Alone “for all practical purposes.”

Two preliminary clarifications are essential before presenting the latest astrophysical results, in particular the discovery of exoplanets, and discussing their dramatic implications for ETI. First, only the existence of intelligent beings is relevant. Primitive life may yet be discovered on Mars; perhaps even multicellular animals will be found on a nearby extrasolar planet. These revolutionary discoveries would help us reconstruct how life on Earth evolved, but unless a species is capable of conscious, independent thought and has the ability to communicate, we will still be alone—with no one to teach or learn from, no one to save us from ourselves, or, in the fanciful extrapolations of Stephen Hawking () (and endless filmmakers), no one to do battle with. ETI implies life able to communicate between stars (e.g., Shklovskii and Sagan ). This implies having radio or similar technology. Since our own society, by this definition, is only about 100 years old, if intelligent life is common in a universe that is 13.7 billion years old we are surely among its youngest forms. As the physicist Enrico Fermi famously observed, however, the fact that there are no other known intelligent life forms signals that the assumption is wrong—intelligent life is not very common (this is called “the Fermi Paradox”).

The second important caveat derives from two features of the world that were unknown to Percival Lowell and all philosophers and scientists before the late twentieth century. The first is relativity—the fastest any signal can travel is the finite speed of light, which sets a practical limit on communication even between stars in our own Milky Way galaxy and its presumed billions of planets. It will take hundreds of thousands of years for any ETI on most of them to receive our signals, and that long again for us to get a reply.

The other feature is the expanding nature of the universe, the result of an inflationary “big bang” creation, with two relevant effects. The first might seem to bolster the chances for ETI: Beyond our cosmic horizon (the distance set by how far light can travel in the age of the universe) the universe may be extremely large. Even more fantastically, there are possibly an infinite number of universes (the so‐called “multiverse”). Therefore, as Stephen Hawking () and other physicists have speculated, even if ETI were infinitesimally rare, in an infinite universe all possible scenarios and life forms will exist in plenitude. While such possibilities may be philosophically amusing, they are irrelevant. We cannot communicate with, or even directly measure anything about, this unlimited vastness. The second relevant aspect of cosmology is the discovery that distant galaxies are actually receding from us at an accelerating rate (e.g., Riess et al. ). As a result, light signals sent from Earth today will never even catch up to galaxies whose light has taken only about 10 billion years to reach us (Loeb ). Although they are well within our cosmic horizon, such galaxies are now beyond our reach forever and receding quickly. Waiting longer times for signals will not help.

The SETI program (Search for Extraterrestrial Intelligence), which undertakes extensive searches for signals, fully acknowledges this science but argues that “we are unanimous in our conviction that the only significant test of the existence of extraterrestrial intelligence (ETI) is an experimental one” (Tarter , 359). I support SETI in principle—if we don't look we won't find. But it is a risky endeavor, and nonprofessionals in particular need to be reminded of the enormous limitations imposed by the finite speed of light. Even if there were a network of advanced civilizations living on the other side of our galaxy, they much are too far away from Earth to converse with. Any signals of theirs that we happen to pick up will already be hundreds of thousands of years out of date, and to be “alone” is to be without anyone to talk to. By the way, indirect statistical evidence, as SETI scientists would agree, is also “experimental,” including the absence of sought‐for signals coupled with careful statistical analyses of geological, astronomical, and biological data sets.

Two possible ways out of these constraints have been proposed. It is possible that some distant alien civilization scans the galaxy's stars for juvenile Earths, predicts their evolution and optimistically sends out greetings eons ahead of time as signals or robotic probes timed to arrive just when intelligent species have evolved and are starting to listen. But it is very hard to imagine any such enterprise being practical either technically or economically (e.g., Goldsmith and Owen , 446–50; Davies ). Another, tactical approach to exploring the cosmos given the limitations of relativity is for civilizations to colonize suitable nearby stellar systems locally, within an achievable volume, and then for each of these new settlements to similarly colonize a spherical volume around itself. As they independently “percolate” outward, civilizations will eventually span the galaxy. Cartin () calculates the probability that spacefaring ETI has arisen in the Solar System's neighborhood (he uses 130 light‐years) based on their not having reached here through this percolation approach. Using assumptions about distance and speed that he considers realistic, he concludes that their absence implies “at most [my emphasis] only one out of every 585 habitable [again, my emphasis] planets within the local Solar neighborhood could be the cradle of an interstellar civilization” (Cartin , 573). He admits that the fraction of worlds on which life has arisen is “unknown.” We show in the detailed discussions below that habitable by no means implies inhabited, and so “at most” is at best an understatement. No wonder there are no signals, nor even faint traces, despite decades of looking. As Fermi argued, they are not there.

