Transcript Chapter 20
Chapter 20 Life in the Universe Introduction We have discussed the nine planets and some of the moons in the Solar System, and have found most of them to be places that seem hostile to terrestrial life forms. Yet a few locations besides the Earth—most notably Mars, with its signs of ancient running water, and Europa, with liquid water below its icy crust—have characteristics that suggest life may have existed there in the past, or might even be present now or develop in the future. Exobiology is the study of life elsewhere than Earth. Introduction In our first real attempt to search for life on another planet, in the 1970s, NASA’s Viking landers carried out biological and chemical experiments with martian soil. The results seemed to show that there is probably no life on Mars (see figure). A small, inadequately funded British probe, Beagle 2 (the first Beagle having been Darwin’s ship), failed as it approached Mars in 2003. (One reason it became so popular among the public may have been that it was supposed to send back tones from the musical group Blur when it landed, though those tones never came.) Introduction NASA continues to explore Mars with robotic spacecraft and one day should have a more sophisticated biology lab landing on it. In the meantime, its Mars Exploration Rovers, Spirit and Opportunity, and the European Space Agency’s Mars Express have found clear signs that water flowed on Mars in the distant past, raising hope that life could have formed there at that time and might even have survived. Although studies of a martian meteorite in 1996 gave some indications of ancient, primitive life on Mars (see Chapter 6), this idea has not been generally accepted, though it is still causing much discussion. Jupiter’s moon Europa and Saturn’s moon Titan (see Chapter 7) are also intriguing places where scientists think it is possible that life has begun. Introduction Since it seems reasonable that life as we know it would be on planetary bodies, we first discuss the chances of life arising elsewhere in the Solar System, as well as the kinds of stars most likely to have planets suitable for the emergence and development of intelligent life. Next we explore attempts to receive communication signals from intelligent extraterrestrials. We also consider a way to estimate the number of communicating civilizations elsewhere in our Milky Way Galaxy, or at least to see which factors most seriously limit our ability to do so. It is possible that humans are the only technologically advanced civilization in our Galaxy, or one of very few. Introduction Finally, we explain why most scientists do not consider reported sightings of UFOs to be good evidence for extraterrestrial visitations to Earth. NASA has formed an institute of “astrobiology” and is making a major push to investigate matters of biology that can be important to understanding the origin of life or to space exploration. The institute is “virtual,” in that it has no actual buildings but rather is made of individuals who communicate by e-mail and by occasional meetings. 20.1 The Origin of Life It would be very helpful if we could state a clear, concise definition of life, but unfortunately that is not yet possible. Biologists state several criteria that are ordinarily satisfied by life forms—reproduction, for example. Still, there exist forms on the fringes of life, such as viruses, that need a host organism in order to reproduce. Scientists cannot always agree whether some of these things are “alive.” 20.1 The Origin of Life In science fiction, authors sometimes conceive of beings that show such signs of life as the capability for intelligent thought (see figure), even though the being may share few of the other criteria that we ordinarily recognize. In Fred Hoyle’s novel The Black Cloud, for instance, an interstellar cloud of gas and dust is as alive as (and smarter than!) human beings. But we can make no concrete deductions if we allow such wild possibilities, so exobiologists prefer to limit the definition of life to forms that are more like “life as we know it.” 20.1 The Origin of Life Life on Earth is based on amino acids—chains of carbon, in which each carbon atom is bonded to hydrogen and sometimes to oxygen and nitrogen. Chemically, carbon is essentially unique in its ability to form such long chains; indeed, we speak of compounds that contain carbon as being organic. But life is selective, incorporating only about 20 of all the possible amino acids. Similarly, long chains of amino acids form proteins, though life utilizes only a small fraction of the multitude of possible combinations of amino acids. The genetic code of any living creature is contained in one extremely long and complex structure: DNA (deoxyribonucleic acid), the famous “double helix.” 20.1 The Origin of Life How hard is it to build up long organic chains? To the surprise of many, an experiment performed in the 1950s showed that making organic molecules is easier than had been supposed. Stanley Miller and Harold Urey, at the University of Chicago, filled a glass jar with simple molecules like water vapor (H2O), methane (CH4), and ammonia (NH3), along with hydrogen gas (H). They exposed it to electric sparks, simulating the vigorous lightning that may have existed in the early stages soon after Earth’s formation. 20.1 The Origin of Life After a few days, long chains of atoms formed in the jar, in some cases complex enough to include simple amino acids, the building blocks of life. Later versions of these experiments created even more complex organic molecules from a wide variety of simple actions on simple molecules (see figures, left and middle). Such molecules may have mixed in the oceans to become a “primordial soup” of organic molecules (see figure, right). 