THE Ĝ INFRARED SEARCH FOR EXTRATERRESTRIAL CIVILIZATIONS WITH LARGE ENERGY SUPPLIES. I. BACKGROUND AND JUSTIFICATION

The fact that we have not been contacted by any extraterrestrial intelligence (ETI) is most famously encapsulated by Enrico Fermi's question "Where is everyone?" (see Jones 1985 for a historical account). Similar observations are "Fact A" of Hart (1975) ("There are no intelligent beings from outer space on Earth now.") and the apparent lack of any evidence of their communication (the "Great Silence" of Brin 1983).

If one assumes that interstellar communication and travel are not so difficult as to be practically impossible, and one holds the Copernican⁶ position that intelligent life is not unique to Earth, then one is led to an apparent contradiction. Given the number of potential sites for intelligent life to emerge in the Galaxy (on the order of 10¹¹ stars) and the amount of time it has had to arise (on the order of a Hubble time, 10¹⁰ yr), then one is led to the so-called Fermi–Hart Paradox or Problem that ETIs should be prevalent throughout the Milky Way, and yet we see no evidence of them.

Resolutions of this problem involve contradicting its various assumptions. The most pessimistic resolution to the paradox is that of Hart (1975): we are alone in the Galaxy, or at least ETIs are sufficiently rare that none has been inclined to engage in widespread interstellar travel. As searches for ETIs become increasingly more sensitive, this resolution becomes increasingly favored. More optimistically, it could be that ETIs are common, but never sufficiently spacefaring to be widespread throughout the Galaxy; or it could be that spacefaring ETIs are common, but we have not yet noticed them for some reason.

In this work, we will review and reinforce Hart's basic thesis, which powerfully argues that once spacefaring ETIs arise, they should populate their galaxy on a timescale short compared to the Hubble time.

The waste heat approach is complementary to other efforts in the search for extraterrestrial intelligence (SETI), especially communication SETI, because it makes different assumptions and can help focus them to likely targets, but might not by itself rule out purely naturalistic phenomena. Among the advantages to a waste heat approach is its small number of assumptions about ETI behavior and technology. Assuming only conservation of energy, the laws of thermodynamics, and that much-faster-than-light travel is impossible, we can test for the existence of ETIs with very large energy supplies in other galaxies, out to great distances.

This work provides background and motivates our search for alien waste heat, which we call Ĝ, the Glimpsing Heat from Alien Technologies survey (or G-HAT), and which we describe in Wright et al. (2014, hereafter Paper II).

We first discuss why civilizations with large energy supplies are plausible and worth searching for. Because SETI is not a common topic in the astrophysical literature or curriculum, this paper is primarily a discussion of previous work most germane to Ĝ.

In Section 2, we discuss the two primary forms of SETI, communication SETI and "artifact" SETI, and some philosophical difficulties in searching for something whose form is uncertain.

To illustrate that galaxy-spanning civilizations are plausible, in Section 3, we review Hart's argument that spacefaring life should rapidly spread throughout the Galaxy.

In Section 4, we extend this argument to the growth of an ETI's total energy supply, and show that it implies that very old, large ETIs in other galaxies should be detectable with today's mid-infrared (MIR) surveys, such as that recently conducted by the WISE satellite, following the waste heat approach of Dyson (1960) and Slysh (1985). We also provide a description of the classes of intelligent spacefaring civilizations a waste heat approach would detect (which we call a "physicist's definition" of intelligent life). Ultimately, this approach will allow humanity to extend our search for ETIs (and, perhaps, Hart's "Fact A") beyond the lack of evidence of ETIs in the Solar System to well beyond the local universe.

In Section 5, we show that previous work on the maximum time it takes such a species to populate its galaxy is in error. In fact, this is actually quite short compared to the age of the Milky Way, which reiterates and amplifies this part of Hart's argument.

In Section 6, we apply our reasoning to some classes of rebuttals to Hart's thesis, in particular those that invoke "sustainability" to explain how the Milky Way could be inhabited with many spacefaring civilizations but not be saturated with them.

In Section 7.1, we discuss the physical plausibility and detectability of Dyson spheres, and in Section 7.2, we discuss past searches for them.

In Section 8, we discuss the consequences of a positive and a null detection of large alien energy supplies, and in Section 9 we give our conclusions to this first paper in the Ĝ series.

Much work in the SETI (see Tarter 2001 for a review) focuses not on finding evidence of ETI travel to the Solar System, but on detecting ETI communication (either deliberate messaging or "leaking" of deliberate signals not intended for us). This approach is in principle sensitive to any ETI that has achieved a level of technology similar to ours, and does not require that ETIs participate in interstellar travel; however, it does require that ETIs produce signals detectable by today's technology. It thus essentially ignores Hart's argument that ETIs would have populated the Galaxy by now (neither denying nor accepting it) and seeks unambiguously engineered signals that could only be the product of an ETI.

Directed programs in SETI have helped to carve out particular niches in the parameter space of ETI behaviors we can imagine. The most popular strategies or proposals for SETI have historically centered on broadcasts by an ETI within particular bands of the electromagnetic spectrum we would consider obvious or desirable (see Sagan & Drake 1975 for examples of recommendations). However, no ostensible signatures of ETI transmissions have been detected in wide radio bands (Latham & Soderblom 1993), in narrow bandwidths centered on the hyperfine transition of H i (Horowitz & Sagan 1993; Lazio et al. 2002), or convenient multiples of this frequency (Blair et al. 1992; Lazio et al. 2002). The same is true for different atomic and molecular transitions of other substances (e.g., H₂O and OH: Cohen et al. 1980; CO and CN: Davidge 1990; positronium: Mauersberger et al. 1996). Siemion et al. (2013) searched for narrow-band radio emission from Kepler-surveyed stellar systems with known exoplanets and extrapolated that the number of radio-loud civilizations in the Galaxy is ∼10⁻⁶ M_☉⁻¹.

Alternatively, Keenan et al. (1999) contended that optical SETI strategies—e.g., hunting for nanosecond pulses from megajoule-scale optical lasers—are superior. These campaigns have thus far been unsuccessful; however, see, e.g., Howard et al. (2004). More exotic strategies have been proposed (e.g., Korpela & Howard 2008); these include hunting for signs of artificial modification of planetary albedo, searching for intentionally generated gravity waves, and researching the utility of quantum entanglement as a communication pipeline. Others employ analyzing the CMB as a potential signal carrier (Kardashev 1979) or hunting for collimated transmissions at millimeter wavelengths (Kardashev 1985 and references therein). These remain achievable in principle, but as of yet are unrealized in practice.