The second way out of the Special Relativity constraint is the question I get most often: Perhaps some physics we think we understand will be overturned with future knowledge! Faster than light travel, in particular, will then turn science fiction into fact, and we will be able to warp‐drive our way even to the most distant galaxies that have so far been excluded from our vision. Unfortunately, Fermi's Paradox—“If they are not uncommon, then where are they?”—implies that we cannot have it both ways. If life were common and if superluminal travel or communication were possible, then we face an insuperable contradiction: the billions of intelligent species in the universe (and we are surely one of the newest and least technologically advanced) should have already used this super‐technology to visit us. That NONE have done so surely means that the basics of relativity as we know it will remain inviolable—leaving us alone. Alternatively, if relativity can some day be overcome, then there cannot be very many civilizations out there, and we are also alone. An interesting implication of this line of argument is that if our scientists were some day to discover a way to travel faster than light, it would put a nail into the coffin of all advanced alien societies. Fermi's observation therefore suggests that advanced beings are not only not living in our galaxy, but there are not many living anywhere in the universe!

To be alone for all practical purposes means to be without any communication—or even the knowledge that any signal is coming—for a very long time. How long before we feel such solitude? For the purposes of a quantitative discussion I choose 100 human generations, practically forever in a subjective sense. This is of course an arbitrary timescale. If we choose a smaller volume, say, that accessible within only one generation, the chances of success go down by a factor of a million (!) because the number of stars is proportional to the volume of space, and scales with time (distance) cubed—but we will have a yes‐or‐no answer one hundred times sooner. If instead we want to improve the probability of success by a factor of one million, we can extend the search volume, but then the wait time goes up correspondingly to ten thousand generations. I stick with 100 generations for now, and because one generation corresponds to 25 years (and at least one round trip of messaging is necessary), in the following Drake equation calculations I constrain estimates to stars closer to Earth than 100 × 25/2 or 1,250 light‐years. We know a lot about the stars in this neighborhood.

Prejudices and expectations

The Drake equation is a set of multiplicative factors tracking the various phenomena thought to be necessary to yield intelligent life. It is not a mathematical formulation of a physical process, and every researcher who uses it breaks down the individual terms somewhat differently, but all estimate the same thing—the number of civilizations around today. At its simplest, the result is a product of five terms: (a) the number of suitable stars, (b) the number of suitable planets around such a star, (c) the probability of life developing on a suitable planet, (d) the probability that life evolves to be intelligent and (e) the typical lifetime of a civilization compared to the lifetime of its star (e.g., Smith ). The individual factors and their values have been hotly debated since Frank Drake introduced the formula in the 1950s because only the first variable could be reasonably estimated from physical evidence or extrapolated from a statistically meaningful sample, the number of solar‐type stars (although the group of “suitable stars” might include more types). The new results from exoplanet searches impact the second term and provide the basis for the following discussion.

Modern astronomers, such as Lowell before them, have generally been outspokenly optimistic that Earth‐like environments are common. Don Goldsmith and Tobias Owen, in their classic book The Search for Life in the Universe (), wrote: “Nothing in our theories for the origin and evolution of our sun is unique to the solar system.…We anticipate that all planetary systems will have a set of rocky inner planets, with atmospheres produced by outgassing, weathering, and escape, for the same reasons that our own rocky inner planets have atmospheres. Judging from our own example, the chances seem good that one of these inner planets will orbit its star at the right distance.… We say one in every two to be conservative” (Goldsmith and Owen , 384).