20.1 The Origin of Life Since the original experiment of Miller and Urey, most scientists have come to think that the Earth’s primitive atmosphere was not made of methane and ammonia, which would have disappeared soon after the Earth’s formation. Instead, it may have consisted mostly of carbon dioxide, carbon monoxide, and water, and such a mixture does not generally lead to a large abundance of amino acids. On the other hand, complex molecules may have formed near geothermal sources of energy under the oceans or on Earth’s surface, or perhaps in vents deep underground, where the right raw materials existed. Also, extraterrestrial amino acids have been found in several meteorites that had been long frozen in Antarctic ice, as well as in some other meteorites (see figure). In any case, some extrasolar planets or moons in our Galaxy may have had primitive atmospheres similar to the mixture used by Miller and Urey, so the results of their experiments are interesting. 20.1 The Origin of Life Also relevant are laboratory observations with conditions mimicking the low density (strong vacuum) of space. Amino acids have been observed to form. However, mere amino acids or even DNA molecules are not life itself. A jar containing a mixture of all the atoms that are in a human being is not the same as a human being. This is a vital gap in the chain; astronomers certainly are not yet qualified to say what supplies the “spark” of life. 20.1 The Origin of Life Still, many astronomers think that since it is not difficult to form complex molecules, primitive life may well have arisen not only on the Earth but also in other locations. The appearance of very simple organisms in Earth rocks that are 3.5 billion years old, and indirect evidence for life as far back as 3.8 billion years (not long after the end of the bombardment suffered by the newly formed Earth), suggests that primitive life arises quite easily. Similarly, the presence of life in what appear to be very harsh environments on Earth (water that is highly acidic or near its boiling point, for example) shows that life can exist in extreme conditions. 20.1 The Origin of Life The discovery of indigenous life on at least one other planet or moon in the Solar System would provide much support for the hypothesis that simple organisms such as microbes and bacteria form readily. But even if life is not found elsewhere in the Solar System, there are so many other stars in space that it would seem that life might have arisen at some location. 20.2 Life in the Solar System Life elsewhere in our planetary system, if present at all, is primitive at best (single cells, or perhaps very simple multi-cellular organisms). Among all of the planets and moons, only a few have nonzero odds for life. Mars has provided the best evidence thus far, but it is still very controversial (see figure). Moreover, even if real, life on Mars might not have been independent of Earth—a meteorite from Mars containing simple life may have contaminated the young Earth, “seeding” it with life, though this idea is of course quite speculative. 20.2 Life in the Solar System Europa, one of Jupiter’s large moons, looks like a promising environment: Below its icy surface there is almost certainly water slush or even an ocean. A Europa orbiter to study this moon in more detail is being considered by NASA, though its recent “Vision for Space Exploration” of “Moon, Mars, and Beyond” is apparently slowing down all other NASA projects, even the Europa orbiter. 20.2 Life in the Solar System Titan, Saturn’s largest moon, has a thick atmosphere of nitrogen molecules. As we saw, in 2004, the Cassini spacecraft sent the Huygens probe into this atmosphere. There is evidence for substantial methane and ethane on Titan, perhaps in the form of lakes; methane can be solid, liquid, or gas at temperatures reasonable for Titan, just as water can have solid, liquid, or gaseous phases on Earth. Huygens, as it approached Titan’s surface, imaged what appeared to be a lake shore. The Cassini orbiter’s radar has found a reflection that seems lake-like, presumably with the liquid being a tarry substance. The absence of liquid water, however, makes it more difficult for life as we know it to form. 20.2 Life in the Solar System Io, another Galilean satellite, has conditions that may be suitable for life resembling that found near volcanic vents on Earth. Note that an intelligent alien who obtains and correctly interprets a spectrum of Earth could deduce the presence of life here. The large amount of free oxygen suggests the continuous production by a process like photosynthesis; otherwise, oxygen would rapidly be depleted because it is so reactive. Again, however, the apparent absence of water is a major problem. In addition, methane quickly reacts with oxygen, so the significant amount of methane (largely from cows—“bovine flatulence,” to quote the late Carl Sagan) in our atmosphere implies a steady production mechanism—the decay of organic compounds. Such signs have been picked up from spacecraft, such as Galileo and Cassini, while they were near the Earth and headed for the outer planets. 