The Eerie Silence: Renewing Our Search for Alien Intelligence
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The Eerie Silence: Renewing Our Search for Alien Intelligence

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by Paul Davies

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One of the world’s leading scientists explains why—and how—the search for intelligent life beyond Earth should be expanded.

Fifty years ago, a young astronomer named Frank Drake first pointed a radio telescope at nearby stars in the hope of picking up a signal from an alien civilization. Thus began one of the boldest scientific projects

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One of the world’s leading scientists explains why—and how—the search for intelligent life beyond Earth should be expanded.

Fifty years ago, a young astronomer named Frank Drake first pointed a radio telescope at nearby stars in the hope of picking up a signal from an alien civilization. Thus began one of the boldest scientific projects in history, the Search for Extraterrestrial Intelligence (SETI). After a half-century of scanning the skies, however, astronomers have little to report but an eerie silence—eerie because many scientists are convinced that the universe is teeming with life. Physicist and astrobiologist Paul Davies has been closely involved with SETI for three decades and chairs the SETI Post-Detection Taskgroup, charged with deciding what to do if we’re suddenly confronted with evidence of alien intelligence. He believes the search so far has fallen into an anthropocentric trap—assuming that an alien species will look, think, and behave much like us. In this provocative book Davies refocuses the search, challenging existing ideas of what form an alien intelligence might take, how it might try to communicate with us, and how we should respond if it does.

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Editorial Reviews

From the Publisher

"Highly recommended for both science fiction and astronomy buffs."—Publisher's Weekly

"All readers interested in astrobiology, SETI, and extraterrestrial life will benefit from this book."
Library Journal

"After half a century of fruitless searching, SETI needs a summing up. This book does the job—you need no other. Davies ranks among our very best science writers, and this book should prompt deep rethinking among the entire SETI community."—Gregory Benford, author of Timescape

"Paul Davies has written a most delightful book, perhaps the most thoughtful, thorough, and comprehensive book ever published on the key question: are we alone in the universe? Davies addresses one of the most pivotal questions facing humanity, and does it with wit, style, and rigor. The Eerie Silence will satisfy the curiosity of anyone interested in big cosmic questions about intelligence in the universe." —Michio Kaku, professor of theoretical physics and author of Physics of the Impossible, Hyperspace, and Parallel Worlds

"Paul Davies gives us a panoramic view of the quickening search for cosmic company—a fascinating tale stuffed with novel ideas about the nature of intelligence far beyond our own."—Seth Shostak, Senior Astronomer, SETI Institute

"Are we looking for life in all the wrong places? This is the deep scientific question that inspires Paul Davies’ The Eerie Silence. With clarity and passion, Davies not only brings you up to speed on the cutting-edge perspective, he also presents his own preferred strategies for making contact. Within may be found some scenarios that even the sci-fi writers haven’t tumbled on yet. A feast for thought about the most fascinating mystery of all." —Ann Druyan, creative director, NASA’s Voyager Interstellar Message, and co-creator, with Carl Sagan, of Contact

"Paul Davies imagines the unimaginable. After fifty years in which radio astronomers have listened in vain for voices from a silent sky, with clarity and authority Davies sets out a stunning new prospectus for the continuing search for intelligence beyond the Earth. A must-read for anybody fascinated by the most profound of questions: are we alone?"—Stephen Baxter, author of Manifold: Time and Flood