The latest results of astronomy show otherwise, and stunningly so. The publication and confirmation so far of over 4,000 exoplanets and their orbital characteristics, and the more detailed analyses of the 366 exoplanets whose masses and radii have been measured, together with other lines of evidence, argue strongly against the assumption of mediocrity. The single most remarkable result from the discovery of extrasolar planets is their variety: some systems have extreme, elliptical orbits, other contain giant planets orbiting very close to their stars (called “hot Jupiters”), among other unexpected properties. It is important to stress again that technology is only just now able to detect Earth‐sized planets, and those in the habitable zone of a Sun‐like star (the range of distances where the temperatures allow surface water to be liquid) will take about a year to orbit, and thus will require several years of monitoring to confirm their repeat transits.

Calculating the odds

The first term in the Drake expression is the number of stars. The Sun lies in a cavity of interstellar gas, called the Local Bubble, which extends over roughly 600 light‐years. It in turn is located in Gould's Belt, a spur of stars, star clusters and molecular clouds between two of the Milky Way galaxy's spiral arms, stretching from the Orion nebula to the Ophiuchus‐Scorpius clouds and on to the Perseus clusters—a distance of about 1,200 light‐years in its longest dimension. The approximate number of stars per cubic light‐year here is 0.004, to within a factor of two, or about 30 million stars of all types in a volume of radius 1,250 light‐years (Smith ). This result provides the first factor in the Drake equation.

Smith () expands on the second criterion above—planetary suitability—and enumerates four essential conditions for life: stability, habitability (including the presence of water), planetary mass, and planetary composition. Stability requires a host star stable in size, age, and radiative output for the billions of years needed to nurture intelligent life. It also implies a system of planets which orbit in a stable, habitable, nonmutually disruptive mode for billions of years. The habitability criterion means that a suitable planet must reside in the habitable zone of its star or else have some other mechanism to maintain liquid water. Its orbit must be stable, and sufficiently circular or otherwise unchanging so that it remains hospitable for billions of years. The third condition, planetary mass, means that a suitable planet must be massive enough to hold an atmosphere, but not so massive that plate tectonics are inhibited, because that would reduce geological processing and its crucial consequences for life. Current estimates (e.g., Ward and Brownlee ) are that planets smaller than about 0.4 Earth masses are unsuited for long‐term atmospheres; if a planet is bigger than about four Earth masses, assuming it is rocky, then planetologists estimate it will be unable to produce the plate tectonics thought necessary to refresh the atmosphere with volcanoes or other processes associated with the carbon cycle. Last is planetary composition. A suitable planet obviously must contain the elements needed for complex molecules (carbon, for example), but it also needs elements that are perhaps not necessary for making life itself but that are essential for an environment that can host intelligent life: silicon and iron, for example, to enable plate tectonics, and a magnetic field to shield the planet's surface from lethal charged winds from its star. The core of Earth remains liquid because of the presence of radioactive elements, whose heat keeps the iron molten and energizes Earth's internal temperature structure.

