Some of the problems that plague SETI research are the problems of the abundance of liquid water planets, the probability of the development of intelligent life, whether or not intelligent life forms develop technology, how long intelligent technological civilizations may survive, and whether or not interstellar travel or colonization are feasible or affordable. These problems lead to extensive and potentially irresolvable debates regarding the various paths species and civilizations may follow from a primitive level to our current level and beyond. This discussion will focus instead on the question of what the characteristics of intelligent technological life should be at the limits of known physical laws. Why do this? Well, because as Scotty observed on the Starship Enterprise, ``Captain, I canna change the laws of physics!''. Even if the laws of physics do not deny the feasibility of a life form, the lack of a practical engineering path to it may prevent its existence. At these limits, the form(s) that life takes may be clearer because convergent evolution could drive civilizations into a very limited set of ecological niches. An architecture for civilizations that hits many of these limits will be proposed. Its characteristics include thought capacities in excess of a trillion trillion times that of an individual human, survival times of trillions of years and astronomical observational capacities trillions of times greater than our civilization. Such civilizations, should, over time, become the dominant population of galaxies. Our own civilization may reach this state within this century. The impact of these conclusions on classical radio and optical SETI verses astrometric and occultation astronomy will be discussed.
Keywords: Dyson shells, evolution, Matrioshka Brains, megascale engineering, nanotechnology, optical SETI, radio SETI, technological civilizations
This paper adopts the perspective that the glass may be much more full than empty, i.e. that advanced technological civilizations may be quite abundant. In the Milky Way, 10 billion stars may have sufficient metallicity to produce terrestrial planets older than 5 billion years [6]. Recent estimates suggest more than 70% of the Earth's in the galaxy should be older than ours [7]. If the characteristics that make our Earth special, such as a comet intercepting Jupiter and a large moon [1], are not too rare or hazardous environments actually accelerate the rate of the evolutionary development of intelligence, then we should expect most technological civilizations to be far more advanced than we ourselves currently are.
Here, we will propose models for advanced civilizations and explore why their capabilities can explain the apparent lack of signals. The reasons the prospects for communications with extraterrestrial intelligence (CETI) are dismal will be discussed. Finally, the contributions that emerging branches of astronomy can make to programs that search for extraterrestrial intelligence (SETI) will be explored.
Technological civilizations, as they develop, expand their capabilities to both the smaller and larger scales. Extrapolations from the current microelectronic industry lithography scale of 0.18 μm, along the trend predicted by Moore's Law, leads to a rather hard limit of atomic scale manufacturing around 2040.
Molecular nanotechnology has a number of important features. These include atomically precise assembly, self-assembly, self-replication, self-motility and the execution of resource acquisition, survival and reproduction programs. Single-celled biological organisms possess all of these properties and constitute an existence proof that molecular nanotechnology is feasible. The primary differences between the naturally evolved nanotechnology found in biological systems and more robust forms envisioned in the previously mentioned references is the use of solution chemistry (so the precise position of all atoms is not always known) and materials that have strengths and operating temperature ranges that are less than the theoretical limits.
A logical progression exists for the development of molecular nanotechnology based on alternate forms of carbon (fullerenes and buckytubes), followed by diamond, sapphire and titanium carbide. Progress along these and related paths eventually leads technological civilizations to develop an extremely large range of devices and structures that are precisely manufactured from individual atoms.
Molecular nanotechnology enables the magical ``replicators'' of the type seen in Star Trek. Anything that can exist should be able to be assembled. It does have limits however. Heat removal constraints will limit the production rate of assemblers operating at the atomic scale [11]. And nanocomputer throughput will be limited by the amount of entropy generating bit-erasure that is done that must removed as heat [13].
Biological self-replicating systems (e.g. bacteria) have mass doubling times as short as 20 minutes [16,17]. These are certainly sub-optimal as these genomes contain several thousand genes while the minimal genome for self-replication may only require ~256 genes [18]. We now have sequenced the genomes of dozens of these microorganisms and as a better understanding of the function of all of the genes they contain develops, the design requirements for robust self-replicating systems will be made clear.