Dwight Garner
…Mr. Davies is an interesting thinker about nearly every aspect of our search for other intelligent life in the universe. The Eerie Silence may reprise material from his earlier books and lean on the work of futuristic thinkers like Freeman Dyson and Raymond Kurzweil. It gets moderately woo-woo at times, too. But Mr. Davies is smart enough to coax you rather slowly out onto the mental gangplank with him, from where the view becomes genially starry and mind-bending.
—The New York Times
Publishers Weekly
In what has become known as Fermi's Paradox, the great nuclear physicist Enrico Fermi once asked, if there are aliens out there, where is everybody? After 50 years of looking, the SETI (Search for Extraterrestrial Intelligence) project has likewise failed to find anybody. Cosmologist Davies (The Mind of God), winner of the 1995 Templeton Prize, believes that SETI's search for narrow-band radio signals from planets around other stars needs to be broadened to look for other possible signs of life. Aliens may be using far more advanced technology than radio to signal the cosmos, such as manipulating pulsars to act as beacons or even neutrino signaling. Davies also puts forth the possibility that alien probes may be silently trolling the solar system. The author surveys popular topics in science fiction such as Dyson spheres, time travel, and wormholes, and decides that they're not feasible under physics as we understand it. He concludes with a far-ranging look at what might happen here on Earth when we make first contact. Highly recommended for both science fiction and astronomy buffs. Illus. (Apr. 13)
Library Journal
Written to mark the 50th anniversary of SETI, the Search for Extraterrestrial Intelligence, as a scientific discipline, this work reviews the current thinking about the likelihood of extraterrestrial life and the best ways to detect it. So far, SETI has found no sign of alien life, so Arizona State University physicist Davies (Cosmic Jackpots) suggests several ways to improve the search: SETI should look for a possible second genesis of life here on Earth as it's possible that not all life is from the same origin. Also, now that most communication is through optical fibers, we know that a civilization's radio exposure is brief, so scientists should expand the search to look for possible beacons left by other civilizations. VERDICT Though speculative, Davies's book is science based and gives an objective and thorough update on the search for alien intelligence. All readers interested in astrobiology, SETI, and extraterrestrial life will benefit from this book.—Jeffrey Beall, Univ. of Colorado at Denver Lib.

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Is Anybody Out There?

Absence of evidence is not the same as evidence of absence.
 Donald Rumsfeld (on weapons of mass destruction)

