•“Intelligent species might reasonably worry about the possible dangers of self-advertisement and hence incline towards discretion” — the “Undetectability Conjecture,” suggested by Beatriz Gato-Rivera.
•Strengthening that argument: “Evolutionary selection, acting on a cosmic scale, tends to extinguish species which conspicuously advertise themselves and their habitats.”
Referring to this new article, the good folks at Kurzweilai.net added:
"In SHOUTING AT THE COSMOS … Or How SETI has Taken a Worrisome Turn Into Dangerous Territory, astrophysicist and science-fiction author Dr. David Brin advises that “people who care about [transmissions from Earth] — preferring a wide-ranging discussion before a few individuals start screaming into space on our behalf — are going to have to do some yelling of their own.” He explores this issue further in A CONTRARIAN PERSPECTIVE ON ALTRUISM: THE DANGERS OF FIRST CONTACT and other thought-provoking articles."
Thanks Kurzweil folks!
Alas, I have been wrassling with the Fermi Paradox since before it was called that! Back in 1985 I named this mystery "The Great Silence" in what is still the only full review article ever published on the subject.*
There I cataloged almost a hundred explanations that people have offered for the silence and the impression that we have - so far - of being alone in the cosmos. Alas, most folks tend to choose one particular answer, over all the others, for reasons having a lot more to do with individual psychology than either logic or evidence!
If you look at the good old Drake Equation (it needs to be expanded by a couple of factors), then it's clear that some factor must be lower-than-expected, in order to make the emptiness that we seem to see around us. But which factor?
Funny thing. Those who *want* the cosmos to be empty of competition (so we can fill it with our descendants) tend to choose factors on the left side of the Drake Equation - e.g. those having to do with the number of potential life-bearing worlds, or the likelihood of biogenesis, or of intelligence or industrial civilization.
Those who are eager for contact - like the SETI folks - tend to choose factors on the right side to blame for the apparent absence of neighbors. Factors that let them say "that just means things are a little sparse; but give us time. Those signals will show up any minute!"
The saddest thing that I've noticed? Once a person picks a favorite explanation, he or she tends to cling to it, vociferously sure that all other theories are utter nonsense. I've seen this happen to some of the smartest guys I know. Such certainty... in a realm that has been called "the only scientific field without any known subject matter."
Almost nobody seems willing to admit "We just don't know; there's too little data. A dozen of the best explanations may be true, maybe even several at once. So let's act accordingly."
As for the theory recently published? The so-called "Darwinian Explanation" amounts to "they're all cowards out there, because some predatory types may be mean. So everybody's hiding." Well, well. It is an old, old, old hypothesis. It's been around a boringly long time, though sadly it seems that the authors think they invented it.Heck that explanation could be true. Indeed, because it might be true, I am part of a growing movement trying for a moratorium on idiotic "message-to-ET" shouting, at least till we learn a bit more and have had a chance to discuss these matters, openly, like adults. Still, that doesn't make the idea original.
(Or even especially likely. The notion of a universe filled with cowards... who stay cowardly FOREVER, no matter how advanced they become... seems no only unimaginative and temporally myopic, but deeply dismal, as well.)
Someday, I hope, some people will enter this field interested in exploring the full scope of ideas, the way grownup scientists do in almost any other field, actually reading the literature and comparing past arguments and progress before blabbing "I just figured it out!"
Perhaps it will happen one day, when curiosity and professionalism outweigh impulsive egotism. Heck, maybe then we'll be worthy and ready for contact.
--See more articles on SETI: Search for Extraterrestrial Intelligence
======
* My "Great Silence" paper about the mysterious Fermi Paradox, the strange lack of signs, in the heavens, of extraterrestrial intelligent life. Quarterly Journal of Royal Astronomical Society, fall1983, v.24, pp283-309 (Downloadable at my website
or at http://adsabs.harvard.edu/abs/1983QJRAS..24..283B )
.
Perhaps advanced civilizations use tight beam or cable transmissions like our fiberoptics so there is little energy leak to detect.
ReplyDeleteALL of them?
ReplyDeleteAlso, why was the Earth never settled or utilized or even visited (by careless aliens who flush a toilet) during 3 billion years of prime real estate?
See the problem isn't just about right now, is it?
Maybe if we understood why it took so long to develop intelligent(ish) life here, we'd have a better handle on it.
ReplyDeleteWhy no Sleestack artifacts from the long ages of dinosaurs?
We don't know what effect a visit might have - if visits tend to be full colonisation, we might have a rule that intelligent species don't develop on colonised worlds. Which might explain our location. (of course, perhaps they would uplift species)
ReplyDeleteCowardly aliens sounds like a cue for Pierson Puppeteers (manipulative bastards that they are!)
