The Drake Equation:  Discussion of Contingencies

Note that although the focus of this chapter is on the possibility that there are other intelligent creatures in the universe besides the human species, the title of this chapter is Darwin's Universe Revisited. A discussion of ICs (Intelligent Civilizations) is usually seen by most people as a flaky subject that does not have much relevance to daily life and the real problems we face on Earth, but our real task is to return to an issue that was raised in Chapter 1. In Chapter 1 an argument for the value of the cosmic perspective and "big" knowledge of the universe was given, called the philosophical spin-off argument. It is very important that you understand that argument and see its connection to the discussion in Chapter 10. Darwin's theory is our best scientific theory about why life is the way it is on Earth. Because of the overwhelming evidence for it, it is assumed to be a universal law similar to gravity. So, we now need to speculate a little about its application throughout the universe. And, this contemplation will shed some light on what it means to be a human being on this Earth in this universe. The objective is to understand that the more we study the possibility of life elsewhere, the more we learn about ourselves. Bottom line: Chapter 9 is really not about ICs. It is about human nature, the problems we face on Earth, our survival, our future, and what we should value to make the right decisions, particularly moral ones, for our future.

That said you should understand the two basic arguments for and against the possibility of ICs. Probably the best summary is the debate between Ernst Mayr and Carl Sagan mentioned in footnotes 11 and 12.

Note that Mayr is a famous biologist and Sagan is a famous astronomer. Most biologists are skeptical of ICs because of the complexity of life and the enormous number of contingent events that have to be just right to repeat anything this complex. Mayr argues that intelligence seems to be a fluke on Earth, so why should we expect it to be repeated in the universe? Sagan, as an astronomer, sees the huge number of stars and possibly the almost innumerable number of planets, and of course reasons that on at least on some of these planets the conditions would be right for evolution to work its magic again and produce an intelligent creature. Note, however, that Sagan is well aware that Mayr is right about evolutionary contingencies -- it is very important that you understand what we learned from Chapter 3 -- so Sagan's main argument does not revolve around the sheer number of stars and possible planets in the universe. His main argument has to do with convergent evolution as possibly applied to the characteristic of intelligence.

Try to see the importance of this argument for what we will discuss in Chapter 10. Think what it would mean if Mayr is right. What if we are totally alone in this gigantic universe? Worse, it may happen that occasionally creatures somewhat like us evolve from time to time. But this characteristic we call intelligence, and its tools of science and mathematics, enable a creature to be very successful for only a limited time. Then it always leads to total species extinction due to the inevitable escalation of weapons development! This scenario would mean that our time as a species on this planet will be relatively short.

Sagan is not unmindful of this possibility. In fact, much of his writing is aimed at helping people become aware of how precarious our existence in the universe is. He believed passionately though that if intelligence were a convergent evolutionary property, there would be many IC-like creatures evolving. Many may become extinct quickly, but not all. Some will be smart enough to recognize how dangerous they are to themselves with their intelligence and survive what he called "technological adolescence." The question is which type will we be?  Our behavior on this planet and what kind of future we creat constitute a big scientific test.



Playing with the Drake Equation helps see the different possibilities for intelligent life in the Universe and to understand thoroughly all the contingencies for human life. I recommend highly that you put in your own calculations. You can play with different scenarios to get a feel for what happens with different assumptions. But, before you do, here are some basic facts and questions you should consider. As an example, I will put in some speculative percentages of my own.

Here is the famous Drake Equation that can be used to estimate the number of possible intelligent civilizations (ICs) in our galaxy :
 
N = N* x Fp x Ne x Fl x Fi x Fc x L
ICs Physical Biological Cultural

Don't be put off by the mathematics and symbols. They will be explained step by step below. It only involves multiplication and percentages.

Anything is possible but in discussing the possibility of ICs, scientists believe it is rational to make the following assumptions:

If other intelligent life is going to be anything like us, if we plan to communicate and interact to any extent remotely like all the science fiction shows, then it will need to be a carbon and DNA based life form.

