SETI and the Cosmic Quarantine Hypothesis

Last Updated: December 5, 2005 9:19

The following article was published in Astrobiology Magazine - October 17 th, 2005

SETI and the Cosmic Quarantine Hypothesis

With this essay by Steven Soter, Astrobiology Magazine presents the first in our series of 'Gedanken', or thought, experiments -musings by noted scientists on scientific mysteries in a series of "what if" scenarios. Gedanken experiments, which have been used for hundreds of years by scientists and philosophers to ponder thorny problems, rely on the power of one's imagination to project these scenarios to logical conclusions. They do not involve lab equipment or, often, even experimental data. They can be thought of as focused daydreams. Yet, as in the famous case of Einstein's Gedanken experiments about what it would be like to hitch a ride on a light wave, they have often led to important scientific breakthroughs.

Soter is Scientist-in-Residence in the Center for Ancient Studies at New York University, where he teaches a seminar on Scientific Thinking and Speculation, and a Research Associate in the Department of Astrophysics at the American Museum of Natural History.

In this essay, Soter examines the Drake Equation, which asks how many technically advanced civilizations exist in our galaxy. He also looks at the Fermi Paradox, which questions why, if there are other technological civilizations nearby, we haven't heard

from them.

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. The Drake Equation has led to serious studies and encouraged the search for extraterrestrial intelligence Michael Crichton recently denounced the equation as "literally meaningless," incapable of being tested, and therefore "not science." The Drake equation, he said, also opened the door to other forms of what he called "pernicious garbage" in the name of science, including the use of mathematical climate models to characterize global warming.

Crichton rightly pointed out that any numerical "answers" produced by the Drake Equation can be no more than guesses, since most of the terms in the equation are quantitatively unknown by many orders of magnitude. But he is utterly wrong to

claim that the equation is "meaningless." An equation describes how the elements of a problem are logically related, whether or not we know their numerical values. Astronomers understand perfectly well that the Drake Equation cannot prove anything.

Instead, we regard it as the most useful way to organize our ignorance of a difficult subject by breaking it down into manageable parts. This kind of analysis is standard, and a valued technique in scientific thinking. As new observations and insights emerge, the Drake Equation can be modified as needed or even replaced altogether. But it provides the necessary place to start.

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?

In Classical antiquity, when Aristarchus conceived the heliocentric view of the solar system and Democritus developed an atomic theory of matter, they had no possible way to test their ideas. The necessary observational tools and data would not exist for another two thousand years. Of course, the Crichtons of antiquity denounced such speculations as pernicious. But when the time finally came, the ancient ideas

were still there, quietly waiting to inspire and encourage Copernicus and Galileo, and the pioneers of modern atomic theory, who took the first steps to test the theories. It may take centuries, but eventually the Drake Equation and all its elements will be testable.

We can express the Drake Equation in several ways, all of which

are more or less equivalent. Here is one form:

N = Rs nh fl fi fc L

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




How to Talk To Aliens

Frank Drake is a renowned astronomer whose principal research activities are directed toward the detection of intelligent life in the universe. He conducted the first radio search for extraterrestrial intelligence and helped construct the Voyager interstellar record, which carried pictures and music from Earth out into the cosmos.

The following article was published by “Forbe’s Magazine”, 24 th October. 2005

On Nov. 16, 1974, astronomer Frank Drake dedicated a new observatory in Arecibo, Puerto Rico, by sending humankind's first deliberate communication to extraterrestrials.

The message, made up of 1,679 seemingly random zeros and ones, was shorter than the first four paragraphs of this article, but it still took three minutes to send. While the message began its voyage to the cosmos--a 24,000 year trip to M-13, a cluster of stars in the constellation Hercules, to be exact--visiting dignitaries listened over a loudspeaker while each bit played as a short, high-pitched tone. Some participants later said it brought tears to their eyes.

