|How many Technological Civilizations are out there?|
There are around a hundred-billion stars (100,000,000,000) in the Milky Way Galaxy.
Let's say that only half the stars in the milky way galaxy have planets (probably a very low estimate, based on recent Astronomical studies). Even though there are probably many planets around each of these stars (our little sun has quite a few), let's say that only one in fifty (1/50) of these stars with planets have one that could support life. Remember that biology has proven that life blooms in boiling waters around lava vents at the bottom of the ocean, as well as survives heartily in constant sub-zero arctic environments.
This low estimate means that one-billion stars (1,000,000,000) in the Milky Way Galaxy may have planets that could support life. Check out National Geographic's top five here.
The oldest star in our Galaxy is estimated to be around thirteen-billion years old, so most of those stars would have at least a billion years to get life going. If only 1/10th of those planets that could support life actually do get life going in their billions of years of development, that still leaves us with around one-hundred-million (100,000,000) planets with life.
How many of those planets with life develop intelligence? This is a bit controversial, but there are clearly many types of intelligence on this planet, but only one that developed what we consider to be "technology." Still, there are estimated to be over 1.5 million species on Earth, and humans have only been around for the past 100,000 years of Earth's development. So on Earth, we can say that 100,000 out of one-billion years of development were necessary for technological life to emerge, which suggests that around 1/10,000 planets with life generates a technological civilization.
That means that there would be around 10,000 technological civilizations in the Milky Way Galaxy!
See that picture to the right? It's taken of a portion of the sky smaller than a pin-head. In it, there are over 1,500 Galaxies! With an average of one-billion stars per galaxy, there would be literally TRILLIONS of technological civilizations in the Universe. But have any come here?