The last three terms in the Drake equation, (c)–(e), are biological. The probabilities associated with all these terms are very uncertain, and astronomy provides no new evidence to evaluate them. Traditional discussions tend to imagine either that intelligence is inevitable on any approximately suitable planet, or that it is unlikely. Spiegel and Turner () recently used a Bayesian statistical analysis to estimate the likelihood that life can form from inanimate matter using priors based on the few known facts: the sample consists only of us, and life arose on Earth relatively quickly (perhaps in under a few hundred million years) once Earth became habitable about 3.5 billion years after forming. Their approach presumes, among other things, that the molecular structures of life originally assembled via stochastic chemical processes in an organic soup. All known life forms, however, even viruses, are chemically large and complex. Even if the basic alphabet blocks of life, amino acids, are easily formed (something that is not entirely obvious), the chances of large numbers of them finding each other and combining to form the simplest macromolecules capable of reproduction as required for Earth‐based life forms, like ribosomes with tens of thousands of carbon atoms, are astronomically small. They conclude that “although terrestrial life's early emergence provides evidence that life might be common in the Universe if early‐Earth–like conditions are, the evidence is inconclusive and indeed is consistent with an arbitrarily low intrinsic probability of abiogenesis [life forming from inanimate matter] for plausible uninformative priors…. Our conclusion that the early emergence of life on Earth is consistent with life being very rare in the Universe for plausible priors is robust against two of the more fundamental simplifications in our formal analysis” (Spiegel and Turner , 1, 6). Other authors also have tackled a broad range of aspects of this question. Schulze‐Makuch, Schulze‐Makuch, and Houtkoopoer () investigate the likelihood of the survival of primitive life forms in hostile environments given the abundance of extremophilic life forms on Earth. All known extremophiles, however, need liquid water, are carbon‐based, and most significantly are alive.  How they came to be alive is not discussed. Irwin et al. () propose a biological complexity index for life forms, based on the astronomical and geological parameters of exoplanets, and rank the potential that some exoplanets or solar system bodies could host complex organisms, but the authors do not try to estimate the biological terms needed to get life started in the first place. Scharf and Cronin () propose a Drake‐like equation to estimate the probability of abiogenesis based on the availability of chemical building blocks and related parameters. They find that, given that there is life on Earth, the probability of an abiogenesis “event” happening in 100 million years on another Earth‐like planet (but only Earth‐sized planets have been found so far), is “poorly understood” but perhaps around 10⁻³² “when all other necessary conditions are met.” All these discussions end up assuming that life‐nurturing places like Earth are abundant in the vast cosmos and that life arises through straightforward chemical reactions, and therefore that even if life is very rare the universe is so large that even low statistics will yield many civilizations. The point of this current article is that truly suitable places for ETI might be rather uncommon, that something more than normal chemistry might be involved, and most crucially, that we are not likely to know one way or the other for a long time, perhaps forever.

Drake himself currently guesses that “only about 1 in 10 million stars has a detectable civilization” (Drake ) so in our 100‐generation volume of space comprising 30 million stars, there might be two others. Readers can make their own estimates. There will be no civilization if a star is too large or too small, if a planet's orbit or obliquity is wrong, if its size or chemical composition is unsuited, if its surface is ill equipped, if its geologic and meteoritic history is too inauspicious, if the powerful chemistry needed to generate the first life forms is too intricate or too slow, if evolution from proteins to intelligence is too often aborted or directed into sterile tangents, or if civilizations die off easily. If we are to have company in our volume of the galaxy, the likelihood on average for each of these conditions has to be pretty high—better than 20 percent. If the probability of some, such as the chances for life to form, evolve or survive, is much smaller, then even if the others are 100 percent certain, it is unlikely there are any stars near us hosting intelligent beings (see Smith , for additional details).

Statistics

One of the most important recent papers on exoplanet statistics is Burke et al.'s () “Terrestrial Planet Occurrence Rates for the Kepler GK Dwarf Sample.” (The Sun is a G Dwarf star.) The group's statistical analysis of the first four years of the Kepler mission predicts that probably 7.7 out of 10 of these stars host an exoplanet with radius between about 0.75 and 2.5 Earth‐radii and with an orbital period of between about 50 and 300 days (that is, overlapping the habitable zones). Much progress has been made on stars smaller than the Sun as well. Smith () notes that over 90 percent of stars are smaller than the Sun, many with less than one‐tenth of the Sun's mass. The MEarth Project (Nutzman and Charbonneau ) focuses on these small M‐dwarf stars which, because they are the most numerous, are possible host stars for the most numerous exoplanet populations. Berta, Irwin, and Charbonneau () completed a statistical analysis of early MEarth results and report that warm (600–700 K), Neptune‐sized exoplanets (about four Earth‐radii) probably lie around about 15 percent of early M dwarfs stars. However, it may be hard for a planet around a small star to evolve intelligent life because small stars are cooler and their habitable zones lie closer to the star. When a planet is in this closer region, it tends to become gravitationally (tidally) locked to the star, with one side perpetually facing the star. (Tidal locking keeps one face of the Moon pointing toward Earth.) But then half of the planet will be in the dark and cold, and the other half at constant noon. A related issue is the lower mass of M‐dwarf stars, which results in their interior circulation being almost entirely convective. Convective flows lead to surface magnetic fields and the production of coronal flares, X‐ray emission, and stellar winds, all of which might be hazardous to nearby exoplanets. Studies suggest that any exoplanets in the habitable zones of M dwarfs with atmospheres will be adversely affected by solar winds (Cohen et al. ). Lalitha et al. () now estimate from a statistical analysis of the X‐ray behaviors of M‐dwarfs that they typically have tens of X‐ray flares per day; the largest of these flares contain as much energy as about 10 percent of the Sun's total radiative output and can occur once every few hundred days (see also Kulow et al. ; Ehrenreich et al. ). In the Sun, energy transport in the inner region is dominated by radiative flow out to about 0.71 solar‐radii, and only beyond that is it convective (and of course the Earth is farther away).