The limits for the mass doubling times are less clear. Estimates as short as 2 milliseconds using assembly line methods have been postulated [19], though these rates are likely to be constrained by heat removal requirements. The difference between the energy levels of high energy state intermediates and the final bonded energy levels determines the amount of waste heat produced in chemical reactions. Initial designs for self-replicating systems are likely to be less efficient and therefore constrained to operate more slowly than systems that are developed over time that utilize optimized reactions that minimize heat production.
| Body | Useful Mass | Orbital Radius | Disassembly
with 1026 W |
Exponential
Self-disassembly |
Disassembled
Areal Density |
| (kg) | (km) | time | time | kg/m2 | |
| Mercury | 3.3×1023 | 5.8×107 | 5 hours | 14 days |
|
| Venus | 4.9×1024 | 1.1×108 | 16 days | 114 days |
|
| Earth | 3.3×1024 | 1.5×107 | 22 days | 179 days |
|
| Mars | 6.4×1023 | 2.3×108 | 12 hours | 176 days |
|
| Jupiter | 1.9×1027 | 7.8×108 | 563 years | 691 years |
|
| Saturn | 5.7×1026 | 1.4×109 | 60 years | 181 years |
|
| Uranus | 8.7×1025 | 2.9×109 | 3.3 years | 223 years |
|
| Neptune | 1.0×1026 | 4.5×109 | 8.2 years | 624 years |
|
| Pluto | 1.3×1022 | 5.0×109 | 2 minutes | 266 years | 0.000037 |
| Asteroids | 5.9×1021 | 4.1×108 | very fast | very fast |
|
These numbers should be used for relative comparison purposes only.
They are subject to change as the simulation models are improved.
The exponential growth allowed by self-replicating nanomachinery and the increasing amount of power that can be delivered back to the planetary body as the solar array grows, allow relatively short disassembly times.1 Once a fully functional star enshrouding collector array is built the other planets can be disassembled at an even faster rate because the full power of the star is available. One conclusion that may be drawn from this is that within our solar system, almost any major body provides sufficient material to enshroud the sun. Even the asteroids may be sufficient if the collectors are moved somewhat closer to the sun. So it seems likely that in most solar systems in which technological civilizations find themselves they will have the resources to harvest the power output of their star. Another conclusion is that when a civilization ``decides'' to make the KT-I (planetary) to KT-II (stellar) transition it may do it very quickly.
Once the material found in solar systems is lifted out of its respective gravity wells, a long process of relocating it into optimal orbits is required. This may involve moving carbon (derived from methane), oxygen (derived from ice) and nitrogen (derived from ammonia) into the inner regions of the solar system and moving metals with desirable magnetic or superconducting properties to the outer regions of the solar system. As this is done, increasing amounts of power are dedicated to the construction and operation of nanocomputers associated with the power collecting satellites orbiting the star. The nanocomputers are interlinked with one another via arrays of VCSEL lasers and CCD detectors. Over time a multi-layer Dyson shell architecture arises where each layer operates at a specific temperature that is related to the elements available and computer architectures chosen to perform calculations. These architectures may range from high-temperature mechanical-logic computers near the star to very cool computers utilizing superconductor based logic at distances light-hours from the star.
This solar system scale supercomputer manufactured from atomic scale
parts has been named by the author a ``Matrioshka
Brain'' (after the nesting Russian dolls). The next table compares
the computational and memory capacities of a human and humanity with a
single node of a Matrioshka Brain and the entire
Matrioshka
Brain.
| Characteristic | Human
Brain |
Humanity | Matrioshka Brain | |
| Node | Solar System | |||
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It can be seen that there are rather large scale differences. How does this impact the probability of CETI? A couple of examples of the gap between a Matrioshka Brain at the Kardashev Type-II civilization level and our pre-Type-I civilization level may be useful:
They may also be a fundamentally different type of intelligence. Between humans, our communication bandwidth is very asymmetric. Our primary input is visual and its capacity may be several megabytes per second. Our outputs, such as speech or typing, use completely different mediums and are limited to around a few dozen bytes per second. In contrast, the innermost internode communication capacities in a Matrioshka Brain are tens to millions of terabytes per second and they may have equivalent input and output bandwidths. So the individual "nodes" are likely to function as a more tightly integrated mega-mind than humanity, in its current form, ever could.
Further details of this are discussed in papers at the Matrioshka Brain Home Page [28].
