Thousands of megastructures in a galaxy with 400 billion stars would be a very low incident rate. This low incidence of megastructure building would itself raise interesting questions. If it is worth building them, why not build as many as possible?

One possible answer is that megastructures actually cause growth to slow down, at least as observed from outside. A megastructure could be a phenomenally complex place - one ringworld could hold the same population as three million Earth-like worlds. And a ringworld is not the largest possible megastructure, by a long chalk. The colonisation of such a megastructure would take a certain amount of time- and the resulting cultural complexity of a world with the same population as an empire of 300 million planets, all in close communication, would be phenomenal.

Add to this the possibilities afforded by miniaturisation, or by virtual realities- given enough infrastructure and technology, an advanced civilisation could grow in complexity for a long time without ever leaving their home system. That could slow down any expansion process considerably.

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Another reason that megastructures might be rare is that they could possibly be used as weapons- see the Nicoll-Dyson Laser as an example.

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It "could" but this is just another soft explanation. It supposes that every one of the millions of hypothesized advanced tool making intelligences (if you agree with Milan M. Cirkovic that Copernicanism etc imply intelligent life should be common in the galaxy) decided to stay at home, playing with their virtual reality helmets. A cogent argument as to why this would necessarily be so would be much more impressive.

And why no Von Neumann machines? The claim that they might not be possible implies new physics, and the idea that every one of the millions of hypothesized advanced tool making intelligences didn't think it was a good idea is very soft.

I don't see why that would make them rare.

I call it 'light-speed paranoia'. Assuming that the speed of light is a real limit, an interstellar empire spread out over several planetary systems would separate into distinct units, isolated from each other by years or decades of light speed delays. This would provoke mistrust and paranoia, as all military intelligence would be years or decades out of date. The possibility of weapons effective at interstellar distances would allow such separate elements within the empire to attack each other, with or without justification.

Circovic, in another paper
http://xxx.soton.ac.uk/ftp/arxiv/pap.../0805.1821.pdf
expresses doubt that interstellar 'empire-state' civilisations will occur, in part because of this light-speed separation of individual parts. Instead he thinks that each planetary system would become a 'city-state', sufficient unto itself and not particularly interested in expansion.

I can see the reasoning behind this; any particular 'city-state' planetary system could develop itself to the limits of miniaturisation and computation, becoming a fantastically complex entity. But the advantage of having another such fantastically complex entity in the next system is comparitively small, and the dangers or perceived dangers may be high.

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It's also just a lot more difficult and expensive. I think the cost of interstellar colonization is often ignored. If the choice is building another O'Neil style habitat in the same system, or attempting to colonize another system, the habitat is much cheaper. If there is a reason to build megastructures at all (perhaps clouds of habitats), they would most likely be found in the home system. But it isn't obvious to me that many cultures would necessarily get around to megastructures, so a few thousand in a galaxy seems as reasonable a number as any other.

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I say there is an invisible elf in my backyard. How do you prove that I am wrong?

Disclaimer: Avatar is not an official NASA image and does not imply any specific interplanetary or interstellar capability.

The Leif Ericson Cruiser

I thought this article was more interesting than the one in the OP. I especially liked this comment:

That's a belief I've long had trouble understanding.

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I say there is an invisible elf in my backyard. How do you prove that I am wrong?

Disclaimer: Avatar is not an official NASA image and does not imply any specific interplanetary or interstellar capability.

The Leif Ericson Cruiser

Well, it's at least better than guessing.

If we assume that we are typical (Copernicanism), then a ratio of 1 civilization-building-species out of a few 100 Mio (macroscopic) species is an acceptable upper limit. If the not (yet) observable, "true" ratio (that certainly exists) was considerably higher (say, 1:10000), we would be one of the very few planets were it took life an enormous amount of time (relative to average) to come up with a civilization-building species - this would make the Fermi paradoxon even worse. If the ratio was considerably lower, we are in "Rare Earth" territory.

I'd say that the "true" ratio is somewhat smaller than 1:~few 100 Mio, but not outlandishly so - maybe 1:~few billion, because this would be in agreement with our observation that mankind appeared towards the end (or at least, more towards the (expected) end in ~200-500 Mio years than towards the beginning. ~600 Mio years ago) of the lifetime of the macroscopic biosphere on Earth.

