
The question of what is life has puzzled us for centuries. A new movie, Chappie, addresses this issue in the context of what is a spirit or a soul. Life is simpler but still can be awkward. Because we’re seeking to find civilizations that send out radio waves, we can limit our ideas of life somewhat. Life could be defined as something that reproduces itself using available energy and material resources. To be useful, this life should also be capable of making reproductive mistakes that lead the way to evolution. Without evolution, that civilization could not appear.
In order to figure out if SETI makes sense, we must gather some sort of estimates of the probability of life starting and of it evolving into something like us. We must also determine how many stars harbor planets capable of supporting such life.
Before beginning this peregrination of thought, consider that our version of life here on Earth consists of organisms spawned in water and built of carbon, oxygen, hydrogen, and nitrogen plus some other elements in smaller proportions. Any life must be capable of a complex chemistry and of building rather extensive molecules. Finally, the basic construction materials should be close at hand and in reasonable abundance.
Hydrogen is the most abundant element in the universe and constitutes nearly all of its normal matter. It is found in important simple compounds: water, ammonia, and methane. These also are the simple compounds in which oxygen, nitrogen, and carbon reside. While some have suggested that alien life chemistry might use silicon in place of carbon, the much greater abundance of carbon argues against that route. Similarly, water is not only abundant on the Earth but also throughout space. It has the advantage of being an excellent solvent and the odd characteristic of expanding upon solidifying so that lakes freeze from the top down. All of these features make water the best medium for harboring life by a large margin. Continue reading
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