Humans have debated the best ways to contact our interstellar neighbors for centuries. In 1820, German mathematician Karl Friedrich Gauss proposed cutting an enormous right triangle into the Siberian pine forest, creating a monument to the Pythagorean theorem big enough to see from outer space. Twenty years later, Austrian astronomer Joseph von Littrow expanded on that idea, suggesting the excavation of huge trenches in the Sahara desert, which would be filled with kerosene and set ablaze. Flaming triangles, circles and squares would be a beacon to our solar neighbors, at least until the fire went out.

To a large extent, modern technologies have made these suggestions irrelevant. Since the 1920s, human radio and TV broadcasts have spammed the galaxy, and anyone listening has already gotten an earful. "In some sense, this is all academic, because we have been broadcasting to aliens for decades," says Seth Shostak, senior astronomer at the SETI Institute, a nonprofit organization dedicated to the search for extraterrestrial intelligence. "They're already watching Kate Smith and Kukla Fran and Howdy Doody."

But what if we decided we wanted to send a message with intent, something that will say more about us than an episode of The Love Boat? What's the best way to create a message that will be received, understood and useful?

The Arecibo broadcast represented one approach. Those 1,679 zeros and ones carried hidden meaning for any intelligent species who noticed that 1,679 is the product of two prime numbers, 73 and 23. Arrange the message in 73 rows of 23 numbers, and you get a picture painted in bits. (Click here to see the decoded Arecibo message.) It was a novel approach, but the message was hidden, and it depended on aliens making leaps of logic in order to decipher it.

Arecibo wasn't the first time Drake pondered how to address an alien audience. In March 1972, a plaque he designed with legendary astronomer Carl Sagan was blasted into space on board the Pioneer 10 spacecraft. (Click here to see the Pioneer Plaque.) A few years later, Drake and Sagan would team up again on a much more ambitious project, attaching a gold-plated record full of music and photos onto the two Voyager probes.

These efforts are notable because so few other attempts have been made to craft a message to alien civilizations. But as actual attempts at communication, the spacecraft fall flat. They're too small to notice and move too slowly. Far better to use a broadcast signal, which we can target at a specific star, and which moves at the speed of light.

We could use the same radio frequencies as the Arecibo message, but why not do something a little more dramatic? The universe is pretty transparent to optical light--that's how we can see far away galaxies. If we used a bank of high-powered lasers, we could beam a high-bandwidth message across the cosmos.

And we could do it with style. "One nice thing about light is that creatures develop eyes, and it would be possible to make optical radiation bright enough to see," says Paul Horowitz, a professor of physics at Harvard University. "That's an unmistakable signature. You look up, and there's a star, blinking in code, and the color's changing, too."

Next comes the question of what the message should say. Drake says if he could do it again, he might convene an international committee of scientists, artists, politicians and religious figures to produce a holographic movie about life on Earth.

Other researchers suggest that the best way to get an alien's attention is to send it a significant numeric pattern, perhaps prime numbers or the value of Pi. "Maybe the most fundamental way to initiate a message would be with mathematics," says Horowitz. "A lot of stuff will surely be understood by anybody, no matter what slime they're made out of, because it’s so basic."

The mathematical approach has its critics. "You're not going to send the value of Pi," says Shostak. "If aliens sent us the value of Pi, wouldn't you be disappointed? You learned that in seventh grade."

Instead, why not transmit everything we've got? "I would just send the entire contents of Google's servers," says Shostak. "To begin with, you don't have to worry about the fact that they don’t speak English, because there's a lot of redundancy, so they'll learn it. And every subject is in there. Sure, there's a lot of pornography, but that's human stuff, too."

Besides, it doesn't make sense to tease an alien civilization with just a "hello," considering that it could take millennia before we hear back from them. "It's like the Romans sent a message [to aliens] and we got the reply," says Shostak. "[The reply] was actually directed at Cicero, not at us. I just think that you would send as much info as the technology would allow on the assumption that you're not going to hear back."

The discussion might seem academic. But many astronomers are confident they'll detect an extraterrestrial intelligence in the next few decades, and when that happens, we better have an official reply ready, or risk being drowned out by the public.

"Once contact's been made, one of the first things that is going to happen is that everyone with a backyard antenna and the ability to wire up a transmitter is going to get online with their personal philosophies," says Shostak. "People will want to reply, and you can't stop them."




