N=R x fp x ne x fl x fi x fc x L

where
R - star formation in the galaxy (approximately 1/year)
fp - stars with planets (nearly every star we have searched)
ne - average number of potentially habitable planets (3 for our Solar System)
fl - those with life forms (=1 since molecules of life seem abundant in Universe)
fi - those with intelligent species (= 0 or 1)
fc - radio technology (=0 or 1)
L - average lifetime of communicating civilization

The last three parameters are currently unknown. The most controversial is the one dealing with intelligence. Is intelligence a natural outcome of life or is it the result of a number of truly unique random events that would not be repeated elsewhere? Of course, intelligent life may evolve without developing radio, so that parameter is either 0 or 1.

It may be that the only factor that really matters is the average lifespan of a communicating civilization. We have been able to communicate with radio waves for only about 100 years and have been capable of destroying our civilization for about 50 years. If L is large there are likely to be lots of ETI to look for. But if L is small then there would be only a few ETI civilizations scattered about the Milky Way. Finally, how long does a technological civilization last? The pessimist in me doesn’t think we will make it through this century without destroying ourselves by overpopulation, pollution or war. The optimist in me thinks we will find solutions to these (and other) problems before they destroy our civilization. So, pick your number and wait and see.

“Where is everybody?”

There are three possible answers to this question. One: extraterrestrials are already here, and despite 50+ years without physical evidence, the UFO stories are true. In fact, some of you who are reading this may be aliens. Two: extraterrestrials know about us but are protecting us from the culture shock of finding we are not alone a`la “Star Trek’s” Prime Directive. Finally, they may be on their way but haven’t reached this rather insignificant part of the universe we call home. I will leave this topic with a fairly accurate prediction – the probability of finding ETI is zero if we don’t look for them.

The Arecibo Signal
In the 1970s the Arecibo Radio Telescope in Puerto Rico sent a radio signal. The radio message was a series of binary digits which look like Figure 1 (page 12). Take a moment to examine this figure and see what information you can discern. Remember you are an alien and have no idea what a human even looks like. Also, the only possible language we would have in common is mathematics. Now let’s see how well you did. The top row is simply the first ten numbers in binary code. The next three rows are a number of chemical formulas in binary. Then we have the DNA double helix and a human figure. The solar system with Earth off centered is shown just below the human figure. Finally the Arecibo telescope itself is shown. Obviously an alien would not necessarily be able to piece this all together! However, it is equally obvious that the alien would be able to recognize that this is not a random radio signal and that it contains an actual message.

Signals in a Bottle
The next signal was really two, which were attached to spacecraft. The first one is the Pioneer plaque (see image to the left). Again the message contained in this plaque is an attempt to create a common language to communicate with whoever discovers it. The human figures are shown to scale against a picture of the pioneer spacecraft. There is enough content (assuming it can be decoded) for a recipient to find Earth. However, it would be tens of thousands or even millions of years before this plaque could possibly be found. The Voyager signal is an even more ambitious attempt to communicate. It consists of a CD with all sorts of information about Earth. In cartoon form, it provides directions on how to build a player to use with the attached disc.