I don't consider "The Fermi Paradox" to be a paradox at all. Considering the breadth of solutions proposed I think that the use of the word paradox is a misnomer. Acknowledging that none of the proposed solutions have been confirmed I would, at best, call the "Fermi issue" a question not a paradox.

It is a paradox because an estimate that does not seem outlandishly wrong is colliding with observations that suggest something completely different.

I read the preprint yesterday, it's well worth reading and a nice review of the topic.

I note that Ćirković has improved upon Brin's overview somewhat, by including all the additional scenarios which I mentioned in an earlier thread; not surprising since he himself devised most of them. However David Brin's methodical presentation of the possibilities remains very useful.

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The only explanation I don't see explored in detail is my highly speculative 'ennui' explanation.

If all civilisations eventually upload themselves into electronic form, and if all electronic minds redesign/evolve themselves towards some hypothetical perfect state, then that state might be free from any of the innate biological goals which give biological life a sense of purpose, however illusory. With no biological nature to satisfy, all perfected electronic minds might fail to see any significant difference between existence and non-existence, between 0 and 1. Having achieved freedom from desire and from the tyranny of the body, these electronic minds might just shut down. END-IF.

Do I believe it? Not in the slightest. But since we know nothing about hypothetical electronic minds, I don't think it can be ruled out.

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eburacum45, that's an interesting proposition. It goes to the question of, to coin a phrase, to be or not to be? What is it that motivates most of us to go on rather than not? I suspect that most people don't think about such existential things until they're unable to function in the manner they've become accustomed to for whatever reason (be it physical or psychological).

Whether a mind is encased in a biological shell or an (artificial) electronic circuit, I would imagine that a desire for new and varied 'inputs' and interactions with others - similar, but different, would serve as a driver for continued existence. Should such stimulation cease or stagnate, than a case may be made for not to be...

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Interesting, but its a "soft" explanation. Unless you could convincingly show why this end were to happen to ALL civilizations, we still would have to explain why there is not a single expanding civilization that averted this fate.

The non-RareEarth scenarios I find most convincing, after giving it a long thought, are the ones called "Phase-transition" and "Deadly Probes".

It's not a paradox when the observations are of an infinitesimal part of what is known to be a very large arena*. Even if we could observe the entire arena then it would only be a semi paradox because it would still be based only on an estimate based on some crucial factors that we do not know. We don't know how life emerged let alone rose to inteligence. We may find that the conditions required for life to arise may be so special that even with the vast numbers of possible places and time for it to arise it is still improbable that the special circumstances could appear more than once if at all.

Even given the fact that the prerequisite for inteligence of having an onboard computer, where our inteligence resides, does not conclusively give rise to human like inteligence. The rarity of inteligence on Earth in spite of all the varieties of specis, capabilities, environments, time, pressures and cirumstances is indicative that inteligence may require very special circumstances indeed. These are all still unanswered questions and the way they are answered will determine if there really is a paradox. The "Fermi Paradox" is reflective of a lack of knowledge not a conflict of facts.


*arena including space and other possible causes of concealment.

Well, I used to think that a Gamma Ray Burst could wipe out vast volumes of an inhabited galaxy, causing a temporary collapse in the local population of intelligent species; but this explanation doesn't seem to hold water.

For a start, the most powerful GRBs are highly directional, so they would only sterilise a long, narrow section of any particular galaxy. Secondly, an advanced civilisation could build shelters that would preserve part of their culture against such a disaster- if built a few tens or hundreds of metres underground, they would be perfectly safe.

Any conceivable natural disaster would be survivable, if you safeguard yourself against it- except for a few very energetic events like galactic starbursts, and they don't occur frequently enough to remove galactic empires consistently.

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I don't much like the paradox label either.

