Drake's equation and Fermi's paradox: "Where are they?"
Stars in the Milky Way.
Stars in the Milky Way.
Are we alone? The search for extraterrestrial intelligence.
Drake's equation.
Fermi's paradox.
What are the odds?
Rate of formation of suitable stars.
Fraction with planetary systems.
Number of planets with suitable environment for life per planetary system.
Fraction where life actually appears.
Fraction of life-bearing planets where intelligence appears.
Fraction of those planets with detectable intelligent signals.
Length, in years of detectability.
A numeric example.
The pale blue dot
The case for human expansion into the galaxy.

Are we alone? The search for extraterrestrial intelligence.

All of us must at some time have asked the question: "Are we alone in the universe?" More precisely, are we the only beings in the universe capable of self-awareness, intelligent reasoning and the development of a technological civilization?

Woodcut from Flammarion.

A woodcut illustration from Flammarion's works. It is now thought that he himself may have created this medieval-looking allegory.

In the 19th century, rapid advances in astronomy and Darwin's discovery of biological evolution led to speculation that alien worlds might harbor life, and even intelligent species.

Writers such as C. Flammarion (La pluralitÚ des mondes habitÚs) and H. G. Wells stimulated popular interest in the subject. When Schiaparelli observed linear patterns on the surface of Mars (an optical illusion, we now know), many thought that there might be a system of canals on a dry planet. In 1908 S. Arrhenius proposed that life had been carried to Earth by spores from outside the solar system (panspermia). In 1938, Orson Welles caused a panic in the U.S.A. with a radio show based on H. G. Wells' War of the Worlds. In the 1940s, reported sightings of "flying saucers" or Unidentified Flying Objects (UFO) started to attract attention.

Science fiction has always been enamoured with the subject, and movies such as E. T. have made the concept of alien beings part of popular culture.

In reality, the possibility of extraterrestrial intelligence is something that most scientists find quite intriguing, even more so, perhaps, than the general public. Yet, up to the present time only very limited resources have been allocated to the "Search for Extraterrestrial Intelligence" (SETI). Part of the explanation is undoubtedly that most scientists are understandably hesitant to invest their time in an endeavor with such uncertain chances of success, not to mention the stigma of being ridiculed as an oddball (notwithstanding the blessing from such prestigious institutions as the American National Academy of Sciences). And the preferred tools are powerful radio telescopes which are also in great demand for "conventional" radio astronomy.

Arecibo radio telescope.

The 305 m radio telescope at Arecibo.

Source: NAIC - Arecibo Observatory

The first serious, if very limited, SETI experiment was carried out in 1960, when Frank Drake turned his radio telescope on two nearby stars during two weeks. Some brief excitement was caused by what turned out to be terrestrial interference, but no alien signals were detected. Another brief flurry of excitement occurred in 1968, when Jocelyn Bell discovered pulsars. It was initially thought that the signals might be evidence of an alien civilization. In the years since then, there have been ebbs and flows in interest and support. More sensitive instruments have been used, including the large telescope at Arecibo, Puerto Rico. The receivers have become more sensitive, and the processing of the data has become many orders of magnitude more powerful. (One money-saving device is distributed processing on personal computers made possible by more than 2 million volunteers - a forerunner of GRID technology.) - A major step will be taken in 2006, when the Allen Telescope Array should become operational with 42 20-foot antennas, later to be expanded to 350 dishes. - It is estimated that within a few decades, hundreds of thousands, possibly millions, of stars in our galaxy will have been monitored at the most promising frequencies. - More on the SETI organization at www.seti.org.

Drake's equation

This brings us to Drake's equation (1961), which is really just a device to facilitate a rational discussion about the long chain of improbable events that must occur for a technological civilization to arise and become detectable through its electromagnetic emissions. Its chief virtue is that it encourages informed debate. - To be sure, trying to assign a probability to the rise of a technological civilization is still just "handwaving", but the handwaving may be a little better informed by discussing each factor in Drake's equation individually.

The equation can be written:

N = Rs* fp * ne * fl * fi * fc * L



The number of civilizations in The Milky Way galaxy whose electromagnetic emissions are detectable.


The average rate of formation of stars suitable for the development of intelligent life.


The fraction of those stars with planetary systems.


The number of planets, per solar system, with an environment suitable for life.


The fraction of suitable planets on which life actually appears.


The fraction of life bearing planets on which intelligent life emerges.


The fraction of civilizations that develop a technology that releases detectable signs of their existence into space.


The length of time such civilizations release detectable signals into space.

The equation only addresses our own Milky Way galaxy. It is thought that there are about as many galaxies in the universe as there are stars in our own galaxy - a hundred billion or more! In total, by extrapolation from the Hubble Space Telescope deep survey, it is thought that there are on the order of 1022 stars in the observable universe (a 1 with 22 zeros)!