20.3 Suitable Stars for Intelligent Life If we seek indigenous (that is, originating locally, not from elsewhere), intelligent life on planets orbiting other stars, what kinds of stars have the best odds? Stars that are either near the beginning or end of their lives are not very good bets. For example, intelligent life may take a long time to form, so very young stars are less suitable than older stars. Stars heading to the red giant or white dwarf stages go through rapid changes, making it difficult for complex life to survive. White dwarfs and neutron stars certainly have passed through stages that would have destroyed life. 20.3 Suitable Stars for Intelligent Life Some types of main-sequence stars are also not especially suitable. The lives of O-type and B-type stars are probably too short for the development of life of any kind. Planets around A-type stars might have life, but probably not intelligent life. Type-M and type-L stars have a small ecosphere (also known as the “habitable zone”)—the range of distances in which conditions suitable for life might be found. There is unlikely to be a planet in this narrow region around a low-mass star. Also, such a planet would be in “synchronous rotation”: The same hemisphere would always face the star, so one side would be very hot, and the other very cold. 20.3 Suitable Stars for Intelligent Life Main-sequence stars of types F, G, and K are the most likely candidates. They live a long time, and models of their ecosphere lead to reasonably large sizes. (In more detail, such models must explain the “Goldilocks effect” in the case of the Solar System: why Venus is too hot, Mars is too cold, but the Earth is just right.) Single stars, stars in very wide binary systems (with planets orbiting close to one star), or closely spaced binary stars (which planets could orbit at large distances) are most suitable. Planets that move from one star to another in a binary system (for example, like a “figure eight”) tend to have unstable orbits and are ejected. 20.3 Suitable Stars for Intelligent Life Over 160 extrasolar planets have already been found (see our discussion in Chapter 9). Generally these planets are gas giants, in many cases very close to the star or on highly eccentric orbits and hence probably inhospitable to intelligent life. In most cases they orbit F, G, and K stars because the searches specifically targeted those stars. A few of them, however, appear to have orbital properties potentially suitable for the emergence of life; perhaps at least simple life exists within their atmosphere or on moons orbiting them. The 2005 announcement of a planet, presumably rocky, with only about 7 to 8 times Earth’s mass, gives hope of our finding closer analogs to Earth, which encourages those trying to discover signs of life there. 20.4 The Search for Extraterrestrial Intelligence How should we look for intelligent extraterrestrial life? Perhaps we can find evidence for such life here on Earth, in the form of alien spacecraft that have landed here. After all, Pioneers 10 and 11 and Voyagers 1 and 2 are even now carrying messages out of the Solar System in case an alien interstellar traveller should happen to encounter these spacecraft (see figures). 20.4 The Search for Extraterrestrial Intelligence The odds, however, seem very small, given the vastness of space. A potentially more fruitful approach is to search for electromagnetic signals, but some waves are more suitable than others. Although spaceships can, in principle, travel between the stars, even rather quickly according to Einstein’s special theory of relativity, such journeys are difficult, expensive, dangerous, and probably rare compared to communication with electromagnetic waves. For example, x-ray and gamma-ray photons have high energy and are therefore expensive to produce, and typical atmospheric gases block them. Ultraviolet photons are absorbed by interstellar dust in the plane of our Milky Way Galaxy. At optical wavelengths, the signal from a planet orbiting a star is very difficult to distinguish from the bright light of the star itself, unless a large amount of energy is concentrated into a narrow range of wavelengths or into a short pulse. And at infrared wavelengths, Earth’s warm atmosphere makes the sky bright. 20.4 The Search for Extraterrestrial Intelligence For many years, radio waves have seemed to be the best choice: They are easy and cheap to produce, and are not generally absorbed by interstellar matter. Also, there are few sources of contamination—although the increasing number and strength of radio and television stations is a threat to radio astronomers in the same manner that city lights brighten the sky at optical wavelengths. At least initially, it seems too overwhelming a task to listen for signals at all radio frequencies in all directions and at all times. One must make some reasonable guesses on how to proceed. 20.4 The Search for Extraterrestrial Intelligence A few frequencies in the radio spectrum seem especially fundamental, such as the 21-cm line of neutral hydrogen (as described in Chapter 15). This wavelength corresponds to 1420 MHz, a frequency over ten times higher than stations at the high end of the normal FM band. We might conclude that creatures on a far-off planet would decide that we would be most likely to listen near this frequency because it is so fundamental. The “water hole,” the wavelength range between the radio spectral lines of H and OH, has a minimum of radio noise from background celestial sources, the telescope’s receiver, and the Earth’s atmosphere, and so is another favored possibility. 20.4 The Search for Extraterrestrial Intelligence Humans have conducted several searches for extraterrestrial radio signals. No unambiguous evidence has been found, but the quest is worthwhile and continues. A telltale signal might be an “unnatural” set of repeating digits, such as the first 100 digits of the irrational number (the Greek letter “pi”). We must verify that the signal could not be produced by a natural, nonintelligent source (such as a pulsar), and also that it is not of human origin (either unintentionally or intentionally). Though there is agreement to verify the veracity of the supposition that a signal received is indeed a message or a sign of extraterrestrial life, and to do so before making a public announcement, the discovery of such an extraterrestrial signal would be so momentous that it would surely be released to the public without major delay. A protocol has been accepted by researchers in the field as to how to announce any believable signal from extraterrestrials, so as not to cause fear and panic. 20.4 The Search for Extraterrestrial Intelligence In 1960, Frank Drake conducted the first serious search for extraterrestrial signals. He used a telescope at the National Radio Astronomy Observatory (in West Virginia) for a few months to listen for signals from two of the nearest stars—tau Ceti and epsilon Eridani. He was searching for any abnormal kind of signal—a sharp burst of energy, for example. He called this investigation Project Ozma after the queen of the land of Oz (see figure) in L. Frank Baum’s stories. 20.4 The Search for Extraterrestrial Intelligence A NASA-sponsored group based at the Search for Extraterrestrial Intelligence (SETI) Institute in California began an ambitious effort on October 12, 1992, the 500th anniversary of Columbus’s landing in the New World. It made use of sophisticated signal-processing capabilities of powerful computers to search millions of radio channels simultaneously in the microwave region of the spectrum (see figure). Consisting of both a sky survey and a targeted study of individual stars, in its first fraction of a second it surpassed the entire Project Ozma. But Congress cut off funds anyway, after about a year. 20.4 The Search for Extraterrestrial Intelligence The targeted search of the project, known as Project Phoenix, is continuing, backed by funds contributed by private individuals. Now led by astrophysicist Jill Tarter (see figure), a real-life model for actress Jodie Foster’s character Ellie Arroway in the movie Contact, it examines about 1000 stars. No unexplained signals have been found so far, though there have been some exciting false alarms, as in 1997 when an intriguing signal turned out to be from the SOHO spacecraft (which was described in Chapter 10)! 20.4 The Search for Extraterrestrial Intelligence A well-known project is “SETI@home,” whose operation is based at the University of California, Berkeley (see http://seti.berkeley.edu). It has established a way for the general public to contribute: During otherwise unused time on your home computer, a special program can automatically analyze data from the giant Arecibo radio telescope in Puerto Rico (see figure, top) for signs of extraterrestrial signals (see figure, bottom). Thus, there is a very small but nonzero chance that the first unambiguous evidence of intelligent life elsewhere in the Universe would be found by your computer, should you choose to participate! 20.4 The Search for Extraterrestrial Intelligence The SETI@home effort already has nearly 6 million participants in over 200 countries, effectively forming the Earth’s largest supercomputer, and the amount of computing time contributed has been over two million computer-years. The scientists conducting the project have taken the most suspicious signals from the widespread data analysis and gone back to Arecibo to observe those sources in detail. Obviously, no confirmation was obtained. 20.4 The Search for Extraterrestrial Intelligence Although most searches for extraterrestrials have been conducted at radio wavelengths, some optical and nearinfrared searches are also underway. Because the plane of the Milky Way Galaxy absorbs visible light, we can’t expect to survey as many stars as at radio wavelengths, but there are still plenty of them. If we search for short, very intense pulses of light emitted by lasers, we can actually see quite far in the plane of our Galaxy, increasing the number of available stars. More importantly, the laser pulses outshine the light from the star that a planet is orbiting. Finally, such laser pulses are very difficult or impossible to produce by any natural phenomena other than life; detection of them could provide strong evidence for the existence of extraterrestrials. 20.4 The Search for Extraterrestrial Intelligence Only some scientists think that there is a reasonable chance of detecting signals from extraterrestrial beings, at least in the near future. Of course, now that we have looked and listened for a while, that possibility seems to be reduced. There are other reasons to think that there may not be any extraterrestrials at all in our neighborhood within the Milky Way Galaxy, or perhaps even in our entire Galaxy. But it would be a shame if there were abundant signals that we missed just because we weren’t looking. For example, the odds are that life would have arisen elsewhere than on Earth first, so why aren’t the aliens already here? Maybe they aren’t here because they don’t exist. Even if we don’t get messages from outer space, there are many scientific spin-offs of the search. Investigating thousands of stars in detail, and mapping the sky in different parts of the spectrum, gives us bits of information that can lead to scientific breakthroughs of other kinds. 