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Peter D. Ward & Donald Brownlee:
'Rare Earth - Why Complex Life is Uncommon in the Universe' (Copernicus
Books, 2000).
Reviewed by Paddy Carroll.
In many people's eyes, this book has shown that SETI* and related activities are most likely pointless. Personally, I am not so sure.
* The telescope search for messages from ET civilizations.
'Rare Earth' is an attempt to calculate the probability of intelligent life existing beyond Earth. It's the best such attempt I've seen. Ward and Brownlee argue that although simple life may be extremely common, complex life is probably far scarcer. They argue that certain factors are essential to the development of intelligent life, and that it was an exceptional coincidence for all those factors to come together on Earth.
This may be so; and the authors marshal some excellent arguments (summarised here, with another copy of the review here) to back up their theory. But something that struck me while reading the book was that even if complex life doesn't come into being very often or in many different places, that doesn't imply that it's scarce. Even if lifeforms only originated once on Earth*, on a single tiny region of the planet's surface, we can't infer that Earth is inhabited by very few organisms. Our planet teems with them. By analogy, it is quite conceivable that intelligent lifeforms might populate a whole galaxy relatively soon after becoming technologically adept. As they stand, Ward and Brownlee's arguments cannot possibly be used to argue that intelligent life is not widespread.
* They may have originated more than once, with earlier versions perhaps being wiped out by gigantic meteorites crashing into Earth and blasting away the atmosphere. But that doesn't affect my argument.
So, is it safe to say that the whole galaxy could be swarming with aliens? Well, no. I am left asking the question posed by the brilliant Italian physicist Enrico Fermi: "where are they?". If the bug-eyes or genius robots exist, why shouldn't they spread around the galaxy? And if they are all over the place, why don't we have any good evidence of them?*
This is a difficult question for someone who, like me, would love there to be other, vastly different intelligences out there. There are several possible answers.
1. There are no technologically-skilled aliens in our galaxy, and possibly not in adjacent galaxies either.
2. There are, but they have not been in our neck of the woods, at least for the last while.
3. They are on Earth, or at least in our solar system (and possibly all over our galaxy, and even nearby galaxies), but for whatever reason choose not to reveal themselves.
4. These could be very smart creatures* indeed. They could be doing things that I can't imagine even in a very general sense. One outcome of their unimaginable activities is that we do not see them.
* or 'creature', or whatever.
Bear in mind that I might have left something out, hard though I've tried. So . . . which of these scenarios is compatible with SETI? Not the first, and probably not the fourth. It is conceivable - though it's a long shot - that in the third scenario, the aliens hold off from making contact until we make the first move (by transmitting our own messages), or until we become technologically developed.
How likely is the second scenario? Well, there are several forms of it.
a) A civilization which has not travelled much from its place of origin.
b) A civilization which has travelled a lot, but has not been to our solar system.
c) A civilization which has been to our solar system, but does not maintain an obvious presence here.
Type a) makes SETI quite difficult, because those aliens could only signal from tiny areas of the sky. Types b) and c) could be very good news for SETI. So, are those types likely? You know, I just think 'speculation, speculation, speculation'. How the hell should we know? Let's just look.
'Rare Earth' may have important implications for SETI, the Terrestrial Planet Finder, Project Darwin, and the rest. Just what implications, though, it's very hard to say. Even if there are very few, however, this is still an incisive and well-thought-out book. I was pleased to read their arguments, because the natural thing for most people in the area of astrobiology is to want there to be intelligent aliens, and to (sometimes unconsciously) do work from that starting point. Ward & Brownlee probably would like there to be aliens, too; but it doesn't affect their work. Theirs is a balancing contribution to the debate.
If their arguments hold up to expert scrutiny, then they've made a valuable contribution to astrobiology; even if intelligent life is to be found just about everywhere (because of extensive interstellar travel), it would still be very scientifically interesting if life only originated on one or two planets in our galaxy or nearby ones. There is also, of course, the possibility that we are the only highly intelligent lifeforms - which would be depressing but also sort of cool.
Worth reading.
Click here to buy 'Rare Earth' ($26.13), or to read some more reviews.
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