However, the total parameter space searched for alien signals remains very small (see, for instance, Papagiannis 1985), so the failure of communication SETI to date rules out only current communications of certain sorts, of certain powers, at certain duty cycles, and in certain places. Thus, today it provides very weak evidence for the non-existence in the Milky Way of electromagnetically telecommunicating civilizations (to use the term of Blair et al. 1992).

An alternative strategy is to identify artifacts or "archaeological" signatures (e.g., Carrigan 2012) of ETIs (somtimes called the "Dysonian" approach, e.g., Bradbury et al. 2011; Vidal 2011).

The most famous example of circumstellar mega-engineering is that of a "Dyson sphere" (Dyson 1960), a structure or suite of structures that block a star's light (as an energy source or a living surface) and reradiate it in the thermal infrared. We find these to be of particular interest: a civilization of sufficient technological sophistication has an obvious source of abundant free energy over ∼Gyr timescales in its host star. A typical Sun-like star provides ≈4 × 10²⁶ W throughout the course of its ∼10 Gyr lifetime (see our current energy generation on the order of 10¹³ W). By building an array of light-gathering structures around the star, the ETI could harness this free stellar power for its own purposes.

Such Dyson spheres need not be single, rigid structures⁷; the actual covering fraction of a swarm of steerable energy collectors constituting the practical "sphere" might vary, and therefore the resulting SED we receive may contain time-variable contributions from the unobscured starlight. "Complete" Dyson spheres would certainly be more easily recognizable, however. Sagan & Walker (1966) and Kardashev (1979) showed that the IR emission of opaque Dyson spheres (with peak wavelengths 15 μm–2 mm) was detectable to ∼ several 100 pc by the technology of the time.

Another approach would be to observe the effects of alien industry on its environment. Freitas (1985) proposed that particularly rapacious civilizations could strip Solar Systems of hydrogen fuel for fusion, creating circumstellar effusion clouds that are detectable in the H i and tritium hyperfine transitions. Other ambitious engineering projects are theoretically detectable (Carrigan 2012), including artificially increasing stellar lifetimes by manipulating stellar rotation, internal pressures, and/or depositing hydrogen fuel (possibly producing blue straggler stars), terraforming planets, or otherwise altering a planet's atmospheric composition with known byproducts of industrial and otherwise intelligent activity. Forgan & Elvis (2011) explored how ETI asteroid tampering could be observable in the SEDs of debris disks, and showed how such engineering projects in progress could be identifiable by anomalous chemical abundances of a mined debris disk, the thermal signature produced by waste dust, or the debris distribution itself as a dynamical signature of ETI intervention.

Another approach is to observe products of alien mega-engineering themselves. For instance, Arnold (2005) postulated that ETIs might construct long-lived beacons in the form of planet-sized shields or louvred structures in short-period orbits around stars.⁸ Such objects would consume little power (and, at any rate, have an abundant supply of energy nearby) but would effectively transmit simple signals across the Galaxy in the form of manifestly non-natural transit light curves (i.e., they would have obviously non-circular aspects). Such structures would thus also be a form of communication SETI.

Efforts to identify known phenomena that have no natural explanation and postulate why ETIs might be the most natural cause have a major difficulty that follows from Clarke's so-called "Third Law" (Clarke 1962), which states, "Any sufficiently advanced technology is indistinguishable from magic."

Science, by definition, assumes that the universe is governed by natural law and seeks to interpret all observations as consequences of these laws. "Magic," in Clarke's sense, means the apparent suspension of natural law, and so any investigation that assumes that an observation is due to a sufficiently advanced technology risks being inherently unscientific. In particular, invoking ETIs to explain any (or all) unexplained phenomena leads to an "aliens of the gaps" approach, which is philosophically problematic. Such ETI hypotheses may be difficult or impossible to disprove, and have little or no justification beyond the facts that an observation remains unexplained by natural causes, and that ETIs may exist.

Of course, the other extreme—assuming that all observations must be the result of purely natural phenomena are not due to advanced technology—is itself patently logically invalid because it assumes that ETIs have no detectable effect on the universe. SETI must thread this philosophical needle if it is to resolve Fermi and Hart's problem in a scientific manner.

Artifact SETI can thus proceed by seeking phenomena that appear outside the range that one would expect natural mechanisms to produce. Such phenomena are inherently scientifically interesting, and worthy of further study by virtue of their extreme nature. The path from the detection of a strange object to the certain discovery of alien life is then one of exclusion of all possible naturalistic origins. While such a path might be quite long, and potentially never-ending, it may be the best we can do.

Communication SETI, on the other hand, shortcuts this path to discovery by seeking signals of such obviously engineered and intelligent origin that no naturalistic explanation could be valid. Together, artifact and communication SETI thus provide us with complementary tools: the most suspicious targets revealed by artifact SETI provide the likeliest targets for communication SETI programs that otherwise must cast an impossibly wide net, and communication SETI might provide conclusive evidence that an extreme but still potentially naturalistic source is in fact the product of extraterrestrial intelligence (Bradbury et al. 2011).

While Hart had to argue that interstellar probes were well within the realm of technical feasibility, barred by neither time nor energy consumption limitations, we can do better today. Voyager 2 and both Pioneer probes have traveled beyond the orbit of Neptune, and once they and New Horizons join Voyager 2 in crossing the heliopause, they will all properly be called interstellar (Kerr 2013). At their current speeds and distances from the Sun, they will require on the order of 10⁵ yr to reach distances comparable to the nearest stars (although they are not traveling toward any of them). Although this time is long, it demonstrates that interstellar travel by human-built probes is certainly possible.

These probes would certainly no longer have functioning power supplies by the time they might arrive at an alien planetary system, but this is a function only of their engineering. Hart correctly points out that nuclear energy could easily provide not just sufficient operational energy, but indeed thrust for deceleration for more advanced probes.

The velocities of our existing interplanetary probes are large (on the order of 10⁻⁴ c), but there is no engineering or physical reason why they could not be at least an order of magnitude larger. Given the possibility of centuries, millennia, millions of years, or hundreds of millions of years of future human technological advancement, it is clear that these spacecraft represent an extreme lower limit to the speed and utility of interstellar probes.

A more ambitious approach would be to send colony ships to other star systems; this additional complication adds considerable engineering difficulty, but, again, no fundamental physical obstacle, and indeed advances in biology and medicine over the last 40 yr, have made such approaches more plausible. Whether the ships would, for the long cruise times of their journeys, contain conscious colonists, humans in some sort of "suspended animation", or merely the machinery for gestating and raising humans from embryos, gametes, or even just genetic material, varies only the degree of complexity of the problem. The fact that we can today imagine and even design ships that could do this implies that the barriers to their construction are a matter of will and achievable technological development, not fundamental limits of physics or engineering.