We will return to the discussion of the numbers in Drake's equation. What is noteworthy at this point is that some distinguished scientists believe that there may be thousands, even millions, of technological civilizations in our own galaxy, based on what they regard as plausible assumptions about the numbers in the equation. But there is also a strongly held opposing view (non-religious), arguing that our Earth is unique in harboring a technological civilization.

Fermi's paradox

Enrico FermiEnrico Fermi was an eminent physicist who developed the world's first nuclear reactor and participated in the Manhattan project. His argument on extraterrestrial intelligence was that, given time, any technological civilization should be capable of colonizing the entire Milky Way. Extraterrestrials should have reached Earth long ago if intelligence were common in our galaxy. "So where are they?"

The argument seems persuasive. The Milky Way extends roughly 100 000 light years from end to end. Even at the relatively modest velocity of one hundredth of the speed of light, the whole galaxy should be accessible within 10 million years. This is just a small fraction of the age of the galaxy.

Of course, a civilization or even a species might not last that long, but there are plausible scenarios where colonization could proceed no matter what happens to the originating civilization. Robotic probes could be designed to seek out new worlds, and upon arrival build a new generation of probes that would be sent out for further exploration, and so on ad infinitum (a concept proposed by the great mathematician John von Neumann). This might spread like a virus across the galaxy, difficult to stop once it is set in motion. Or genetic material could be sent in "unmanned" space ships, to be incubated and transformed into living creatures upon arrival. Living beings might hibernate aboard spaceships for centuries and be resuscitated upon arrival. In each case, the colonists would be completely self-reliant and unaffected by any event on their native world.

Many would consider space colonization a waste of resources, but if intelligent life were really common in the galaxy, one might expect that at least one civilization would have embarked upon a program of colonization. After all, our own civilization went from heavier-than-air flight to footsteps on the moon in just two thirds of a century. Few would dare claim that interstellar flight is out of our reach in a multi-century perspective.

It is interesting to have a look at some of the attempts that have been made to reconcile the "thousands of space-faring civilizations" theory with Fermi's observation. There are two schools of thought:

  • Extraterrestrials have been here (or are here now).
  • There is no good reason to come here, so they have all stayed at home.
  • Häpna!

    A Swedish science-fiction magazine from 1955. The cover shows Clarke's hypothesized sentinel on the Moon.

    In the first case, they may have decided that our planet is unsuited to their needs. "Earth is a great place to visit but I would hate to live there". Or they may have brought life to Earth, or they may have engineered the evolution of multi-cellular forms of life, or they may have steered the evolution of apes into humans, or they may have brought us religion. Who knows, we might find a greeting from ET (or even a "user's manual" encoded in our "junk" DNA ;-) Even less credible theories suggest that we are being monitored now by aliens in our midst, whether they include abductions by UFOs or not. A more plausible scenario, the hypothesis by Arthur C. Clarke on which the movie 2001 was based, suggests that ET may have left monitoring devices in our solar system or its vicinity to report to their distant owners on the possible development of intelligence on Earth. In that case, the alarm bell may already be ringing: our radio transmissions and TV broadcasts (clearly identifying ourselves as a potential threat to the galaxy) have travelled 70 - 80 light years by now (and would be easily detectable at that distance using present Earth technology). Or perhaps they visit us periodically at fairly long intervals to check on the progress of evolution .

    In the second case, one of the leading theories is the "zoo" hypothesis: Earth is kept as a preserve to study the emergence of a primitive civilization. Perhaps there is an agreed ethos among mature civilizations in the galaxy not to interfere with, or disrupt, primitive societies? Perhaps we are being tested. - Other theories suggest that galactic colonization may simply not be justifiable on rational grounds, and any stable civilization will be extremely rational. ET sees better use for his/her (its?) tax euros. Or technological civilizations may quickly reach a stage where the meaning of life is not sought or expressed in outward expansion. Lord Rutherford famously declared: "All science is either physics or stamp collecting." Perhaps ET takes a similar dim view of the study of alien life forms?

    It is sobering to realize, that any detectable alien civilization is likely to be much more advanced technologically than we are. We have been broadcasting electromagnetic signals for less than a century. We are the new kids on the block. [There - that should be good for a few Google hits!] If we try to imagine what we would have been able to predict about our present technology 500 years ago, some modesty seems appropriate concerning our ability to predict our own achievements even as little as 500 years into the future. When it comes to theorizing about the motives and achievements and rate of progress of a completely alien civilization, perhaps tens of thousands of years old, we must admit that we are stumped. "Wovon man nicht sprechen kann, darŘber mu▀ man schweigen." (Wittgenstein) And, as pointed out by Arthur C. Clarke, any sufficiently advanced technology is indistinguishable from magic.