What if ET calls tomorrow?
On a cold and misty morning in April 1960, a young astronomer named Frank Drake quietly took control of the 26-metre dish at the US National Radio Astronomy Observatory in Green Bank, West Virginia. Few people understood that this moment was a turning point in science. Slowly and methodically Drake steered the giant instrument towards a sun-like star known as Tau Ceti, eleven light years away, tuned in to 1,420 MHz, and settled down to wait. His fervent hope was that alien beings on a planet orbiting Tau Ceti might just be sending radio signals our way, and that his powerful radio dish would detect them.
 Drake stared at the pen and ink chart recording the antenna's reception, its fitful spasms accompanied by a hiss from the audio feed. After about half an hour he concluded there was nothing of significance coming from Tau Ceti - just the usual radio static and natural background from space. Taking a deep breath, he carefully reoriented the big dish towards a second star, Epsilon Eridani. Suddenly, a series of dramatic booms emanated from the loudspeaker and the pen recorder began frantically flying back and forth. Drake almost fell off his chair. The antenna had clearly picked up a strong artificial signal. The astronomer was so taken aback he remained rooted to the spot for a long while. Finally, getting his brain in gear, he moved the telescope slightly off target. The signal faded. But when he moved the antenna back, the signal had disappeared! Could this really have been a fleeting broadcast from ET? Drake quickly realized that picking up a signal from an alien civilization on the second attempt was too good to be true. The explanation must lie with a manmade source and, sure enough, the signal turned out to be produced by a secret military radar establishment.
 With these humble beginnings - whimsically called Project Ozma after the mythical Land of Oz - Frank Drake pioneered the most ambitious, and potentially the most significant, research project in history. Known as SETI, for Search for Extraterrestrial Intelligence, it seeks to answer one of the biggest of the big questions of existence: are we alone in the universe? Most of the SETI programme builds on Drake's original concept of sweeping the skies with radio telescopes for any hint of a message from the stars. It is clearly a high-stakes endeavour. The consequences of success would be truly momentous, having a greater impact on humanity than the discoveries of Copernicus, Darwin and Einstein put together. But it is a needle-in-a-haystack search without any guarantee that a needle is even there. Apart from one or two intriguing incidents (of which, more later) all attempts have so far been greeted with an eerie silence. What does that tell us? That there are no aliens? Or that we have been looking for the wrong thing in the wrong place at the wrong time?
 SETI astronomers say the silence is no surprise: they simply haven't looked hard enough for long enough. To date, the searches have scrutinized only a few thousand stars within 100 light years or so. Compare this to the scale of our galaxy as a whole - 400 billion stars spread over 100,000 light years of space. And there are billions of other galaxies. But the power of the search is expanding all the time, following its own version of Moore's Law for computers, doubling every year or two, driven by surging instrument efficiency and data-processing speed. Now the scope is set to improve dramatically, with the construction of 350 interlinked radio dishes at Hat Creek in Northern California. Named after the benefactor Paul Allen, the Allen Telescope Array will enable researchers to monitor a much larger fraction of the galaxy for alien signals (see Plate 1). The facility is operated by the University of California, Berkeley, and the SETI Institute, which is where Frank Drake now works. The Institute remains upbeat about the prospects for success, and keeps champagne permanently on ice in anticipation of a definitive detection event.
 It's easy to picture the scene if the optimism is right, and something is found soon. An astronomer sits stoically at the controls of the instrument, his feet stuck up on a desk cluttered with papers. Absently, he thumbs though a mathematics textbook. So it has been for him, and dozens of others engaged in SETI, for decade after decade. But today is different. Suddenly the bored astronomer is startled out of his reverie by the shrill, distinctive sound of an alarm. The screech is generated by a computer algorithm designed to spot 'funny' radio signals and separate them from the clutter continually being received from outer space. At first, the astronomer assumes it's just another one of those false alarms, usually a manmade transmission that slips through the net designed to filter out obvious artificial signals coming from mobile phones, radar and satellites. Adhering to the time-honoured protocol, the astronomer keys in some simple instructions and moves the telescope slightly off the target star. The signal immediately dies. He moves the instrument back on target and the signal is still there. After carefully studying the radio wave form and determining that the source remains at a fixed location relative to the stars, the astronomer quickly places a telephone call to a companion observatory involved in the project and simultaneously e-mails the coordinates of the mystery signal.
 Five thousand miles away, another astronomer is called out of bed to investigate. Drowsily she wanders to the control room and pours herself a coffee. Then, shaking the sleep from her head, she checks her e-mail and enters the given coordinates. Within a minute the second radio telescope has locked on to the target and immediately picks up the same signal, loud and clear. Her pulse begins to race. Is it conceivable that this time the alert is for real? After decades of unrewarded search, might she be the first person on Earth to confirm that an alien civilization really exists and is transmitting radio signals? She knows that many more checks will be needed before leaping to that conclusion, but the two astronomers, now in excited telephone conversation between different continents, systematically eliminate one mundane possibility after another until, with 90 per cent certainty, they infer that the signal is indeed artificial, non-human and originating far, far out in space. As the radio telescopes continue to track in synchrony and record every minute detail, the dazed pair behave as if in a dream, stunned, awed and euphoric, all at once. What next? Who to tell? What can be gleaned from the data already gathered? Will the world ever be the same again?
 The story so far (which I admit involves some literary licence) does not demand any great leap of imagination. The basic scenario was well enough portrayed in the Hollywood movie Contact, in which Jodie Foster plays the role of the lucky, overawed astronomer. What is far less clear is the next step. What would follow from the successful detection of an alien radio signal? Most scientists agree that such a discovery would be disruptive and transformative in myriad ways. Even contemplating a signal received out of the blue raises many questions: how and by whom would it be evaluated? How would the public get to learn about it? Would there be social unrest, even panic? What would governments do? How would the world's leaders react? Would the news be regarded with fear or wonderment? And in the longer term, what would it mean for our society, our sense of identity, our science, technology and religions? On top of these imponderables is the vexed issue of whether we should respond to the signal, by sending our own message to the aliens. Would that invite dire consequences, such as invasion by a fleet of well-armed starships? Or would it promise deliverance for a possibly stricken species?
 There are no agreed answers to any of these questions. The narrative of Contact parted company with established science once the signal was received, and lurched off into the speculative realms of wormhole space travel and other dramatic themes. That was science fiction, deriving from the fertile imagination of the late Cornell University astronomer Carl Sagan, author of the book on which the film was based. In the real world, it is completely unclear what would follow the discovery that we are not alone in the universe. In 2001 the International Academy of Astronautics established a committee to address 'what next?' issues. Known as the SETI Post-Detection Taskgroup, its job is to prepare the ground in the event that SETI suddenly succeeds. The rationale is that once a signal from an alien source is confirmed, things would move too fast for the scientific community to deliberate wisely. I happen to be the current Chair of the SETI Post-Detection Taskgroup, and this unusual position has prompted me to give considerable thought to the subject of SETI in general, and post-detection in particular.