3.2.1. The "Monocultural Fallacy"

3.2.2. Sustainability

Given the vastness of the distance between the stars, and the apparent technological implausibility of sending a probe to even the nearest star within the span of even a long-lived human, it may seem absurd to consider a galaxy-spanning supercivilization plausible, but the timescales for galactic colonization are actually quite short compared to the age of the Galaxy, even for civilizations with "slow" travel capabilities. Indeed, optimistic assumptions lead to very short minimum colonization times: for instance, Ćirković (2009) and Tipler (1980) calculated that galactic colonization should only take $\mathcal {O} (10^6)$ years, and Armstrong & Sandberg (2013) argue that entire galaxy groups can be populated in much less than a Hubble time.

There are two primary timescales to consider: that for the first civilizations to arise in a galaxy, and then for colonization of the galaxy.

3.3.1. Timescale to the Rise of Civilization

3.3.2. Timescale for Galactic Colonization

Hart's original argument centered on his "Fact A," that we have no evidence of ETIs here in the Solar System today. That is, that the Solar System has not yet been colonized. His fourth class of resolutions to the Paradox, "Perhaps They Have Come," is that they have colonized the Solar System (or at least visited it), but we have not noticed. Hart assumed that it is unlikely that the Solar System has been visited but not colonized: in the history of the Galaxy it would only take one passing colony ship to permanently inhabit the Solar System (or, at least, permanently scar it with its civilization), and given the timescales involved, we should have been visited many times over in the history of the Solar System. Indeed, it is hard to imagine how we might miss evidence of ETI colonization of the Solar System, even if they happened to go extinct before present day (but see, for instance, Haqq-Misra & Kopparapu 2012).

Waste heat searches may be sensitive to what we call to be a "physicist's definition" of life: matter that processes resources and energy to produce more of itself. We acknowledge that this definition is so broad as to potentially include abiological self-replicating phenomena such as crystals, but it needs to be broad so that we do not exclude exotic forms of life that might exist in the universe that we merely have not yet considered. Despite this, even this definition may not be broad enough to encompass all forms of life in the universe.

Our "physicist's definition" of intelligent life is life that can overcome local resource limitations through the application of energy. This roughly tracks with our intuitive understanding of intelligence. A barnacle on a whale has no volition or ability to acquire food; it can only instinctively collect food that comes to it. A whale, on the other hand, can expend energy to hunt for additional food. Octopuses can apply torque to a lid of a jar to extract the food within; birds can drop shells to break them and acquire the food inside; humans can desalinate sea water and fertilize crops to create new sources of fresh water and food.

Taken literally and applied over long timescales, this definition of intelligence might be so rough as to encompass non-intelligent behavior, especially if applied to a group of organisms as a whole (such as ant colonies, or even a grove of trees that spreads to cover a hillside, thus gathering additional solar flux), and it can be unclear to what degree instinctive behavior in an individual can be properly called intelligence.

Of course, intelligence is not a binary proposition: humanity's capacity for overcoming local resource limitations far exceeds that of cetaceans, and certainly dwarfs that of trees. So despite their lack of rigor, these definitions are useful from the perspective of the search for intelligent life in the universe. If an intelligent species is spacefaring, then it will, by definition, consume resources with the possibility of exponential growth, and it can overcome a local lack of resources by acquiring additional energy by leaving its home planet (or other place of origin).

We argue that if a species is spacefaring, then its level of intelligence is such that there is no practical resource limitation that it cannot overcome, except that of energy. On Earth, apparent limitations of arable land, fresh water, and energy have been consistently pushed back as technology has advanced. Desalinization and hydroponics illustrate how, in principle, our food supply is limited only by our energy supply and our will to apply it. While it is true that Earth's population may stabilize in the near- or medium-term future for sociological reasons, this does not necessarily have any implications for the long-term future of the species as it expands into space.

For brevity and specificity, we will use these definitions of "intelligence" and "life" for the rest of this paper, but we acknowledge that there may be other forms of intelligence in the universe not captured by our "physicist's definition." Naturally, a waste heat approach will not be sensitive to such life.

In the limit that a species or civilization uses its entire local energy supply, we can refer to it as a free-energy-limited species. "Free energy" here refers to the amount of work that can be extracted from an energy source, when that work is performed at a given operating or ambient temperature. Since energy must be conserved, all energy used to do this work must be expelled, usually as waste heat, with a higher entropy than it had before it performed the work, unless it is stored.¹⁰

There are many definitions of free energy, reflecting the complication that the precise amount of work that can be done with a given energy source will depend on the nature of the mechanism doing the work, and how it changes its environment. Because we are making an order-of-magnitude argument, and to avoid committing ourselves to particular models of alien technology, we will use the concept generally.

The term "energy-limited life" is usually applied to parts of the Earth's biosphere with barely enough free energy to sustain any terrestrial life at all, and so what life there is has its metabolic rate limited by free energy, not some other resource.

Intelligent life, being able to apply energy to overcome every other resource limitation, has the potential to become free-energy-limited on a much larger scale. Such a species could still grow its energy use (by increasing its efficiency), and it may be able to expand to acquire additional free energy (from other stars, for instance), but we do not expect to be able to detect much low entropy energy from it (because it is using as much as it can), and we should expect to detect nearly its entire energy supply as it is reradiated as waste heat.

It is perhaps likely that no species can ever become truly free-energy-limited, but any species that approaches this limit (say, consuming more than a few percent of the available free energy) will have a marked and detectable effect on the MIR luminosity of its host planet, star, or galaxy. As we will see, a truly free-energy-limited species would have a profound and easily detected effect on its host star(s), and a thorough waste heat search with currently available data can rule out certain classes of free-energy-limited species out to intergalactic distances.

Life on Earth has spread nearly everywhere. A majority of its surface has life, so a test for metabolic processes would succeed on most randomly selected samples of Earth's soil or water (in some cases even the ice, air, and deep Earth would pass such a test). The sunlight that lands on Earth is used directly by vegetation across the globe (and indirectly by organisms higher on the food chain), and even more of this energy is used passively by life because it warms the environment and drives the wind, weather, and climate upon which many species rely.

