The likelihood of extraterrestrial
intelligence
Vast distances and long travel times
It has been said that the discovery of an extraterrestrial intelligence will be the most important event in mankind's history. For millennia, humans have been looking at the stars at night and wondering whether we are alone in the universe. Only with the advent of large-dish radio-frequency antennas and ultra-sensitive receivers in the late-twentieth century did it become possible to attempt a search for extraterrestrial intelligence (SETI).
The search at radio frequencies continues and has even involved the public (see SETI@home) by allowing home PCs to analyze some of the received noise. With so much data collected, it becomes easier to examine if pieces of the data are divided up and dispersed to many individual computers. A home PC can analyze the data at a time it is otherwise idle. The fact that tens of thousands of people signed up to participate illustrates the strong public interest in SETI. Whilst a very successful promotion, it has had no success in finding an extraterrestrial signal.
On the other hand, look at what we have accomplished in less than 200 years: we have progressed from essentially being limited to communicating within earshot or by messengers traveling on foot or riding horses, to communicating at the speed of light with space probes millions of kilometers away.This fantastic accomplishment illustrates the exponential growth of our technology. In this context, several decades spent on SETI is a mere drop in the bucket of time. The disappointment of SETI to date is, I believe, due to the overoptimistic expectation of there being an advanced intelligence in our immediate neighborhood. Less than 100 years ago it was widely believed that there might be beings on Mars or Venus, the nearest planets to us. We now know this is not so.
Indeed, we have come to realise that whilst intelligent life on planets orbiting other stars is feasible, its development is dependent on a number of conditions that may not occur in combination very often .In spite of there being several hundred billion stars in our Milky Way galaxy, the likelihood of an intelligent society sending signals our way is thought to be low. The recent discovery of over 300 planets orbiting relatively nearby stars lends hope that there are many planets that can sustain life, some of which will develop intelligence that is willing to communicate. But the equation developed by Frank Drake in 1960, the hypothesis advocated by Peter Ward and Donald E. Brownlee in their book Rare Earth: Why Complex Life is Uncommon in the Universe, published in 2000 (Chapter 3), and the study by Stephen Webb using the Sieve of Eratosthenes in his book If the Universe is Teeming with Aliens. . .Where is Everybody, published in 2002 (Chapter 6), all highlight the many probabilities in play. Depending on how optimistic one is in assigning probabilities to each factor, one can reach either very low probabilities or much better odds. A probability of one in a million would still mean 400,000 stars in our galaxy have intelligent life - and there are hundreds of billions of galaxies. So where are they? Either intelligence is scarcer, or we have not been looking in the right places using the right instruments at the right time.
The failure of SETI to-date raises the intriguing question of whether our search at radio frequencies was naive, since no intelligent society would use radio frequencies to transmit over distances of hundreds of light-years if other wavelengths were more useful. Is a technology which we ourselves have only recently acquired likely to be favored by a far more advanced society? In fact, a good argument can be made that radio frequencies are an unlikely choice for an advanced society, and that if we must select just one part of the electromagnetic spectrum to monitor then visible, infrared or ultraviolet offer better prospects for SETI. In essence, the case against radio is that it is a high-powered transmission whose wide beam washes over many stars. In contrast, lasers in the visible, infrared or ultraviolet require less power and the energy is aimed towards a particular star system. A civilization seeking to establish contact with any intelligences around stars in its neighborhood might aim such a laser at a star which shows characteristics likely to support life. As so few star systems have such characteristics, we would probably be included in a targeted search by a nearby civilization. If we were fortunate, we might spot such a laser probing for a response from any life in our system. Although many papers have been written showing why and how laser signals could be present, early studies by radio-frequency engineers compared continuous-wave laser signals with continuous-wave radio frequencies and drew conclusions that may not actually be correct. It was clear from the physics and from the noise and background light that the most efficient modulation method at optical wavelengths was high-peak-power short-pulse low-duty-cycle pulses.