In my opinion he wastes virtual paper on nonsense (what he calls "Solipsistic Explanations") and hand-waving/wishful thinking ("soft" explanations). I'm fine with an exception to gradualism, but he doesn't suggest a plausible mechanism. Either the galaxy was just generally uninhabitable for some reason we don't know about until relatively recently, or we're in the period after some astrophysical catastrophe that wiped all advanced life out. The latter seems particularly unlikely: a civilization millions of years ahead of us should know about such potential disasters and would have had plenty of time to build shelters (as ebu points out).

The explanations I prefer are either the road from protoplanetary disks to intelligent life is extraordinarily difficult and unlikely, or all intelligent life evolves according to laws we do not know into something that makes no visible impression on the universe. That something could be "dead" or something like ebu's 'ennui' explanation.

Though I would agree that there is no positive evidence for this class of explanations, and I do not consider them very likely, it does not feel right to me to exclude them a priori by calling them "nonsense".

Why would you say that? A soft explanation has nothing to do with hand-waving and wishful thinking - it's just an explanation of the fermi paradox that depends on local developpements and that can therefore not be assumed to work for all civilizations (like "they all destroy themselves in a nuclear war").

Therefore, if "Rare Earth" (which I would prefer) and "zoo" are rejected, we are left with the "deadly probes" scenario.

Guess what I would use against gamma ray bursts? :P Yep, von Neumann probes. I think they would be better than getting bored in a shelter. Parking billions of wide but thin probes between planet and potential source of burst would probably do. They could be even used as window blind: normally they are in position that doesn't cover our view: but they turn to covering position immediately when the burst is detected, thus forming one huge shield.

Bynaus, don't you think we could safeguard ourselves against "deadly probes" scenario?

A shield made only of self-replicating devices would be thin, and inadequate to stop penetrating radiation such as gamma rays from a GRB.

A few tens of metres of rock would be a better shield. Many designs for space habitats include a rocky shield on the outside to protect against space radiation hazards; silicate rock is a common enough material in space, so using rocky shields to protect sensitive biology or electronics is likely to be a universal rather than a parochial discovery. And any planet with a rocky surface would make a good shield too- you could live deep underground, using geothermal energy for power.

Self-replicating technology would be very useful if you used it to gather shielding material and increase the thickness of the shield around each habitat - assuming you had enough warning, and that SRT in fact works (not yet proven, unless you count biological processes).

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OK, I'm not a material or radiation specialist so I can't comment what kind of thickness or material would be enough. But that layer of rocky shields sounds reasonable and material would be easy to find.

The good thing in "remote shield" approach would be that you could leave the nature intact - it wouldn't be necessary to haul it to the depths of Earth.

Because shields would be operated relatively near Earth I think also "semi" or "aided" self-replication would be enough.

Similar shield devices could also be used to cool certain areas of Earth (and if you add mirrors, also warm some other areas) and to drag threatening asteroids off their path.

But now I'm starting to drift away from the topic....

No. Say we battle 10 of the self-replicating probes, and we win. Prior to destruction, the probes signal to all neighboring star systems that they will be destroyed by a new kid on the block... So all the sister probes in the neighboring star systems produce a 100 new probes and send them to intercept. Additionally, they hand on the message to star systems even farther away, which also produce backup-probes in case the second wave fails... In time, a civilization with the resources of only one star system at hand would have to battle wave after wave of deadly probes which have essentially the entire resources of the galaxy at their disposal.

So the first civilization to establish a self-replicating probe network has a huge advantage over all the others. The only strategy to counter this would be to sneakingly establish another probe network, without the first network noticing.

I completely agree that you can't battle self-replication without self-replication. But I'm not so sure the first one who builds probe network will deny that right from others. If that civilization fails in it's attempt to destroy all the other probe families (emerging a bit later), it will be probably isolated and hated for the rest of the time left in the universe. Also, they might find their destroyer probes morally problematic.

When looked at from a purely scientific and conventional viewpoint, the FP certainly seems illogical. Looked at in a slightly less conventional way, it can appear less so.

When asking why (it would appear) there are no other intelligences in the universe, one might contrast it to the question of why there are no other intelligences of the human 'type' on our individual planet. Certainly I would not suggest there are no other species on Earth which possess intelligence of any kind...but I do believe it is clear that in this regard, human beings are unique.