Obviously, exponential growth is limited in individual species by a variety of factors, but the biosphere considered as a whole has managed to expand the amount of solar energy captured for metabolism to around 5% (Paper II), limited by the nonuniform presence of key nutrients across Earth's surface—primarily fresh water, phosphorus, and nitrogen. Life on Earth is not free-energy-limited because until recently, it has not had the intelligence and mega-engineering to distribute Earth's resources to all of the places solar energy happens to fall, and so it is, in most places, nutrient-limited (see, for example, discussions in Reich et al. 2006; Peñuelas et al. 2011).

An intelligent species by our "physicist's definition" could apply some of the unused solar energy to redistribute these limiting resources, and so expand life's footprint beyond what 4 billion years of evolution has managed. Indeed, artificial fertilization and irrigation are exactly this redistribution, and we rightfully identify the development of these methods as a turning point in human history and a hallmark of civilization. Today, humanity has made photosynthetic activity common in many nominally inhospitable parts of the American southwest and Middle East, for instance. If we consider our own energy generation as part of the biosphere's metabolism, then photovoltaic arrays, wind farms, and other forms of solar power stations represent further extensions of this trend.

Non-intelligent life on a planetary scale is not directly detectable from its waste heat because the ground beneath it would absorb and reradiate thermal energy even in the absence of life. However, spacefaring intelligent civilizations, by our definition of them, are able to extend beyond the spatial limits where planets naturally re-radiate starlight to their circumstellar and pan-galactic environments, and so are susceptible to searches for their waste heat.

Humanity's ever-increasing energy demands drive the exploitation of increasingly more sources of energy with increasingly higher degrees of difficulty. Even if one country, today, cannot increase its energy supply, or simply declines to increase it, this does not mean that humanity as a species will forever cease to increase it. Indeed, a major difficulty today is not how humanity might continue to increase its energy supply, but how to manage sufficient international cooperation even to limit its fossil fuel consumption in the face of ecological catastrophe.

It is instructive to note along these lines that humanity's energy supply has doubled in the past 30 yr. Left unchecked, such a doubling rate would imply an energy supply equal to the total incident solar flux on the Earth in ∼400 yr, at which point direct heating of the Earth would create a significant increase in temperature. If this energy generation were accomplished entirely from non-solar sources, then the equilibrium temperature of the Earth would increase by ∼50 K, rendering it essentially uninhabitable. If accomplished from solar sources, the interception of such a large fraction of sunlight would have dramatic consequences for climate and the biosphere.

From this we can conclude that if no other limits or global cooperation intervene, humanity's energy supply on Earth must maximize at some point in the next few centuries, having effectively saturated the planet's capacity for waste heat. However, it would be incorrect to conclude that this somehow invalidates the conclusion that energy supply growth may be inevitable. On the contrary, this calculation illustrates how rapidly, on a cosmic timescale, humanity's energy needs can approach fundamental limits (see, e.g., von Hoerner 1975). As humanity expands into space, these limits will expand dramatically, and as humanity's technology level increases, its ability to approach them will only improve.

As we have discussed, percolation and automata theory has been a popular approach for estimating the timescale for Galactic colonization (Hair & Hedman 2013; Landis 1998; Vukotić & Ćirković 2012). In these approaches, one typically considers a static network of stars, and so many of these models do not account for Galactic shear and the physics of halo orbits in their calculations. In particular, Newman & Sagan (1981) found that colonization times could reach ∼10¹⁰ yr for slowly growing civilizations because they were limited to expand along their frontiers, and that they eventually "outgrow" their colonization phase (the latter is also an example of the monocultural fallacy).

The static model of stars, in which a supercivilization can be said to occupy a compact and contiguous region of space, is a reasonable approximation for short times (≲ 10⁵ yr) and in the case of fast ships (with velocities in significant excess of the typical thermal or orbital velocities of the stars, so ∼10⁻² c). In such cases, the stars essentially sit still while the ships move at a significant fraction of c and populate a small region of the Galaxy in some small multiple of the region's light-crossing time.

However, for longer times or slower ships, the model fails badly because the stars in the Galaxy are not static, even in a rotating Galactic frame.

Let us consider conservative timescales involved for a "slow" civilization in the Milky Way. In the interest of calculating a maximum Galaxy colonization time, we will assume a single spacefaring civilization arises, and that it is confined to launching colony ships traveling at 10⁻⁴ c (i.e., comparable to the orbital speeds of the planets in the Solar System, and to that of our interplanetary probes). To be further conservative will assume zero technological development for the homeworld and the diaspora after the launch of the first ship. Finally, we will further conservatively assume that such ships are launched at a very slow rate, every 10⁴ yr, and have a maximum range of 10 pc.

Travel time to the nearest stars is then on the order of 10⁻⁴ c/1 pc ∼ 10⁵ yr, in which time ∼10 ships will be launched. Since this travel speed is also comparable to the velocity dispersion of stars in the Galactic midplane, this timescale also describes the time it takes for stars to mix locally, bringing new stars into range of the colony ships. To first order, the stellar system can thus continue to populate the 10 nearest stars every 10⁵ yr, without immediately saturating its neighborhood with colonies or the need to launch faster or longer-lived colony ships to continue its expansion. Further, arrival of the colony ships at the nearby stars should not be modeled as a pause in the expansion of the civilization. Rather, the colonies themselves will continue to travel at these speeds with respect to the home stellar system, and themselves encounter fresh stars for colonization every 10⁵ yr, during which time they can also launch 10 colony ships.

Since the time to launch a new colony ship in our model is shorter than the cruise times, the expansion rate of the supercivilization is set by the velocity dispersion of the stars, not the speed of the ships or the time it takes a colony to generate a new ship. Halo stars further shorten galactic colonization timescales because of their high relative velocities to the disk (of order 10⁻³ c). Halo stars, being significantly more metal-poor than the disk, may have too few planets or other resources to make effective colonization destinations, but they can still be used to provide gravity assists to colony ships, giving them speeds relative to the local standard of rest of 10⁻³c. Thus, the travel time to any point in the Galaxy for a ship is the travel time to the nearest suitable halo star (10⁵ yr in our example), plus the length of a cruise phase at ∼10⁻³ c, plus the time required for deceleration (also 10⁵ yr, if another halo star near the destination is used). This scheme would allow the ships in our example to extend their range to 100 pc.

If we drop the assumption of a maximum range for our colony ships, halo stars provide the capability to travel 1 kpc in 3 Myr, dominated by the long cruise time. Since the typical velocity of halo stars with respect to a disk star is simply the orbital speed of stars at a given galactocentric radius, use of halo stars for gravitational assists allows a ship to cross the Galaxy on a rotational timescale, by definition.

