Originally Posted by Brian Rush

I know I sound like a broken record here, but there are some compelling illusions about how rare intelligence must be, and it's an interesting exercise to penetrate and dispel those illusions.

It doesn't have to be rare are all to not be present here. Nobody residing further that around 50 light-years from Earth can both know about us and have had the time to do somehting about it (and this only if they acted immediately). That is, only a tiny piece of our galaxy containing less than 200 G-type stars would possess any reason to come here, as opposed to other closer worlds that that essentially the same. Based on our existing surveys of stellar systems associated with G-class stars, ones resembling our own are not common (none have been found so far). So the probability that any of these <200 closeby stars have systems like ours with terrestial-type planets is not large, much less that such planets would harbor technological civilizations. And this would be true even if a million such civilizations exist right now within our own galaxy.

Mike, you're assuming the "slow boat" explanation for why they're not here, which I agree is the most likely one. I chose Prime Directive in answer to the poll because that's actually what I consider the second-most-likely explanation, the one that must be true if Slow Boat is false. Just about everything I've posted on this thread is designed to show that Rare Earth is not true.

I would like it much better if Slow Boat is false. I prefer to believe that it is false. I recognize that's wishful thinking on my part. But just the same, I don't think we should assume that it's false. It probably is, but since we don't know everything there is to know about the physics that might impact the question, we can't say for certain that it is.

I disagree with one thing you said. "Much less" doesn't apply. If these G-type stars have Earth-like planets, those Earth-like planets probably have life, and if they have life they almost certainly have intelligent life, or will before long (in evolutionary terms), or did and it self-destructed.

Originally Posted by Brian Rush

Mike, you're assuming the "slow boat" explanation for why they're not here..

This is correct.

I would like it much better if Slow Boat is false. I prefer to believe that it is false. I recognize that's wishful thinking on my part. But just the same, I don't think we should assume that it's false. It probably is, but since we don't know everything there is to know about the physics that might impact the question, we can't say for certain that it is.

I would too. But then there is the fact that the aliens are not here, which is easily explainable by slow boat.

I disagree with one thing you said. "Much less" doesn't apply. If these G-type stars have Earth-like planets, those Earth-like planets probably have life, and if they have life they almost certainly have intelligent life, or will before long (in evolutionary terms), or did and it self-destructed.

I was not referring to intelligent life, but to interstellar-capable civilization. Also of the ~400 known planets, only one is Earth-like (Earth) and even that one does not have interstellar-capable civilization. Not only that, but a few million years ago (a short-time ago evolutionarily speaking) it did not even have intelligent life.

Originally Posted by Mikebert

I would too. But then there is the fact that the aliens are not here, which is easily explainable by slow boat.

And that's the most likely explanation of course. But it's fun to play "what if." It isn't the only explanation, and therefore not an automatic.

I was not referring to intelligent life, but to interstellar-capable civilization. Also of the ~400 known planets, only one is Earth-like (Earth) and even that one does not have interstellar-capable civilization.

Yet.

I was also referring to interstellar-capable civilization assuming that Slow Boat is false.

Not only that, but a few million years ago (a short-time ago evolutionarily speaking) it did not even have intelligent life.

That's a point in favor of believing that interstellar-capable civilization is common: once intelligence is developed, it doesn't take long for it to produce high-tech civilization. (Of course, that's social/technological evolution not biological evolution, so we should expect it to be faster.) As for biological evolution though, each step along the road to intelligent life was taken in a very short time once the prerequisite was achieved. So I'm convinced that high-tech civilization will occur wherever life occurs, in a few hundred million years.

So really there are only three possible explanations (IMO) for their absence from human discovery: slow boat, prime directive, and big bang.

Originally Posted by Mikebert

I would too. But then there is the fact that the aliens are not here....

Sure they are.

I was not referring to intelligent life, but to interstellar-capable civilization. Also of the ~400 known planets, only one is Earth-like (Earth) and even that one does not have interstellar-capable civilization. Not only that, but a few million years ago (a short-time ago evolutionarily speaking) it did not even have intelligent life.