20.5 Communicating with Extraterrestrials All of the searches described above are passive—astronomers are simply looking for signals from intelligent, communicating extraterrestrials. If such a signal is ever found and confirmed by several cross-checks, we might choose to “reply”—but not until some global consensus has been reached about who will speak for Earth and what they will say. Nevertheless, we have already intentionally sent our own signals toward very distant stars that may or may not be orbited by planets containing intelligent life. 20.5 Communicating with Extraterrestrials For example, on November 16, 1974, astronomers used the giant Arecibo radio telescope in Puerto Rico to send a coded message about people on Earth (see figure, right) toward the globular star cluster M13 (see figure, below) in the constellation Hercules. The idea was that the presence of over 200,000 closely packed stars in that location would increase the chances of our signal being received by a civilization in the planetary system of one of them. 20.5 Communicating with Extraterrestrials But the travel time of the message (at the speed of light) is about 24,000 years to M13, so we certainly cannot expect to have an answer before twice 24,000, or 48,000 years, have passed. If any creature is observing the Sun at the right frequency (2380 MHz) when the signal arrives, the radio brightness of the Sun will increase by 10 million times over a 3-minute period. A similar signal, if received from a distant star, could be the giveaway that there is intelligent life there. 20.5 Communicating with Extraterrestrials In retrospect, M13 may not have been the best choice as a target, despite its large number of stars. The problem is that the stars in globular clusters, being very old, were formed from gas that did not have a large proportion of heavy elements; it had not gone through many stages of nuclear processing by massive stars and supernovae. Rocky, Earth-like planets are thus not as likely to have formed there as they would have around younger stars. 20.5 Communicating with Extraterrestrials A quarter century later, in 1999, a new message was sent by Canadian astronomers toward several relatively nearby (50 to 70 light-years away) Sun-like stars, including 16 Cygni B, which is known through Doppler measurements to have at least one planet orbiting it (see our discussion in Chapter 9). The complete message, which is much larger in size, duration, and scope than the one sent in 1974, was transmitted three times over a 3-hour period in the direction of each star. Still later, starting on July 5, 2003, some much more complicated messages were sent out toward five nearby stars (32 to 46 light-years away) from a 70-m radio telescope in Evpatoriya, Ukraine. One would hope to get answers in fewer than 100 years instead of tens of thousands, but nobody is betting on a return message. 20.5 Communicating with Extraterrestrials Even had we not sent these few directed messages, during much of the 20th century (and so far in the 21st century) we have been unintentionally transmitting radio signals into space on the normal broadcast channels. Aliens within a few tens of light-years could listen to our radio and television programs. A wave bearing the voice of Winston Churchill is expanding into space, and at present is about 60 light-years from Earth. And once a week or so a new episode of Fear Factor is carried into the depths of the Universe. Do you think aliens would get a favorable impression of us from most of what they hear? 20.5 Communicating with Extraterrestrials Is it reasonable to expect that there are any signals out there that we can hope to detect with projects such as that at the SETI Institute? What if no intelligent creatures exist elsewhere in our Milky Way Galaxy or even in the observable parts of the Universe? 20.6a The Drake Equation Instead of phrasing an all-or-nothing question about extraterrestrial life, we can use a procedure developed by Frank Drake, then of Cornell University, and extended by Carl Sagan and Joseph Shklovskii, among others. The overall problem is broken down into a chain of simpler questions, the results of which are multiplied together in what is called the Drake equation to give the final answer. The formulation is unusual foran equation because it is really a guide to thought rather than a real way of calculating; there is no right answer or solution to the Drake “equation.” 20.6a The Drake Equation In the standard formulation of the Drake equation, we first estimate the rate at which stars form in our Galaxy. Second, we consider the probability that stars at the centers of planetary systems are suitable to allow intelligent life to evolve. For example, as we have already discussed, the most massive stars evolve rather quickly, remaining stable for too short a time to allow intelligent life a good chance to evolve. Third, we ask what the probability is that a suitable star has planets. With the new detection of planets orbiting other stars, most astronomers think that the chances are likely to be pretty high. 20.6a The Drake Equation Fourth, we need planets with suitable conditions for the origin of life, so we multiply by the average number of such planets per suitable star. A planet like Jupiter might be ruled out, for example, because it lacks a solid surface and because its surface gravity is high. (Alternatively, though, one could consider a liquid region, if it were at a suitable temperature, to be as advantageous as the oceans on Earth may have been to the development of life here. Or a moon of the