Among the many issues to consider when evaluating the suitability of exoplanets of any kind to host intelligent life, Smith () uses as an example the case of eccentricity. The orbital eccentricity of a planet is a measure of its closest distance to the star compared to its largest distance—and thus determines the annual variations the planet receives in stellar illumination, as well as its susceptibility to gravitational orbital perturbations and disruption by other planets whose orbits might cross nearby. Thankfully, the Earth's elliptical orbit is nearly circular with an eccentricity of only about 0.017. The Exoplanet Encyclopedia () now lists the eccentricities of 754 exoplanets, almost double the number at the time of Smith (), and the percent of exoplanets with eccentricities less than or equal to the Earth's is 5.3 percent, up from the 2.2 percent in the earlier paper though still a relatively small number. The point here is that continued research on exoplanets and exobiology is essential, and there is a lot more to be learned.

Earth‐sized planets and their habitable zones

There about 391 exoplanets known with radii between about 1–2 Earth‐radii, about 30 percent. However, 98 percent of these orbit closer to their star than does Mercury in our solar system. Of particular excitement is the exoplanet Kepler‐186f which is both Earth‐sized and in the habitable zone of its M‐dwarf star (Quintana et al. ). Kepler‐186f is the fifth exoplanet found around this star, and interestingly the other four, 186b‐186e, are also Earth‐sized although not in the habitable zone. Another interesting case is Kepler‐296, a binary star of two M‐dwarfs and five orbiting exoplanets, two of which seem to be Earth‐sized and possibly in their habitable zone (Barclay et al. ).

Atmospheres

Any exoplanet hosting life would have to have some kind of an atmosphere, and detecting atmospheres has become an exciting sub‐specialty of exoplanet research. One of the earliest detections of an exoplanetary atmosphere was made with infrared measurements by the Spitzer Space Telescope, by noticing asymmetric behavior in the lightcurve of the exoplanet HD189733b as it crossed the face of its star and then passed behind, inferring that an atmosphere was responsible. A more powerful technique is transmission spectroscopy—as the exoplanet passes across the face of the star, molecules in its atmosphere absorb some of the starlight and reveal their presence. Four exoplanets have so far had their atmospheres measured in this way, revealing that haze and/or clouds may cover the surfaces. Naturally, there is keen interest to see if any biomarkers can be spotted; these are molecules like oxygen, nitrous oxide, and methane; other important molecules (though not particularly indicative of the presence of life) are water, carbon dioxide, and nitrogen (e.g., Seager ). DeWit () estimates that there are likely to be hundreds of Earth‐sized exoplanets in habitable zones with atmospheres accessible to such spectroscopic measurement in the next decade. Heng and Showman () review the atmospheres of hot‐Jupiters, which as noted in Smith () are easier to detect but perhaps less likely to host life. The results so far find atmospheres relatively free of clear molecular indicators, suggesting that scattering by clouds dominates.

In summary, the field of exoplanet research has exploded in the past few years, and is likely to continue to grow in drama as well as in important data. In this author's view, one unfortunate aspect of this recent progress, especially as typified in the popular reporting of Earth‐sized planets or others in their “Goldilocks” habitable zones, is the nonsequitur hyperbole about ETI, as for example in the New Scientist headline: “What if … We are not alone?” (5 August 2015). As the new results reviewed in this section illustrate, the prospects of finding ETI remain (at best) low.

