Interstellar travel and biosphere reboots means there are a lot of aliens in the galaxy

In 1961, radio astronomer Frank Drake developed a pedagogy for analyzing the question of the frequency of extraterrestrial civilizations. Robert Zubrin shows a couple of significant mistaken assumptions by Drake. Robert Zubrin wrote this for Centauri Dreams.

Drake equation defines a “civilization” as a species possessing interstellar communication capability. This means radiotelescopes. By this definition, civilization did not appear on Earth until the 1930s. Although, Earth does not really have the means to usefully broadcast and had limited means to interpret interstellar radio communications. Also, we may need to look at laser or other forms of interstellar communication.

L is the average lifetime of a technological civilization.
N/L, is the rate at which such civilizations are disappearing from the galaxy.
R∗, the rate of star formation in our galaxy;
fp, the fraction of these stars that have planetary systems;
ne, is the mean number of planets in each system that have environments favorable to life;
fl the fraction of these that actually developed life;
fi the fraction of these that evolved intelligent species; and
fc the fraction of intelligent species that developed sufficient technology for interstellar communication

If we estimate L=50,000 years (ten times recorded history), R∗ = 10 stars per year, fp = 0.5, and each of the other four factors ne, fl, fi, and fc equal to 0.2, we calculate the total number of technological civilizations in our galaxy, N, equals 400.

Four-hundred civilizations in our galaxy may seem like a lot, but scattered among the Milky Way’s 400 billion stars, they would represent a very tiny fraction: just one in a billion to be precise. 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.

The Drake equation is wrong. The equation assumes that life, intelligence, and civilization can only evolve in a given solar system once. This is manifestly untrue. Stars evolve on time scales of billions of years, species over millions of years, and civilizations take mere thousands of years.

Current human civilization could knock itself out with a thermonuclear war, but unless humanity drove itself into complete extinction, there is little doubt that 1,000 years later global civilization would be fully reestablished. An asteroidal impact on the scale of the K-T event that eliminated the dinosaurs might well wipe out humanity completely. But 5 million years after the K-T impact the biosphere had fully recovered and was sporting the early Cenozoic’s promising array of novel mammals, birds, and reptiles. Similarly, 5 million years after a K-T class event drove humanity and most of the other land species to extinction, the world would be repopulated with new species, including probably many types of advanced mammals descended from current nocturnal or aquatic varieties.

Estimating the Galactic Population
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. Almost all of these probably have planets, and it’s a fair guess that 10 percent of these planetary systems feature a world with an active biosphere, probably half of which have been living and evolving for as long as the Earth. That leaves us with two billion active, well-developed biospheres filled with complex plants and animals, capable of generating technological species on time scales of somewhere between 10 and 40 million years. As a middle value, let’s choose 20 million years as the “regeneration time” tr.

Using average lifespan technological civilization at 50,000 years then there are probably 5 million technological civilizations active in the galaxy right now and the nearest civilization is probably about 185 light years away.

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.

If a typical civilization has been spreading out at 0.25 light speed for 25,000 years and taking 1000 years to consolidate a solar system, the radius, R, of its settlement zone would be 62.5 light years (R = VL/2 = 62.5 ly), and its domain would include about 3,000 stars. The nearest outpost of extraterrestrial civilization could be expected to be found at a distance of 185-62.5 = 122.5 light years.

The value of L completely dominates our picture of the galaxy.
If L is “short” (10,000 years or less), then interstellar civilizations are few and far between, and direct contact would almost never occur.

If L is “medium” (~50,000 years), then the radius of domains is likely to be smaller than the distance between civilizations, but not much smaller, and so contact could be expected to happen occasionally. Remember, L, V, and S are averages; particular civilizations in various localities could vary in their values for these quantities.

If L is a long time (over 200,000 years), then civilizations are closely packed, and contact should occur frequently.


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