Think of it as a game with 3 dices - who ever gets 3 sixes wins (1:216), but you are only allowed to try 20 times. Over many such games, you would clearly see that the typical winner is found towards the end of the 20-tries series - the smaller the chances of winning, the closer to the end (this is because we see only the lowest part of the tail of the probability distribution peaking at ~216 tries).

If the ratio mentioned above now really is in the 1:~few billions ballpark, obviously there will be hundreds of millions of Earth analogs (macroscopic biospheres) where no civilization-building species ever arises.

It only takes one, so in order to maintain a belief in millions of alien civilizations in this galaxy you need to assume strong limits on the probability that any one civilization will do so. Given that every second thinker of our species who considers the future seems to think that interstellar expansion is likely and imperative, it doesn't seem particularly naive to believe that at least, say, one in a million, alien intelligences would have the same idea. Remember the Copernican Principle? we're nothing special, they say¹. Our dreams are ordinary dreams.


1. I don't give the CP much weight in this domain, but it is central to Cirkovic's reasoning so I'm playing along.

I don't have any particular belief about the number of civilizations in the galaxy. However, I see no reason to assume that any one technological species must conquer the galaxy. Among other things, it isn't clear to me that there would be any strong motivation to do so, it does appear to be expensive and difficult, and it requires an unremitting effort to continue.

People have many ideas. So what? Also, interstellar expansion isn't automatically the same thing as conquering the galaxy.

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I say there is an invisible elf in my backyard. How do you prove that I am wrong?

Disclaimer: Avatar is not an official NASA image and does not imply any specific interplanetary or interstellar capability.

The Leif Ericson Cruiser

Yes indeed.

As a quick perusal of this thread will confirm, and a reading of the paper cited will make utterly clear, we have no model that can be trusted to predict the behavior of any civilization let alone an alien civilization with unknown technology and unknown knowledge of science. Note that the cited "paper" contains no quantitative statements, no clear premises, and no application of deductive logic.

All that is evident is wild speculation, lacking any foundation and therefore lacking any pretense of a logical conclusion.

A paradox requires a logical contradiction based on a set of acceptable assumptions and the application the usual rules of logic. Here we have neither. The only fact in evidence is that there is no documented incidence of visitation by an alien civilization. Period.

There is no paradox.

It must have been a "slow news day" for the editors of the Serbian Astronomical Journal.

This paper, and David Brin's earlier paper it cites, are attempts to be exhaustive. The fact that we are not members of an interstellar civilisation is an observation which is a little puzzling. Without any other data we can't determine the reason for this singular fact, but we can make lists of possible reasons. I would be curious to know what conclusions could be reached by the application of logic alone.

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Without reasonable premises and a reasonable model there are no conclusions that can be reached regarding alien behavior. That is precisely the point.

You cannot possibly make a claim to being exhaustive without knowledge of the set of possibilities that you purport to be exhausting. We have zero basis on which to judge the behavior, technology and scientific knowledge of an alien civilization. It is, after all, alien.

We don't even know if interstellar travel and communication is feasible. Based on our current knowledge of physics it is not. Trying to make an exhaustive list of possibilities based on things that we know that we do not know is futile.

The only thing that those papers exhaust is the reader.

What you can conclude based on the application of logic alone is that the arguments in the papers in question have no logical foundation at all and that no conclusions can be justified. Those papers are not science papers at all. They are more along the line of pointless philosophical musings.

Whether or not they are pointless is subjective, but I do agree about their characterization as philosophical musings.

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Conscious reasoning is an attempt to justify the choice after it has been made.

I agree, it is more of an observation than a paradox, and an unpopular one too (to the SETI fans).

How about both? The road from protoplanetary disks to intelligent life is extraordinarily difficult and unlikely because catastrophes such as gamma ray bursts and comet impacts periodically knock life back to lichen and cockroaches stage. Does not have to be "sterilizing", or even "back to bacteria stage". Just enough to keep anything with a big brain from getting a toehold.

And Earth just got lucky.

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Fiction has to be plausible. Reality is under no such constraint.

My comments were in the contect of Cirkovic's proposal of galactic scale effects preventing intelligence from evolving until recently. I agree local bad stuff (comets etc) could be part of the reason intelligences are rare. The corollary of the "we got lucky" hypothesis is that our luck could run out at any time.