The term short-pulse low-duty-cycle refers to the fact that the signal is not continuous, but is active only for a small fraction of the time. For example, the transmitted pulse may be on for one nanosecond, and the pulse rate may be once per millisecond. As the duty cycle is the pulse width multiplied by the pulse rate, we have 1 nanosecond multiplied by 1,000 pulses per second for a duty cycle of one part in a million. This means that the system is transmitting one-millionth of the time. Thus the peak power can be 1,000,000 times the average power, or the continuous power in this example.Other issues in determining the best choice for such communication are discussed in later sections.
In retrospect, it is evident that SETI began searching at radio frequencies because extraterrestrial intelligence was initially believed to be plentiful and we had systems for receiving weak radio signals from probes operating in deep space, whereas laser technology was not at the same level of development.
The likelihood of radio frequencies being used in lieu of lasers is diminished if nearby star systems are not transmitting. This is due to the much larger antennas that would be required at the receiver site to receive signals from much greater distances. The received power is proportional to the area of the antenna.
A light-year is 9.46 x 10^12 kilometers , and stars are many light-years apart.Owing to the inverse square law in which the area irradiated increases by the square of the distance, there is a factor of 400 difference in the signal power lost in space between a source that lies 10 light-years away and one 200 light-years away. If the same transmitter is used, the area of the receiving antenna must be increased by a factor of 400 in order to detect a source 200 light-years away compared to 10 light-years away (i.e. 20 x 20). This may well be impracticable. And this is only one argument against using radio frequencies for interstellar communication. It is more likely that the stars will be far away because of geometry. That is, imagine the Sun to be located at the center of a sphere in which the other stars are assumed to be more or less equally distributed (Figure 1.1), then the fact that
How far are the stars and how do we know?
A 'parsec' is 3.26 light-years, and is based on the distance from Earth at which the annular parallax is one second of arc. The angles are very small because the distance across Earth's orbit of the Sun is extremely small in comparison to the distances of the stars . Indeed, the nearest star, Proxima Centauri, lies 4.3 light-years away and has a parallax of only 0.76 seconds of arc.
The accuracy of angular measurements made from Earth's surface is limited by distortions in the atmosphere. Telescopes in space therefore have an advantage.
In 1989 the European Space Agency put a satellite named Hipparcos into orbit around Earth to employ the baseline of Earth's orbit around the Sun to accurately measure parallaxes for stars as far away as 1,600 light-years. There are methods which do not use geometric parallax and facilitate measurements at greater distances. These are more difficult to implement, but can yield reasonably accurate results. In 1997 NASA began a study of a Space Interferometry Mission (SIM). Progress was slow due to budget constraints. As currently envisaged, the renamed SIM Lite will be launched at some time between 2015 and 2020 and be put into solar orbit trailing Earth . It will have a number of goals, including searching for terrestrial planets in nearby star systems . Optical interferometry will enable the positions of stars on the sky to be measured to within 4 millionths of a second of arc. This will facilitate measuring distances as far away as 25 parsecs to an accuracy of 10 per cent, which is many times better than is possible from Earth's surface.By a variety of techniques the parallax effect can provide acceptable results out to about 1,000 light-years, with the distances to the nearer stars being more accurate than those farther away. Such a volume of space includes a large number of stars. It can therefore be assumed that an advanced civilization will accurately know how far we are from them, and hence can calculate the transmitter power needed to reach us.
Of course, another issue is the time involved in communicating across interstellar distances, because an electromagnetic signal traveling at the speed of light takes one year to travel a light-year. A civilization might be willing to try to prompt a response from a nearby star system, but reject waiting hundreds of years for a response from a distant star. The volume of space within which communication is practicable might therefore be quite small.
Stars, their evolution and types
In the last few years we have been able to detect a number of extra-solar planetary systems, but we cannot tell much about them. Our knowledge will improve in the next decade or two, however. It is likely that an advanced extraterrestrial civilization will know which star systems in its neighborhood are good candidates to host intelligent life, and which are not. The primary selection criteria are the type of the star, which is related to the temperature of its surface, and the size and location of its planets. As we learn more about planets and their characteristics, we should be able to apply a variety of other constraints. Once an advanced society has made such an analysis, the resulting list of nearby stellar systems likely to harbor life may well be very short.