This is so, because if we are going to speculate about extraterrestrial intelligent life, then we have to use our best science (astronomy, chemistry, biology, and physics) as a basis for this speculation. Life elsewhere could be very different than life on Earth, but given all that we know about chemistry and biology, no other molecule but carbon allows for the combining potential that we find in DNA. So if life elsewhere also needs to be based on DNA, then that life will have to live in an Earth-like temperature range and need lots of water, because without these temperatures and water, DNA cannot exist and the necessary energy transformations cannot take place.

Before you plug in some percentages for each variable, contemplate the following factual details:

N = # of Current Intellignent Civilizations (ICs) in Our Galaxy
 

N* = Number of Stars in Our Galaxy

Estimated to be between 100 and 400 billion. I am going to be optimistic and put in 300 billion.
 

Fp = Percentage of Stars that Develop Planetary System in a galactic habitable zone (GHZ)


This function is most likely the number of single system sun-like stars estimated to exist in our galaxy. From Chapter 1 you should remember that it is estimated that our galaxy contains between 100 to 400 billion stars. They are not all like our sun. Why is our sun important? Aren't we being egotistical in thinking that an Earth-like planet has to have a sun-like star? No. Here is where our best astronomy and physics enter the picture. Evolution takes a long time. Large stars don't last very long. They are so big and the gravitational pressures so immense that they eat up their hydrogen fuel quickly and go supernova. Some last as little as 10,000 years. Small stars last almost forever, but they have very little energy output and probably would have only gas planets (if any at all). Heavier elements such as carbon, iron, and gold are created in the explosion of big stars. Because small stars have been around since the beginning of the early stages of the universe, there would not have been enough time when they formed for big stars to blow up and seed the universe with the elements needed for life. Also, applying a little physics, we would expect that any planet revolving around a small star would be tidally locked with only one side facing the star. Such tidal locking would not produce a global environment conducive to life. Our sun appears to be the best type for evolution. It is just the right size for a lifetime of about 10 billion years. Stars 1.4 times larger would have life times that are too quick.

Another problem is that astronomers estimate that 50% of the right type of sun-like stars are not solitary stars, but are part of binary or multiple (2 or more) systems. This would most likely produce extreme temperature swings and severe gravitational stresses for any planets. The closest star system to us is a tertiary system (Alpha, Beta, and Proxima Centari). Plus, it is likely that in multiple star systems planets do not exist at all, since the mass that would have made planets was incorporated into the stars.

Next there is the problem of location in a galaxy. Stars too close to the center of our galaxy would probably have planetary systems that suffer orbital instabilities, dangerous bursts of radiation, and collisions from debris (comets). For example colliding neutron stars would produce more energy released in 10 seconds than our sun will in 10 billion years. Any planet located within 3,000 light years would be completely sterilized.  Radiation from supernovas could destroy any protective ozone layer and increase to dangerous levels secondary radiation cascades from particle reactions in the atmosphere.   Because there are more stars and a great deal more activity toward the center of our galaxy, it might be a very unhealthy place for life. It is probably much safer to live in the outskirts of a galaxy (where we live) than toward the center.  In other words, just as our solar system has a habitable zone for life – neither too close nor too far is good – so a little reflection reveals it matters where a star is in our galaxy.

Our sun also has an apparently rather unique circular orbit around the center of our galaxy.  A more elliptical orbit would cause the sun to move in and out of dense spiral arms, causing gravitational disruptions to the trillions of comets orbiting our sun and bombarding the solar system with collisional debris.  There would also be an increase in the level of radiation bursts due to more star formation and death.