Is SETI stuck in a rut?
I've been associated with SETI one way or another for most of my career, and have enormous admiration for the astronomers who operate the radio telescopes and analyse the data, as well as for the technical staff who design and build the equipment. I hope the eerie silence is indeed due to the fact that the search has been limited, and I am a strong supporter of the Allen Telescope Array. But I also think, for reasons I shall come to later, that there is only a very slender hope of receiving a message from the stars at this time, so alongside 'traditional SETI,' of the sort pioneered by Frank Drake, we need to establish a much broader programme of research, a search for general signatures of intelligence, wherever they may be imprinted in the physical universe. And that requires the resources of all the sciences, not just radio astronomy. There is, however, another factor that has to be addressed. By focusing on a very specific scenario - an alien civilization beaming detectable so-called narrow-band (sharp-frequency) radio messages to Earth - traditional SETI has become stuck in something of a conceptual rut. Fifty years of silence is an excellent cue for us to enlarge our thinking about the subject. Crucially, we must free SETI from the shackles of anthropocentrism, which has hampered it from the very beginning. To help spur this process, I convened a special SETI workshop in February 2008 at Arizona State University's Beyond Center for Fundamental Concepts in Science, with the goal of fostering a lively exchange of ideas between mainstream SETI researchers and a handful of quirky out-of-the-box thinkers, including philosophers, science fiction writers and cosmologists. The upshot was a blueprint for 'new SETI', with some great ideas I shall describe in the coming chapters.
 How could something as bold and visionary as SETI become conservative? A major part of the reason is the tendency of humans to extrapolate from their own experience. The very basis for SETI is, after all, an assumption that our civilization is in some respects typical, and that there will be other earths out there with flesh-and-blood sentient beings not too different from us, who will be anxious to communicate. Given that predicate, it is reasonable to take human nature and human society as a model for what an alien civilization will be like - we don't have much else to go on, after all. In the early days of SETI, when the basic strategy was being planned, there were a lot of questions along the lines, 'What would we do in those circumstances?' The result, inevitably, is an inbuilt bias towards anthropocentrism.
 Here is a classic example. SETI began with the realization that radio telescopes have the power to beam signals across space. Therefore it's possible that alien signals are coming our way. The image popularized by Carl Sagan was that of an alien civilization directing a message at Earth in the form of narrow-band radio signals. The specifics soon fell into place: the message would be folded into a carrier wave and transmitted from an antenna at a fixed frequency and with enough power to loom above naturally produced radio noise. That is the way terrestrial radio stations do it. It's easy to detect narrow-band signals, once the receiving antenna has been tuned to the right frequency (and, in the case of radio telescopes, pointed in the right direction). There are many other ways to encode and transmit radio messages which require more sophisticated receiving procedures, but SETI astronomers assume that an alien civilization anxious to attract our attention would adopt the simplest method appropriate to entry-level radio technology.
 Back in the 1960s, a major preoccupation among SETI researchers was to decide which particular frequency ET might choose, given that there are billions of possibilities. Not all radio frequencies penetrate Earth's atmosphere effectively, so the hope was that the aliens would customize their signals for Earth-like planets by using a frequency that doesn't get greatly attenuated by its passage down from space. But that still left a huge number of potential radio channels. It would be the supreme irony to turn a radio telescope on the right star but tune into the wrong frequency and miss the message. Researchers argued that the aliens would anticipate our dilemma and pick a 'natural' frequency - one likely to be known to all radio astronomers. A popular guess was 1,420 MHz, the emission frequency for cold hydrogen gas. Radio astronomers are very familiar with this pervasive 'song of hydrogen', and it is in some sense a good choice. At any rate, that was the frequency Frank Drake picked for Project Ozma in 1960. Other astronomers suggested multiplying the hydrogen frequency by π, that number being what humans would take to be a 'signature of intelligence' because it enters into both geometry and the equations of fundamental physics, so would surely be familiar to any alien scientist. But there are other special numbers too, like exponential e and the square root of 2. In addition, there was a conundrum about whether the aliens would insert a correction to compensate for the motion of their planet and/or our planet. Very soon, the list of possible 'natural' frequencies became depressingly long. However, this battle of the wavebands went away, because technology became available that enables radio astronomers to monitor millions and even billions of radio channels (typically between 1 and 10 Hz wide) simultaneously. As a result, not many SETI researchers worry these days about second-guessing the aliens' choice of frequency. My point is that modest advances in human technology have led within just a few decades to a change in thinking about likely alien communication frequencies. There is a major lesson in this example. It is wise to view the situation through the eyes of the civilization setting out to communicate with us, on the assumption that it has been around for a very long time - at least one million years, and maybe 100 million years or more. Although the aliens may well settle on radio as the medium (perhaps for our benefit), they can hardly be expected to discriminate between 1950s and 1980s levels of human technology: what are a few decades in a million years?