The slow expansion of an ETI should thus be modeled not as an expanding circle or sphere, subject to saturation and "fronts" of slower-expanding components of the supercivilization. A better model is as the mixing of a gas, as every colonizing world populates the stars that come near it, and those stars disperse into the Galaxy in random directions, further "contaminating" every star they come near. If halo stars are themselves colonizable, then those that counter-rotate and remain near the plane will provide even faster means of colonization, since they will encounter ∼10 times as many stars per unit time as disk stars. Non-circular orbits also provide significant radial mixing, and Galactic shear provides an additional source of mixing that is comparable to that of the velocity dispersion of the disk stars once the colonies have spread to v_rot/σ_v ∼ 1/10 of the Galaxy's size, or ∼1 kpc from the home stellar system.

We conclude that once a civilization develops and employs the technology to colonize the nearest stars, it will populate the entire Galaxy in no more than ∼10⁸–10⁹ yr, given our deliberately conservative assumptions of colony ship launch rate (10² Myr⁻¹), cruising speeds (30 km s⁻¹), and technological advancement rate (zero). More "realistic" values for these parameters will only decrease the galactic colonization timescale.

Thus, we see that galactic colonization timescales are likely to be at least one order of magnitude shorter than the ages of galaxies, and rotational shear and the thermal motions will disperse and "mix" any Fermi bubbles on a rotational timescale. Indeed, we find that the maximum timescale for Galactic colonization for a spacefaring civilization is on the order of a Galactic rotation (10⁸ yr), even for present-day probe speeds.

It is therefore only for a brief period of a galaxy's history that an ETI would have populated only single contiguous part of a galaxy, and we should expect only a brief transition period between the rise of the first spacefaring civilization(s) and a galaxy-spanning supercivilization. To first order, one would expect the fraction of galaxies that have been only partially populated to be the ratio of the colonization timescale to the galaxy's age, or roughly 10⁸/10¹⁰ ∼ 1%. Until we have discovered 100 galaxy-spanning supercivilizations, we should not expect to find any Fermi bubbles (Pagagiannis 1980).

There are important implications for extragalactic waste heat SETI, as well. Because stars in a galaxy will thoroughly "mix" a civilization with their random motions, especially in an elliptical galaxy where there are fewer strongly correlated velocities, the waste heat of an alien supercivilization should be smoothly distributed across the galaxy. This is in contrast to interstellar dust, which, being self-gravitating and dissipative, will tend to clump and collect into a disk. Morphology could thus be a powerful discriminant between MIR emission from dust and ETI's (Chandler 2013).

In the case of the possibility of extinction, Hart's temporal reasoning appears at first to work against him: while the Earthlings of the twentieth century may have avoided global thermonuclear war, this does not mean that the Earthlings of the 21st century will avoid extinction by a manufactured supervirus plague, or that those of the 22nd century will avoid catastrophic climate change from runaway greenhouse warming, or that those of the 23rd century will avoid becoming fodder for runaway, exponentially self-replicating nanobots.¹¹ In other words, even for a fixed extinction probability per unit time, extinction is inevitable, and it is reasonable to suppose that extinction probability per unit time would grow (or at least not decrease) with technological development.

This argument has merit, but it only applies during the narrow time frame during which a civilization has the capability and propensity to destroy itself and all of its colonies. This certainly applies to Earth today, but once a self-sufficient lunar or Martian colony exists, the colony can act as a "lifeboat" that can repopulate the Earth after the effects of any disaster. Once nearby star systems are colonized, humanity's expansion into the Galaxy can proceed even without the Earth, even if colonies occasionally extinguish themselves. As long as the periods between extinction are typically longer than the periods between the launch of successful colony ships, the expansion of life into the Galaxy can proceed apace.

Thus, the existential threats to any spacefaring species, including both externally and internally induced extinction or cultural collapse, decrease with its size. When confined to a single planet, many dangers exist, but a single self-sufficient colony will immunize the species from intra-planetary war, climate change, and even asteroid or comet impact. Spreading to a nearby star eliminates extinction from interplanetary warfare or a catastrophe with its host star. Once the civilization grows across a significant fraction of the Galaxy, either from fast colony ships or because the host stars have spread apart naturally, supernovae and gamma-ray bursts (GRBs) will also fail to be effective sterilizers.

This defense is quite robust: even turning the same mechanisms that make the spread and persistence of spacefaring life robust against life does not defeat this line of reasoning. For instance, a fleet of self-replicating machines ("von Neumann" machines, Freitas 1980; Neumann 1966) that attempted to exploit the powers of exponential growth could rapidly colonize the Galaxy to perform some task. However, even if this task were to scour the Galaxy of life ("berserkers," e.g., Saberhagen 1967), they would eventually become ineffective, since the technology of colonies on the far side of the Galaxy will have advanced by millions of years by the time the berserkers arrive. Even if one invokes berserkers that can grow and adapt to such advancement and succeed in their task, the berserkers themselves would constitute an intelligent, spacefaring, galaxy-spanning supercivilization by our definition of it, which defeats the purpose of the argument.

The monocultural fallacy includes treating a single species as a single culture. Even if a civilization is not generally inclined to colonize or communicate, this does not mean that it will not happen. To use a human analogy, it was certainly true that "European culture" before the 15th century had little or no inclination to cross the Atlantic and colonize the New World, but this did not prevent a sub-culture within Europe from accomplishing the task (twice).

Indeed, the number of distinct civilizations within a species will increase as the size of the civilization grows. This is because "culture" is a phenomenon born of coordination and collective action, which become more difficult when communication becomes slow, or the possibility of coercion becomes remote because of the distances involved. To continue the European analogy, long communication times and very slow exchange of population meant the colonies of the European powers had distinct cultures, which in some cases were kept in line only through the great expense of extending military force over long distances.

While radio communication can, in this sense, make a planet more culturally integrated, and so potentially more uniform, interplanetary and interstellar distances make even this solution inadequate. Once colonies are separated by light travel times that are a substantial fraction of a lifetime or significantly longer than the timescales over which cultures evolve, the coordination and collective action that characterizes a single culture become impossible. An interstellar Napoleon could not hope to bring under his sway colonies beyond a radius of one light lifetime.

We should then anticipate that extraterrestrial civilizations that span more than one planetary system should not be characterized by a single set of cultural characteristics, but as a supercivilization. We use the term "supercivilization" to describe the set of a large number of widely separated civilizations with distinct cultures, whose shared features result from their common origin, not any social coordination (which would be impossible due to the large light travel time involved). Supercivilizations are thus resistant to many sociological resolutions to the Fermi–Hart Paradox.