Previous papers[32,33] discuss the megascale engineering limits in more detail, particularly with regard to Matrioshka Brains. Here we will simply summarize some conclusions that have been reached:
This possibility means that we can foresee a logical progression in the development of technological civilizations. First they develop to the point of understanding the laws of physics and chemistry. Then they comprehend their basic biology and their information carrier (e.g. DNA). Then they develop computing machinery and a means for transferring their minds into those devices. If they choose to distribute their intelligence over a large enough physical volume (planetary volumes for example) or simply make backup copies of their intelligence (on opposite sides of a solar system), they have effectively immortalized themselves because local accidents cannot destroy their ``minds''. Over time, accidents will eliminate individuals and civilizations that do not follow this path. We can thus predict that that civilizations that do follow this path, should, at some point, become the dominant populations in galaxies.
Those signals produced by accident would be similar to those that we ourselves currently produce. The low power levels of these signals makes them difficult to detect at moderate interstellar distances. In addition, terrestrial interference conspires against radio searches for signals similar to our own. Leakage signals would presumably decrease as a civilization matures. As a civilization develops more optimized means of transmitting information such as microwave, local lasers, fiber optics, low power multi-hop radio, etc. wasteful practices such as high power broadcast signals should diminish. Because leakage signals come from civilizations around our level, and are not designed to be received and understood, they are presumably of less interest than intentional communications from more advanced civilizations.
Most radio CETI searches to date have been conducted with the assumption that an advanced civilization would be transmitting at frequencies at which others might be listening and that they are advanced enough and/or kind enough to continue transmitting for year after year until we hear them. This cost of this effort is usually justified with some hand-waving that we have transmitted signals and we would want to do it continuously if we could afford it. But would we? If we had a choice of transmitting to a more advanced civilization or a less advanced civilization which would we choose? Our current CETI strategy seems to suggest we are looking for a Galactic Club handout. Situations where we communicate with those less advanced than us are cases where the difference is not that great - probably only a few orders of magnitude for human infants and such animals as chimpanzees, dogs and cats. What rationale is there for extraterrestrials many orders of magnitude higher on the intelligence scale behaving any differently towards us than we do towards most of the other species on the planet?
Due to the lack of success in CETI at radio frequencies and the possible advantages offered by the visible frequencies, CETI searches are now conducted in both regions. However both radio and optical CETI operate by pointing their telescopes at visible stars! The civilizations that would seem best able to afford the power costs of interstellar communication would be those who have the greatest amount of power at their disposal, i.e. those that have enshrouded their stars with power harvesting devices. This process causes the star to grow dim or even disappear (at visible wavelengths). The lack of success in previous CETI programs is not surprising because the stars being examined most likely contain no civilizations or civilizations below our level of development.
Civilizations that develop much beyond our level should make the KT-I to KT-II transition. After that they have all the power output of a star at their disposal. Using their telescopes they can see any civilizations with whom they might want to communicate. Civilizations that value their time and energy will attempt to communicate with more intelligent civilizations or civilizations with access to information that is otherwise unavailable to them. There seems to be little justification for directing time or energy towards civilizations that are lower than ``worms'' from their viewpoint.
The phase space of devices that may be constructed using nanotechnology on solar system scales and the variety of underlying computing architectures that may be adopted by advanced civilizations may produce a strong motivation for leaving developing civilizations untainted to promote galactic ``diversity''. The development of a civilization along its own path is likely to foster such diversity because as the saying goes, ``Necessity is the mother of invention''. Premature exposure of developing civilizations to advanced extraterrestrial technologies could drive such civilizations onto previously explored development paths resulting in a reduction of the exploration of the phase space.
Thus the Zoo Hypothesis[2] may be correct if a Matrioshka Brain is using our planet as the subject of a controlled experiment. Alternatively, if the extraterrestrials desire to maximize the information content of the universe[3] then the Interdict Hypothesis[4] is the likely explanation for the lack of signals or extraterrestrial artifacts.
These lines of reasoning argue that CETI at our level of development is unlikely to succeed. If that is the case what are the prospects for SETI?
Efforts to combine the microlensing observations with IR and occultation astronomy (discussed below) could yield valuable information with regard to what the microlensing objects in really are.
