Activity 9
Communication
Through Space

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Background Information

Our solar system contains some 100 billion stars. Many of these stars have systems of planets, and some of those planets probably have the conditions that can support life. Perhaps life has evolved in our own galaxy. Will we be able to make contact? Oddly enough, our chances of success are limited by the lifetime of our own civilization. Imagine that we search for life by sending out radio signals. Imagine we begin by searching the nearby stars and then work our way out progressively from the Earth. If we search in a sphere of radius r, the volume of that sphere is 3/4πr³, which increases more rapidly the further out we go. Consequently, as we search further, we encounter far more stars that might harbor life, so the odds strongly favor searching as far away from the Earth as possible. We could look out at least 30,000 light-years just within our own galaxy. What would happen if we sent a signal and another civilization replied? The reply might arrive 50,000 years after we sent the signal. And suppose we sent a signal to Andromeda, the nearest galaxy, at a distance of 2.2 million light-years. Would there still be a technological civilization on Earth in 4.4 million years to receive a reply? We get little help with this question from our own history, since our civilization has been technological for fewer than 100 years. How long will our civilization last? The answer to this question may determine whether or not we make contact with extraterrestrial life.

Here are some important distances:

distance across the solar system: 11 light-hours
distance to the nearest star: 4.2 light-years
distance across the galaxy: 100,000 light-years
distance to the nearest galaxy: 2.2 million light-years
distance to the edge of the universe: 15 billion light-years

And here are some important times:

the Big Bang: 15 billion years ago
the formation of the Earth: 4.5 billion years ago

Originally, the entire universe was confined to a single point. The Big Bang was the explosion that created the universe in a blast of radiation, which was so hot that at first, matter could not exist. As the radiation expanded and cooled, subatomic particles acquired a stable existence and, later, atoms as well. Matter collected into galaxies, which are still rushing apart today and producing the characteristic red shifts—one important piece of evidence supporting the Big Bang Theory. The other piece of evidence is the microwave background, residual microwave radiation from the beginning of the universe, when the temperature was so high that all the energy in the universe was radiation.