SETI and the Cosmic Quarantine Hypothesis (Where is Everybody?)

If civilizations exist in our galaxy with levels of technology at least equal to our own, we might be able to detect some of them using radio telescopes. And if civilizations exist with technologies far in advance of our own, we might expect them to have colonized millions of habitable worlds in the Milky Way, and even to have visited our own planet. Yet there is no evidence in the astronomical, geological, archaeological, or historical records that extraterrestrial civilizations exist or that visitors from other worlds have ever been to Earth. Does that mean, as some have concluded, that ours is the only civilization in the galaxy? Or could there be a natural self-regulating mechanism that limits the intensive colonization of other worlds?

In 1961 radio astronomer Frank Drake devised an equation to express how the hypothetical number of observable civilizations in our galaxy should depend on a wide range of astronomical and biological factors, such as the number of habitable planets per star, and the fraction of inhabited worlds that give rise to intelligent life.

When Drake first proposed his equation, we had no way to estimate any of its terms beyond the first one, representing the rate of star formation in our galaxy. Then in 1995, astronomers began to discover planets in orbits around other stars. These results now promise to sharpen our estimates for the second term in the equation, denoting the number of habitable worlds per star. Who knows what unforeseen discoveries will tell us about the other terms in the equation?

We can express the Drake Equation in several ways, all of which are more or less equivalent. Here is one form:

where N is the number of civilizations in our galaxy, expressed as the product of six factors: Rs is the rate of star formation, nh is the number of habitable worlds per star, fl is the fraction of habitable worlds on which life arises, fi is the fraction of inhabited worlds with intelligent life, fc is the fraction of intelligent life forms that produce civilizations, and L is the average lifetime of such civilizations.

The rate of star formation in our galaxy is roughly ten per year. We can define habitable worlds conservatively as those with liquid water on the surface. Many more worlds probably have liquid water only below the surface, but any subterranean life on such worlds would not be likely to produce an observable civilization. Recent discoveries of other planetary systems suggest that habitable worlds are common and that nh is at least one such planet in a hundred stars.

The remaining terms in the equation depend on the biology and social development of other worlds, and here we are profoundly ignorant. Our local experience may provide some guidance, however. We know that life on Earth arose almost as soon as conditions allowed - as soon as the crust cooled enough for liquid water to persist. This fact suggests that conditions for the origin of life on other habitable worlds are not restrictive, and that the value of fl is closer to one than to one in a thousand. But that is merely a guess. No one knows how life began on Earth, and we cannot generalize from a single case.

The conditions for intelligent life are probably more restrictive. On Earth this step first required the evolution of complex animals, which began about three billion years after the origin of life, and then the development of brains capable of abstract thought, which took another half billion years. Among the millions of animal species that have lived on Earth, probably only one ever had intelligence sufficient to understand the Drake Equation. This suggests that fi might be a small fraction.

The probability that intelligent life develops a civilization depends on the evolution of organs to manipulate the environment. On Earth, whales and dolphins may well have intelligence sufficient for abstract thought, but they lack the means to make tools. Humans, with dexterous hands, began making tools over a million years ago. Starting about ten thousand years ago, civilizations based on agriculture arose several times independently, in Mesopotamia, Egypt, China, Mexico, Peru, and New Guinea. This suggests that the value of fc is large, but again we should not generalize from the experience of only one intelligent and manipulative species.

We now come to the most intriguing term, the average lifetime L of a civilization. The Drake Equation assumes that, whatever the other factors, the number of civilizations presently in our galaxy is simply proportional to their average lifetime. The longer they live, the more civilizations exist at any given time. But what is the life expectancy of a civilization? On Earth, dozens of major civilizations have flourished and died within the last ten thousand years. Their average lifetime is about four centuries. Few if any civilizations on Earth have ever lasted as long as two thousand years.

History and archaeology show that the collapse of any given civilization causes only a temporary gap in the record of civilizations on Earth. Other civilizations eventually arise, either from the ruins of the collapsed one or independently and elsewhere. Those civilizations also eventually collapse, but new ones continue to emerge.