Since supercivilizations are the consequence of large light travel time, faster-than-light communication (or travel) would make it possible to avoid cultural diversity. Indeed, if travel and communication time can be made arbitrarily short (a convenience adopted, for instance, in much science fiction), then a highly organized species might be able to impose a single culture across a large region of space; one could rule the Galaxy (or the universe) from the White House. The spatial portion of the monocultural fallacy, then, is subject to the (reasonable) assumption that the speed of light is an inviolable maximum. The temporal aspect, similarly, is subject to the assumption that (backwards) time travel is not possible.

Thus, we should not expect spacefaring civilizations to go extinct (even from external causes) once they form supercivilizations so large that the timescale for communication across the supercivilization is larger than the timescale for technological advancement and cultural evolution.

6.3.1. Lack of Universal Mechanism

6.3.2. Colonization Need Not Be Driven by Overcrowding or Resource Depletion

6.3.3. Implications for Waste-Heat SETI

If there is a large hole in Hart's argument, it is in his assumption that at least one visitor to the Solar System would have stayed. Arguing that the Galaxy is filled with ETIs and that none of them colonize nearby stars is an example of the monocultural fallacy; it is much less fallacious to argue that none of the ETIs that has happened to visit the Solar System has decided to colonize it. Alternatively, an anthropic argument can be made that our very existence here, pondering this question, is predicated on the Solar System having remained unmolested up to the present day, despite a galaxy teeming with ETIs. In either case, the other stars in the Milky Way would host inhabited worlds, and we have simply not yet noticed. To use an earthly analogy, most individual animals on Earth have probably never seen a human, despite our ability (in principle) to colonize every corner of it.

So would we necessarily have noticed a Galaxy filled with ETIs? Given that the age of the Galaxy is consistent with ETIs being billions of years more advanced that we are, we must anticipate that we might not recognize many ETIs if we were to encounter them, any more than an ant colony might recognize humans as a fellow organized species (von Hoerner 1975). This concern must also be applied to any attempts they might make to engineer deliberate signals for us to detect. Sagan (1973) considered the probable timescales involved for the development (and destruction) of ETIs and argued that, even if our Galaxy is saturated with intelligent life, much of it has most likely survived catastrophe, evolved beyond this communication horizon, and is thus invisible to communication SETI.

One way in which alien civilizations would be very recognizable at any level of technological development would be in their use of energy. By searching for evidence of energy use, we would free ourselves of the guesswork of communication with unknown intelligence and the assumptions of what form alien life or advanced technology would take, and root ourselves in fundamental science.

Hart's thesis combines the short colonization timescales with the other aspects of his argument to claim that the Milky Way should contain either zero space-faring civilizations, or be filled with them. Since the latter option appears not to be the case, he concluded that we are alone in the Milky Way.

However, Hart's arguments apply equally well (or better) to other galaxies: every other galaxy represents an essentially independent realization of Hart's thought experiment. We would therefore expect that all galaxies are either uninhabited by spacefaring civilizations or host (at least one) galaxy-spanning supercivilization. Indeed, if such galaxy-spanning civilizations could be detected, the relative numbers of inhabited and sterile galaxies would then provide a rough guide to the frequency of advanced ETI on a cosmic scale.

Thus, Hart's pessimism toward the utility of a search for ETIs in the Milky Way actually translates into an optimism for the existence of galaxy-spanning civilizations in other galaxies. However, if a thorough search for such civilizations in galaxies across the local universe reveals nothing, then one of the following is true.

• 1.  
  Galaxy-spanning ETIs are universally undetectable (perhaps simply because their energy supplies are universally below our detection threshold).
• 2.  
  Hart's thesis is fundamentally flawed.
• 3.  
  Spacefaring life in the universe is so vanishingly rare that it is effectively unique to Earth.

If the first of these is the case, then SETI efforts can proceed by improving our detection thresholds. If the second of these is the case (perhaps because faster-than-light communication is possible, or interstellar colonization is fundamentally difficult in some way not considered here), a search for within the Milky Way is much more likely to succeed than Hart asserted, and so should be pursued more vigorously.

We will argue in Paper II that starlight contains most of the useful free energy in a planetary system, so we expect that any long-lived ETIs with large energy supplies will primarily be driven by stars' energy. There are two obvious ways to convert a star's mass to useful energy: by collecting the starlight naturally generated by stellar fusion, and by attempting to extract mass-energy at a faster rate (and perhaps higher efficiency) through something like the Penrose mechanism using a black hole (Penrose 1969). Both methods require that the extracted or collected energy be ultimately reradiated as waste heat.

At first glance, it may seem plausible that the complete collection of stellar photons could pose insurmountable engineering problems, but in fact the minimum physical requirements of such an endeavor are not so severe as to be physically impossible. A Dyson sphere need not be considered as a solid object, and the collection, operating, and radiation surfaces need not be coincident. Perhaps the simplest model would consist of a swarm of collectors (a "Dyson swarm") orbiting very close to a star, converting stellar photons into useful energy (and thus reducing the amount of starlight that escapes). This energy could then be transmitted (for instance, with lasers) to the locations where the alien civilization does its work. Without specifying their exact nature, we might call these locations "factories," or imagine that they are parts of a "stellar engine" (Badescu & Cathcart 2000). These factories would radiate the waste heat from the work they do.

Basic energy balance reveals that these factories would require an outward surface area equal to a sphere 1 AU in radius to radiate a solar luminosity of 255 K waste heat. Such a structure or series of structures would certainly require mega-engineering of almost inconceivable complexity, but the amount of available mass in a stellar system does not necessarily prohibit such a structure: even beyond the mass available in the star itself, a single giant planet contains enough material for such a radiating surface (the volume of Jupiter is equivalent to a solid shell 1 AU in radius and over 5 m in thickness). The construction of such surfaces, then, requires "just" engineering, the desire for more energy use, time, and the application of sufficient energy. Indeed, a Dyson swarm may not even require central planning, but might arise naturally from many independent subcultures' ever-increasing energy demands.

We can only imagine (probably unsuccessfully) what kind of projects an ETI could complete with such an energy resource. However, regardless of their intentions, thermodynamics dictates that a significant fraction of that energy must be expelled as waste heat corresponding to the operating temperature of their technological efforts. If an ETI prefers terrestrial conditions like ours, this operating temperature would likely then be measured in hundreds of Kelvins, which implies strong radiation in the MIR. Therefore, one promising SETI approach would be to look for the spectral signatures of this process. If the star were perfectly encased by such a shell of structures orbiting at about 1 AU, the resulting spectrum would then appear to be a good, few-hundred-Kelvin blackbody, with a temperature depending on the thermodynamic efficiency (Badescu 1995; Slysh 1985).

