IntroductionWe live in this incredibly vast universe, the story goes that, one day back in the 1940’s, a group of atomic scientists, including the famous enrico Fermi, were sitting around talking, when the subject turned to extraterrestrial life. Fermi is supposed to have then asked, “So? Where is everybody?” What he meant was: If there are all these billions of planets in the universe that are capable of supporting life, and millions of intelligent species out there, then how come none has visited earth? This has come to be known as “The Fermi Paradox”. Fermi realised that any civilisation with a modest amount of rocket technology and an immodest amount of imperial incentive could rapidly colonise the entire Galaxy. While interstellar distances are vast, perhaps too vast to be conquered by living creatures with finite lifetimes, it should be possible for an advanced civilisation to construct self-reproducing, autonomous robots to colonise the Galaxy. The idea of self-reproducing automaton was proposed by mathematician John von Neumann in the 1950’s. The idea is that a device could; 1) perform tasks in the real world and 2) make copies of itself (like bacteria). The fastest, and cheapest, way to explore and learn about the Galaxy is to construct Bracewell-von Neumann probes. A Bracewell-von Neumann probe is simply a payload that is a self-reproducing automaton with an intelligent program (AI) and plans to build more of itself. Such a probe could travel between the stars at a very slow pace. When it reaches a target system, it finds suitable material (like asteroids) and makes copies of itself.Growth of the number of probes would occur exponentially and the Galaxy could be explored in 4 million years. While this time span seems long compared to the age of human civilisation, remember the Galaxy is over 10 billion years old and any past extraterrestrial civilisation could have explored the Galaxy 250 times over. Within a few million years, every star system could be brought under the wing of empire. A few million years may sound long, but in fact it’s quite short compared with the age of the Galaxy, which is roughly ten thousand million years. Colonisation of the Milky Way should be a quick exercise. This prompted Fermi to ask what was an obvious question: “where is everybody?” If one considers the amount of time the Galaxy has been around (over 10 billion years) and the speed of technological advancement in our own culture, then a more relevant point is where are all the super-advanced alien civilisations.Kardashev scale – the energetic increaseRussian astrophysicist, Nikolai Kardashev proposed a useful scheme to classify advanced civilisations. But looking around, he didn’t see any clear indication would posses one of three levels of technology. A Type I civilisation similar to our own, one that uses the energy resources of a planet. A Type II civilisation would use the energy resources of a star, such as a Dyson sphere. A Type III civilisation would employ the energy resources of an entire galaxy. A Type III civilisation would be easy to detect, even at vast distances. But, we have encountered nothing at all, so maybe we, us humans are at fault.Our civilisation uses more and more energy. energy is all-purpose, so we don’t even need to understand the how or the why of this energy use to see that this trend is robust. extrapolating this exponential increase of energy consumption, Kardashev (1964) showed that this would lead our civilisation to type KII in year 5164 and to type KIII in 7764. Although Kardashev’s original scale is an energetic one, it has often been interpreted as, and extrapolated to a spatial one. This is probably because the order of magnitude of the energy processed is as follows.Type KI harnesses the energy of a earth-like planet; type KII harnesses the energy of a star and type KIII the energy of a galaxy. We are currently a KI civilisation. Let us examine, as a typical example, our possible transition from type KI to type KII. What motivations could we have to harness the energy of the Sun? There are essentially two reasons. First, simply to meet our growing energy consumption needs; second, to avoid the predictable death of our Sun, associated with the destruction of life on earth. Let us first consider how to meet a civilisation’s growing energy needs. einstein famously formulated the matter-energy equivalence formula e=mc². If we consider our solar system, where can we find most of its mass-energy? It is above all in the Sun, since 99.8% of our solar system’s mass is in the Sun. That is, 99.8% of the energy in our solar system is to be found in the Sun. For any long-term use, the Sun is thus the obvious resource to harness energy from.exploiting the energy of a star is an explorative engineering field known as star lifting, also called stellar mining, stellar engineering or astero-engineering (see Reeves 1985; Criswell 1985; Beech 2008). The second incentive to engineer our Sun is to avoid its red giant phase which will begin in about 5 billion years. This enterprise is vital if we are concerned by saving life on earth. Various processes have been proposed for this purpose, resulting in an elimination of this red giant phase. The topic is treated extensively by Martin Beech (20083). From a SeTI perspective, this leads to concrete and observable predictions. Beech (2008, 190-1913) proposes 12 possible signs of stellar rejuvenation in