ReplyDeleteingish: the universal lingua franca
Maybe there is more grit in between the stars than people realize? If you get your starship up to a sizeable fraction of the speed of light, a grain of sand can cause quite a bit of damage.
ReplyDeleteLet's see: At one tenth light speed, a kilogram rock is a kiloton weapon, roughly using Newtonian math and Wikipedia's definition of a kiloton. A one gram pebble is thus the same as a ton of TNT. A grain of sand I guess is somewhere on the order of a kilogram of TNT. Need some pretty think walls up front if there is much grit out there.
As for the listening part, I wonder if most people can comprehend just how large of a difference there is between the 60 odd years we've been listening compared to the 10-15 billion years age of the universe.
ReplyDeleteBeing a visual person, I picture it as an microbe size person crawling over a .5mm grain of sand and then wondering why they're not finding any life on Earth. The odds that the first grain that we searched would turn out to be a crumb from a Girl Scout Thin Mint is pretty high.
If natural selection is at work, then there would need to be a selection agent that detects intelligent technological races and then wipes them out or silences them in some other way.
ReplyDeleteIt is likely that they too would be intelligent. Let's assume something like Saberhagen's Berserkers or Baxter's Xeelee are behind the silence.
Since silence is assumed to be the primary survival strategy, successful antagonists will start to employ countermeasures to encourage civilizations to break their silence. Call it the False Friend strategy. Bad guys pretend to be a peaceful intelligent race and start broadcasting in an attempt to prompt a response.
So not only do we have the potential problem of us broadcasting our location, but responding to an alien signal might be just as big a threat.
Of course if natural selection is in play, the "environment" will quickly become quite complicated with various civilizations filling certain niches. Examples:
- Liar civilizations that send signals from remote locations to lure the predator civilizations away from them
- Parasite civilizations that also send a signal, but do so in such a way that they attach themselves to the predator civilization in order to survive
- Hunter civilizations that intentionally attract predators into traps
- Altruist civilizations that broadcast warnings to everyone else about the predators.
And so on.
All the natural selection hypothesis does is suggest that the Drake Equation should be a bit more complex and broaden the "where is everybody?" question. It doesn't actually answer the question.
Still, an interesting idea. It is ripe with SF story possibilities.
Also, why was the Earth never settled or utilized or even visited (by careless aliens who flush a toilet) during 3 billion years of prime real estate?
ReplyDeleteBecause FTL travel is impossible? I know there's all kinds of theoretical physics that says that it might be possible, but those proposals all seem to have precious little proof.
My current standard is "assume FTL is impossible until absolutely proven otherwise".
Now, if FTL is possible, it would also mean that other fancy telecommunication methods are possible. Subspace and Amisibles and fiber optic cables strung through wormholes.
I can see the argument for 'spore probes', that travel at .01 C and seed life, maybe even eventually sentient life, onto worlds they encounter. My guess is that while there are many planets, and there are many planets with life, the living conditions on those planets are all different. Immagine worlds where life uses solvents like liquid ammonia or methane, or molten salts, or supercritical CO2. Each of those species is nicely adapted to its own planet, but while the universe teems with life, there aren't many species who can find other planets suitable for their own species. This seems especially likely, since life changes the planet it sits on (makeing an O2 atmosphere, for example).
If you then posit that terraforming is actually really hard, maybe even impossible to start with a little bitty spore probe, then there would be very few occasions to use a spore probe.
Why don't we notice the myriad other life forms? Because they all just use radio waves, and their signals dissolve into background static, just like ours do. Sure, they probably have their SETI equivalents, and some of them do focused broadcasts. However, for that to work, someone on the other star would have to be equally focused on listening to that particular star. In an infinite universe, that has probably happened at least a few times. Generally though? The aliens never make a connection and then get bored after a few hundred years.
And once more I put forward my own theory: the conditions for a planet of the proper size existing in the proper place developing intelligent life is fairly rare. There are plenty of planets out there with life on them. However, many are planets with gravity wells too massive for the civilizations to achieve orbit (or break orbit) from the planet, while smaller planets lose their atmospheres too quickly to support life long enough for intelligence and civilization to become established.
ReplyDeleteEarth lucked out in that it's just the right size, has a massive-enough moon to stabilize its rotation and axis and to act as a gravitational slingshot, and is in a fairly sweet zone allowing for life to flourish.
Evidence for my theory can be found one planet away in both directions: Mars lost its atmosphere, possibly after a large asteroid impacted it, while Venus proved too close to the Sun and has conditions lethal for life as we know it... and also lacks a moon to stabilize its rotation (ie, with a moon of its own, Venus might have been able to establish a viable atmosphere with liquid water for life to flourish... if perhaps life rather unlike what we'd consider normal due to the higher temperatures).
Robert A. