Colonizing Galaxy With Self Replicating Probes
April 18, 2010 8 Comments
Replicating systems technology is the key to exploration and human habitat expansion beyond the confines of the Solar System. Although these kinds of missions necessarily are highly speculative, and admittedly exceed the limits of current or projected technology in many areas, a consideration of possible interstellar and galactic applications is nonetheless a useful exercise because it serves to illustrate the fantastic power and virtually limitless potential of the SRS [Self Replicating System] concept.
Before humankind can move out into interstellar space, automated probes will scout the way ahead. The distances are so large and the volumes so vast that self-replicating probes are highly desirable, even essential, to adequately and efficiently perform a reconnaissance of extrasolar star systems in a search for human habitable worlds and extraterrestrial life. A preliminary design for a self-reproducing interstellar probe has been presented in the scientific literature (Freitas, 1980a), and another study of the comparative benefits of reproducing and nonreproducing galactic exploration strategies by unmanned probes suggests that search patterns using semi-intelligent automata involving more than about the nearest 100 stars would probably be optimized (in terms of economy and productivity) if self-replicating systems are employed. Reproductive probes could permit the direct investigation of the nearest million stars in about 10,000 years and the entire Milky Way galaxy in less than 106 years, starting with a total investment by humanity of a single self-replicating exploratory spacecraft.
The problems in keeping track of, controlling, and assimilating data returned by an exponentially growing number of self-reproducing space probes are staggering. Part of the solution may lie in the use of an extremely high level of autonomy in operations management and reasoning; part may lie in the utilization of high levels of abstraction in the information returned to Earth after the fashion of the World Model sensing and data-processing philosophy. Another major piece of the solution is the development of a hierarchical command, control, and information-gathering architecture in which any given probe communicates directly only with its own parent and offspring. Control messages and exploration reports would pass up and down the chain of ancestral repeater stations erected by earlier generations. Certain highly critical but low probability- signals might perhaps be broadcast in an omnidirectional alarm mode to all members of the expanding network (and to Earth) by individual probes which encountered specific phenomena or events – such as the discovery of an extrasolar planet suitable for human habitation or a confrontation with intelligent alien lifeforms or their artifacts.
Before mankind can venture out among the stars, his artifacts and replicating machines must blaze the trail. Ultimately, however, one can envision freeflying space colonies journeying through interstellar space . Upon reaching some new solar system or other convenient source of raw materials, these mobile habitats would reproduce themselves with the human passengers redistributed among the offspring colonies. The original space habitats would serve as extraterrestrial refuges for humanity and for other terrestrial lifeforms that man might choose to bring along. This dispersal of humankind to many spatially separated ecosystems would ensure that no planetary-scale disaster, and, as people travel to other stars, no stellar-scale or (ultimately) galactic-scale event, could threaten the destruction of all mankind and his accomplishments. Replicating systems may be the only rational means to attempt large-scale astroengineering projects usually relegated to the domain of science fiction, such as the construction of “Dyson Spheres” which enclose and utilize the energy output of entire suns.
The limits of expansion. The expansion of a population of replicating systems in any environment is restricted largely by two factors: (1) replication time, and (2) maximum velocity of the outer “envelope” which defines the physical extent or dispersion of the population. No population can accrue at a faster rate than its components can reproduce themselves. Similarly, no population can disperse faster than its medium will permit, no matter how fast components are manufactured – assuming number density remains essentially constant, corresponding to continuous maximum utilization of the environment. Neither factor may be ignored during any phase of population growth.
If envelope expansion velocity does not constrain a population because components are produced only relatively very slowly, then that population will experience exponential multiplication according to:
N(T) = exp(T/t) ……..(1)
where N(T) is the number of replicating units comprising the population at time T (replication starts at T = 0) and t is the replication time per unit, assumed constant. On the other hand, if unit reproduction is so swift that multiplication is not constrained by replication time, then the population can grow only as fast as it can physically disperse that is, as fast as the expansion velocity of the surface of its spherical outer envelope – according to:
N(T) = 4/3 π d(VT)3 …….(2)
where V is peak dispersion velocity for individual replicating units at the periphery and d is the number density of useful sites for reproduction. Expansion cannot exceed the values for N(T) given either by equations (1) or (2) at any time T, provided all replication sites receive maximum utilization as stipulated (e.g., constant number density of units).
Populations of machines expanding across the surfaces of worlds with replication times on the order of 1 year will not achieve mean envelope growth speeds in excess of a few meters per hour, even in later phases of extreme enlargement when the population of SRS covers a large fraction of the available planetary surface. This figure is well within anticipated nominal ground transport capabilities, so exponential extension should remain largely velocity-unconstrained on such bodies if replication time remains constant at greater population sizes.
Similarly, three-dimensional populations of replicating systems in interplanetary space using Solar System materials and solar energy ultimately are restricted to spherical circumstellar shells where SRS units can collect virtually all energy radiated by the Sun. If a “Dyson Sphere” of 100-ton replicating “seed” units is assembled near the orbit of Earth, approximately one terrestrial mass is required to manufacture the more than 1019individual units needed to completely enclose the star. But maximum expansion velocity even in this case never exceeds about 100 m/sec, hence interplanetary replicating systems as well in theory may spread at purely exponential rates.
In the interstellar realm, however, the situation is far more complex. Depending on the maximum dispersal velocity and interstellar probe replication time, either equation (1) or (2) may control. Figure 5.24 compares pure exponentiation and dispersal speed effects for t = 1 year and t = 500 years (Freitas, 1980a), and for V = c (since the theoretical maximum envelope expansion rate is the speed of light) and V= 10%c (Martin, 1978) for an assumed homogeneous stellar distribution of “habitable” star systems (taken as 10% of the total) in the galactic disk. In most cases, exponential multiplication soon is halted by the speed-of-light barrier to dispersion, after which the SRS population expansion proceeds only polynomially.
Technology requirements. In order to sustain the expansion of a potentially infinite replicating system, new dispersal mechanisms must be developed. Initially, self-replicating machines or their “seeds” must be capable of motion across a planetary surface or through its atmosphere or seas. Later, interplanetary, interstellar, and, ultimately, intergalactic dispersal mechanisms must be devised. Supplies of energy, stored and generated, must be established if extrasolar spacecraft are to survive in the depths of interstellar space far from convenient sources of power (such as stars) for a major portion of their lives. The technologies of command, control, and communication over stellar and galactic distances ultimately also must be developed.