    Pioneer plaque designed by Carl Sagan.
    The Pioneer plaque designed by Carl Sagan.

    I am personally skeptical about the idea that our nice, friendly SETI professors will find nice, friendly colleagues eager to teach them everything they know, in case we make contact. - Of course, even if extraterrestrials should be willing to act as our tutors, any dialog is likely to require hundreds, or thousands of years (or millions in the case of other galaxies) between question and answer, so the best we can hope for is that somebody is broadcasting the entire Encyclopedia Galactica in our direction even before we make ourselves known.

    By the way, the wisdom of announcing our presence is open to doubt. The cat is out of the bag in the form of our radio and TV broadcasts. In 1974, a directed message was sent from the Arecibo radio telescope to a globular cluster of stars 21000 light years away. However, there is no systematic effort to attract attention, at least for the time being. One of the early Pioneer space probes on its way out of the solar system contains a greeting to "whomever it may concern", but it will take some 300 000 years before it approaches another star.

    In summary, no credible evidence exists of visits by aliens in the past. The absence of evidence would seem to make it unlikely that we have been visited. On the other hand, as pointed out by Carl Sagan: "Absence of evidence is not evidence of absence."

    In particular, molecular biologists have thus far found no evidence that would suggest that "intelligent design", whether by aliens or through divine intervention, has interfered with the evolution of terrestrial life (with the minor exception of our own efforts in that direction).

    For some interesting comments concerning Fermi's Paradox, see Shostak (2012 Ted talk 18:23 min), Freiheit IV, Wikipedia, Myers (blog with 88 responses, many of them excellent food for thought), or any number of web sites and news groups through a search on "Fermi's paradox".

What are the odds?

Let us go back to Drake's equation and discuss the factors one by one.

Rs - The average rate of formation of stars suitable for the development of intelligent life.

Note. By convention, the average rate (stars per year) is used rather than the total number of suitable stars. This leads to an estimate of the average number of new civilizations per year, which, when multiplied by their expected "lifetime", gives the number of civilizations available for detection.

We have a fairly good estimate of the age and the total number of stars in the galaxy. The Big Bang occurred about 14 billion years ago, and the Milky Way is one of the oldest galaxies among those that have been studied. It probably formed within a billion years after the Big Bang. It has some 200 - 400 billion stars. (The range of uncertainty reflects the effects of dust in the center of the galaxy, but also, I think, the number of red dwarves and the exact definition of a star.) Not all stars are suitable for the development of complex life. Some burn out too quickly. Some are deficient in heavy elements. Recent studies indicate that only certain regions of the galaxy are suitable for the development of terrestrial planets (assumed to be necessary for the development of complex life). This is due to radiation from supernovae in the dense inner regions of the galaxy, gravitational disturbances, bombardment from comets, too little or too much metal content, etc. Perhaps there are 100 billion stars that could potentially support (or have supported) the development of life?

If we assume that 100 billion suitable stars have formed in 10 billion years, we arrive at a star formation rate of the order of 10 per year. (This rate is probably lower today, but our interest centers on stars with an age comparable to that of the sun - some 5 billion years.)

fp - The fraction of those stars with planetary systems.

Planetary system being formed.

The Hubble Space Telescope's edge-on view of a planetary system being formed in the Orion nebula.

Source: NASA/Space Telescope Science Institute

This is one parameter where great progress has been made in just the last decade. Before the 1990s, there was only one recorded planetary system - our own. As of May 2024, 5662 extrasolar planets have been discovered. Due to limitations in the sensitivity of the measurements, nearly all are Jupiter-sized planets in close orbits around the central star - quite bizarre objects in the light of our previous understanding. It is expected, however, that as the sensitivity of measurements improves, smaller planets will be discovered, as well as giant planets in more "normal" orbits. In particular, satellites will be launched in the next decade with the aim of detecting Earth-sized planets. They may even be able to detect signs of biological activity through spectroscopy.

Recently, there have also been observations of planetary system formation in progress (the Hubble and Spitzer space telescopes).

Based on our present understanding, the fraction of stars with planetary systems is significant, perhaps as high as 50 percent.

ne - The number of planets, per solar system, with an environment suitable for life.

This parameter is more difficult to assess. First of all, we do not know what is required for life to exist. For all we know, exotic lifeforms based on a completely different chemistry than ours may be possible. We do not even know for sure that the surface of a terrestrial planet is the only suitable habitat. Perhaps life could arise in the atmosphere of a gas giant or in the interior of a comet?

To have some basis for speculation, however, it seems natural to look at what has made life possible on our planet. Here carbon-based organic chemistry and liquid water seem to have been key ingredients. In addition, conditions need to be reasonably stable over geologic time scales for life to take hold. This means, inter alia, a rocky planet in a stable circular orbit in a temperate zone neither too close nor too distant from the central star (or stars - we should not exclude double stars, which are very common in the galaxy). The planet needs to be the right size to maintain an atmosphere as a shield against ultraviolet radiation and energetic particles, and to regulate temperature.

Not very long ago, many scientists thought that the odds were good that there should be at least one terrestrial planet hospitable to life in a typical planetary system. After all, in our own system there is a distinct possibility that subsurface life exists on Mars (although any life on Mars may have originated on Earth - or perhaps even more likely the other way around - through contamination via meteorites), and perhaps on Jupiter's moon Europa.

Water ice in Martian crater.
Photo: ESA/DLR/FU Berlin (G. Neukum)
Jupiter's moon Europa.
Water ice in a Martian crater.

A deep ocean is thought to exist beneath the water ice covering the surface of Jupiter's moon Europa .

More recently, however, several aspects have been pointed out that may make it less likely that our solar system is a typical "run-of-the-mill" planetary system. It is believed that water on the inner planets comes from comet impacts, and that Jupiter has had a key role in this process. A gas giant in the right place and with the right mass may thus be required but absent. Conversely, gas giants in the wrong places may preclude a stable circular orbit in the temperate zone (liquid water) due to gravitational effects. The surprising finding that out of the extrasolar planets detected up to now, a fair number are in a highly elliptical orbit, contributes to the pessimism.

The central star must also be the right size. If it is too large, it will burn too quickly and emit too much harmful radiation. If it is too small, the temperate zone will be too close to the star and the planet will experience tidal locking, always turning the same side towards the star. The energy output must be stable over eons, otherwise the liquid water will freeze or evaporate.

For these and other reasons, it is now commonly believed that the number is much smaller than 1, perhaps more like 0.001. This still leaves us with some 50 million planets in our galaxy where life could potentially arise.

fl - The fraction of suitable planets on which life actually appears.

Once conditions are right, what is the probability that life will actually occur? Quite high in the opinion of most scientists. Experiments carried out by Stanley Miller and Harold Urey in 1953 demonstrated that many of the building blocks of life could arise in a "soup" of chemicals thought to be present on young planets, in a water solution when energy was added. However, the road to more complex molecules remains obscure, and in particular there is no credible detailed scenario for how DNA evolved. What we do know is that life developed fairly quickly on Earth once the conditions were right. The oldest fossils are 3.5 billion years old, and the solar system itself is 4.5 billion years. There was heavy bombardment of the Earth until 3.8 billion years ago, so life may have emerged just a few hundred million years after conditions made it possible.

An enjoyable conversation on the roots of "the tree of life"between AI scientist Lex Fridman and astrobiologist BetŘl Kašar can be found in this 2 h 40 min video. In particular the critical aspect of translating the genetic code into metabolism (and vice versa) is discussed. Molecular biology has come a long way since the double helix structure of the DNA molecule was mapped in 1953! - And here is one recent "opinion" that seeks to summarize the current state of affairs. - And this is a series of nine 1½ h lectures on the "Origins of Life" focusing on pre-biotic chemistry. (Note added March 2024.)

It may well be that the emergence of life is a statistical certainty once all ingredients are in place. It is of course also possible that this event is so rare that it takes a miracle for it to occur (figuratively or literally - take your pick). The prevailing opinion among scientists seems to be that there should be millions of life-bearing planets in the galaxy. And once primitive life takes hold, it could be extremely difficult to stamp out.

fi - The fraction of life bearing planets on which intelligent life emerges.

This parameter is probably where mainstream scientists most strongly distance themselves from SETI enthusiasts. Many modern biologists do not see evolution as an inevitable march from "lower" to "higher" organisms with intelligence an important attribute for genetic success. To them mankind is an "unforeseen" byproduct of environmental adaptations that have been going on for billions of years. To the unbiased eye, insects have been at least as successful as vertebrates - at least until humans came along and invented pesticides. Insects are likely to still be around long after mankind goes extinct. If history were repeated from a few hundred million years back with a new roll of the dice, in the opinion of many scientists, it is quite unlikely that another intelligent species would evolve.

Evolution from the most primitive lifeforms occurred in several big leaps. It took two billion years for life to evolve from simple anaerobic bacteria to single-celled eukaryotic organisms. Multi-cellular algae and sexual reproduction were established 1.2 billion years ago, and fossils of multi-cellular animals have been found from 600 million years ago. See timeline.

Precambrian organisms.

Reconstruction of organisms from the very end of the Precambrian. (Diorama at the Smithsonian Institution.)

Copyright: Smithsonian.