planet could provide the solid surface.) Also, planets probably must be in orbits in which the temperature does not vary too much. 20.6a The Drake Equation Fifth, we have to consider the fraction of the suitable planets on which life actually begins. This is a very large uncertainty, for if this fraction is zero (with the Earth being the only exception), then we get nowhere with this entire line of reasoning. Still, the discovery that amino acids can be formed in laboratory simulations of at least some kinds of potential primitive atmosphere, and the discovery of complex molecules (such as ammonia, formic acid, and vinegar) in interstellar space, indicate to many astronomers that it is relatively easy to form complicated molecules. As mentioned previously, amino acids, much less complex than DNA but also basic to life as we know it, have even been found in some meteorites. 20.6a The Drake Equation Moreover, life is found in a wide range of extremes on Earth, including oxygen-free environments near geothermal sulfur vents on the ocean bottom or surface hot springs, under rocks in the Antarctic, and deep underground in some other parts of the Earth (see figures). These bacteria do not survive in the presence of oxygen; their existence supports the idea that life evolved before oxygen appeared in Earth’s atmosphere. So environments on other planets may not be as hostile to life as we had thought, even though they couldn’t support the types of animal and plant life with which we are most familiar. It is these hardy examples of life on Earth that give us hope that the possible discovery of primitive life on Mars will someday be confirmed. 20.6a The Drake Equation Sixth, if we want to hear meaningful signals from aliens, we must have a situation where not just life but intelligent life has evolved. We cannot converse with algae or paramecia, and certainly not with the organic compounds reported on Mars. Furthermore, the life must have developed a technological civilization capable of interstellar communication. So we consider the fraction of stars with life on which intelligence and communication actually develop. 20.6a The Drake Equation Finally, such a civilization must also live for a fairly long time; otherwise, its existence would be just like a flashbulb going off in an otherwise perpetually dark room. Humans now have the capability of destroying themselves either dramatically in a flurry of hydrogen bombs or more slowly by, for example, altering our climate, lessening our ozone shield (which keeps harmful ultraviolet radiation out), or increasing the level of atmospheric pollution. Natural disasters must also be avoided. That an Earth-crossing asteroid or comet will eventually impact the Earth with major consequences seems statistically guaranteed on a timescale of a few hundred million years, unless we take preventive action. 20.6a The Drake Equation It is a sobering question to ask whether the typical lifetime of a technological civilization is measured in decades, or whether all the problems that we have— political, environmental, and otherwise—can be overcome, leaving our civilization to last for millions or billions of years. So, to complete our calculation, we must multiply by the average lifetime of a civilization. We can try to estimate answers for each of these simpler questions within our chain of reasoning, though some of them are actually more like wild guesses. (Consequently, some scientists don’t find the exercise useful.) We can then use these answers together to figure out the larger question of the probability that communicating extraterrestrial life exists. 20.6a The Drake Equation Fairly reasonable assumptions can lead to the conclusion that there may be thousands, or even tens of thousands, of planets in our Galaxy on which technologically advanced life has evolved. (Handy “Drake equation calculators” can be found at several websites, including that of the SETI Institute, http://www.seti.org). Perhaps overly optimistically, Carl Sagan estimated that there might be a million such planets. On the other hand, adopting more pessimistic (but possibly more realistic) values for the probabilities of intelligent and technologically advanced life, and for the typical lifetime of such a civilization, gives a much bleaker picture. Indeed, humans may be at the pinnacle of intelligence and technological capability in our Galaxy, with few (if any) comparably advanced civilizations. 20.6b Where Is Everyone? One interesting argument is as follows: If there are many (say, a million) intelligent, communicating civilizations in our Galaxy, why haven’t we detected any of them? Where are they all? Surely most must be far more advanced than we are, and have sent signals if not spacecraft that have reached Earth, yet there is no evidence for them. This reasoning, promoted among others by the late Enrico Fermi of the University of Chicago, suggests that advanced creatures such as us might be very rare, even if primitive life is abundant in our Galaxy. (Note that various surface features claimed by some people to have been built by extraterrestrials are either nonexistent, such as the “face” on Mars—see figure—or have more conventional explanations, like the huge drawings on the Peruvian desert known as the Nazca lines.) 