To understand the search for intelligent extraterrestrial signals, it is necessary to consider the hundreds of billion stars in our galaxy which are possible hosts, and the means of transmission and reception of a signal over such large distances.
Consider the problem of a civilization which wishes to contact another intelligent society. How do they proceed? They appreciate that conditions for intelligent life are quite restrictive, but conclude that there are so many stars that perhaps all they need to do is to make a thorough search. But the galaxy is approximately 100,000 light-years across, and communication across that distance would be impracticable. It would be better if they were to find a society within about 500 light-years . Although small in relation to the galaxy as a whole, this volume is likely to include in excess of a million stars, which is a reasonable basis for applying the 'habitability' selection criteria.
To better understand the likelihood of advanced intelligence in our galaxy, it is worth reviewing the types and evolution of stars, and the chance of one possessing a planet with characteristics suitable for the development of an advanced intelligence. However, much of what we have inferred is based on the only intelligent life that we know of, namely ourselves and our environment in the solar system, and there is the possibility that we are in some way atypical. Nevertheless, with this caveat in mind it is possible to estimate the likelihood of other stars having planets that are in this sense 'right' for the development of advanced intelligence.
In what follows, we will examine the constraints imposed on stellar systems as suitable abodes of intelligent life. Some constraints seem certain, some seem likely, and others are simply possibilities about which cosmologists argue. As we discover more about stellar systems, the individual constraints may be tightened or loosened. In general, as we have learned more, the probability of there being another advanced society nearby has reduced. Indeed, if the constraints are applied harshly it becomes unlikely that there is another intelligent civilization anywhere near us.
In the ancient past, Earth was considered to lie at the center of the universe, with mankind being special. The work of Copernicus and Galileo in the sixteenth and early seventeenth centuries showed that the planets, including Earth, travel around the Sun. This weakened man's perception of being centrally located. The discovery that there are hundreds of billions of stars in the galaxy and hundreds of billions of galaxies provided a sense of immensity that reinforced man's insignificance. But the possibility that we are the only advanced civilization puts us center-stage again. To assess the chances of there being many societies out there, we need to know more about stars and planets. Figure 2.1 shows the number of stars within a given radius of us.
A galaxy such as ours comprises a spherical core and a disk that is rich in the gas and dust from which stellar systems are made. The interstellar medium is typically composed of 70 per cent hydrogen (by mass) with the remainder being helium and trace amounts of heavier elements which astronomers refer to as 'metals', Some of the interstellar medium consists of denser clouds or nebulas.
Much of the hydrogen in the denser nebulas is in its molecular form, so these are referred to as 'molecular clouds'. The largest molecular clouds can be as much as 100 light-years in diameter. If a cloud grows so massive that the gas pressure cannot support it, the cloud will undergo gravitational collapse. The mass at which a cloud will collapse is called the Jeans' mass. It depends on the temperature and density, but is typically thousands to tens of thousands of times the mass of the Sun. As the cloud is collapsing, it may be disrupted by one of several possible events.
Perhaps two molecular clouds come into collision with each other.
Perhaps a nearby supernova explosion sends a shock wave into the cloud.
Perhaps two galaxies collide. By such means, clouds are broken into condensations known as Bok globules, with the smallest ones being the densest.
As the process of collapse continues, dense knots become protostars and the
release of gravitational energy causes them to shine. As the protostar draws in material from the surrounding cloud, the temperature of its core increases. When the pressure and temperature in the core achieve a certain value, nuclear fusion begins. Once all the available deuterium has been fused into helium-3, the protostar shrinks further until the temperature reaches 15 million degrees and allows hydrogen to fuse into helium, at which time radiation pressure halts the collapse and it becomes a stable star.
The onset of hydrogen 'burning' marks the initiation of a star's life on what is called the 'main sequence' of a relationship derived early in the twentieth century by Ejnar Hertzsprung and Henry Norris Russell. They plotted the absolute magnitudes of stars against their spectral types, observational parameters which equate to the intrinsic luminosity and surface temperature. The resulting diagram (Figure 2.2) shows a high correlation between luminosity and surface temperature among the average-size stars known as dwarfs, with hot blue stars being the most luminous and cool red stars being the least luminous. Running in a narrow band from the upper left to the lower right, this correlation defines the main sequence. Its importance is that all stars of a given mass will join the main sequence at a given position. But stars evolve and depart the main sequence. If a star becomes a giant or a supergiant, it will develop a relatively high luminosity for its surface temperature and therefore move above the main sequence. If a star becomes a white dwarf, its luminosity will be relatively low for its surface temperature, placing it below the main sequence. The stars that lie on the main sequence maintain a stable nuclear reaction, with only minor fluctuations in their luminosity. Once the hydrogen in its core is exhausted, a star will depart the main sequence. The more massive the star, the faster it burns its fuel and the shorter its life on the main sequence. If the development of intelligent life takes a long time, then it might be limited to low-mass stars. The actual ages of stars are known only approximately, but it is clear that whilst very massive stars can remain on the main sequence for only several million years, smaller ones should do so for 100 billion years. Since the universe is 13.7 billion years old, it is evident that many low-mass stars are still youthful. The Sun is believed to have condensed out of a nebula about 5 billion years ago and to be half way through its time on the main sequence.
At 1.99 x 10^30 kg, the Sun is 333,000 times the mass of Earth . Astronomers find it convenient to express stellar masses in terms of the solar mass. The range of stellar masses is believed to result from variations in the star formation process. This theory suggests that low-mass stars form by the gravitational collapse of rotating clumps within molecular clouds. Specifically, the collapse of a rotating cloud of gas and dust produces an accretion disk through which matter is channeled 'down ' onto the protostar at its center. For stars above 8 solar masses, however, the mechanism is not well understood. Massive stars emit vast amounts of radiation, and it was initially believed that the pressure of this radiation would be sufficient to halt the process of accretion, thereby inhibiting the formation of stars having masses exceeding several tens of solar masses, but the latest thinking is that high-mass stars do indeed form in a manner similar to that by which low-mass stars form. There appears to be evidence that at least some massive protostars are surrounded by accretion disks. One theory is that massive protostars draw in material from the entire parent molecular cloud, as opposed to just a small part of it . Another theory of the formation of massive stars is they are formed by the coalescence of stars of lesser mass. Although many stars are more massive than the Sun, most are less so. This is a key issue in estimating the prospects for the development of life, as the lower surface temperature of a smaller star sharply reduces the number of photons with sufficient energy for the process of photosynthesis. The color of a star defines its spectral class, and by assuming that it acts as a 'blackbody' and radiates its energy equally in all directions it is possible to calculate the temperature of its surface. The hottest stars have their peak wavelength located towards the ultraviolet end of the visible spectrum, but the coolest stars peak in the infrared. When astronomers in the early twentieth century proposed a series of stages through which a star was presumed to pass as it evolved, they introduced an alphabetical sequence. Although further study prompted them to revise this process, the alphabetical designations were retained and the ordering was changed. Hence we now have O-B-A-F-G-K-M, where- O stars are blue,
- B stars are blue-white,
- A stars are white,
- F stars are white-yellow,
- G stars are yellow,
- K stars are orange, and
- M stars are red.
- Other letters were added later. For example, R, Sand C are stars whose spectra show specific chemical elements, and L and T signify brown dwarfs.
In general, a star will spend 80% of its life on the main sequence but, as we have noted, more massive stars do not last very long. If they do possess planets, these probably do not have time for intelligence to develop. Once the hydrogen in the core is consumed, the star will evolve away from the main sequence. What happens depends on its mass. For a star of up to several solar masses, hydrogen burning will continue in a shell that leaves behind a core of inert helium. In the process, the outer envelope is inflated to many times its original diameter and simultaneously cooled to displace the peak wavelength towards the red end of the visible spectrum, turning it into a red giant of spectral classes K or M. When more massive stars evolve off the main sequence they not only continue to burn hydrogen in a shell, their cores are hot enough to initiate helium fusion and this additional source of energy inflates the star into a red supergiant. Such stars may well end their lives as supernovas. Stars which have left the main sequence are rarely stable, and even if life developed while the star was on the main sequence, this will probably be extinguished by its subsequent evolution. Certainly when the Sun departs the main sequence it will swallow up the inner planets.