There are approximately 1.5 million species on our planet. Only one of these would appear to embody the particular set of attributes we humans do. Certainly none of them save our own feel the need to develop technological societies of the type we are seeking evidence of in the universe beyond our planet. Whether they lack the level of sophistication, desire or capability to do so, it is a fact that we are unique out of that 1.5 million.

Compare this to the silent universe...or in terms of the drake equation, our own galaxy. Our one out of 1.5 million makes the silence in this context somewhat less remarkable or unusual. Of course, the two examples are quite different...but it does give a different perspective on the question.

We humans sometimes set ourselves apart from the other species on this planet by proposing that we alone possess a soul, or some special aspect not found in other forms of life. It is often remarked that humans beings could truthfully be described as bits of the universe that have achieved self-awareness. Just as our planet has only one apparent such form of life, and just as we human beings have only one brain, perhaps there is some mechanism fundamental to the nature of the universe that determines only one human-like intelligence can exist at any one time....

I myself would have to say I do not believe this is the case. If it were determined to be so however, given the above, I would not find it unnatural, in the context of the way things operate in our universe. Just as our individual consciousness cannot occupy more than a single body at any one time, I could accept that any consciousness the universe might manifest can only do so in one species.

In any event, again I do not hold this view - I do believe there might be another explanation for FP (such as cosmic bureaucracy!), but it came to mind in the wee hours this night as a possibility.

Probably because we displaced them. We co-existed with the Neanderthals for quite some time until we displaced them.

My take is that while simple life may be prolific, complex life is less so and technologically capable life even rarer. Furthermore, by all accounts, species have a finite lifespan, space travel is very hard and the cosmos is vast. All this means that the chances of finding other intelligent life that happens to exist at the same time that we do and are near enough to be detectable, are vanishingly small. So, if anything, I would turn the 'paradox' on its head and say that it would be amazing if we did in fact discover ETs.

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The basic idea is valid. We simply can not take for granted that evolution will bring up a civilization-building species in the lifetime of a biosphere of a typical Earth-like planet. The odds are even lower than 1:1.5 million, as there have been millions and millions of species in the history of the biosphere, and only a single one of them (us) has formed a civilization. So it seems, the chances of a biosphere to come up with a species able to form a civilization and eventually travel to the stars are no higher than ~1 to ~a few 100 million (nr of species that have appeared on Earth so far) - because of the anthropic principle, this has to be considered a lower limit. So even if there were 1 million full-blown biospheres (i.e., containing macroscopic lifeforms, a rather high estimate (?) ) in the galaxy, only every ~few 100 million years we would expect one of them to come up with a civilization-building species (at least, one with the potential to build one).

That's no revolutionary insight. It is the ratios that matter.

Nonsense. The proportion of species at one specific point in time (now) on one specific planet (Earth) that have formed civilizations tells us nothing about the probability that a random Earth-like planet will ever (over its lifetime) give rise to a civilization.

I never claimed it to be revolutionary, just likely IMHO.

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One solution I didn't see listed is one I find very obvious. Signs of other civilizations are out there, we just aren't seeing them for what they are.

We're busy looking for radio signals, because that's what we use at this stage of our culture. What are the odds of another civilization at our level of technology using radio waves close enough that we would be able to catch it? How long will we be using radio waves? another 50-100 years? So any civilization out there would have about a 200 year "window" to spot us? We are not likely to spot any other using this technology, the odds are against it.

So what will they be using? What type of radiation would an advanced Civilization be "leaking"? How would we recognize any advanced technological species? Will we be able to see or detect a Dyson Sphere that encapsulates an entire solar system? What type of technology will they be using? Bioengineered Spaceships? Heck, there could be VN machines now, harvesting parts of the Oort cloud and we would not likely know. Not only is there not enough evidence, we don't know what the evidence will look like.

The answer to "where are they?" is "All around us...we just don't know at his point what they look like."