Frank Drake on Ambiguity


The following article was published by “Forbe’s Magazine”, 24 th October. 2005

If you're sending a message to extraterrestrials, what you want to send is what's special about us and our planet--what is unusual. Now that’s not basic chemistry or mineralogy, it’s pretty much the cultural stuff and the consequences of evolution. The consequences of evolution will be different everywhere.

So that’s why on the Voyager record, there is a little chemistry and some mineralogy given, but much of it has to do with our physiology, our way of life, our culture, the things that are special to us and will not be exactly duplicated anywhere in the universe.

But the problem with that information is when you try to send it, ambiguity arises because it is very hard to express ideas like joy or fear in pictures alone.

In the Voyager record, we tried to figure out how to express death. And there's one picture that attempts to do that by showing a picture of a family where the age of each person is given. The fact that there is a maximum age is a clue that typically, that's how old we get to be. That’s how our age and our longevity is expressed, though it’s sort of indirect. That same picture also clarifies another point that people never think of, which is whether we are born big or small. We’re born as babies, but one could conceive of a biology where creatures are born out of eggs, essentially full-sized, and as they get older they get smaller. It’s kind of bizarre, but it could be. But that one picture on the Voyager clarifies that issue because the small people have smaller ages and the large people larger ages. So that’s how we clear that one up.

There’s another picture on the Voyager record, which in retrospect was a big mistake. It shows a woman in the grocery store buying groceries, and she’s eating some grapes. That picture was there to show where we get food and how we eat. But what we didn’t even notice was that in that same picture, on an upper shelf, there are some toy trucks. They look just like real, full-size trucks that appear in some of the other pictures. It can give the false impression that you buy baby trucks in the grocery store, and you feed them, and they grow into big trucks. That doesn't make sense at all to us, but it could totally confuse the extraterrestrials.

You can’t assume anything. I tell people that you’ve got to try to imagine that you’re a 14-legged spider looking at these things.



Desmond Morris on Close Encounters


The following article was published by “Forbe’s Magazine”, 24 th October. 2005

Desmond Morris is a zoologist and author, and one of the world’s leading authorities on human and animal behavior. He is the author of books including The Naked Ape and People Watching, and host of numerous television documentaries.

I would be scared to offer extraterrestrials any kind of communication. Many people would go up and sort of raise a hand and say hello. But the fact is that if you have the opportunity to meet such people or creatures, the secret is to watch them and see what they do. They might be doing the same with you, of course. If they were friendly, they might be watching you to see what you were doing. The only way you can tell that a gesture is friendly is if that gesture is not associated with a hostile act. You can’t simply smile because smiling is a uniquely human gesture--it has a completely different meaning in chimpanzees.

We think that we would smile at the aliens--I'm thinking of Close Encounters of the Third Kind, where the aliens come out of their spaceship and the humans greet them. If I remember correctly, the humans mostly stood still, and one or two of them raised a hand.

If I were faced with that situation, I would remain immobile. I wouldn’t make any gestures until I could see what kind of body language was present between the aliens. Unfortunately, if it was just one to one, you couldn't see that. Generally speaking, gentleness is the key to friendliness. So I wouldn’t make any sudden gestures or sharp movements, but rather I’d almost move in slow motion, making my movements very gentle, with no power attached to them.

I think gentleness as opposed to power, and slowness as opposed to sharpness, would have a pretty universal meaning. Any life form should, generally speaking, consider a gentle or slow movement to be non-threatening. But as for any other sort of signal, I simply don’t think anything else would mean anything to them.

I know that some of the space capsules have contained strange sorts of messages, but I don't think it would mean anything to aliens.

I daresay that out of all the thousands or millions of galaxies in the universe, somewhere something could understand our messages, but I think the chances are that the aliens we meet will not be even bipedal and bilaterally symmetrical. We may meet something which is an amorphous blob, and there wouldn't be any form of communication possible. I think the chances of meeting men with slightly funny faces, the kind you get in space operas and science-fiction movies, is utterly remote.