Very wide-field infrared surveys are clearly needed to systematically search for these structures. The Infrared All-Sky Survey (IRAS; Neugebauer et al. 1984) provided a hallmark survey for homogeneously investigating infrared sources and identifying Dyson sphere candidates; Slysh (1985) reasoned that the survey could detect (mostly complete) Dyson spheres out to ∼1 kpc. The survey immediately showed several potential Dyson sphere candidates, but the infrared colors and low-resolution spectra of candidate objects can be confused with stars of many ages obscured by reddened and dusty objects such as stars behind large amounts of extinction, protostars, Mira variables, stars in the AGB phase of evolution and beyond, and planetary nebulæ.

Nonetheless, Jugaku et al. performed a series of follow-up searches on catalogs of IRAS sources, concentrating on those with excess flux around 12 μ (Jugaku & Nishimura 1991; Jugaku et al. 1995; Jugaku & Nishimura 1997, 2000). These studies hunted for objects with anomalously red (K − 12μ) colors, using both archival and new K-band photometry. These studies combined determined that there are unlikely any partial to full Dyson spheres around the 365 solar-type stars analyzed within 25 pc. Further results using 2MASS photometry indicated the same with an additional sample of 180 stars within the same distance (Jugaku & Nishimura 2004).

Later work (Carrigan 2009b; Carrigan 2009a) examined sources observed with the IRAS low-resolution spectrometer (LRS) to determine whether their SEDs could be fit by the blackbody spectra of full or partial Dyson spheres (T = 100–600 K). Very few of the 11,000 sources investigated with the LRS were weak Dyson sphere candidates; the spectra of these typically had characteristics of carbon stars with 11.3 mm SiC emission features. The most promising candidate, IRAS 20369+5131, had the most accurate blackbody fit to its spectrum (T = 376 K), but is more likely a distant red giant star with no visible 11.3 mm emission peak. Carrigan (2012) noted that no obvious H i emission of an artificial and intentional origin was detected from this or other Dyson sphere candidates, again underlining the importance of this archeological SETI.

These source lists do not overlap with another study of Planck spectrum fits to IRAS data, specifically broadband measurements (Timofeev et al. 2000). Carrigan (2009a) argued that fitting LRS data is fundamentally superior to fitting the sparser broadband data, discriminating against filter measurements that only approximately follow a Planck distribution, but have distinct discrepancies in their SEDs. In any case, an observed deficit of OH and SiO emission for the more promising (however unlikely) sources from any of these programs would be needed to more concretely identify proper Dyson sphere candidates (Slysh 1985) and rule out more customary astrophysical phenomena.

Unlike the equivalent searches for these effects on individual stars, few SETI programs to hunt for their accumulated effects of collections of stars have been completed. Annis (1999b) looked for outliers of the Tully–Fischer relation for a sample of 57 spiral galaxies, and found that the scatter intrinsic to the plots can be attributed to natural causes (e.g., dust extinction) and would only correspond to brightness discrepancies of ≈50% or less. He considered magnitude differences of 1.5 mag or more—stellar optical obscuration by 75% or more—as possible candidates for homes of expansive supercivilizations. The same was true for his sample of 106 elliptical galaxies on the fundamental plane. He found no inexplicable outliers on that diagram either.

Assuming that the emergence of galaxy-wide supercivilizations can be modeled with Poisson statistics, Annis (1999b) inferred that, without evidence of these supercivilizations in the Milky Way, M31, or M33, the timescale of this evolution could be about 7 billion years, and could rise to hundreds of billions of years if the other galaxies of his sample are considered. This is an important first attempt to evaluate our isolation in the universe based on observable characteristics of nearby galaxies, constraining the number of supercivilizations there may well be in the low-redshift universe. Of course, the assumption that their development is a random process and independent of time is highly uncertain, and seems to contradict, e.g., the time-dependent phase transition arguments of Ćirković & Vukotić (2008).

This type of statistical analysis of galaxy properties, while elegant, has its disadvantages. It is limited to galaxies already discovered and classified in optical catalogs, and so would necessarily miss galaxies with little to no optical emission (i.e., completely obscured by Dyson spheres). H i surveys are starting to uncover H i consolidations like VirgoH i 21 (Minchin et al. 2005) that would lie well off either the Tully–Fischer relation or the Fundamental Plane and almost certainly have astrophysical explanations. VirgoH i 21 in particular has ≈10⁸ M_☉ of H i spread across 16 kpc, and given its velocity dispersion, has a dynamical mass-to-light ratio of ≈500 M_☉/L_☉. There is no presently observed optical component, placing it 12 magnitudes off the Tully–Fischer relation and a clear initial candidate for that methodology. However, VirgoH i 21 is likely a dark halo devoid of a central bright galaxy (Minchin et al. 2005) and not a fully inhabited galaxy. Many unusual galaxies off the traditional relations may well be surrounded by different masses of halos or simply be very dusty. To help eliminate these cases, additional observations and a larger sample size of homogeneously surveyed galaxies (preferably in the infrared) are needed.

An effective search for ETIs with large energy supplies would image a large sample of stars and galaxies in mid- and far-infrared wavelengths, and look for anomalous colors or SEDs that would indicate the presence of an advanced civilization. Such a search for ETIs has been performed by Timofeev et al. (2000), Jugaku & Nishimura (1991), Jugaku et al. (1995), Jugaku & Nishimura (1997), and Carrigan (2009a). A search for galaxy-spanning civilizations would operate on similar principles, but on the integrated starlight of entire galaxies.

IRAS (Neugebauer et al. 1984) performed photometry at 12, 25, 60, and 100 μ and low-resolution spectroscopy from 7.5 to 23 μ. IRAS offered sensitivity of around 0.2 Jy (1 Jy at 100 μ) (Moshir et al. 1990), and angular resolution of 30'' at 12 μ, and 2' at 100 μ. This has been the primary data set used in previous searches for alien waste heat.

High hopes for the sensitivity of IRAS to Dyson spheres were dashed by the unexpected discovery of the infrared cirrus (Weiland et al. 1986). The high backgrounds made the point source sensitivity of the relatively large IRAS beam significantly lower than anticipated at 60 and 100 μ, as did the corresponding PAH emission in the shorter-wavelength bands.