progress.Dokuchaev has also said, “The naked central singularity illuminates the orbiting internal planets and provides the energy supply for life supporting,” he adds. “Some additional highlighting during the night time comes from eternally circulating photons.” So any civilisation capable of doing so, which would probably rank as a type III advanced civilisations on the Kardashev scale, would derive light and heat from orbiting photons and energy from within the singularity itself. More interestingly, such a civilisation would be completely closed off to the rest of the universe beyond the event horison, just as we too, can not see anything inside of.Barrow scale – the inward manipulationJohn Barrow (1998) classified technological civilisations by their ability to control smaller and smaller entities. This trend leads to major societal revolutions. Biotechnology, nanotechnology and information technologies are progressing at an accelerating pace and all stem from our abilities to control and manipulate small scales entities. This pivotal and overwhelming trend toward small spatial scales is largely overlooked in SeTI, resulting in the Barrow scale being not well-known. Barrow estimates that we are currently a BIV civilisation which has just entered nanotechnology. Another argument for the importance of the Barrow scale is that, from the relative human point of view, there is more to explore in small scales than in large scales.As counter-intuitive as it is, space exploration offers more prospect in small scales than in large scales. That humans are not in the center of the universe is also true in terms of scales. This implies that there is more to explore in small scales than in large scales. Richard Feynman (1960) popularised this insight when he said “there is plenty of room at the bottom”. In contrast with large cosmological scales, manufacturing, testing, exploring and exploiting small scale technologies is easier, cheaper and more controllable. It is also more efficient energetically.Since the development of such technologies is not hampered by the finiteness of the speed of light, its accelerating progress has no reason to slow down until we reach the Planck scale. Futurist and systems theorist John Smart (2009) characterised this trend as Space-Time-energy-Matter (STeM) efficiency and density, or “STeM Compression”. It can also simply be summarised with the motto of “doing more with less”.Black holes as attractors for intelligenceIf we take seriously the Barrow scale, we do not even need to speculate on any particular black hole technology. We can simply assume that an intelligent civilisation will develop to type B?, whatever its purpose is in using black holes. The reader averse to scientific and philosophical speculation might thus jump directly to section 3 for an application of the two-dimensional metric. However, black holes are fascinating attractors, not only because of their staggering gravitational field, but also because they are an intelligence’s greatest potential. Let us see why with a short adventure on the speculative topic of black hole technology.Black holes are the densest objects in the universe. If we want to face the needs of consuming more energy, it might be beneficial to store or extract energy from black holes. Roger Penrose (1969, 270-272) imagined the following extraction mechanism. It consists of injecting matter into a black hole in a carefully chosen way, thereby extracting its rotational energy (Misner, Thorne, and Wheeler 1973, 908). Blandford and snajek (1977) suggested a similar process with electrically charged and rotating black holes. Other proposals suggest collecting energy from gravitational waves of colliding black holes. Misner imagined this in 1968 as a personal communication to Penrose (19696). Frautschi (1982) also proposed to merge black holes as a way to produce a power source.Which other societal functions could black holes fulfill? Louis Crane (2010, 370) has suggested that black holes are the perfect waste-disposal, although they should be manipulated with great care. He has also conducted an extensive study with Westmoreland on the possibility of black hole starships (Crane and Westmoreland 2009). Furthermore, general relativity leads to the fascinating topic of time travel via worm holes, theoretical cousins of black holes. Although no evidence of their existence is available, they could in theory provide shortcuts for traveling in spacetime (for popular accounts see Thorne 1994; Randall 2005).Let us assume that terrestrial and eTIs are curious and continue to develop science. Black holes, especially their interiors, currently challenge our knowledge of the three fundamental physical theories: quantum mechanics, general relativity and thermodynamics. For scientific purposes, there might be an incentive to artificially produce black holes to better understand them. Indeed, since the 80’s scientists have considered the possibility of making “universes in the lab” (Ansoldi and Guendelman 2006 for a review). Although improbable sources of danger, some concerns have been raised regarding the accidental production of micro black holes in particle accelerators (Giddings and Thomas 2002). Still, we might want to produce them intentionally in the future. A more concrete scientific application of black hole technology is to use them as telescopes or communication devices.How is it possible? An established consequence of