between the stars is too great to cross,
[10:40.03]or they have reached us and decided to let us develop in peace,or have failed to appear for any number of other reasons.
[10:48.57]We can't be sure that simply because no alien is here,there are no aliens somewhere out there.
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