And that is the major problem. Even if there were traces of large scale/macro engineering projects, they wouldn't be obvious as such from interstellar distances. The data would be open to interpretation with many mundane explanations competing with the extraordinary. They would have to be literally in our backyard to be easily discriminated.

It's a lot easier to detect some king of a modulated signal and determine that it could not be natural in origin.

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Of course that's possible, but still simply conjecture.

So what point are you making? That we should give up trying? Or that we somehow do a quantum leap (perhaps literally ) in our knowledge and technology to improve the odds? Or perhaps we should do what we're doing right now -- the most we can with the resources and technology currently available and do a lot of brainstorming on what the next step should be. I'd be interested in hearing what your ideas are on what that next step might realistically be.

No, the answer is that we don't know where they are or even if they are, at this point. That's what we're still trying to answer. Remember, we're still doing baby-steps. If there's anyone out there, we may not know within your or my lifetime, but we're progressing, and I'm hopeful it will lead to an incredible discovery.

Luckmeister

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Waste heat, and active radar systems of some kind, might be most the detectable emissions.We might not recognise all possible types of advanced species. but any species which uses megaengineering and stellar engineering could be detectable using current techniques. Megastructures would eclipse the local star in the same way that planets eclipse the local star- we are detecting planetary transits on a regular basis, now. Megastructures would have different characteristics but would be detectable, unless they are smaller than certain limits-= in which case the question is why do such advanced civilisations limit the size of their megastructures to below detectable levels? Are they scared of detection? Oh yes. It would still emit waste heat. To hide it completely it would have to be fantastically large, so that the outer shell re-radiates the star's luminosity at the same temperature as the CMBR. Once again that would suggest a deliberate act of concealment. Even bioengineered spaceships would probably emit detectable energy. How else would they move? Through the power of thought? Yes. That has also been suggested elsewhere by Robert Freitas. Trouble is, why would a civilisation equipped with VN machines stop them from replicating at an undetectable level? They could be used to convert the Oort Cloud, the Asteroid Belt and Earth itself into habitable megastructures or computation substrate. Why would they stop at undetectable levels? The lack of evidence is evidence itself.

They appear to look like an empty universe. Is this deliberate?

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Fermi's question, "Where are they?" stated as a reaction to a lunchroom conversation about the existence of possible advanced alien life is based on an assumption that advanced civilizations are in some way, like us or are far-flung versions of us, if not physically at least psychologically. There us nothing in nature that dictates any alien lifeforms would think or be like us in any way.

There is however a simple extrapolation that the basic chemical components necessary for life to evolve can be detected in the galaxy and this infers that life elsewhere is possible. We don't know if alien lifeforms exist yet we often unconsciously apply our concepts of a human 'civilization' as a necessity to such lifeforms.

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Good point. Stanislaw Lem, the Polish science fiction writer, came up with an interesting concept that I find useful in this sort of speculation. He coined the term toposophy: the 'shape' of 'wisdom', meaning the many forms which intelligence might take. One can imagine intelligent swarms of insects, intelligent cultures of bacteria, intelligent fungi and plant-like organisms, as well as various kinds of intelligent animals and sentient computers of a great many kinds.These would all have different 'toposophies', and their civilisations would all manifest in different ways.

Some of the diverse species one might expect to emerge might be expansive and be disposed to building megastructures, while others might not. We would tend to observe the ones which are expansive and which build on a large scale - even though their toposophy might be very different from human minds, these expansive and big-building examples would be more widespread and more visible than others, so there would be a self-selecting process at work. The ones which build big would be more successful, if the criterion used to judge their success is total biomass, or perhaps total processing power.

Unless there is some selection process which prevents the emergence of expansive and big-building species, of course.

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Only if they were relatively close. As I've pointed out in other threads, there could be thousands of megastructures in this galaxy. We would have trouble identifying all but the largest types we've imagined, unless they were very close in galactic scales.

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It's unusual, but I'm in complete agreement with your statement.

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

The Leif Ericson Cruiser