A habitable planet also needs basic chemical building blocks as well as relative seclusion from cosmic debris and threats.  Scientists have discovered that the probability of forming a terrestrial (Earth—like) planet depends on the “metallicity” of the parent star.  For various reasons, large sections of our galaxy have a metallicity either too low or too high for sun-like, and hence, Earth-like formations.  The further away one moves from the galactic center, the less gas there is for star formation.  The less star formation, the less processing of hydrogen into heavy metals via supernovas.  So,  planets forming around stars far from the galactic center are far less probable.   On the other hand, stars closer to the center can have not only too high a metallicity, but any planets will also be subject to many cosmic threats noted above.

To make a long story short, our sun resides in a thin disk of our galaxy about 28,000 light-years from the center.  At about this general distance there appears to be a Galactic Habitable Zone (GHZ) that is very important for establishing some necessary conditions for life.  The highest probability for life would be within this zone.

This GHZ concept also applies to time.  Our Milky Way galaxy would have been full of unpleasant supernova events during the early stages of its existence.  These supernovas are not only necessary to produce the basic chemical building blocks of life, but their existence is potentially deadly to life once it has formed.  Many astrophysicists believe that only within the past 5 billion years or so could ICs safely evolve.  Hence, the much vaunted science fiction possibility of the existence of many civilizations far advanced from ours runs into a wall of skeptical scientific facts.

In short, it looks like for life to exist we need a Goldilock's star in terms of size, location, and time!

As noted above, binary systems may be the norm in creation of solar systems rather than planetary systems; 50% of the stars in our galaxy seem to be part of binary systems. In our solar system only 1% of the mass of the entire solar system is taken up by planets, and most of this in Jupiter and Saturn. Now it is true much excitement has been generated recently by the apparent discovery of planets in other star systems. But planets have not been discovered around every star system we have checked. In fact the percentage is fairly low, but this result may be due to the vast distances involved and the relatively imprecise detection technology that we use to do the searches.

But Fp just asks us to estimate the number of any stars that have any type of planetary systems in the GHZ.  I am going to put a fairly optimistic number. Given the high probability of multiple star systems (and low probability of planets) and planetless stars, and the relatively thin slice of the GHZ, I am going to put in 20%.

 

Ne = Number of Earth-like Planets that Evolve Within Planetary Systems


For evolution we also need a Goldilock's Earth. Almost all the planets detected in other star systems to date appear to be giants like Jupiter, and some may not be planets at all. They may be brown dwarfs -- massive gas-like objects that are not large enough to become stars. Some are sometimes referred to as "failed stars." It may not be sufficiently appreciated that even with a positive interpretation of the evidence for other planets, how strange and possibly unique our planetary system is. However, that almost all the planets detected so far are huge could be an artifact of our limited measuring techniques. Given the huge distances these stars are from us, it may be that the only planets we can detect are ones that are huge. For an example, see the article on "hot Jupiters."

But we can't have it both ways. In science we can't say that we should be encouraged by the evidence and then say our evidence techniques are poor when they don't give us the picture we want.

Here is a basic fact that no one disagrees with: No Earth-like planet had been detected outside our solar system. Computer simulations, however, do show that when stars such as our sun form, a range of different sized planets form also.

The late Carl Sagan thought that around each sun-like star that formed there would be 1-2 Earth-like planets per star.   I think that is too optimistic. The planet has to be the right size, be the right distance from its star, and have the right temperature for water. Consider some disturbing contingencies. If our Earth were just 5% closer to our sun, it would probably be like Venus, too hot. Also if our Earth was just 10% larger, but right where it is now, there would be no oceans and it would also be like Venus. If it were just 10% larger the gravitational force would produce more volcanoes and more out-gassing, leading to a greenhouse effect and very high temperatures. On the other hand, if our Earth were just 1 % farther away from the sun when it formed, it would have experienced a severe glaciated state during its early formation. Because of an albedo effect (most of the light is reflected and not absorbed as heat), it may never have recovered from an early frozen state. Also, if our Earth were just 6% smaller, there would not be enough gravitational force to hold a protective ozone layer.

