|MadSci Network: Astronomy|
Doug, The question about life in the galaxy (I'll restrict the discussion to just this galaxy since any signals from a civilization in even the closest galaxy outside our own would be too weak to detect) is a hotly debated one. So far we have no evidence from SETI or any other study that there are any other life forms but us. There are several possible explanations for this. First, it may be that advanced life is not all that common. Life started very early in the history of the solar system, which is 4.6 billion years old. The earliest evidence for life on Earth is 3.8 billion years old and life could have arisen earlier than that. We can't be sure because there are very few rocks on Earth that are that old and those that are have mostly undergone metamorphism which would destroy most evidence for life. However, multicellular organisms did not arise until about 700 million years ago and humans with the ability to communicate or send signals into space have only come on the scene in the last century. That would indicate that it is very time consuming and difficult to reach a point of intelligence and technological advancement. Any number of things, such as an asteroid or comet impact, disease or predation, or a natural disaster, could have wiped out the precursor to humanoids before the population rose, preventing or further delaying the rise of an intelligent species. Many scientists believe that microbial life may be common in the universe, but that animal life may be very rare (a good popular science book which addresses this is "Rare Earth" by Peter Ward and Donald Brownlee). Microbial life has adapted to every niche on earth that has been investigated. They are in hot springs, at ocean bottoms, volcanic vents, the rocks of Antarctica, and buried deep in crustal rock. They have even been resuscitated after flying on spacecraft in high radiation, frigid temperature conditions. Animal life, on the other hand, has more restrictive requirements for life. We require liquid water and much more stringent temperature restrictions. This must be the case throughout the evolution of animal life. Venus and Mars may have had these conditions early in the solar system's history, but have since lost them, preventing the rise of complex animal life. That leads to the question of how many planets can sustain conditions conducive to animal life for the time needed to evolve a technological species? That is unknown. It may very well be that intelligent, technological societies are very rare in the galaxy. It may be that we just haven't looked in the right place yet (there are billions of stars and a huge wavelength range to investigate). It may be that another species would not wish to communicate. We just don't know the answer to that question. My personal speculation is that advanced life is rare and that if it is out there and trying to communicate, it may take a while to find it. After all, we've only been looking for about 1/100,000,000 of our planet's history! That means any civilization looking for us would have to be within 70 light years or so (since we've only been sending substantial radio signals for about that long). The galaxy is about 100,000 light years across--that means only a very small percentage of the galaxy would even have the ability to tell that we are here to communicate with. The galaxy is a very large place! Erika
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