20.6b Where Is Everyone? Indeed, a more extreme version of this argument points out that there could be self-sustaining colonies voyaging through space for generations. They need not travel close to the speed of light if families are aboard. Even if colonization of space took place at a rate of only 1 light-year per century, our entire Galaxy would still have been colonized during a span of just a hundred million years, less than one per cent of its lifetime. The fact that we do not find extraterrestrial life here indicates that the Solar System has not been colonized, which in turn may imply that technologically advanced life has not arisen elsewhere in our Galaxy. 20.6b Where Is Everyone? Another possibly relevant fact suggesting that life having our capabilities is very rare in our Galaxy is that more than 1 billion species have ever lived on Earth, yet apparently we are the only species to have developed space communication or even acquired technology. Similarly, it is sobering to realize that a communicating civilization developed on Earth only in the last century, despite evidence for primitive life on Earth for the past 3.5 to 3.8 billion years. 20.6b Where Is Everyone? Recently, astronomers and other scientists have carefully considered the many factors that affect complex life on Earth. For example, the stability of the Earth’s axis of rotation depends on the presence of our rather massive Moon. If we had no moon, or only a small one, then Earth’s axis would undergo rather rapid, random changes in its orientation, causing large variations in the seasons and climate, and presumably making it more difficult for complex life to develop. Similarly, the presence of the very massive planet Jupiter in the Solar System, yet fairly far from Earth and having a nearly circular orbit, is a blessing: Jupiter’s gravitational tugs have cleared out the Solar System, making collisions between Earth and large “killer meteoroids” infrequent. 20.6b Where Is Everyone? There are many other relevant factors. The presence of plate tectonics on Earth is very important, for instance, because it allows carbon to be recycled in a steady manner through Earth’s atmosphere, oceans, interior, and surface. This “carbon cycle” is important for maintaining global climate stability and for certain aspects of life itself. Having an abundant supply of heavy elements during the formation of the Solar System was also crucial; most regions of our Milky Way Galaxy were quite deficient in such elements billions of years ago, when the Sun formed. These arguments, and others, have led many astronomers to conclude that Earth-like planets capable of developing complex, technologically advanced, communicating civilizations really are rare in our Milky Way Galaxy, though primitive life such as bacteria might be very common indeed. However, other astronomers (such as those at the SETI Institute) argue that there are major potential flaws in the reasoning for this “rare Earth” hypothesis, and that communicating civilizations may be common. 20.6b Where Is Everyone? It is not clear, for example, whether the apparently “special” conditions of Earth are essential to the development of intelligent life; maybe our view has been too highly skewed by the single example we know— ourselves. Moreover, is it necessarily true that if other intelligent creatures evolved, they would choose to colonize space, or have the ability to do so? The rather late appearance of technologically advanced life on Earth may also be a statistically unlikely fluke. Finally, some of the arguments relied on the unproven assumption that life elsewhere is quite similar to that on Earth in its properties and evolutionary path. 20.6b Where Is Everyone? In any case, unless we make the effort to actually look for signs of intelligent extraterrestrial life, we might never know whether humans are indeed alone in our Galaxy, or simply one of many such creatures. Thus, many astronomers support the search for extraterrestrial intelligence, especially using telescopes that are simultaneously doing other, more conventional types of research projects. 20.6b Where Is Everyone? Some major radio-telescope projects are being built for SETI purposes, with more ordinary astronomy to be carried out as a bonus (see figure). Perhaps someday we will be scanning the skies with radio telescopes on the far side of the Moon, shielded from Earth’s radio interference by the Moon’s bulk. The chances for success might be slim, but all agree that the actual detection of extraterrestrial signals would be one of the most important and mind-blowing discoveries ever, if not the greatest discovery of all time. 20.7 UFOs and the Scientific Method If some or many astronomers believe that technologically advanced life exists elsewhere in our Galaxy, why do they not accept the idea that unidentified flying objects (UFOs) represent visitations from these other civilizations? The answer to this question leads us not only to explore the nature of UFOs but also to consider the nature of knowledge and truth. The discussion that follows is a personal view of the authors, but one that is shared to a greater or lesser extent by most scientists. 20.7a UFOs The most common UFO is a light that appears in the sky that seems to the observer unlike anything made by humans or any commonly understood natural phenomenon. UFOs may appear as a point or extended, or may seem to vary in size or brightness. Most of the sightings of UFOs that are reported can actually be explained in terms of natural phenomena. But the observations are usually anecdotal, are not controlled as in a scientific experiment, and are not accessible to study by sophisticated instruments. The Earth’s atmosphere can display a variety of strange effects, and these can be used to explain many apparent UFOs. When Venus shines brightly near the horizon, for example, we sometimes get telephone calls from people asking us about the “UFO”— especially if the crescent moon happens to also be in that direction. It is not well known that a bright star or planet low on the horizon can seem to flash red or green because of atmospheric refraction. Atmospheric effects can affect radar (radio) waves as well as visible light. 