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Here's an interesting excerpt from:
Carl Sagan Takes Questions
More From His ‘Wonder and Skepticism’ CSICOP 1994 Keynote
www.csicop.org/si/2005-07/sagan.html
QUESTION: A question concerning the search for extraterrestrial intelligence. It seems that we as skeptics, there’s an argument that seems very disappointing and maybe a bit persuasive in the Fermi paradox, the idea that if civilizations were to arise at any significant level, that even given a very extremely slow rate of expansion in the galaxy, that there’s been more than enough time for them to have populated the galaxy several times over. What’s your view on the Fermi paradox?
SAGAN: The Fermi paradox essentially says, as you said, that if there’s extraterrestrial high technology intelligence anywhere they should have been here because if they travel at the speed of light, the galaxy is 100,000 light-years across, it takes you 100,000 years to cross the galaxy. The galaxy is 10 billion years old, they should be here. And if you say you can’t travel at the speed of light, take a tenth of the speed of light, a hundredth of the speed of light, still much less than the age of the galaxy. William Newman and I published a paper on this very point, in which we point out: Imagine there is a civilization that has capable interstellar spacecraft and now they start exploring. What are we talking about? That they’re sending out 400 billion spacecraft, all at once, simultaneously, to every star in the galaxy? Not at all. Interstellar space flight is going to be hard, you’re going to go slow, you’re going to go to the nearest star systems first, you’re going to explore those stars. It is not a straight line but a diffusion question. And when you do the diffusion physics with the appropriate diffusivity, that is, the time to random walk, there are many cases in which the time for an advanced civilization to fully explore the galaxy in the sense of visiting every star system is considerably longer than the age of the galaxy. It’s just a bad model, we claim, the straight line, dedicated exploration of every star in the galaxy.
Carl Sagan Takes Questions
More From His ‘Wonder and Skepticism’ CSICOP 1994 Keynote
www.csicop.org/si/2005-07/sagan.html
QUESTION: A question concerning the search for extraterrestrial intelligence. It seems that we as skeptics, there’s an argument that seems very disappointing and maybe a bit persuasive in the Fermi paradox, the idea that if civilizations were to arise at any significant level, that even given a very extremely slow rate of expansion in the galaxy, that there’s been more than enough time for them to have populated the galaxy several times over. What’s your view on the Fermi paradox?
SAGAN: The Fermi paradox essentially says, as you said, that if there’s extraterrestrial high technology intelligence anywhere they should have been here because if they travel at the speed of light, the galaxy is 100,000 light-years across, it takes you 100,000 years to cross the galaxy. The galaxy is 10 billion years old, they should be here. And if you say you can’t travel at the speed of light, take a tenth of the speed of light, a hundredth of the speed of light, still much less than the age of the galaxy. William Newman and I published a paper on this very point, in which we point out: Imagine there is a civilization that has capable interstellar spacecraft and now they start exploring. What are we talking about? That they’re sending out 400 billion spacecraft, all at once, simultaneously, to every star in the galaxy? Not at all. Interstellar space flight is going to be hard, you’re going to go slow, you’re going to go to the nearest star systems first, you’re going to explore those stars. It is not a straight line but a diffusion question. And when you do the diffusion physics with the appropriate diffusivity, that is, the time to random walk, there are many cases in which the time for an advanced civilization to fully explore the galaxy in the sense of visiting every star system is considerably longer than the age of the galaxy. It’s just a bad model, we claim, the straight line, dedicated exploration of every star in the galaxy.
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