Then came the big breakthrough about 540 million years ago: the Cambrian explosion, when multi-cellular animals evolved at a tremendous rate within a very short period. (At least their morphology did, it now seems that considerable genetic diversification took place earlier - but without great fanfare.) Within as little as ten million years a spectacular variety of body plans appeared. As many as twenty basic designs evolved, of which only a few remain today. Today we have many more species, but they are all variations of these few basic themes (vertebrate, exoskeleton, jellyfish?). Thus, a rich macroscopic fauna has only existed for the past 500 million years or so.

To the undiscerning eye, not much appears to have happened during the first three billion years of evolution. (This reminds me of a historian who tongue-in-cheek claimed that the 8th century had never taken place - it was a construction. It turns out that it is not all that simple to disprove his thesis.) All that Nature had been able to come up with until 600 million years ago was what most of us would regard as very primitive life forms: bacteria, algae, perhaps some worms and sponges. It is tempting to think of the eons of pre-Cambrian evolution as "wasted time" that could have been much reduced with some luck. However, one should not underestimate the complexity at the molecular level of the genetic machinery needed to evolve multi-cellular animals. By the time of the Cambrian explosion, an impressive arsenal of biochemical building blocks had been assembled. - If we look at a newborn baby, not much seems to be going on during the first few weeks, but in fact its mental capacity is evolving at a tremendous rate. It is building an internal map of the world, learning to interpret sounds, smells, colors, shapes, faces, motion, gravity. In a similar way, complex life could not arise without a lot of prior basic development at the molecular level. In the absence of a Designer, it should not come as a surprise that billions of years passed before the Cambrian explosion.

It is not well understood how the Cambrian explosion was triggered. It may be that the slow buildup of atmospheric oxygen passed a critical threshold, or some catastrophic event "reshuffled the cards", or some other environmental change occurred, or perhaps some mutation affected the genetic machinery in a way that facilitated the evolution of new organisms.

About 248 million years ago, the greatest mass extinction in history occurred, when over 90 percent of all marine species disappeared. The dinosaurs emerged 230 million years ago and went extinct 60 million years ago, most likely as a result of the famous meteor impact in the Yucatan peninsula at that time. Since then, mammals have taken over as the dominant form of animals. Primates have been around for some 20 million years, hominids for some 4 million years, recognizable humans for 300 000 years. Our own Cro Magnon brand of humanity appeared just 40 000 years ago, and we have not evolved in any significant way since then. Einstein, Mozart and Shakespeare could easily have been born and raised in an ice age hunter society!

What can we learn from all of this? There does not seem to be anything inevitable in the rise of intelligence. Life emerged rapidly, once conditions permitted, but then 3 billion years passed before anything more interesting than bacteria, algae and tiny worms appeared on the scene. Contrary to popular belief, the history of life has not been a steady, gradual progression from "lower" to "higher" forms. The modern view is - or was until recently - that there have been long periods of equilibrium with minor fine-tuning of life forms, interspersed with the rapid evolution of new life forms when the environment has changed - sometimes as a result of catastrophic events (asteroid impacts, volcanism, glaciation), sometimes when gradual change has crossed a certain threshold (build-up of oxygen, plate tectonics rearranging the continents). - It is interesting in this context that humans evolved during an atypical epoch of repeated glaciations during the past 4 million years.

This theory of "Punctuated equilibrium", by the way, does not fundamentally challenge Darwin's views on evolution. Darwin was no stranger to the concept:

But I must here remark that I do not suppose that the process ever goes on so regularly as is represented in the diagram, though in itself made somewhat irregular, nor that it goes on continuously; it is far more probable that each form remains for long periods unaltered, and then again undergoes modification. (Darwin, Ch. 4, "Natural Selection," pp. 152)

"It is a more important consideration ... that the period during which each species underwent modification, though long as measured by years, was probably short in comparison with that during which it remained without undergoing any change." (Darwin, Ch. 10, "On the imperfection of the geological record," p. 428)

Source: Douglas Theobald

The main driver of evolution seems to be environmental change. Periods of environmental stability do not seem to correlate well with rapid evolution. Catastrophic events causing mass extinctions may well have been the key factor leading to intelligence, by allowing a new "roll of the dice" from time to time. It is noteworthy that, at least as far as we know, the dinosaurs did not evolve intelligence over a period of 170 million years. If intelligence by itself had been important for survival, there is no obvious reason why they should not have been able to do so. (On the other hand, among mammals, dolphins have evolved a certain level of intelligence, and they are not that closely related to our species.) Besides, the possibilities of hard-wired specialized "intelligence" should not to be sneezed at. Consider the social insects, or the web-weaving and nest-building capabilities of spiders and birds. - The Earth may well be unusual in having had the right mix of environmental stability and rapid change to create favorable conditions for intelligence to finally evolve.

Earth and Moon to scale.

The Earth and the Moon. The Moon may have played an important role in the evolution of complex life on Earth.