general relativity theory is that light is bended by massive objects. This is known as gravitational lensing. For a few decades, researchers have proposed to use the Sun as a gravitational lens. At 22.45AU and 29.59AU we have a focus for gravitational waves and neutrinos. Starting from 550AU, electromagnetic waves converge. Those focus regions offer one of the greatest opportunity for astronomy and astrophysics, offering gains from 2 to 9 orders of magnitude compared to earth-based telescopes. Over the years, Claudio Maccone (2009) has detailled with great technical precision such a scientific mission, called FOCAL. It is also worth noting that such gravitational lensing could also be used for communication.If we want to continue and improve our quest for understanding the cosmos, this mission is a great opportunity to complete our fussy astronomy with a focused one. In other words, the time may be ripe to put on our cosmic glasses. But other eTIs may already have binoculars. Indeed, it is easy to extrapolate the maximal capacity of gravitational lensing using, instead of the Sun, a much more massive object, i.e. a neutron star or a black hole. This would probably consitute the most powerful possible telescope. This possibility was envisioned — yet not developedby Von eshleman in (1991). Since objects observed by gravitational lensing must be aligned, we can imagine an additional dilating and contracting focal sphere or artificial swarm around a black hole, thereby observing the universe in all directions and depths. Maybe such focal spheres are already in operation.What is the maximal information that can be processed by an advanced eTI? Visionary scientist Robert A. Freitas (1984) introduced the sentience quotient, which is a “scale of cosmic sentience universally applicable to any intelligent entity in the cosmos”. At its limits, we have the maximal computational density of matter, what Seth Lloyd (2000) more recently called the “ultimate computer”. What does such a computer look like? Lloyd argues that it is a black hole. Interestingly, if Moore’s law is extrapolated, we attain such a maximal computational power by 2205 (Lloyd 2005, 162). But black holes can be even more than ultimate computers. At the edge of theoretical computer science, some models of computation outperform Turing’s original definition. Such devices are called hypercomputers (earman and Norton 1993). They are theoretically possible assuming particular space-time structures or with slowly rotating black holes (see etesi and I Nemeti 2002; Andreka, I Nemeti, and P Nemeti 2009). If the construction of such hypercomputers is successful and indeed possible, this would bring qualitatively new ways to understand and model our universe. A breakthrough perhaps comparable to the invention of our ubiquitous computing machines.Intelligence is the capacity to solve problems. It is by focusing on universal and longterm problems that we have the highest chances to understand the purpose of presumed eTIs. I see only two such serious problems. The first is the already mentioned red giant phase capable of wiping out life in a solar system like ours. This is a fundamental challenge any civilisation born on the shore of a Sun-like star will have to face. A promising SeTI strategy is thus to search for civilisations refusing this fate, by looking at artificially modified stars. According to Criswell (1985, 832) star lifting can considerably extend a civilisation’s time with matter to energy conversion, up to 2 millions times the present age of the universe, assuming the civilisation stays at KI. Yet, even this runs out in the long term because the star will ultimately run out of usable energy.What is the next level? Possibly migration, but that also cannot continue forever, because new star formation comes to an end in the very long term (Adams and Laughlin 1997). After realising that the fate of stars is doomed, the longest term and truly universal problem is the continuation of the universe as a whole, to avoid its inevitable global entropy increase and death (?irkovi? 2003 on physical eschatology). The second challenge is thus, “How can we make life, intelligence and evolution survive indefinitely?” Answering this question is of course beyond the scope of this paper, but let us mention two proposals which include a role for black holes. Freeman Dyson proposed in his landmark (1979) paper that a civilisation could hibernate and exploit the time dilation effects near black holes, to survive forever. This is utilising the idea, of conserving energy in the best possible way, which would mean that this intelligent life no longer requires the primitive idea of “conquering” stars, galaxies, entire universes, etc. which is not sustainable at all. However, this scenario doesn’t work if the universe continues its accelerated expansion (Dyson 2004, xv). Yet, the core of the argument can be maintained if we replace digital computers by analog ones (Dyson 2007).Another speculative solution is to reproduce the universe (Harrison 1995; Gardner 2003; Balás 2005; Smart 20095; Gribbin 2009; Stewart 2010). This scenario combines the origin and future of the universe with a role for intelligent life. It is also worth noting that the future discipline of Artificial Cosmogenesis (Vidal 200834), analogous to Artificial Life but extended to the cosmos, would benefit the power of ultimate computers, to run simulations of whole