Speaking of the albedo effect, even if another planet were exactly the size of the Earth and the same distance from a perfect sun-like star, consider how the arrangement of continents adds another major contingency. It is now believed that 750 million years ago on Earth a condition existed called a Snowball Earth. Throughout their history the continental plates have been moving. At this time, the continental plates happened to be arranged around the Earth's equator. This resulted in an ordinary ice age turning into a global catastrophe. Why?

Global warming is primarily caused by having a lot of carbon dioxide (CO2) in our atmosphere. Ironically having a lot of exposed rock (not covered by ice) draws CO2 out of the atmosphere, which causes global temperatures to fall and more ice to be produced. So by having all the continents near the equator and not covered by ice produced a runaway ice age. By the time the expanding ice sheets reached the equator and began to cover the continents, it was too late for an ordinary waxing and waning of massive ice sheets proceeding and retreating from the poles. If it were not for the slow release of more CO2 from volcanoes, the Earth could have remained locked forever into a frozen state and the Cambrian explosion several hundred million years latter that produced complex life would never have occurred.

So even having a perfectly sized Earth-like planet in the perfect location does not guarantee perfect environmental conditions for life, especially complex life.

Our Earth also has an unusually strong magnetic field and this field has a vital connection to the type of environment of Earth. Finally, there is Earth/moon relationship. Our moon has had a unique effect on evolving life on Earth. Most notable is the tides created by having only a single moon. Having a single moon appears to be very rare. Plus our moon appears to be the result of an early chance collision between the early Earth and a Mars-sized object.

Given all these contingencies, I am going to argue that .33 is an optimistic number. Meaning that when star systems have planets an Earth-like planet exists only 33% of the time.

 

Fl = Perfect Earth-like Planets that Develop Life



Sagan argued that once you have a perfect planet it may be inevitable that life will evolve. According to Sagan, our Earth shows that once you get a perfect Earth-like planet "life pops up." It is true that it is easy to produce the basic building blocks of life (amino acids) and that nucleic acids have also been identified in cosmic dust. In other words, the basic seeds for life would probably exist on a perfect Earth-like planet.

But DNA has never been observed to be created naturally from just the building blocks. The creation of DNA requires a huge jump, what microbiologists call polymerization. In other words, scientists can place a bunch of chemicals in a vat in a laboratory, send some electricity through (possibly mimicking lightening strikes on an early Earth), and produce the building blocks of DNA in the vat. But so far no DNA has ever formed. DNA is a very complex molecule and it must reproduce itself. How did it start? There are lots of theories and controversies about its formation.

Consider also some more disturbing contingencies. Ironically, given how important oxygen is to life on Earth today, it was crucial that early Earth contained very little oxygen. Oxygen destroys naked molecular chains. Also, without oxygen, and hence a protective ozone layer, the early Earth would have been constantly zapped by strong ultraviolet radiation from the sun. But ultraviolet light also destroys long molecular chains. How was early forming DNA protected?

On the plus side we find life today living in the most extreme environments. Recall from Chapter 1 the discussion of thermophile bacteria that can live in temperatures up to 165 degrees F.

Also, note that as a biologist Mayr is optimistic about the existence of bacterial and simple life being common throughout the universe. It is not an exaggeration to say that simple single-celled life is not only the most prevalent life on Earth but controls life on Earth as well. As you read this there is a good chance that there are over 6 billion bacteria on your body (depending on whether you have taken a shower recently or not!), and you will never get rid of them.  In your gut alone, there are more bacteria than all the human beings that have ever lived.  In the end, all our bodies will be devoured by microbial life when we die.

So on a perfect Earth-like planet how often would life get started? Given the major message of contingency, it would probably not be 100%. Since we know life exits on Earth, we know the percentage is not 0%. Still the formation of DNA looks somewhat miraculous. This appearance could be due just to a lack of knowledge on our part, but I am going to put down an optimistic 50%. So given the formation of perfect stars and perfect planets, I speculate that at least 50% of the time life gets started on these perfect planets.

 

Fi = Percentage of Perfect Earth-like Planets that Develop Life that also Develop Intelligence


Now we reach the heart of the debate between Mayr and Sagan, the place where biologists and astronomers disagree the most.

Sagan argues that intelligence is a convergent evolutionary property. Put simply, in the game of natural selection, according to Sagan, it is better to be smart than stupid. Although the development of big brains would not be inevitable, the tendency would favored on at least many planets. Mayr argues that life on Earth refutes this claim. There may have been up to 50 billion species on Earth and very few have developed big brains.

Mayr and Sagan don't disagree that evolution does tend to repeat optimal survival characteristics. They don't disagree that convergent evolution is a fact of nature and that it would be operative on other life-bearing planets. The crucial question is, "Is intelligence an optimal survival characteristic?"  Sagan says yes.  Mayr says no.

The examples given for life on Earth are eyesight and flight. But these evolved many times in many different types of species. Why have big brains only evolved in mammals (whales, dolphins, apes, and humans)? Any biologist will tell you that as a whole, mammals constitute an extremely small number of species compared to the total number of species on Earth. What we see in our Zoos is actually an example of egocentrism. Most of the animals we put there are closely related to humans, and they are not very representative of the whole of life on Earth.

One possibility is that once large brains develop, the creatures that possess them suppress any other chance of large brain development. In other words, big brains are so successful that they pretty much get the rest of life on the planet to stay simple.

Mayr is the biologist and I would have to give him some authority points here.  His arguments are persuasive. Some biologists would put 0% here and the argument is over. As soon as you put a 0 anywhere the computation ends in 0.   However, Mayr would probably put a very small percentage. Something like .0001%. By that standard, I am going to be very optimistic, but far less so than Sagan, and put 1%.

This would mean that around perfect stars and the revolving perfect planets where life begins, only 1% of the time does the evolving life produce big-brained mammal-like creatures. It is easy to disagree with this reasoning and put in a higher number. Please do so and see what happens. But what would the argument be for the higher number?
 

Fc = Percentage of Intelligent Life that Develops Technology and a Technological Communication Ability


This function raises the question, "Even assuming you might get large-brained creatures from time to time, what is the probability that these creatures will learn to do science and mathematics and develop technology?" Think of the possibilities. Large-brained creatures such as whales and dolphins evolve, but they don't live a human-like scientific-technological life-style at all. Dolphins have been around for 30 million years. They seem to have a lot of fun with life. Although they are not totally peaceful, they are much more peaceful than humans are. They don't develop nuclear bombs and other examples of weapons of mass destruction.  Plus, human-like creatures could evolve, but not want to, or never learn,  to develop science, mathematics, and technology.

Chapter 9 raises the issue, "Is it really a good thing to be smart?" It discusses the indirect symbolic re-presentation of reality that we have with science and mathematics v. direct-instinctual method used by the vast majority of successful creatures on Earth. This fact is part of what made Mayr extremely skeptical about evolution favoring creatures like us. Plus, a large brain may not be favored by evolution because of very high-energy requirements. On Earth human intelligence evolved due to physical limitations. Would this be repeated?

Here is where Sagan brings up his evolutionary convergence and Cosmic Rosetta Stone arguments. Think about it he says, "If two different human-like large-brained creatures evolved and one learned science and mathematics and the other stayed simple and concentrated on poetry, which one would natural selection favor?" Sagan argues that it would be obvious that natural selection would favor the scientific culture in the long run, because the scientific civilization would be better able to protect itself from comets and meteors that could destroy its planet.

Implied in this argument is the view that any version of relativism is totally wrong. The laws of nature and the basic principles of mathematics are not just some sort of cultural creation. We can see, Sagan argued, that gravity is universal throughout the universe. Mathematics works everywhere, not just in "haole" cultures. If a civilization of poets arose they would be eventually eliminated by natural selection long before a scientific culture that would not only be able to protect itself from asteroids and comets, but get in space ships and move if need be. The one environment that is the same everywhere in the universe is that the laws of nature apply everywhere. As the ancient Greeks argued, any being who understood them and learned how to apply them technologically would have the ultimate in survival power.

Sagan might be right but his arguments involve major philosophical assumptions that we will not be able to discuss. His arguments cross the boundaries of science into metaphysics and ontology.  Read carefully the section in this Chapter on the "Ontological Status of Mathematics" for a little introduction.

Again, I find Mayr's arguments more convincing, although I wish and hope that Sagan's scenario will be true. It would be wonderful if the scenario described in the Jodi Foster movie Contact would happen. (Sagan wrote the novel from which the movie was made.) But we have been listening for many decades now with our radio telescopes for any sign of other intelligent life with radio communication.  A few unexplained signals, but basically nothing.  There are lots of reasons to believe that any conclusion is premature and that we have not listened long enough yet, but the apparent silence has been deafening so far.

Most of the percentages Sagan put in for the values of the Drake Equation end up giving us a million or more advanced civilizations in our galaxy alone.   But here I am going to put in a very skeptical number, .001%.

Such a number means that given the development of perfect stars, that have perfect planets, that develop life, and that even develop intelligent life, only .001% develop our type of modern scientific-technological life-style.

Using Sagan's comparison above between a civilization of poets and scientists, now the even greater question is, "What is our most immediate threat -- a threat from comets and asteroids or the threat that stems from the products of science and technology and the terrible violent uses we put them to against each other?"

 

L = Lifetime of a Technological Civilization



The biggest question of all. It is the main focus of Chapter 10. Even if there are perfect stars and around these stars develop perfect planets, and on these planets life begins and evolves not only some big-brained creatures but also those that discover science and mathematics, how long do such creatures live?

Human life on our Earth today is basically a big scientific test for this variable. Our species has only been around for about 200,000 years. The average vertebrate species lasts about 10 million years. Simple life forms tend to live much longer. Chapter 10 asks you to contemplate objectively the obvious virtually irrefutable fact -- "The human species is clearly the most violent creature the Earth has ever produced." In a very short amount of time relatively speaking, our species has learned the tools of mass destruction. And the weapons will only get more and more destructive.

Chapter 10 raises the very real possibility that even if intelligent scientific creatures do evolve from time to time in the universe, they may not last long because they fairly promptly destroy themselves. As followers of the philosopher Nietzsche (discussed in C10) might say, such creatures could be a mere "disease" that the universe catches from time to time.

In some of Sagan's optimistic scenarios, he argues that if IC creatures have life times of 10 million years, there would be 100,000 ETIs in our galaxy alone. But why then have we not heard anything yet? The counter to the negative implications of this question is that there are millions of radio frequencies to check in listening for ICs, and there are billions of stars to point our radio receiver antennas at. Some scientists argue that a negative reception will not really be negative until we have listened for 5,000 years.  Plus, given the constraints of the GHZ discussed above, most ICs may have evolved at roughly the same time as humans, and thus there has not been enough time yet for their or our radio transmissions to travel very far.

I am going to put in a very hopeful number here. In spite of how bad the situation looks on our planet. In spite of how stupid the world leaders appear to be. In spite of the possibility of nuclear, biological, or environmental destruction, I am going to be hopeful that in the end we will make it through this period of "technological adolescence" and survive for 10,000,000 years. So for the last variable, 10 million years.

With the numbers that I have put in, what is the result? What does the number mean? (Use the drop down menus to put in the values; be sure to scroll right to see the entire list of variables.)

ICs Physical Biological Cultural
N = N* x Fp x Ne x Fl x Fi x Fc x L
= x x x x x x

It is time though to remind ourselves that this Chapter was not really about ICs. The Drake equation helps put into perspective the uniqueness and preciousness of human life. It dramatically underscores the relevance of contingency again and the implications of Darwin's theory of natural selection. Humans will most probably exist once and only once; evolution is nondirectional and nonrepeatable. After almost 14 billion years of evolution, what will we do with this chance at life?