For example, in the eastern Mediterranean at the end of the Bronze Age, the prevailing Mycenaean civilization suffered widespread catastrophic collapse around 1100 BC. During a few centuries of "darkness" that followed, the population was illiterate, impoverished and relatively small -- but not extinct. Classical civilization gradually arose and flourished, and gave rise to the Roman Empire, which itself collapsed in the fifth century AD. Another period of impoverished Dark Ages followed, but eventually trade and literacy revived, leading to the Renaissance. Each revival of civilization was stimulated in part by the survival of relics from the past.

Our global technological civilization, with its roots in the Mediterranean Bronze Age, is now arguably headed for collapse. But that will not be the end of civilization on Earth -- not as long as the human species survives. And the biological lifetime of our species is likely to be several million years, even if we do our worst.

We should therefore distinguish between the longevity of a single occurrence of civilization and the aggregate lifetime of a sequence of civilizations. Almost all discussions of the Drake Equation have overlooked this distinction and therefore significantly underestimated L.

The proper value of L is not the average duration of a single episode of civilization on a planet, which for Earth is about 400 years. Rather, L is much larger, being the sum of recurrent episodes of civilization, and constitutes a substantial fraction of the biological lifetime of the intelligent species. The average species lifetime for mammals is a few million years. Suppose the human species lasts another million years and our descendants have recurrent episodes of civilization for more than 10 percent of that time. Then the average effective lifetime of civilization on Earth will exceed 100,000 years, or 250 times the duration of a single episode. Other factors being the same, this generally neglected consideration should increase the expected number of civilizations in our galaxy by at least a hundredfold.

While the aggregate lifetime of civilization on a planet may be only a hundred thousand years, we should allow the possibility that a small minority of intelligent life forms, say one in a thousand, has managed to use their intelligence and technology to survive for stellar evolutionary timescales -- that is, on the order of a billion years. In that case, the average effective lifetime of civilizations in our galaxy would be about a million years.

If we now insert numbers in the Drake Equation that represent the wide range of plausible estimates for the various terms, we find that the number N of civilizations in our galaxy could range anywhere from a few thousand to about one in ten thousand. The latter (pessimistic) case is equivalent to finding no more than one civilization in ten thousand galaxies, so that ours would be the only one in the Milky Way. In the former (optimistic) case, the nearest civilization might be close enough for us to detect its radio signals. Estimates for N thus range all over the map. While this exasperates critics who demand concrete answers from science, it does not invalidate the conceptual power of the Drake Equation.

If many civilizations have arisen in our galaxy, we might expect that some of them sent out colonies, and some of those colonies sent out still more colonies. The resulting waves of colonization would have spread out across the Milky Way in a time less than the age of our galaxy. So where are all those alien civilizations? Why haven't we seen them? The physicist Enrico Fermi first posed the question in 1950. Many answers have since been proposed, including (1) ours is the first and only civilization to arise in the Milky Way, (2) the aliens exist but are hiding, and (3) they have already been here and we are their descendants. In his book Where is Everybody? Stephen Webb considers fifty proposed solutions to the so-called "Fermi Paradox" but he leaves out the most thought-provoking explanation of all, one that I call the Cosmic Quarantine Hypothesis.

In 1981, cosmologist Edward Harrison suggested a powerful self-regulating mechanism that would neatly resolve the paradox. Any civilization bent on the intensive colonization of other worlds would be driven by an expansive territorial impulse. But such an aggressive nature would be unstable in combination with the immense technological powers required for interstellar travel. Such a civilization would self-destruct long before it could reach for the stars.

The unrestrained territorial drive that served biological evolution so well for millions of years becomes a severe liability for a species once it acquires powers more than sufficient for its self-destruction. The Milky Way may well contain civilizations more advanced than ours, but they must have passed through a filter of natural selection that eliminates, by war or other self-inflicted environmental catastrophes, those civilizations driven by aggressive expansion. That is, the acquisition of powerful technology ultimately selects for wisdom.

However, suppose an alien civilization somehow finds a way to launch the aggressive colonization of other planetary systems while avoiding self-destruction. It would only take one such case, and our galaxy would have been overrun by the reproducing colonies of the civilization. But Harrison proposed a plausible backup mechanism that comes into play in the event that the self-regulating control mechanism fails. The most evolved civilizations in the galaxy, he suggested, would notice any upstart world that showed signs of launching a campaign of galactic conquest, and they would nip it in the bud. Advanced intelligence might regard any prospect of the exponential diffusion throughout the Milky Way of self-replicating colonies very much as we regard the outbreak of a deadly viral epidemic. They would have good reason, and presumably the ability, to suppress it as a measure of galactic hygiene.