 Another case in point: in the 1960s, the laser came to be seen as a powerful alternative means of human communication, and very soon some SETI researchers began to argue that ET, being so much more advanced, would surely prefer to use this fancy tool rather than oldfashioned radio. As a result, optical SETI was born (and still flourishes): astronomers search for a signal in the form of very short-duration, high-intensity pulses of light that with suitable equipment can be distinguished from the overall much brighter but unvarying light of the parent star. Laser communication came less than a century after the invention of radio communication, so once again I ask, what does a century matter to a million-year-old civilization?
 A greater degree of parochialism occurs when SETI gets influenced by human politics and even economics. One of the main unknowns is the longevity of a communicating civilization. The challenge is to guess whether ET will be on the air for centuries, millennia or even longer. During the Cold War, many SETI proponents reasoned that the development of advanced radio communication would be paralleled by similar-level technological developments, such as nuclear weapons. Because our own society was at that time in grave danger of nuclear annihilation, it was fashionable to argue that alien technological civilizations likewise wouldn't last long. They would have their own Cold War which, after a few decades, would turn hot, and knock them off the air. When the (terrestrial) Cold War ended, human political concerns shifted to the environment, and SETI thinking duly shifted with it. The hot-button issue now, in many people's eyes, is no longer nuclear war, but sustainability. Transmitting powerful radio waves across the galaxy would require large-scale engineering and soak up a lot of energy. Surely an advanced alien civilization would tailor its technology so as to minimize the environmental impact? Well, maybe. But that line of reasoning would have been received sceptically in the 1960s political atmosphere, and may well be regarded as irrelevant in another hundred years, when environmental problems may be replaced by other concerns. There is no reason to suppose that a million-year-old super-civilization would have 'a sustainability problem'. It might, of course, have some other problem, maybe one we couldn't anticipate, or wouldn't understand even if we were told. SETI is the quintessentially long-term project, and it is foolish to base too much of our search strategy on flavour-of-the-month political fashion. Guessing the political priorities of an alien civilization is futile.
 Equally futile is guessing alien economics. Take, for example, H. G. Wells's novel The War of the Worlds, in which the Martians, saddled with an inferior planet, consider decamping to Earth. Wells portrays a creepy image of covetous aliens, technologically far ahead of humans, eyeing our planet with malice, '. . . across the gulf of space, minds that are to our minds as ours are to those of the beasts that perish, intellects vast and cool and unsympathetic, regarded this earth with envious eyes, and slowly and surely drew their plans against us.' Wells wrote his story in the 1890s, at the height of the British Empire, when wealth and power were measured in acres of land, tons of coal and iron, and head of cattle. The richest men built railways and owned big ships, mined coal or copper or gold, and purchased vast tracts of grazing land. In short, wealth in Victorian times meant physical stuff. So it was natural to think of alien civilizations similarly valuing real estate and mineral resources, and making plans to spread across space in search of more once their own planet was mined out. Such was the prime motive of Wells's Martians. However, barely a century later, the global economy had transformed out of all recognition. By the 1990s, Bill Gates was the new Rockefeller, making money not from 'physical stuff' but from bits of information. Microsoft had more financial clout than most countries. With information age economics came information age SETI. Surely, it was reasoned, the aliens would not be so primitively rapacious as to scour the galaxy for iron ore, still less for gold or diamonds. An advanced extraterrestrial community would value information - that would be their currency, their source of wealth. Information and knowledge - those more noble incentives - would come to dominate the alien agenda. Lust for information may drive them to send out probes, not to acquire material, but to explore and observe and measure, and to compile a database, a veritable Encyclopedia Galactica. It seems reasonable enough today, but I wonder how the information argument will play out in the 2090s, when the economy may revolve around something that hasn't yet been imagined, let alone invented. If human priorities can change so dramatically in a mere century, what hope have we of guessing the priorities of a civilization that may have enjoyed a million or more years of economic development?
 The same general criticism can be levelled at most theorizing about what an alien civilization would be like and how its members would behave. It's true that the history of human civilization gives a clue, and certain general principles might apply to all intelligent life. The problem is, we have only one sample of life, one sample of advanced intelligence, and one sample of high technology. It is really hard to untangle the features that may be special to our planet from any general principles about the emergence of life and intelligence in the universe. In these circumstances there is an inevitable temptation to fall back on analogy with humanity when trying to second-guess ET. But that is almost certainly fallacious. Asking what we would do is largely irrelevant. The narrow focus and parochialism inherent in traditional SETI has not been lost on Frank Drake. 'Our signals of today are very different from the signals of 40 years ago, which we then felt were perfect models of what might be radiated from other worlds of any state of advancement,' he writes. 'We were wrong. If technology can change that much in 40 years, how much might it change in thousands or millions of years?' And that's it in a nutshell. However, this clear acknowledgement by the founder of traditional SETI has yet to translate into radical new approaches on the research front. In my opinion, the way forward is to stop viewing alien motives and activities through human eyes. Thinking about SETI requires us to abandon all our presuppositions about the nature of life, mind, civilization, technology and community destiny. In short, it means thinking the unthinkable.