ETI in the future universe

Astrophysicist Avi Loeb has noted that about 10 million years after the creation of the universe in the Big Bang, the temperature in the cosmos cooled to room‐temperature and was suitable for life in the same sense as the habitable zone requirement for temperature: water can be liquid (Loeb ). Loeb does not suggest, however, that intelligent life could exist in the early universe. Even people wary of presuming that all intelligent beings resemble us, for example in their atomic composition, and so on, generally agree that, at the very least, all intelligence requires complexity. This is the reason why almost certainly atomic carbon will be the building‐bone of such beings, because of all the elements only carbon can form a wide array of extremely complex molecules, DNA being just one example. But however it is structured, complexity takes time to develop (in our own case it took a few billion years) and there is not enough time in the early universe for ETI to form. Allowing enough time for complexity to develop is precisely why a planet's long‐term stability is such a critical requirement for ETI.

The universe is vast. The size of the universe that we can see (our cosmic horizon, set by the distance light could travel in the age of the universe, 13.8 billion years) is currently about 80 billion light‐years (the universe has expanded significantly since the light left these distant regions headed toward us). If there is ETI scattered throughout the universe, we have shown above why we probably will not know one way or the other for a very long time. Indeed, for most of the universe—the part on the other side of our horizon—we will never know even if we wait forever because it is accelerating away from us. Moreover, the longer we do wait, the more of the universe accelerates across the horizon and is lost to us into the unknowable beyond. In the context of considering cosmic time, however, we can also consider (albeit in a very cursory fashion) the possibilities of ETI developing locally (for example, in the Milky Way) in the future. Our Sun and stars like it are slowly exhausting the hydrogen gas they burn. As has been well described in the literature (e.g., Sackmann, Boothroyd, and Kraemer ) the Sun will swell in size as it ages, eventually enveloping all the planets out to Mars, and in about 5 billion years will leave the so‐called main‐sequence and evolve rapidly towards its eventual death as a white dwarf star in another billion years or so. There is time, therefore, for new life to evolve elsewhere in the outer solar system, perhaps in subterranean water that might be located in the moons of Jupiter or Saturn. The rapid changes in illumination as the Sun ages, however, make the stability requirement for ETI very difficult to achieve. The situation is roughly the same for other Sun‐like stars elsewhere in the Milky Way. Less massive stars will remain on the main‐sequence for very much longer times, but also as noted above the closer‐in habitable zones of these M‐dwarfs pose risks of their own for ETI. New stars are being formed in the galaxy, currently at a rate of about one per year (the current stellar population is about 100 billion stars). In another few billion years, if current trends hold, there could be a few billion new stars and their planetary systems are possible hosts for ETI.

The Milky Way is part of a local group of galaxies, bound together by their gravity, with the most massive other member of the group being the Andromeda Galaxy. Our two galaxies appear to be gradually falling together in a humongous collision (Cox and Loeb ). Within the current lifetime of the Sun these two galaxies will merge—well before the Sun makes it to old age—and observations (and simulations) of other currently merging galaxies suggest the merger process is extremely disruptive, though not necessarily fatal. Individual stars are so widely spaced that they will practically never collide, but gravity will sling them out of their normal galactic orbits onto unfamiliar paths. Our own collision with Andromeda will cast the Sun out to the outer halo of the galaxy (Cox and Loeb ). Although our planetary system, and even some neighboring stars, might perhaps remain bound together, the long, stable era of steadily orbiting our spiral galaxy's center will be ended. Simulations show that after a few billion years more the merger settles down, and some new stars will continue to form, although as the hydrogen gas is used up or disbursed the rate of star formation drops steadily.

Complex elements are a requirement for life, as noted above. Several authors (e.g.,Ward and Brownlee ; Smith ) point out that these “metals” are not uniformly available across the galaxy, depending on the local star formation activity that makes them. A collision with Andromeda will enhance the star formation rate substantially for a few hundred million years and certainly will result in the production of more of these needed elements. After a few billion years more, however, the rate of making new stars drops substantially and then stays low. There is a catch to this initial enhanced production of elements: the Earth relies on radioactive elements to heat its interior, a key factor in the development of life here. Elements like uranium‐238 and thorium‐232 are made in supernovae, and supernovae are also a product of star formation. Fortunately one went off in the neighborhood of our Sun just before it formed. But unlike elements that accumulate, these radioactive elements decay and disappear; they are the result of the star formation rate, which drops dramatically after a merger. After 10 billion years or so, these isotopes will no longer be available to heat planetary interiors. Ultimately, after hundreds of billions more years, our galaxy and its neighbors will increasingly be made up of black holes and the cold ashes of dead stars, while meanwhile the expanding universe sweeps other systems even farther away or beyond our horizon altogether. In this cold and lonely future, ETI is not necessarily excluded from developing, at least for a while, and current ETI may even find ways to survive it, but the long‐term future prospects for ETI are even less auspicious than the current ones.