There may be many highly evolved civilizations in our galaxy, and some of them may even be the interstellar colonies of others. They may control technologies vastly more powerful than ours, applied to purposes we can scarcely imagine. But Harrison's regulatory mechanisms should preclude any relentless wave of colonization from overrunning and cannibalizing the Milky Way.

By most appearances, the dominant civilization on our planet is of the expansive territorial type, and is thus headed for self-destruction. Only if we can intelligently regulate our growth-obsessed and self-destructive tendencies is our civilization likely to survive long enough to achieve interstellar communication.

" Only if we can intelligently regulate our growth-obsessed and self-destructive tendencies is our civilization likely to survive long enough to achieve interstellar communication.

This is a typical BS conclusion made by someone who thinks they are thinking.

One of humanity's most self-destructive tendencies is the attempt to regulate human freedom through the regulation of growth and so-called self-destructive tendencies.

The most truly self-destructive human tendency is the need to control others.

Bilge.

The creator of "Dilbert" (I forget his name) once said that mankind's last invention would be the holodeck. Once people can create a private fantasy paradise for themselves they will never leave. Who would choose the pain and suffering and disappointments of real life if there were a holodeck to fulfil your every fantasy ?

So I think technologically advanced civilizations invented holodeck technology before they invented warp drives. And they had no reason to colonize or emigrate.

Our global technological civilization, with its roots in the Mediterranean Bronze Age, is now arguably headed for collapse......

By most appearances, the dominant civilization on our planet is of the expansive territorial type, and is thus headed for self-destruction.

Good to know someone can predict with such certainty the future of the human race. < /sarc >

The conclusion bothers me too.
What about the premise tho'?
"the dominant civilization on our planet is of the expansive territorial type"

It is ironic that his conclusion just seems to be another expression of the postulated problem.

Another alternative: civilizations exist which do not communicate using radio or even the electromagnetic spectrum as we know it. Lots of possibilities out there.

In fact, the lack of radio communications may actually suggest other civilizations are using different, and more advanced, methods. Just as we discovered the radio some 100 years ago, perhaps we will discover the next method of communication in a few years.

Who would choose the pain and suffering and disappointments of real life if there were a holodeck to fulfil your every fantasy ?

The fact that it is not real might deter some.

In terms of colonization and why we haven't been visited, it may simply be impossible to travel between the stars.

The best spaceships we can currently think of would take at least 1,000 years to travel to the nearest star. What if they actually got there and found out there was nothing particularly interesting in this new solar system, certainly no habitable planets. Start out on another 1,000 year trek to next star, only to find the same thing?

There are other civilizations out there. We will eventually hear one of them through SETI. Then we can send back a message that will take 5,000 years to get there. Maybe there is no interstellar communication as well no interstellar travel. It might just be that facts are facts.

> This is a typical BS conclusion made by someone who thinks they are thinking.

They've been around for centuries. They even have a name: Malthusians.

Reading this article, I fell asleep and hit my head on my keyboard...a regular occurance.

But the issue is not the detection of life that might not want to be detected. Isn't it the resolution of the issue that thousands...maybe even millions...of people believe they have been abducted since childhood.

Do I believe? Actually, I have no experience in this matter so I have not formed an opinion. But many traumatized humans have the same story of abduction by the same creatures doing the same sexual probing and testing.

I have an idea what might be happening...but it is just a theory.

But why do scientists look where evidence IS NOT... and ignore where evidence IS.

If this were a crime, these detectives would be laughed out of town.

For long haul comm, EM is the easy way of doing it. Fast (speed of light) and does not take the energy required to produce gravity waves. I would place money on the table that EM is still the best method for long haul comm even 10,000 years from now.

"By most appearances, the dominant civilization on our planet is of the expansive territorial type, and is thus headed for self-destruction."

What dominant civilization could they be talking about????? Gee...let me guess...

He makes a logical fallacy assuming an advanced civilization would prohibit an aggressive destructive civilization from advancing. Just look at the West vs. the Arab world for proof of the opposite.

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