A fact that really indicates how fast life evolves, which argues for many similar cases elsewhere, and that we too might continue to evolve quite quickly to today-unimagined levels. And Earth-like planets are being discovered now, or close to it. It takes really good observation to be able to see or prove one. We are just now starting to get that capability.

An argument against self-replicating probes.

Perhaps life mostly appears in non-earthlike settings? Consider gas bag creatures floating in the atmosphere of a gas giant. Perhaps a dolphin like intelligence might appear. Scroll down.

Life under the Europa ice?

Strange planets.

This is essentially my view. The idea is that the distance over which a civilization will expand is limited, and over time cultures change. Every time the culture changes to a stay-at-home type, that line of expansion stops and eventually the entire growth spurt ends. The underlying assumptions are (1) space travel is difficulte and length (this is slow boat) (2) many cultures will be stay-at-home (this is my cost argument). The percolation result is intuitive (at least to a chemical engineer like me).

Originally Posted by Brian Rush

I was also referring to interstellar-capable civilization assuming that Slow Boat is false.

This makes your case weaker. If slow boat is false then interstellar travel becomes easy, and the assumption than the range over which a civilization can expand is limited collapses. If you remove this, the self-limiting percolation* doesn't happen and so the Earth would very likley have been colonized long before the arrival of humans unless space-faring civilizations are extremely rare.

*See Tim Walker's probes link.

Originally Posted by TimWalker

Strange planets.

I am fascinated by this topic. I hope to live long enough for us to detect another Earth on a nearby star. To directly visualize an Earth orbiting Alpha Centauri would require a 15 meter telescope (largest in the world is currently 10 meters) so it's not out of the realm of possibility.

Originally Posted by Mikebert

This makes your case weaker. If slow boat is false then interstellar travel becomes easy, and the assumption than the range over which a civilization can expand is limited collapses.

Not really. We naturally (but wrongly) use terrestrial models of expansion to understand interstellar expansion. Interstellar expansion is different from planetary expansion in fundamental ways. Even if FTL travel is possible we are NOT going to be expanding across space the way our ancestors expanded across the Earth.

When humans emerged from Africa and colonized Europe, Asia, Australia, and the Americas, ninety percent of the job was already done for them. They could breathe the air. They could drink the water. They could eat (and be eaten by) other living things. They were still in the same biosphere, which nourished them, recycled their wastes, and ultimately recycled them. We are not a self-sustaining organism; we are sustained by a biosphere and that's how we are designed by evolution.

Right now, without FTL, we have the ability to get to another potentially-habitable planet, Mars. We've sent machines there already. But at this point, any colony established there would be supported by resources from Earth. The colonists could certainly produce their own energy and probably their own water and maybe grow their own food, but in doing so they would be creating a micro-biosphere, and I believe it's axiomatic that no micro-biosphere can be self-sustaining. It can only be sustained by importing more resources from an actual biosphere (from Earth, in this case) than it would have cost to support the colonists if they'd stayed home. Our distant ancestors could just hop in their outrigger canoes and go colonize Polynesia. We can't do that.

In order to colonize other planets on a permanent basis, we must create on them a self-sustaining biosphere compatible with our own life form. That could probably be done on Mars, but it would have a huge capital cost and take a long time. It would involve transporting more water to the planet (not from Earth; there's ice out in the solar system), genetically-engineering basic bacterial life to form a bedrock layer, then engineering more advanced plants and animals to create an ecosystem, and working out the kinks. We don't have the necessary biological and ecological knowledge to so this yet, but we're not too far off and there's no reason to believe it couldn't be done. Once it was done, then humans could move there and establish a Martian civilization. Until then, the only people who can live on Mars will not be colonists but scientists and others doing a job on the planet but being supported from Earth. The job has to be valuable enough to be worth the cost of support.