20.7a UFOs For many of the effects that have been reported, the UFOs would have been defying wellestablished laws of physics. Why haven’t we heard the expected sonic booms, for example, from rapidly moving UFOs? Scientists treat challenges to laws of physics very seriously, since our science and technology are based on these laws and they seem to work extremely well. 20.7a UFOs Most professional astronomers feel that UFOs can be so completely explained by natural phenomena that they are not worthy of more of our time. Although most of us do not categorically deny the possibility that UFOs exist (after all, the Voyager spacecraft might someday pass by a planet orbiting another star), the standard of evidence expected of all claims in science has not yet been met. Some individuals may ask why we reject the identification of UFOs with flying saucers, when—they may say—that explanation is “just as good an explanation as any other.” Let us go on to discuss what scientists mean by “truth” and how that applies to the above question. 20.7b Of Truth and Theories At every instant, we can explain what is happening in a variety of ways. When we flip a light switch, for example, we assume that the switch closes an electric circuit in the wall and allows the electricity to flow. But it is certainly possible, although not very likely, that the switch activates a relay that turns on a radio that broadcasts a message to the president of the United States. The president then might send back a telepathic message to the electricity to flow, making the light go on. The latter explanation sounds so unlikely that we don’t seriously consider it. We would even call the former explanation “true,” without qualification. 20.7b Of Truth and Theories We usually regard as “true” the simplest explanation that satisfies all the data we have about any given thing. Science is based on Occam’s Razor, though we don’t usually bother to think about it. This principle is known as Occam’s Razor; it is named after a 14thcentury British philosopher who originally proposed it. Without this rule, we would always be subject to such complicated doubts that we would accept nothing as known to be true. Sometimes something we call “true” might be more accurately described as a theory (see Chapter 1). An example of a theory is the Newtonian theory of gravitation, which for many years explained almost all of the planetary motions. Albert Einstein’s 1916 theory of gravity, known as the general theory of relativity, provided an explanation for a nagging discrepancy in the orbit of Mercury, as we described in Chapter 10. 20.7b Of Truth and Theories Is Newton’s theory “true”? Is Einstein’s theory “true”? We may say so, although one day a newer theory may come along that is more general than Einstein’s in the same way that Einstein’s is more general than Newton’s. Indeed, as we discussed in Chapter 19, superstring theory is a leading candidate for the unification of general relativity and quantum physics, and hence may someday be a more complete theory than Einstein’s. How does this view of truth tie in with the previous discussion of UFOs? Though we know it is “false,” it is a good approximation of the truth in most regions of space; it is generally an accurate model for what we observe. Scientists have assessed the probability of UFOs being flying saucers from other worlds, and most have decided that the probability is so low that we have better things to do with our time and with our national resources. We have so many other, simpler explanations of the phenomena that are reported as UFOs that when we apply Occam’s Razor, we call the identifications of UFOs with extraterrestrial visitation “false.” 20.8 Conclusion You have covered a lot of material in this book, and learned much about the Universe. We, the authors, hope that this new knowledge increases your awe and fascination for nature. The understanding of a phenomenon should enhance its beauty, not detract from it. The magnificence of the Universe comes in part from its logical structure—and the foundation is perhaps unexpectedly simple. Indeed, Einstein remarked that “The most incomprehensible thing about the Universe is that it’s comprehensible.” 20.8 Conclusion The actual consequences of the basic laws can be extraordinarily complicated and varied. The best example is life itself: It is the most complex known structure. Even the simplest cell is more difficult to understand than the formation of galaxies or the structure of stars. Our highly advanced brains and dexterity are what sets humans apart from other forms of life. We are able to question, explore, and ultimately understand the inner workings of nature through a process of observation, experimentation, and logical thought. 20.8 Conclusion It is almost as if the Universe has found a way to know itself, through us. We are the observers and explorers of the Universe; we are its brains and its conscience. This makes each one of us special to the Universe as a whole. Perhaps it need not have been this way: Alter the physical constants ever so slightly and the Universe may have been stillborn, with no such complexity. But here we are, enjoying life in this beautiful, amazing, mind-blowing Universe.