The Misanthropic Principle

The cumulative results from exoplanet studies summarized above reinforce the basic conclusion reached in earlier analyses (Smith ): we are most probably alone—at least there is probably no other intelligent life within 100 generations reach to talk with. For all intents and purposes, we and our descendants for at least 100 generations are very likely living in solitude. I call this the Misanthropic Principle (Smith ). The Anthropic Principle expresses the observation that the physical constants in the cosmos are remarkably finely tuned to make it perfect for hosting intelligent life (e.g., Barrow and Tipler ; Davies ). The Misanthropic Principle expresses the idea that the multiplicity of possible environments in this suitable cosmos is so varied and uncooperative (or hostile) either always or at some time during the roughly 3–4 billion years intelligent life needs to emerge, that it is extremely unlikely for intelligent life to form and thrive.

I have only considered a volume of space that is 100 generations of light‐travel‐time across. As noted earlier, by expanding the volume of space by a factor of a thousand, or more, the chances for finding intelligent life are of course correspondingly higher, but then neither we nor our children are likely to know for even longer. In this sense the term Misanthropic Principle reflects not only the inhospitable nature of the universe to intelligence, but also the way it seems we must live, alone with our uncertainties, our doubts, and ourselves. However, opposite to the dour connotations of misanthropy, the Misanthropic Principle is joyous. We should rejoice in our good fortune. Atheists and religious people alike can also identify in it an expression of pride in humanity and (as I will argue later) even an acknowledgment of our cosmic competence.

It used to be thought, until science proved otherwise, that we were the center of the universe and everything orbited around us. This is no longer anyone's way of thinking, even metaphorically. From the Copernican revolution onward, people have gradually come to realize that in most ways we are utterly ordinary, neither at the center of the universe nor even of our solar system. Our bodies are made from the chemical detritus of stars, and we (like all life) evolved from simpler organisms through apparently contingent processes that in our case took billions of years. One consequence of this aspect of self‐awareness has been the popular nonsequitur presumption (even hope!) that ETI exists, and should exist. The theologian and astronomer Thomas Dick succinctly expressed this logic in 1826 when, after presenting his theological reasoning, he summed up his logical argument: “There is an absurdity involved in the contrary supposition” (quoted in Crowe , 261). The Misanthropic Principle, the result of current research and basic physical constraints, offers a new and more honest perspective: ETI is almost certainly nowhere nearby, and its possible existence elsewhere is for all practical purposes unknowable to us (at least for a very long time, if not forever). Speculation about its nature is consequently specious and irrelevant. Very likely we are alone.

The Misanthropic Principle raises three acute dilemmas that have not previously been carefully explored: epistemological, theological, and ethical. The epistemological dilemma is clear: Not knowing about the existence of something does not mean it does not exist. Until we hear a clear signal from beyond, or until our science has progressed far enough to provide some kind of all‐embracing and conclusive answer (although the nature of such absolute evidence is hard to imagine), humanity is left in an existential quandary. Religion of course makes the point that we are not spiritually alone (although religion has its own ontological uncertainty). In this environment of necessary ignorance, how should scientists and theologians respond to the many people for whom the prospect of being “alone,” without hope for salvation or comfort from a super‐intelligent species abiding in heaven, is frightening?