It's great - but is it science?
Although the scientific community is on the whole fairly comfortable with SETI these days, members of the public have a hard job positioning it in the scientific landscape. People want to know why it's okay to look for aliens but not for ghosts, why messages from the stars are scientifically respectable, but messages from the dead are not. Where does one draw the line between science and pseudoscience? It is an important but subtle point that goes right to the heart of the scientific method, and it's impossible to understand how SETI works without an explanation of this distinction. So here goes.
 Carl Sagan once declared, 'extraordinary claims demand extraordinary evidence.' He made the remark in the context of UFO stories (for which, see the final section of this chapter), but the dictum applies quite generally. Sagan was expressing colloquially what is formally known as Bayes' rule for inference based on the statistical evaluation of evidence. Thomas Bayes was an eighteenth-century English clergyman who appreciated that the weight attributed to evidence will depend on how plausible the hypothesis to which it pertains is deemed beforehand (its so-called prior probability). Let me give an everyday example. I wake at 6 a.m. to find a bottle of milk on my doorstep. What do I conclude? There are two hypotheses. The first is that the milk has been delivered by the milkman, as it is every day except Sunday, because I have a contract with the local company, Express Dairy. Normally the milkman comes at 7 a.m., but perhaps today he came early. The second hypothesis is that the milk has been left there by an altruistic neighbour, Mrs Jones, who might have had a spare bottle. The second hypothesis is obviously a long shot, so it has a much lower prior probability than the first. To believe it, I would require 'extraordinary evidence'. What might that be? Well, Mrs Jones subscribes to the rival company, United Dairies. Their bottles of milk have the brand name 'United' embossed on the side, whereas Express Dairy has 'Express'. If today the bottle displays 'United', I would re-evaluate the odds on the Jones explanation. But I see 'Express'. Do I eliminate Hypothesis 2? Not entirely. It could be that Express Dairy delivered to Mrs Jones by mistake the day before, for example. But the more contrived and extravagant the hypothesis, the greater the weight of evidence needs to be before I will take it seriously. Actually, the probability of either hypothesis being correct is essentially zero, because nobody seems to deliver milk to the doorstep in bottles any more, at least they don't in the countries in which I have lived. So this example is just a bit of nostalgia. (Accurate as of London, circa 1960, for those who are interested. My best friend Brian was the milkman's son, and would occasionally help his father with deliveries. He even recalls turning out on Christmas Day, such was the level of service in the Good Old Days. The milk bottles were originally conveyed to the customer on a horse-drawn cart, and the horse would often get a carrot as a Christmas present. Then the horses were decommissioned in favour of a soulless electrical vehicle. Then the milkman himself was decommissioned, along with the bottles and the vehicle, in favour of horrid supermarket cartons. Such is progress.)
 Applied to science and pseudoscience, Bayes' rule helps us assign credibility factors to competing claims. Thomas Jefferson famously said, 'I would sooner believe that two Yankee professors lied, than that stones fell from the sky', when he was told of an eyewitness report of falling meteorites. Like many nineteenth-century intellectuals, Jefferson pooh-poohed meteorite claims on the basis that the deemed prior probability of there being stones in the sky is tiny, whereas the prior the eerie silence probability that a scurrilous professor might make up a story for reasons of fame is not that small. Today we know that the solar system is replete with rubble left over from its formation, so the prior probability we would now assign to a story of a meteorite fall is much greater. We should therefore be inclined to take such reports seriously. (Though still cautiously: a geologist friend of mine has investigated several eyewitness reports of meteorite falls, and they all turned out to be mistaken interpretations.)