Still, as Cirkovic apparently notes*, there appears to be some evidence pointing towards our luck being slightly less likely to run out just now and in the future than it would've been earlier on in the history of the Solar system and the Milky Way.

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*) Didn't read much beyond synopsis, instead decided to actually print a hardcopy of the paper. Will have to peruse it at my leisure during my vacation.

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The dog, the dog, he's at it again!

Circovic's main argument seems to be that we can't trust the Copernican Principle to assure us that life is widespread, although he provides many counter-arguments.

Perhaps the best counter-argument is that given in his other paper about 'city-states' and 'empire-states'; an advanced civilisation might be expected to consistently create a complex civilisation within a single system, rather than bother with colonising elsewhere and simply duplicating itself with little net increase in complexity.

The complexity of a civilisation within a single solar system is limited by the amount of available matter and energy in that system; each star would have a similar limit, and even if you had many separate colonies they could not become much more complex when considered together as a civilisation, because their intercommunications would be limited by light-speed delays and distance.

The only real advantage to colonising nearby stars is that an interstellar civilisation is no longer dependent on a single star; a local catastrophe might wipe of a stay-at-home civilisation, and eventually the star would itself leave the main sequence (unless some sort of stellar engineering is possible). So it would make sense to colonise at least a small number of nearby stars.

Note that there isn't any detectable starlifting going on in any of the stars we can see, unless we don't recognise it.

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Agree with those that feel it is not really a paradox. There is simply too much not known (not enough information) for it to be a paradox.

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Don of Borg - Cool, Calm, Collective.

"Within the next generation I believe that the world's leaders will discover that infant conditioning and narco-hypnosis are more efficient, as instruments of government, than clubs and prisons, and that the lust for power can be just as completely satisfied by suggesting people into loving their servitude as by flogging them and kicking them into obedience." -- Aldous Huxley

Spectroscopic observations provide hard evidence of the abundances of the elements that comprise CHON centered life. These abundances are sufficient to support the rise of life within each galaxy for which spectroscopic data indicate adequate abundances. The durability and flexibility of the CHON centered molecules and their profusion into the intragalactic medium from successive generations of supernovae suggest an abundance of life's building blocks and the initiation of pre-biotic evolution (self-organization) to produce molecules that support life. Once biological evolution is initiated by the tendency of these molecules to continue self-organization, the forces that drive natural selection produce ever greater complex living forms leading beyond cockroach types to technically competent critters capable of building telescopes and of accomplishing interstellar travel. They quickly become informed by their musing that the universe is capable of destroying them, and, having nothing better to do, they consider how much safer it would be to live around long-lived, highly stable, stars that supply enough energy to support them and are sufficiently far from novae and supernovae pre-cursors. They migrate to locations where they would not likely have originated and build infrastructures to support the comfort and safety of their lifestyle. Having reached Kardashev Class V level they merrily begin configuring the universe to their liking---let us hope we are among them.

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For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?

Fermi captures space-time theory evidence


WASHINGTON, Oct. 28 (UPI) -- NASA says its Fermi Gamma Ray Space Telescope ended its first year of operation by obtaining a measurement that is evidence of the structure of space-time.

....

But capping those achievements is a measurement that provides rare experimental evidence about the very structure of space and time, unified as space-time in the theories of the late theoretical physicist Albert Einstein.

On May 10, Fermi and other satellites detected a short gamma ray burst that scientists think occurs when neutron stars collide, NASA said. Of the many gamma ray photons Fermi detected from the 2.1-second burst, two possessed energies differing by a million times. Yet, scientists said, after traveling some 7 billion years, the pair arrived just nine-tenths of a second apart.

"This measurement eliminates any approach to a new theory of gravity that predicts a strong energy dependent change in the speed of light," said Peter Michelson, Fermi's principal investigator at Stanford University. "To one part in 100 million billion, these two photons traveled at the same speed. Einstein still rules."

http://www.upi.com/Science_News/2009...2321256758304/

I totally get the paradox, and it is a paradox - plugging in even very conservaitve numbers into the Drake equation allows for many thousands of technological civilizations (with the ability to be detected by various means, including SETI, visitation, stellar artifacts, etc) to exist or have existed in the past. Fermi was saying that, because of the overwhelming odds that there are many intelligences out there, we should have heard or at least seen evidence by now. Yet we havent. This is in essence, the paradox.

You may have too liberal an interpretation of "very conservative numbers."

especialy since we have almost no hard numbers to place anywhere within a rigorous analysis of the Drake guideline.

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Iyam what Iyam

While I agree that we don't have any hard numbers to plug in yet, and any number for thr Drake equation above 1 is speculative, but we are beginning to find that our Solar system is indeed common. There are over 400 exo planets known today, and I expect Kepler to find at least a 1000 within the next 5 years; and its expected results are that 40-200 of those will be earthlike (inner, rocky planets in the HZ, possiblity of liquid water.) The models they used to predict average systems are showing that an average solar system with planets will have between 1 and 3 inner rocky worlds with at least one gas giant > Jupiter Mass, which many scientists think is a necessary item for complex life to evolve on inner rocky worlds.

If we are a common world, in a common solar system, then it does stand to reason that the likelyhood of life evolving is , well, common, or at least plausible. Therefore, if we take this assumption, and plug in very conservative numbers for main sequence stars that have middle aged rocky planets with liquid water, plate tectonics, and gas giants keeping the extinction-level asteroids off their backs, and even more conservative numbers for intelligence arising from life bearing worlds, we should still have thousands of civilizations.

the operative word is should, by our reasoning. Also, by Fermi's reasoning. Which is why its a paradox: If there are so many civilizations, then where the heck are they?

I dont believe my numbers are too liberal, but of course, no one can really say. Its an opinion of a well read and (overly) optimistic layperson. Obviously something is wrong with my numbers, or we would have heard from ET by now. which is why its a paradox.

Personally, I think life is common but our level of intelligence is rare. Or, perhaps they all went machine intelligence on use and they dont care for meat intelligences anymore.

Even a number of 1 is speculative. A number >1 would mean that every galaxy contains (on average) at least 1 civilization. It could be even lower than that, of course - 1 civilization per 1000 galaxies, for example.

I think we are simply overestimating the chances of the formation of life, macroscopic life, intelligence, civilization, as well as the average life expectancy of civilizations. We can always reasonably argue as if we were typical observers of the universe: it seems the typical observer lives on the home planet his species formed on, does not see any large space colonies or interstellar spaceflight. If we really are typical observers of our universe, then the Fermi paradox should not surprise us: we will never make it to the stars, whatever the reasons (put the other way round, if we HAD already crossed interstellar distances and built space colonies, the Fermi paradox would be even deeper). But off course, no one wants to hear that.

The Drake equation estimates the number of technological civilations in the Milky Way. The '1' is us.

The equation, and nearly everybody who discusses it, leaves out the question of how many of those rocky planets in the HZ also have a large (in relation to the planet) moon orbiting them?

How necessary is the moon, in terms of life forming on Earth? With no large satellite to produce tides and keep the water sloshing around, do you get life? Can the necessary processes occur in still, stagnant bodies of water? Would tectonic activity alone churn the water sufficiently?

Our Sun may very well be a common star, Earth may be a common planet, and our solar system may be a common configuration, but the precise impact that formed our moon could very well be rare.

That is a good point. The tidal pull of the moon is postulated to be an important reason on why life evolved. They dont have to be created from primary impact, though, they could be captured objects.

However, we also forget life that can develop on moons such as Titan, where a larger body such as Saturn constantly stresses the mantle and provides energy for life.

The Drake equation and the Fermi paradox make a lot of extra assumptions; to me the most interesting assumption is to disregard the possibilities of interstellar travel. If you consider interstellar travel to be possible, then it's possible that one civilization has already colonized and dominated the galaxy. At that point, there are any number of plausible reasons why a single civilization might not feel like contacting us.

Many Fermi paradox "solutions" work fine for one ET civilization; they only strain credulity when they have to apply to thousands of ET civilizations.

Actually, they don't disregard it. They postulate that there has been ample time for civilizations to colonize the galaxy without the need to introduce FTL travel. Von Neumann Machines could have already done it easily, if there were some number of space-faring civilizations around a few million years ago. I think the last estimate I saw was it would take about 100,000 years to do so. This is one of the reasons Fermi asked the question, "Where are they?"

Why would interstellar travel be impossible? Interstellar space isn't much different to space near a star.

The only problem humans have with it is the long time is takes to complete.