Because terraforming followed by colonization is SLOW, it can't be used to relieve population pressure. That means that before we can do it, we must stabilize our population within the limits imposed by our planetary biosphere. Until we do that, we're not going to be able to afford the cost of terraforming another planet, so there will be no Martian colonies unless and until we have a sustainable civilization on Earth. Once that's accomplished, one of the great impellers to migration (population pressure) will no longer exist.

So colonization in space will not be rapid. It will be limited not by the speed of travel but by the pace of terraforming. That means it could be done just about as easily without FTL travel, but in that case each colony would be politically independent, since collective decision-making is limited by the pace of travel and communication. There can be no interstellar nation or empire without FTL, but there can be interstellar colonies.

Originally Posted by Brian Rush

So colonization in space will not be rapid.

No it IS rapid. Suppose it takes 25,000 years for a colony to become established and then grow to a point where someone in the colony decides to found a new colony elsewhere (say for cultural reasons). In just 1 million years, the expanding species would go through 40 such iterations. In another post you talks about the short time (evolutionally speaking) needed to develop life and to development intelligent life. One million years is certainly a short time on this basis.

Each of these iterations is akin to a microbial division or budding. Forty generations would increase the number of settled systems from 1 to 1.1 trillion stars, ten times more than are in the Milky Way. You can argue it would take 250,000 years to establish a single colony (this seems excessive) but it still fills up the galaxy in less than 10 million years, still a short time on a astronomical timescale.

Originally Posted by Brian Rush

Because terraforming followed by colonization is SLOW, it can't be used to relieve population pressure. That means that before we can do it, we must stabilize our population within the limits imposed by our planetary biosphere.

Brian, long before we develop any signficant space capability we will have had to solve this problem (or the issue of space travel is really moot). You know that peak population is going to be reached this century. Solutions to this problem will emerge (or not) during the lifetime of people now alive. Interstellar capability is not going to come on that time scale.

Our population will long have stabilized and probably will be falling long before signficant numbers are able to venture off of our home world, even into interplanetary space. There was never a rational case to be made for space travel to relieve population pressure, the idea is ludicrous. You cannot outrun an exponential with a cubic--simply apply L'Hopital's rule to determine the limit of exp(t)/t^3 as t goes to infinity. As you can see it blows up.

Originally Posted by Mikebert

Brian, long before we develop any signficant space capability we will have had to solve this problem (or the issue of space travel is really moot). You know that peak population is going to be reached this century. Solutions to this problem will emerge (or not) during the lifetime of people now alive. Interstellar capability is not going to come on that time scale.

I agree, this was actually my point, and you seem to have missed the significance of it. The real point is that we won't colonize space as quickly as we can. There will be no reason to do so. Why did humans expand across the Earth? Mainly because of population pressure. Too many people in one place to be supported easily and all get along, so Gronk, disgusted with Snort's incompetent leadership, says "Y'all come with me," and some do and they go off somewhere else that doesn't have any people yet. So it continues until there are no good hunting grounds for Gronk to migrate to, and then instead they take up farming.

With a sustainable society, there's no population pressure and thus no clear reason to expand. Expansion will be a planned thing. Also, think about this. How many planets are available to expand to? We've got at least two potentially habitable planets in this solar system and maybe a a lot more, depending on whether anything could be done with Venus or some of the Jovian moons. This may be a common feature of G type stars; planets that can be terraformed may be much more common than those that are capable of spawning life on their own. Without any real pressure to expand, and with quite a lot of room to expand into, the logic of Fermi fails.

Originally Posted by Mikebert

This makes your case weaker. If slow boat is false then interstellar travel becomes easy, and the assumption (that) the range over which a civilization can expand is limited collapses. If you remove this, the self-limiting percolation* doesn't happen and so the Earth would very likley have been colonized long before the arrival of humans unless space-faring civilizations are extremely rare.

I agree with Brian's point that advanced civilizations will not expand as fast as they can, because they would have sustainable populations. I add (assuming FTL travel is possible) that many claim Earth has been visited long before earthlings became civilized, and that ETs may indeed have colonized Earth; and that we humans are hybrids. If you don't buy that one, it is still possible for an advanced civilization to visit us, but not colonize. There are likely laws against colonization that are upheld by the galactic federation which we are not yet qualified to join.

