Crazy Kiwi
One of my WI Ps works is based on the premise that while life is commonplace, intelligent life of our level is exceptionally rare due to there being fewer places where intelligent, tool-building life can exist than places where extremophiles can exist.
Even if intelligences comparable with ours have arisen, if they happen to be aquatic cephalopod-like organisms in a liquid water haven beneath a kilometre of solid ice with no evolutionary advantage to developing tool-using limbs and no metal mining or industry, then we're not going to get them building cities or radios - just intelligent creatures that are very good at out-thinking their prey.
And such intelligences would be far outnumbered by worlds where the only life that can exist is some simple, extremely hardy organisms that don't notice the extreme heat or extreme cold or highly "toxic" (to us) environment.
Life, I think, is ubiquitous. Life that has had the opportunities to develop as we have could well be extremely rare.
Natural extinction events might not prevent intelligent life from evolving, but it can set it back from time to time.
Who's to say that some of the worlds out there haven't only recently (geologically speaking) become capable of sustaining intelligence - more recently than us - since the last natural Reset Button?
There may well be an intelligence equal to ours on the far side of the galaxy, but we're not going to be hearing about it any time soon, if ever, and closer worlds may be mostly populated by unintelligent extremophiles, maybe a few intelligences (say greater than that of a dog) that cannot, due to their development, affect their surroundings and an even fewer number of human-level intelligences, of whom we may well currently be the most advanced. With perhaps the occasional remnant of a greater, more advanced civilisation that died out millenia ago.
And that's just within the scope of stars that are the right age to support a civilisation at or near our level - say, your G and K class stars - that also have rocky, Earthlike planets in the HZ.
If all the planets in a system are Jovian gas giants and life has had to evolve on frigid moons with a smidgeon of liquid water beneath the ice, then we're not likely to be seeing visitors or radio signals coming from there.
Ave Imperator
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Do note that a gas giant in the habitable zone of a star would have moons theoretically capable of sustaining liquid water and, somewhat further out, liquid ammonia, the most plausible alternate solvent to water.
Depending on how thick the atmospheres of said moons were and the composition of said atmosphere, radiation might not be that severe of an issue.
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Except is doesn't. The K-T extinction was caused by an asteroid impact going by the iridium anomaly while the P-Tr mass extinction event (aka The Great Dying) was most likely caused by the eruption of the Siberian Flood Basalts, a massive volcanic event on a scale that has not been repeated since. (There is debate whether it was a direct cause or by intruding on coal deposits or methane clathrates or some mix of the three). One terrestrial cause and one extra-terrestrial cause localised to the solar system utterly destroys any degree of regularity.
Actually Gamma Ray Bursts can be pretty bad news out to nearly 10,000 light years which is a decent chunk of distance (about 10% of the galactic radius)
Two problems. One is that while GR Bs are often formed from hypernovae, there are other ways for them to form. Two neutron stars colliding to form a black hole could cause one, as could a black hole eating a neutron star and both of those are considerably harder to detect.
The other is... giant elliptical galaxies may lack much in the way of large, hot stars to form hypernovae, but that's because those stars tend to have brief, intense lives. This means that they're probably enriched in things like neutron stars. Also elliptical galaxies get that large largely by absorbing other galaxies which tends to trigger bursts of star formation. All this means that stars with habitable planets are likely to be getting on in years and been exposed to multiple periods of star formation.
Ave Imperator
Actually, I just checked my facts, and it turns out the actual reason for the hypothesized regularity of major extinctions is due to the inclination of the suns orbit around the galactic nuclei, not a cycle of galactic activity, which obviously isn't applicable to the Fermi Paradox.
edited 30th Jul '13 4:43:48 PM by Archereon
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I think that a mighty fine solution to the Fermi Paradox goes: "For every ten planets in a habitable zone around a stable star, there's one with native life. For every ten planets with native life, there's one with multicelled life. For every ten planets with multicelled life, there's one with a full biosphere. For every ten planets with a full biosphere, there's one with intelligence. For every ten planets with intelligence, there's one with civilization. For every ten planets with civilization, there's one with advanced technology. And for every ten planets with advanced technology, there's one with starfaring and nine burned-out husks."
Numbers like that if we had one star in 10 has a planet in the habitable zone would put 40 species in the milky way. 1 in 200 would put it at two species in the milky way.
40 species, 36 of them are ooga-booga cavemen and 4 Bronze Age. I'm fine with that.
Ndf27
In my own sci-fi, the solution that I thought up for this problem, inspired by the idea of resource limitation, is that most spacefaring civilisations reach a point where they may use FTL, for travel between star systems that negates time dilitation, but all matter that passes through FTL experiences something known as "temporal entropy". What this means is that all alien life and materials decay over time at a faster rate than normal, with the only artefacts of alien life being things made up of materials that hasn't had to pass through. Given that most colonies, even the ones set during the story's timeframe, are constructed off-world this means there is little evidence.
“You have your way. I have my way. As for the right way, the correct way, and the only way, it does not exist.” Friedrich Nietzsche
Trans Siberian Anarchestra (it/they)
Furthermore, assuming that GR Bs mostly originate from the deaths of massive stars, the fact remains that the largest galaxies in the Universe (giant elliptical galaxies) have no or almost no massive stars. Life that evolved there would be relatively safe from GR Bs. The closest one to us is Maffei 1, about 10 million light years away, well within flying distance if we assume that civilizations see fit to engage in extragalactic colonization.
On the other hand, anthropic reasoning suggests that some other aspect of giant elliptical galaxies probably inhibits the development of intelligent life there.
If this is a proposed explanation for the alleged periodicity of mass extinctions during the Earth's natural history, I would point out that the mass extinctions did not kill all life on the planet and ultimately did not prevent the development of intelligent life; thus, similar events occurring to an advanced interplanetary civilization would presumably pose no great threat to its survival.
As I recall, there is some indication that younger galaxies are more likely to have active nuclei and eventually 'go dormant' as they age. At any rate, a type 3 civilization (which ought to take less than 100 million years to develop) would have ways of manipulating the material in the galactic nucleus to keep things from getting out of hand. So if the cycle takes billions of years to complete, there is plenty of opportunity for it to be put under intelligent control.
One is the discovery that evolution on Earth seems to take place faster closer to the poles (source
), apparently due to greater competition over resources. Humans, on the other hand, evolved in tropical Africa, just about right on the equator, where resources are at their most abundant. This suggests that intelligence is not the result of more evolution per se, but of a higher availability of resources. This is good for the Fermi Paradox, because we suspect there are a lot of places in the Universe (such as Europa's subsurface ocean) where conditions capable of supporting simple life exist and have done for billions of years, but where resources are still much more scarce than in equatorial Africa. Locations where resources are truly abundant may be very rare.
Another is the fact that not only has there been a general trend for the most advanced organisms to increase in intelligence over the past 500 million years or so, but that the development of advanced brains has actually happened twice here on Earth. Once in vertebrates, and once in cephalopods, two lineages that supposedly diverged all the way back in the Cambrian (and possibly Precambrian) before their common ancestor had developed anything remotely like an advanced brain. This suggests that, by the time life gets to the stage that it was at in the early Cambrian or late Precambrian, intelligence is a relatively easy step to make.
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