 A persistent complaint among my non-scientist friends is that modern physics touts all sorts of mind-bending ideas about extra dimensions, unseen dark matter, invisible strings, parallel universes, evaporating black holes, wormholes, etc., in spite of the fact that most of these proposals have little or no experimental or observational evidence to support them. Yet phenomena like telepathy and precognition are experienced first hand by thousands of people, and immediately rejected by scientists as nonsense. Is this not a glaring case of double standards? 'How can you deny the existence of ghosts,' I was once challenged, 'when you accept the existence of neutrinos, which are far more ghostly and have never been seen directly by anybody?' (Neutrinos are elusive subatomic particles that mostly pass right through solid matter, making them exceedingly hard to detect.)
 The short riposte to the above complaint is 'Bayes' rule.' The point about modern physics is that weird entities like dark matter or neutrinos are not proposed as isolated speculations, but as part of a large body of detailed theory that predicts them. They are linked to familiar and well-tested physics through a coherent encompassing mathematical scheme. In other words, they have a place in well-understood theory. As a result, their prior probability is high. The job of the experimenter is to test the theory. If you build an experiment to make an accurate measurement of such-and-such a quantity, the precise value of which is predicted in advance, then the level of evidence we require to believe that the said entity is real is much less than if someone simply found it by chance in the absence of any theoretical underpinning. Regarding the paranormal, telepathy is not obviously an absurd notion, but it would take a lot of evidence for me to believe in it because there is no properly worked out theory, and certainly no mathematical model to predict how it works or how strong it will be in different circumstances. So I assign it a very low (but non-zero) prior probability. If someone came up with a plausible mechanism for telepathy backed by a proper mathematical model which linked it to the rest of physics, and if the theory predicted specific results - for example, that the 'telepathic power' would fall off in a well-defined way as the distance increases, and would be twice as strong between same-sex subjects as mixed-sex subjects - I would sit up and take notice. I would then be fairly easily convinced if the experimental evidence confirmed the predictions. Alas, no such theory is on the horizon, and I remain extremely sceptical about telepathy in spite of the many amazing stories I have read.
 Turning now to SETI, how does it measure up as science versus pseudoscience? Well, we immediately hit the core problem in the whole enterprise. What prior probability should we assign to the existence of a communicating extraterrestrial civilization? Nobody knows. If you already have good reason to believe ET is out there, and a definite idea about the nature of the signal, then you are, so to speak, 'primed' for the evidence and likely to be easily won over. But if you think the very notion of an alien civilization is incredible, you would need very strong evidence indeed. In Chapter 4 I shall argue that either advanced alien civilizations are very common or they are exceedingly rare: a middle position of a few here and there is intrinsically unlikely. So those who find the notion of alien civilizations a wild and unjustified speculation place SETI in the realm of pseudoscience, while others who find the idea plausible regard it as real science. You, the reader, must make up your own mind. What is not in question, however, is that the methodology of SETI is real science. The research is conducted with state-of-the-art technology by highly trained scientists using rigorous techniques of enquiry and analysis, and the results are subject to the usual scrutiny of peer-review. There is no question that the research groups are doing quality science. But are they chasing a chimera? Well, read on . . .

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From the Publisher
"Paul Davies imagines the unimaginable.... A [must-listen] for anybody fascinated by the most profound of questions: are we alone?" —-Stephen Baxter, author of Manifold

Meet the Author

PAUL DAVIES is an internationally acclaimed physicist, cosmologist, and astrobiologist at Arizona State University. He is the author of more than twenty books, including The Mind of God, About Time, How to Build a Time Machine, and The Goldilocks Enigma.

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