Reasons for pessimism on the chances for intelligence, or even complex life, to arise have been forcefully expressed by Ward and Brownlee in their book "Rare Earth". (94 reader reviews, some of them quite interesting.)

At the heart of the matter, there are different views of evolution itself. Gould argued that evolution has no inherent direction. It just adapts organisms to fit the environment. Evolution could just as easily lead to decreased complexity as the opposite. It proceeds by random mutations as a "drunkard's walk" (Markov chain) with no preference for any particular direction. There is a "wall" on one side (life cannot go below a certain level of simplicity), and purely by chance some life forms at the other end will become increasingly complex, so there will be a slow drift toward increased complexity on average, but there is no reason why in the long run those life forms should be more successful than simpler forms.

The opposite view (religious beliefs aside) is that evolution has an inherent component in the direction of increased complexity. Even in a stable environment there is room for improvement through the development of new capabilities based on increased complexity. - There is some evidence for this view in the form of simulations of artificial life forms.

To me it would seem very optimistic to assign a value close to 1 to the factor fi, meaning that every life-bearing planet will ultimately evolve intelligent life. Even fi = 0.001 may be optimistic.

fc - The fraction of civilizations that develop a technology that releases detectable signs of their existence into space.

This is another very uncertain number. Surely, a portion of intelligent species would find it impossible to develop the capability to signal their presence. Not only would tool-making be required, which puts certain demands on the body design; it must be possible to extract and shape metals and other materials and assemble them into devices, and to extract and manage energy from suitable sources. We are fortunate to have had easy access to fire, for instance.

Even given our own mental capabilities, it does not appear inevitable that we should have succeeded to move from stone age technology to broadcasting electromagnetic signals before our species became extinct. And our form of intelligence may, purely by chance, be especially well suited to develop science and technology. Most theorists believe that the development of our intelligence had to do with language and social interaction, and perhaps tool manipulation. Our ability to discover Maxwell's equations was an unforeseen side effect, so to speak. Other forms of intelligence may develop in other directions.

On the positive side, we managed to move from our present level of intelligence to radio technology in just 40 000 years. As long as an intelligent species does not go extinct, there is no particular reason for hurry from the perspective of Drake's equation.

Still, it appears optimistic to believe that a large fraction of "intelligent" (self-awareness, intelligent reasoning, language) species would be capable of signalling their presence into space. (Think of dolphins again.)

L - The length of time such civilizations release detectable signals into space.

This is perhaps the number about which there is the widest divergence of views. The argument has centered on how long our own technological civilization may be expected to last. There are extremely optimistic views: "We will evolve into a stable society that could last for millions of years. Human history has known many civilizations. Even a thermonuclear war will not mean the end. Humans are resilient. After a dark age, another civilization will inevitably arise.", just as there are equally pessimistic views: "Mankind is on the brink of extinction less than a century after starting to emit electromagnetic signals. In addition to war, there are the threats of a global plague - man-made or natural - or of a global environmental disaster. Any technological civilization will be unstable and short-lived."

Even if a technological civilization should achieve a good long-term perspective, it is far from obvious that it will "leak" electromagnetic radiation into space, except for the specific purpose of trying to contact other civilizations. Our own radio and TV transmissions are already being switched to more efficient means of distribution (cable and short-range microwave).

It is quite possible that an advanced civilization would want to announce its existence to the rest of "the galactic community", but is is equally probable that it would prefer to remain inconspicuous, listening rather than transmitting. As Fermi pointed out, any sufficiently advanced civilization should be able to spread across the galaxy. A corollary might be that any advanced technological civilization represents a potential threat to other galactic civilizations.

Therefore, even an optimistic estimate of the average lifetime of a technological civilization may need to be tempered with a more cautious estimate of the length of time when such a civilization will emit detectable signals.

Another factor that should perhaps be considered, is that a technological civilization may become capable of creating artificial intelligence (not limited to smart chess programs, but actually able to pass the Turing test), perhaps including an artificial evolution process millions of times faster than any natural process, which could lead to a "robot civilization" that might be more stable than the originating civilization.

As a final observation, already during the present century, mankind will probably start tinkering with its own genetic machinery. Initially, research will be directed toward curing a variety of genetically influenced diseases, but as knowledge is gained (also from non-human genetic engineering) and confidence increases, it is quite probable that ambitions will rise despite ethical considerations and religious objections. This suggests that mankind may evolve (or devolve) substantially in directions that are difficult to foresee, even in a time perspective of just thousands of years. - It is anyway a myth that human evolution has come to a standstill just because the struggle for survival has become more lenient in modern society. For example, I am convinced that strong selection for parental instincts is going on right now, as child-bearing has become facultative in Western society. - All of this may not have any direct bearing on Drake's equation, but it should reinforce the need for humility when we indulge in speculation. Not even our own species may be as stable (in the sense of retaining all its present characteristics) as we like to think over the course of the next few thousand years (assuming we survive that long).