Civilizations must go through a development stage before they reach this very cool state. In solar systems where civilizations choose to dedicate much of the early material harvested to computer construction rather than power collection, in systems that are metal resource poor, and in systems where large amounts of power must be dedicated to material relocation to optimum temperature ranges - the process of the star being enshrouded may not occur quickly. It may take decades, centuries or even millennia. In these systems we will observe the star slowly growing darker and darker, radiating increasingly more amounts of its radiation in the infrared, until finally it ``disappears''. We can use measurements from astrometric and other long term survey missions to identify stars with these characteristics and analyze their IR emissions for this type of process. The final state of the system will depend on the amount of construction material available. It does not appear that we could currently fill the outermost shells of a Matrioshka Brain and we would end up radiating at a temperature below that of liquid nitrogen, but above that of the microwave background radiation.
So there is a possibility of SETI catching civilizations when they are making the KT-I to KT-II transition. Stars possessing the slowly varying brightness characteristic are known as Long-Period Variable stars. The General Catalog of Variable Stars [41] lists thousands of stars in these categories and more are being discovered every day. Astronomers who study these stars have recently reported that of the stars in this group, there are apparently more stars getting dimmer than are getting brighter [42]. While this may be a characteristic of the stars in this class, these stars should examined in greater detail, particularly at infrared wavelengths, for signs of astroengineering activity. As the number of stars observed by current and planned surveys increases from the millions to the billions, we will begin to accumulate the data necessary to determine the frequency at which this transition occurs and therefore begin to set limits on the abundance of civilizations slightly ahead of our own level in the galaxy.
It appears clear that if advanced civilizations do exist and expand their capabilities in the ways described in this paper that occultation astronomy may be an excellent way to conduct SETI. Ultimately a combination of the three approaches - gravitational microlensing, infrared and occultation may be needed to provide concrete identification of advanced technological civilizations.
Highly evolved individuals of advanced technological civilizations may decide to sever their connection with their ``parent'' Matrioshka Brain in which they ``live'', even though this would require sacrificing much of its tremendous computational capacity and memory. They may choose to wander the galaxy in search of novel sources of information[45]. During their long interstellar voyages most of the systems on their ship may be suspended with little waste heat being produced. Until we attain the level of a KT-II civilization ourselves our chances of detecting these ships is virtually zero unless one shows up directly above the Earth tomorrow. We are at liberty to suspect their possible existence due to the lack of robust explanations for the missing baryonic dark matter.
Advanced civilizations based on an optimized computronium infrastructure have little need for conversations with mere humans or even human civilizations whose thought capacities are trillions of times less than their own. In contrast, they may have an interest in leaving our civilization to its own unique development path so as to increase the potential diversity of, and information content in, the galaxy. This is due to the large phase space of what can be constructed using molecular nanotechnology and the difficulties in proving that the computational architectures previously adopted to support advanced civilizations are, in fact, ``optimal''. Advanced civilizations may need less developed civilizations for the ``dumb luck'' they may have in developing an unexplored quadrant of the phase space of what may be designed and assembled in support of the evolution of intelligence.
The billions of large telescopes advanced civilizations may construct allow them to observe the observable regions of their galaxy at very low cost. They may also identify and communicate with civilizations that have information regarding the locations of objects that are invisible from their location. Such information is of great value in calculations of the long term motion of objects in the galaxy and is essential for civilizations seeking the lowest cost sources of additional matter and energy as well as seeking to avoid galactic hazards such as black holes or supernovas. This is due to the large energy cost and long time periods required to alter the course of entities with the mass of solar systems.
The accelerating pace of technology development seems to be driving us towards the singularity[46]. An ultimate manifestation of this will be the conversion of our solar system into a Matrioshka Brain. Even if Rare Earth[1] is correct and we are one of a few rare intelligent technological civilizations in the galaxy, implying that both CETI and SETI will fail, and even if the missing baryonic dark matter and gravitational microlensing observations have perfectly ``natural'' explanations - the development path outlined here merits further study as our civilization seems to be following it.
I would like to acknowledge Anders Sandberg for his groundbreaking explorations of megascale intelligent superentitites and thank the many Extropians and transhumanists who have made fruitful contributions to these ideas.