As a middle value, let’s choose 20 million years as the “regeneration time” t, the product of these last four factors, is the number of active mature biospheres in the galaxy.
If we stick with our previous estimate that the lifetime, L, of an average technological civilization is 50,000 years, and plug in the rest of the above numbers, equation (2) says that there are probably 5 technological civilizations active in the galaxy right now.
In our own region of the galaxy, (known) stars occur with a density of about one in every 320 cubic light years.If the calculation in the previous paragraph were correct, it would therefore indicate that the nearest extraterrestrial civilization is likely to be about 4,300 light years away.This is much faster than the 4 billion years required by nature to produce a brand-new biosphere in a new solar system.Furthermore, the Drake equation also ignores the possibility that both life and civilization can propagate across interstellar space. There are 400 billion stars in our galaxy, and about 10 percent of them are good G and K type stars which are not part of multiple stellar systems.(In other words, the Drake equation defines a “civilization” as a species possessing radiotelescopes.
By this definition, civilization did not appear on Earth until the 1930s.) By plugging in numbers, we can use the Drake equation to compute N.
Robert Zubrin thinks so, and in the essay that follows, he explains why, with a particular nod to the possibility that life can move among the stars.
Although he is well known for his work at The Mars Society and authorship of back in 2000, which led me to his scientific papers, including key work on the Bussard ramjet concept and magsail braking.
In our own region of the galaxy, the typical distance between stars is five or six light years.
So, if we guess that it might take 1,000 years to consolidate and develop a new solar system to the point where it is ready to launch missions of its own, this would suggest the speed at which a settlement wave spreads through the galaxy might be on the order of 0.5 percent the speed of light.
For example, if we estimate L=50,000 years (ten times recorded history), R∗ = 10 stars per year, f equal to 0.2, we calculate the total number of technological civilizations in our galaxy, N, equals 400.