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Papers on the Fermi Paradox, aka "the universe is huge and old, space is transparent, colonization doesn't seem that hard, where are all the aliens or signs of their existence?"
http://www.kschroeder.com/weblog/the-deepening-paradox/
Karl Schroeder linking to the next paper, and briefly mentioning his own idea of Rewilding, that advanced technology for some reason ends up looking like nature.
http://arxiv.org/PS_cache/arxiv/pdf/...111.6131v1.pdf
13 page PDF by Keith Wiley, "The Fermi Paradox, Self-Replicating Probes, and the Interstellar Transportation Bandwidth", reviewing the potential impact of SRPs and flaws in various arguments against them, including Landis's percolation model, and Sagan's "mutations would be just too dangerous". Percolation stoppage requires unrealistic assumptions, and modern tech shows that we can reduce viable error rates to very low levels. For that matter, the number of replications in our own bodies is at least comparable to that involved in sweeping a galaxy -- he actually cites a much higher number, 10,000 trillion -- without having all that many cancers.
Comments to the blog post include David Brin pointing out that his 1983 review paper on the Great Silence is online:
http://adsabs.harvard.edu/abs/1983QJRAS..24..283B
27 page PDF, oldie but goodie. Modifies the Drake Equation, as does Wiley.
Me, I've long been in the camp of what Brin calls the Uniqueness Hypothesis, allied with the Anthropic Principle. Someone has to be first, and if von Neumann probes can sweep the galaxy easily, then the first can be the last. If we get as far as being able to envision such probes, probably no one else has yet, and we will. Unless we do ourselves in first, but that sort of thing doesn't help the number of aliens be larger than 0 either. As for why we might be first: planets are common, and life might be fast to develop where it can (but see Hanson on hard steps), but *stable* planets might be a lot rarer; we could be unusual in not having had total mass extinctions. Or human level intelligence is rare. Or industrial civilization -- heck, humans were around for maybe 90,000 years without developing agriculture; why?
Other papers
By Robin Hanson
on the Great Filter
http://hanson.gmu.edu/greatfilter.html
Burning the Cosmic Commons
http://hanson.gmu.edu/filluniv.pdf
how early lucky success makes hard steps look easy, or why we really can't extrapolate from the data point of Earth
http://hanson.gmu.edu/hardstep.pdf
Relatedly: detecting Kuiper Belt cities via artificial lighting
http://www.universetoday.com/91915/a...-their-cities/
One could quibble (wouldn't such cities be covered?) but the possibility of detecting artificial lighting and its spectral oddities strikes me as another useful SETI tool, and constraint on what anyone out there is doing.
How long for replicators to sweep the galaxy?
Say stars are 4 light years apart on average. (This is probably low, but that actually slows things down.) Say travel time is 10,000 years (still fast by our standards, 120 km/s), and that it takes 10,000 years to build up to the next probe, as might fit if you're sending a small band of colonists who have to build up a new systemwide civilization[1]. 20,000 years, 4 light years, 5000 years per light year on average. So, 500 million years to cross the Milky Way. That sounds like a long time... but then you consider life being 3 billion years old on Earth, and second(?) generation stars developing 8 billions years ago, and it's not that big a fraction of the available time.
Also note you have to improve both limiting factors to make a difference in speed. So for 50 million years, less than the time since the K-T dinosaur extinction, you need 1000 year travel time, or 0.004 c, and 1000 year development/doubling time. 4% c and 100 year doubling gives you 5 million years for the galaxy, less time then it took for humans to evolve from our last ancestor with chimps. 4% c is still well within nuclear energies, if beyond our ability to build an actual engine. 40% c (via huge mass ratios, or beamed propulsion, or antimatter) and Drexlerian nanotech doubling of 10 years gives 500,000 years, maybe 2-5x as long as the age of Homo sapiens sapiens.
If distance is 6 light years, and you spend 15,000 years in travel and 10,000 in doubling, that's 25,000/6 or 4166 years per light year, which is why I said taking a low number slows things down. And of course that assumes sending a minimal number of probes to exactly the two nearest neighbors, rather than spamming everything within reasonable range, e.g. sending probes to 4 and 8 ly away, in which case it'd be the longest legs which limited your expansion time; 8 ly jumps would mean 375 million years, or 37.5 million years, etc.