Dwarfs of class K or M have surface temperatures of between 4,900K and 2,000K. They will last a very long time, longer indeed than the universe is old. This explains why they are so numerous. It may be that many red dwarfs possess planets, but the low temperature has its peak emission in the red and infrared, with the result that most of the photons are weak, possibly too weak to drive photosynthesis. If a planet is located sufficiently close to the star for its surface to be warm enough for life, the gravitational gradient will cause the planet to
Threats to life
Figure 2.6 shows the 'great extinctions' of life on Earth, known as the:
- Ordovician,
- Devonian,
- Permian,
- Triassic-Jurassic and
- Cretaceous-Tertiary.
- The worst is thought to have been the Permian, where 96% of all marine species and 70% of terrestrial vertebrate species died off.
- The second worst, the Ordovician, could well have been caused by a supernova 10,000 light-years away that irradiated Earth with 50 times the solar energy flux, sufficient to destroy the chemistry of the atmosphere and enable solar ultraviolet to reach the surface. Far worse would be a supernova at 50 light-years. The atmosphere would suffer 300 times the amount of ionization that it receives over an entire year from cosmic rays. It would ionize the nitrogen in the atmosphere, which would react with oxygen to produce chemicals that would reduce the ozone layer by about 95% and leave the surface exposed to ultraviolet at an intensity four orders of magnitude greater than normal. Lasting for 2 years, this would probably sterilize the planet. Astronomer Ray Norris of the CSIRO Australia Telescope National Facility estimated that a supernova should occur within 50 light-years once every 5 million years. In fact, they occur rather less frequently. Nevertheless, a nearby supernova would pose a serious threat to intelligent life.
Magnetars are neutron stars which emit X-rays, gamma rays and charged-particle radiation. There are none in our part of the galaxy, but in the core a vast number of stars are closely packed and neutron stars are common. Intelligent life in the galactic core must therefore be unlikely.
On a more local scale, there is always the threat of a planet being struck by either an asteroid or a comet. Most of the asteroids in the solar system are confined to a belt located between the orbits of Mars and Jupiter, but some are in
elliptical orbits which cross those of the inner planets. The impact 65 million years ago which wiped out half of all species, including the dinosaurs, is believed to have been an asteroid strike. We have a great deal to learn about other star systems, but if asteroid belts are common then they could pose a serious threat to the development of intelligent life there.
Life might wipe itself out! Some studies have suggested that the Permian-Triassic extinction was caused by microbes. For millions of years beforehand, environmental stress had caused conditions to deteriorate, with a combination of global warming and a slowdown in ocean circulation making it ever more difficult to replenish the oxygen which marine life drew from the water. According to this theory, microbes saturated the oceans with extremely toxic hydrogen sulfide that would have readily killed most organisms. Single-celled microbes survived, and indeed may well have prospered in that environment, but everything else was devastated. It took 10 million years for complex life to recover. And intelligent life, even if it avoids extinction by nuclear war, could develop a technology that causes its demise. Nanotechnology, for example. This is at the forefront of research and development in so many technical areas. How could it pose a risk? The term refers to engineering on the nanoscale level, which is billionths of a meter. This offers marvelous developments, but also introduces threats. On the drawing board, so to speak, is a proposal for a nanobot as a fundamental building block that can replicate itself. If self-replication were to get out of control, the population of nanobots would grow at an exponential rate. Let us say that a nanobot needs carbon for its molecules. This makes anything that contains carbon a potential source of raw material. Could nanobots extinguish life on Earth? Also, whilst there are many possible advantages to medicine at the nanoscale level, anything that enters the body also represents a potential toxin that could be difficult to eradicate. To be safe, we might have to control nanotechnology much as we control the likes of anthrax, namely by isolating it from the world at large. This is a problem that will face civilization in the next decade.
It is therefore thought unlikely that intelligence could develop on any planet that is subjected to extinction events on a frequent basis.
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