The theological problem is particularly grave for scientific reductionists, but most people will share some of the angst. We moderns are nearly convinced by the Epicurean argument that we are a randomly evolved collection of atoms. But if we might be unique (at least as far we will know) then must we not reconsider the possibility that we are not an accident but were created by some kind of intent, even for some purpose? When the Anthropic Principle directly challenged the Epicurean approach, scientists responded by positing a multiverse. There has not yet been a scientific rejoinder to the Misanthropic Principle. I imagine that it will take the form of a wait‐and‐see attitude, even if it takes a few millennia of living with nagging doubt. The theological dilemma is also present for religious believers, who have spent centuries showing that Divine potency or grace should naturally imply many extraterrestrial civilizations, but who now must justify the need for the vast, excess cosmos beyond human reach (e.g., Crowe ). For both groups, the default explanation is that a vast universe is just the consequence of natural laws. Religious believers, like the Epicureans, might also decide that a wait‐and‐see attitude is the best response.

The ethical dilemma, however, cannot wait. The Earth itself is under stress, and humanity faces growing misery. If we are merely a collection of evolved atoms, then these issues are of no great concern: there is life elsewhere, distributed among the stars, along with many salubrious, Earth‐like planets. Possibly some will survive; perhaps that is enough. We have no special status and no particular purpose. But if—as far as we are likely to know for eons—we are alone, then perhaps, just perhaps, the above is not true, and neither we nor our planet are products of common happenstance. The Earth and its life have value. The prospect brings great urgency to the cause of protecting this rare planet and all of its precious inhabitants.

What are we going to do?

But we might be unique! And even if we are not unique in the universe, even if intelligent aliens thrive throughout the galaxy, there are probably none nearby and we are unique in our cosmic sphere of influence. We are unusual, and we are certainly blessed. Since Biblical times, added blessings or favors entail added responsibilities, with consequences when those responsibilities are shirked. In particular, responsibility includes the obligation to deal compassionately with other beings and to attend to the welfare of community and its environment.

The Jewish view of the state of being blessed (having gifts) or even of being chosen (having special gifts) offers some insights into the three dilemmas raised earlier. The epistemological issue has no practical implications and is not particularly problematic: Whether or not we know about others, we know about our own blessings. Indeed, says the prophet Amos, urging humility: “Are ye not as children of the Ethiopians unto me, O children of Israel? saith the Lord. Have not I brought up Israel out of the land of Egypt? And the Philistines from Caphtor, and the Syrians from Kir?” (Amos 9:7). There might be other intelligent beings, but our obligations are independent of theirs. The theological issues are similarly unproblematic. The absence of ETI in our sphere of influence only enhances our self‐awareness of our peculiar status, and their possible presence does not diminish it.

The ethical dilemma is the one for which I think a Jewish perspective is the most helpful. Our exceptional status on Earth, and our new‐found awareness of this probable good fortune, should make us more sensitive of our task “to serve the Earth and to protect it” (Genesis 2:15). “When you live in the land that flows with milk and honey…you shall therefore obey the voice of God and keep his commands” (Deuteronomy 27:3). The Jewish perspective not only asserts that we should try, it emphasizes that we have the skills to succeed. Our task is possible. Maimonides (1135–1204) presents the case in his Guide for the Perplexed: The very fact that a loving God has commanded us to behave ethically proves that we have been empowered to succeed if we so choose (Friedlander , 386). The Kabbalists and Jewish mystics wove a deeper layer of meaning into this perspective, arguing that responsibility and caring were not only important to self and society, they were essential to the very welfare of the cosmos. Humanity actually plays a role in perfecting the world—called tikkun olam (e.g., Matt ). Quantum mechanics include the still incompletely understood implication that the world and its matter are composed of wave functions of probability that only become real entities upon being measured. Physicists have long speculated that measurement by a conscious observer is what leads to this “collapse of the wave function.” Some, most famously John Wheeler, have suggested that the universe created conscious beings in order to observe it and thus bring it into reality (e.g., Smith ). It is in this quantum mechanical sense (even when considering the important process of decoherence) that our consciousness is much more than a mere chemical accident. If we are truly alone in the observable universe, then we play a crucial—not a peripheral—role in the cosmic order, and moreover in a Maimonidean sense the cosmos has empowered us to succeed in this task if we so choose. As for ETI, if intelligent extraterrestrials exist, they like the nations mentioned by Amos presumably have their own moral codes with intrinsic value. If their codes happen also to include, for example, an injunction about a Sabbath, that would certainly be a remarkable suggestion of universal ethics, implying that we are partners in a larger cause.