universes. Finally, if we assume that our universe is a black hole (Pathria 1972), the pussling fine-tuning of universal constants could itself be interpreted as an intelligent signal from previous universe makers (Pagels 1989, 155-156; Gardner 200330). This is a radical proposal and the “Search for extra Universal Intelligence” field has yet to emerge.Black hole star liftingIn a SeTI mindset, considering seriously that black holes are attractors for intelligence, we can now start to ask the following questions. What are the observable manifestations of a black hole when it’s used as an energy source? as waste disposal? As a time-machine? As a starship engine? As an ultimate or hyper computer? As a universe production facility? The exercise is highly speculative, and raises the efficiency objection. We saw that the Barrow scale trend makes civilisations develop with more and more efficiency. This would make small black holes more useful and thus hard or impossible to detect. It would be like trying to detect from earth the existence of nanotechnology on the Moon.This is the essence of Smart’s (20095) response to Fermi’s paradox. We don’t see other eTIs because they are confined inside black holes. However, the two trends of more energy use and more energy efficiency need not be incompatible. Roughly speaking, our civilisation has always been more efficient yet always using increasing amounts of energy. The key lies in the availability of energy. If it is poor, efficiency will strongly constrain civilisation development. If energy is largely available, then efficiency matters less and civilisations can also grow on the Kardashev scale. In his seminal paper, Dyson (1966, 643) assumed that eTIs would use technology we can understand.He qualified this assumption as “totally unrealistic”. However, there is a profound dilemma here. If we respect this rule, we restrict our search to civilisations roughly at our developmental level, not really higher. The search for eTIs more advanced than us is unlikely to succeed. But if we release this rule, this brings a paradox. It will be hard, if not impossible, to argue that a phenomenon we don’t understand is artificial, since its technology will, by definition, be alien to us.Signs of KII-B? civilisationsWe don’t need to imagine or to wait because such configurations already exist. Indeed, 18 systems composed of a black hole accreting gas from a star have been found today (e.g. GRO J1655-40, 1659-487, GRS 1915+105, SS433, etc.). They are part of the family of binary systems, called X-Ray Binaries (XRB) because of their emissions in the X-Ray electromagnetic spectrum. Since a few decades, they are actively studied as natural -though sometimes intriguing- astrophysical systems. Importantly, researchers have concluded that a thin accretion disk around a rotating black hole is the most efficient power source in the universe , a process up to 50 times more efficient than nuclear fusion occurring in stars (Thorne 1974; Narayan and Quataert 2005).If any civilisation is to climb the Kardashev scale, it would certainly at some point want to master that energetic source. We call such an endeavor black hole star lifting. Let us call such an hypothetical civilisation KII-B?. It is of Type II on Kardashev’s scale, because it is able to harness the energy of a star; and type ? on Barrow’s scale, since it manipulates space-time’s structure with black hole technology. In fact, some XRB even display the main features of non-equilibrium systems. They have a strong energy flow from the star to the accreting black hole; at irregular intervals, plasma jets are ejected at relativistic velocities, which may be interpreted as entropy production; and, the black hole may be a structurally and informationally rich entity, if we assume that it could be a technology like an ultimate computer. We also can note that the black hole is not primarily used as an energy source, but as a technology which needs energy.Accreting binaries are found in a great variety of configurations. Our two-dimensional metric allows to also speculate on the existence of other less advanced civilisations than KII-B?. A young type KII might be accreting the energy from a white dwarf star, which is nothing else than a burnt-out Sun-like star. A more mature KII may be using a neutron star accreting system. We can also hypothesise that some accreting neutron stars are in fact artificial black holes. The main observational technique to decide whether the very dense accreting object is a neutron star or a black hole is to estimate its mass. If the object has a mass superior to Chandrasekhar’s limit of 3 solar masses, it can only be a black hole. Otherwise, it is a neutron star.Therefore, the finding (by future methods) of a black hole less than 3 solar masses may corroborate its artificial origin. What about more than KII? The Chandra X-Ray spatial telescope provided data showing that Low-Mass XRB (LMXB) are overabundant within 1 parsec of the galactic center (Muno et al. 2005). Could these be civilisations migrating toward the supermassive black hole? Although it sounds like a science-fiction novel, Vyacheslav I. Dokuchaev (2011) recently suggested that stable periodic orbits are theoretically possible inside supermassive black holes, and therefore, may be habitable. Frank Tipler (1997) also envisioned with great details the possibility of a K? civilisation mastering huge computational capacity.