A numeric example

Based on the discussion above, let us finally propose a set of probabilities that appear "reasonable" and see where that leads us. I have picked numbers that seem plausible to me, perhaps on the optimistic side:

Rs 10
fp 0.5
ne 0.001
fl 0.5
fi 0.001
fc 0.01
L 10000

N = Rs* fp * ne * fl * fi * fc * L

If I have counted the zeros right, this would give us a number of just 0,00025. This is very far from even a single civilization in our galaxy emitting detectable signals.

Of course, I could easily be off by a factor of a billion or more, either way. That would still only leave 250000 detectable civilizations at best, spread out over a hundred thousand light years. We would in that case have to inspect on the order of a million stars for each positive result.

In the next 30 years, we should gain a much better understanding of the first four parameters through direct observations from space telescopes. Atmospheric measurements of extrasolar planets indicative of biological processes (presence and ongoing replenishment of gases such as oxygen and methane) would be of great interest and might form the basis for a statistical assessment of the chances for the emergence of life.

Unfortunately, the last three parameters are likely to remain very uncertain unless the SETI program turns out to be successful, something that is entirely possible, of course. But a negative result even after picking the "low hanging fruit" - searching the most promising and accessible one million stars, say, in the next few decades - would be discouraging, even though they would just represent a sample of one in a hundred thousand.

The reason is Fermi's paradox. It would seem reasonable to believe that a large portion of all detectable civilizations should also be able to develop interstellar space flight. If there were many detectable civilizations in our galaxy, we would expect at least one of them to have expanded across the Milky Way. In that case, surely at least one planetary system in a million should host a detectable civilization.

My conclusions, no strike that, my tentative opinions until I talk to the next persuasive scientist with a different view, are:

1) There are probably millions of planets in our galaxy teeming with simple life forms.

2) There is probably only one detectable technological civilization in our galaxy at present - our own.

3) There are probably millions of advanced technological civilizations in the universe.

The pale blue dot

The case for human expansion into the galaxy

A man half asleep at an astronomy lecture suddenly became wide awake and asked: "Could you repeat what you just said, please?" - "Certainly. Our sun will expand and engulf the Earth five billion years from now." - "Thank God. I thought you said five million!"

Messier 83.

At a distance of 15 million light years, Messier 83 has approximately the same size and structure as the Milky Way. It gives a good idea of what our own galaxy may look like from the outside. We live in a spiral arm about 28000 light years from the center of our galaxy.

Copyright: European Southern Observatory.


Back in 1969, when Apollo 11 landed on the moon, even jaded journalists remarked: "Incredibly, after billions of years of evolution, it is within our own short life span that creatures from our planet have for the first time visited another world!"

Even though the same event may have occurred many times in the history of our galaxy, it may very well be that at present we are the only space-faring civilization among billions of worlds in the Milky Way.

I do not know which vision fills me with more awe - the Milky Way with many advanced civilizations, or the one where we are unique. But undoubtedly the latter scenario places the heaviest responsibility on our shoulders, for whether we are here by design or by pure chance, our demise would mean the loss of something very precious, not only to Mother Earth, but to the whole Milky Way.


As a species of mammals, we could normally look forward to an existence of a few million years before replacement by a new species better adapted to the environment. But ordinary rules do not apply to us. We are ourselves modifying the environment at an unprecedented rate. In the long run - if there is one - this capability could work to our advantage, but in the near term (the next few centuries) we may well trigger irreversible processes that spell our doom. A run-away greenhouse effect is just one of the possibilities. The rapid spread of diseases made possible by modern modes of transportation is another worry. Ecological disaster as a side effect of genetic engineering is yet another possibility.

All unintended disasters seem less threatening than our tendency to willfully inflict damage on ourselves, however. Even if my generation has enjoyed a long period without a major war, it would be naive to think that wars are a thing of the past. A thermonuclear war could well bring us back to the stone age. A "nuclear winter" causing famine after a major war remains a possibility. What worries me even more is the potential for creating dangerous pathogens. This would not necessarily mean that biological warfare is a strong threat, but the technology is being developed by governments as a "precautionary measure", and once the technology exists, there is always the danger that it may fall into the hands of extremist groups "on a mission from God" or whomever. I fear that the technological barriers to developing means of mass destruction may well be lowered in the future. There are people who regard mankind as a "cancer" on Mother Earth and would not hesitate to perform an operation if given the opportunity.