[1] Note that broadly speaking, colonizations and civilizations, or generation ships, are "self-replicating probes" of their own. It's not like you're limited to mechanical clanking replicator von Neumann machines or dubious nanotech; replicators are free to include biology as needed, whether frozen bacteria for use in bioreactors for mineral separation or colonists for intelligence. Granted, full colonies are probably erratic enough for the percolation model of non-propagating colonies to fit, but anyway.
http://www.kschroeder.com/weblog/the-deepening-paradox/
Karl Schroeder linking to the next paper, and briefly mentioning his own idea of Rewilding, that advanced technology for some reason ends up looking like nature.
http://arxiv.org/PS_cache/arxiv/pdf/...111.6131v1.pdf
13 page PDF by Keith Wiley, "The Fermi Paradox, Self-Replicating Probes, and the Interstellar Transportation Bandwidth", reviewing the potential impact of SRPs and flaws in various arguments against them, including Landis's percolation model, and Sagan's "mutations would be just too dangerous". Percolation stoppage requires unrealistic assumptions, and modern tech shows that we can reduce viable error rates to very low levels. For that matter, the number of replications in our own bodies is at least comparable to that involved in sweeping a galaxy -- he actually cites a much higher number, 10,000 trillion -- without having all that many cancers.
Comments to the blog post include David Brin pointing out that his 1983 review paper on the Great Silence is online:
http://adsabs.harvard.edu/abs/1983QJRAS..24..283B
27 page PDF, oldie but goodie. Modifies the Drake Equation, as does Wiley.
Me, I've long been in the camp of what Brin calls the Uniqueness Hypothesis, allied with the Anthropic Principle. Someone has to be first, and if von Neumann probes can sweep the galaxy easily, then the first can be the last. If we get as far as being able to envision such probes, probably no one else has yet, and we will. Unless we do ourselves in first, but that sort of thing doesn't help the number of aliens be larger than 0 either. As for why we might be first: planets are common, and life might be fast to develop where it can (but see Hanson on hard steps), but *stable* planets might be a lot rarer; we could be unusual in not having had total mass extinctions. Or human level intelligence is rare. Or industrial civilization -- heck, humans were around for maybe 90,000 years without developing agriculture; why?
Other papers
By Robin Hanson
on the Great Filter
http://hanson.gmu.edu/greatfilter.html
Burning the Cosmic Commons
http://hanson.gmu.edu/filluniv.pdf
how early lucky success makes hard steps look easy, or why we really can't extrapolate from the data point of Earth
http://hanson.gmu.edu/hardstep.pdf
Relatedly: detecting Kuiper Belt cities via artificial lighting
http://www.universetoday.com/91915/a...-their-cities/
One could quibble (wouldn't such cities be covered?) but the possibility of detecting artificial lighting and its spectral oddities strikes me as another useful SETI tool, and constraint on what anyone out there is doing.
How long for replicators to sweep the galaxy?
Say stars are 4 light years apart on average. (This is probably low, but that actually slows things down.) Say travel time is 10,000 years (still fast by our standards, 120 km/s), and that it takes 10,000 years to build up to the next probe, as might fit if you're sending a small band of colonists who have to build up a new systemwide civilization[1]. 20,000 years, 4 light years, 5000 years per light year on average. So, 500 million years to cross the Milky Way. That sounds like a long time... but then you consider life being 3 billion years old on Earth, and second(?) generation stars developing 8 billions years ago, and it's not that big a fraction of the available time.
Also note you have to improve both limiting factors to make a difference in speed. So for 50 million years, less than the time since the K-T dinosaur extinction, you need 1000 year travel time, or 0.004 c, and 1000 year development/doubling time. 4% c and 100 year doubling gives you 5 million years for the galaxy, less time then it took for humans to evolve from our last ancestor with chimps. 4% c is still well within nuclear energies, if beyond our ability to build an actual engine. 40% c (via huge mass ratios, or beamed propulsion, or antimatter) and Drexlerian nanotech doubling of 10 years gives 500,000 years, maybe 2-5x as long as the age of Homo sapiens sapiens.
If distance is 6 light years, and you spend 15,000 years in travel and 10,000 in doubling, that's 25,000/6 or 4166 years per light year, which is why I said taking a low number slows things down. And of course that assumes sending a minimal number of probes to exactly the two nearest neighbors, rather than spamming everything within reasonable range, e.g. sending probes to 4 and 8 ly away, in which case it'd be the longest legs which limited your expansion time; 8 ly jumps would mean 375 million years, or 37.5 million years, etc.
[1] Note that broadly speaking, colonizations and civilizations, or generation ships, are "self-replicating probes" of their own. It's not like you're limited to mechanical clanking replicator von Neumann machines or dubious nanotech; replicators are free to include biology as needed, whether frozen bacteria for use in bioreactors for mineral separation or colonists for intelligence. Granted, full colonies are probably erratic enough for the percolation model of non-propagating colonies to fit, but anyway.