The High Frequency Instrument of the Planck mission (Planck Collaboration et al. 2011) detected galaxies at 350 μ as compact sources with 45 resolution and a sensitivity of 3 Jy, and might be used for the (non-)detection of far-infrared (FIR) thermal emission from dust from bright sources (to rule out extraterrestrial origin of the MIR emission).

In 2009, the Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010) launched and began its all-sky survey. WISE provides photometry at 3.4, 4.62, 12, and 22 μ (the W1, W2, W3, and W4 bands), with angular resolution of 61, 64, 65, and 12'', and sensitivities of 0.07, 0.1, 0.9, and 5.4 mJy, respectively (with ∼3% photometric accuracy; Cutri et al. 2012; Jarrett et al. 2011) WISE thus provides five times better angular resolution and 1000 times better sensitivity than IRAS. WISE thus affords us a deeper and more accurate glimpse into our own Galaxy for finding evidence of Dyson spheres, and a significantly improved ability to accurately detect, resolve, and image approximately 10⁵ galaxies across the sky in four infrared photometric bands and hunt for galaxy-spanning civilizations among them.

Additional surveys, such as the SWIRE stare, GLIMPSE, and MIPSGAL of the Spitzer Space Telescope, provide superior sensitivity and resolution to WISE, albeit with more limited spatial coverage. Together, these surveys provide a new and unprecedented glimpse into the universe of reprocessed starlight, including, potentially, the starlight that has been repurposed by ETIs.

The primary advantage to a waste-heat search for alien civilizations is that it makes no assumptions about how or whether an alien civilization would attempt communication. Further, since it can be extended to searches for supercivilizations in other galaxies, it can address resolutions to the Fermi-Hart paradox that allow for the ubiquity of spacefaring ETIs in the universe but excuse our lack of detection (for instance, many "sociological" explanations for their unwillingness to contact us).

This is in contrast to contact SETI, where the number of frequencies, duty cycles, and transmission methods of ETIs is large, and potentially inexhaustible. A null result in contact SETI is thus a failure to detect a certain class of ETI communication, while a thorough search for waste heat that comes up empty has the potential to be a successful demonstration of the absence of large ETI energy supplies with few hundred Kelvin waste heat temperatures.

If the number of ETIs per galaxy is as high as optimistic solutions to the Drake Equation suggest (Drake 1980; Pagagiannis 1980), then it is plausible that some fraction of galaxies will show evidence of large-scale civilizations.

A null detection of galaxy-spanning ETIs with waste heat luminosities above some upper limit in a large sample of galaxies would thus correspond to an upper limit on alien energy supplies in general, and individual upper limits on every galaxy in the sample. For elliptical galaxies, this upper limit might be similar to current limits for nearby stars (on the order of a few percent).

This would itself be an interesting result; to wit, at least one of the following must be true about galaxy-spanning alien civilizations in the search volume.

• 1.  
  They do not exist, or are sufficiently rare that they are not in our search volume.
• 2.  
  They exist, but their total energy supplies are universally below the search's detection threshold.
• 3.  
  ETIs with large energy supplies universally expel waste heat at low temperatures (i.e., wavelengths longer than the capabilities of the search), perhaps because their energy supply is not starlight (see Paper II).
• 4.  
  Spacefaring ETIs inevitably discover and universally employ physics that makes their civilizations effectively invisible in the MIR despite having large energy supplies (for example, expelling their waste heat as neutrinos, efficiently using their energy supply to emit low-entropy radiation, employing energy-to-mass conversion on a massive scale, or by violating conservation of energy).

Indeed, since MIR-bright ETIs with energy supplies in excess of their host galaxy's luminosity would be all but trivial for a survey such as WISE to detect and distinguish out to great distances (it would resolve them out to z ∼ 0.1), the absence of any such galaxies would effectively rule out the possibility that physics allows for an easily tapped source of "free" energy (e.g., zero-point energy) at high effective temperatures, if galaxy-spanning ETIs exist (and such a possibility would make it significantly easier for them to exist in the first place). The absence of such civilizations would thus rule out an entire class of either exotic physics or ETI's. We quantify these statements in Paper II.

The last two or three options on our list have little overlap with the resolutions to the "contact" version of the Fermi paradox, demonstrating the complementary nature of waste-heat SETI with communication SETI. To wit, our uniqueness as an intelligent, spacefaring civilization would explain the null detections in both contact SETI and artifact SETI, but most of the other options on this list would not. The most parsimonious explanation for a null result from both methods, then, would simply be that we are alone in the universe. Other explanations would need to invoke multiple reasons for our failure to detect ETIs.

Another perspective is that a null detection of galaxy-spanning ETIs in the local universe is in conflict with Hart's thesis that spacefaring civilizations will quickly colonize their galaxy. Unless humanity is alone in the universe, such a null detection would lend evidence to the hypothesis that spacefaring life rarely spreads far beyond its home star system, in which case searches within the Milky Way are much more likely to succeed than Hart argued.

The Rare Earth hypothesis of Ward & Brownlee (2000) states that "primitive" life is common in the universe but that animal life is unique to Earth. While null detections of waste-heat from ETIs would be consistent with this hypothesis, we stress that, unlike Ward & Brownlee, we have not assumed that ETIs need be animal. Any spacefaring agent that satisfies our "physicist's" definition of intelligent life should generate detectable waste heat.

So, while a null detection of artifact SETI supports the conclusion of the Rare Earth hypothesis, it does not necessarily support the reasoning and evidence presented by Ward & Brownlee in support of it.

A positive detection of waste heat from an alien civilization would have several consequences. Beyond demonstrating that alien life exists, it would also allow us to essentially "peek ahead" at the nature of engineering of a vastly more advanced species, and perhaps allow us to test our current theories of fundamental physics. Such a detection would:

• 1.  
  demonstrate that there are no insurmountable obstacles to energy supplies comparable to the luminosities of stars or galaxies;
• 2.  
  demonstrate that conservation of energy and the laws of thermodynamics, as we understand them, are not circumvented on a large scale by at least some very advanced civilizations, and so may be absolute. This might imply that attempting to overcome them at our level of technology would be fruitless; and
• 3.  
  allow for the analysis of the extent of the alien supercivilization, and so a measure of characteristic travel speed, potentially revealing the practicalities of travel near the speed of light.

In short, it would validate the assumptions that underlie the search for alien waste heat.

The fraction and extent of obscuration and reradiation by artificial constructions would certainly highlight important constraints on the unknown engineering of individual Dyson spheres, e.g., the fraction of stellar coverage and operating temperatures. They would also point to the colonization habits of ETI that have been thus far so difficult to anticipate, e.g., the extent, patterns, and timescales of their conquests.