Overall, I think that there is a significant probability that Man, "king of ashes", will selfdestruct within the next few millenia, if not before. To safeguard our cultural heritage, to facilitate a comeback after a possible collapse, and perhaps to protect our very existence as a species, we should avoid "putting all our eggs in the same basket". This, in my opinion, is a strong argument for establishing self-sufficient outposts in the solar system with some urgency (although it will not be easy), or perhaps O'Neill type space colonies. But it is not yet a compelling argument for expansion into the galaxy due to the time scales involved.

In the long run, however, I can think of no better way to ensure the survival and the success of humanity than to establish a human presence in other planetary systems than our own. The major disadvantage of having to bridge the huge gaps that separate the stars in space, and therefore also in time, is also the great virtue of interstellar colonization. The colonies would be much more isolated from us than America and Australia ever were from Europe. Trade and war and imperialism would become impractical, but we could still share all the elements of culture - science, novels, poetry, films, music, art, games - albeit with a huge time lag. Our descendants on Earth would in effect become observers of many human cultures, without the opportunity to interfere in their affairs, for better or for worse.

In time, colonies would evolve into human civilizations in their own right, launching new waves of colonization. Within a few thousand years, the risk that our race will expire could be dramatically reduced. Within a few million years, each of a million human civilizations could enjoy the knowledge and the creative accomplishments of all the others. The benefits would be literally unimaginable. - Of course, by then the different populations might have developed traits that would make them seem quite alien to us - but so might any civilization left on Earth.


Diversity is beneficial to the longterm prospects for success of any species or grouping of species (phylum). I would argue that cultural diversity is important too, and has played an important role in the development of our civilization.

Star field in Sagittarius.
A star field in Sagittarius.
Source: NASA/Space Telescope Science Institute

Yet, as technological advances have led to global trade and instant communications, our culture seems to be growing more uniform. A hotel room looks the same in Sydney as in Berlin or Buenos Aires. The cars look the same, the fast-food restaurants look the same, the music and TV shows and films played on radio and TV and in movie theaters are largely the same. "Do you have what we Americans call pizza?" Breakthroughs such as the Internet or mobile telephony sweep across the world virtually overnight. True, a lot of this is less obvious to the poor in large parts of the world. Still, our technological society seems to be heading toward convergence rather than diversity. We are becoming more like an ant hill and less like a meadow. - All of this may be a good thing if it promotes international understanding and economic development worldwide, but it still seems a little worrisome to me.

One of the attractive features of the idea of a human expansion to other planetary systems is that cultural diversity would be guaranteed and could grow beyond imagination. Thanks to the built-in time lag between widely dispersed civilizations, they would all develop in different directions, without however being completely cut off from the rest of humanity.

Sense of purpose

The final point to be made in favor of galactic expansion is the sheer grandeur of the concept. - There is a story about two stone cutters living in the 12th century. One of them was swearing and generally indignant. "I have been given the [expletive deleted] job of cutting this [expletive deleted] stone into a square shape." The other one was whistling. "I am building a cathedral." - It is hard to imagine a more glorious undertaking than the settlement of the galaxy with its hundreds of billions of stars, and it is one that could give a sense of purpose to all future generations. - It is also in line with religious teachings (Genesis 1:28).


What about cost? What about implementation? Is it even feasible?

I do not believe that such an undertaking will be realistic for several centuries. We must learn to walk before we can run, and the exploration of our own solar system should keep us busy for quite a while. Before this century is out we may be able to send unmanned probes to the nearest promising stars. More likely, we will build giant space telescopes allowing us to examine neighboring star systems in much greater detail than will be possible initially.

Another thing to consider is when our technology is good enough to aim for the stars. - There is an amusing science fiction story by A. E. van Vogt, where humans travel to our nearest star system Alpha Centauri for 500 years in suspended animation. When they arrive there, they are welcomed by an advanced human society! - It turns out that technology has progressed to the point where faster-than-light travel has become possible. The target planet was reached many years before our heroes arrived! - I consider it extremely unlikely that faster-than-light travel will ever become possible, but obviously it will make a great deal of difference just how fast an interstellar ship (or robotic probe) will be able to travel.

Another possibility when we think in terms of thousands of years is planetary engineering ("terraforming"). We may be able to develop bacteria that can provide a planet with a breathable atmosphere within a "reasonable" amount of time, or at least an atmosphere that will moderate temperatures. We may also want to seed sterile planets in order to "jump start" biological evolution. If our scientific understanding becomes sufficiently advanced, we might be able to guide biological evolution of extant life in the direction of increased complexity. Something for an ethics committee to ponder...

So, for the foreseeable future, human expansion into the galaxy must remain just a vision. But it is a vision that can give us hope for the long-term survival of our species, and give us hope that great things await our descendants in the distant future!

  Last edited or checked June 7, 2013. Note on the origin of life added March 16, 2018. Broken links fixed or replaced May 30, 2024.

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