In general, I don't think it makes sense to ignore the evidence that ETs are indeed visiting us, just because such an idea violates our own assumptions and our currently most-accepted science about the universe.

A number of nations - in the developed countries - appear on the verge of a population decline. So, in addition to Big Bang and Slow Boat, I offer this as another argument against exponential expansion.

Slow boat strategies - scroll down

Slow boat discussed. Will our solar system keep us occupied for a thousand years? A few thousand years?

Regarding Mikebert's comments...perhaps an exoplanet will be found that will capture the public's imagination, as did Mars.

Originally Posted by Brian Rush

How many planets are available to expand to? We've got at least two potentially habitable planets in this solar system and maybe a a lot more, depending on whether anything could be done with Venus or some of the Jovian moons. This may be a common feature of G type stars; planets that can be terraformed may be much more common than those that are capable of spawning life on their own. Without any real pressure to expand, and with quite a lot of room to expand into, the logic of Fermi fails.

Not to mention various types of artificial habitats that could be built from asteroid materials (or habitats in hollowed-out asteroids), which could all be placed in orbits where they would receive plenty of energy from the Sun (unless they are powered by other means). You could get a lot more living density per volume of material used (compared to the millions of cubic miles of material "wasted" under the thin biosphere of a terraformed planet or moon), and spin them for terrestrial gravity rather than having to adapt to the local gravity (or microgravity) of a smaller planet or moon. (Then again, I've read some interesting novels by Paul McAuley based on genetic "tweaking" of humans, animals and plants to adapt to such uncontrollable local conditions as microgravity--where habitats were built even on the smaller moons of Saturn and the other outer planets.)

I don't believe orbital habitats can be viable without drastic alteration of the human genotype. The problem isn't living space but total environmental footprint. I suggest considering what happened with Biosphere II concerning the difficulty of building micro-ecosystems that are self-sustaining.

I'm convinced the way nature does it is by redundancy. You can see this in the way reproduction works in every species, with a lot more reproductive cells produced than are needed. Failure is allowed for. The same thing happens with the biosphere and its symbiotic support system. There is enough redundancy built in that the system is very robust. We think of nature as a "delicate balance," but it's really not delicate at all. If it were, it wouldn't exist today, because we've had five going on six huge mass extinctions and a delicate system wouldn't have survived any of them. There's no way to reproduce that redundancy on a small scale.

Originally Posted by Brian Rush

I don't believe orbital habitats can be viable without drastic alteration of the human genotype. The problem isn't living space but total environmental footprint. I suggest considering what happened with Biosphere II concerning the difficulty of building micro-ecosystems that are self-sustaining.

I'm convinced the way nature does it is by redundancy. You can see this in the way reproduction works in every species, with a lot more reproductive cells produced than are needed. Failure is allowed for. The same thing happens with the biosphere and its symbiotic support system. There is enough redundancy built in that the system is very robust. We think of nature as a "delicate balance," but it's really not delicate at all. If it were, it wouldn't exist today, because we've had five going on six huge mass extinctions and a delicate system wouldn't have survived any of them. There's no way to reproduce that redundancy on a small scale.

The habitats don't have to be perfectly self-sustaining, they just have to be self-sustaining enough, for, say, 3,000 years rather than a billion, because there will be AIs making regular corrections.

Originally Posted by Odin

The habitats don't have to be perfectly self-sustaining, they just have to be self-sustaining enough, for, say, 3,000 years rather than a billion, because there will be AIs making regular corrections.

With what?

The point here is that such a system requires constant input from a self-sustaining biosphere in order to be maintained. AI making corrections is fine, but it can only do so using resources from Earth. That's assuming of course that we have a habitat for human beings. If it's ALL AI, i.e. a machine settlement, that changes the logic entirely.

Axiom: to support biosphere-dependent organisms outside the biosphere always requires more resources from the supporting biosphere than would be required to support the same organisms within the biosphere.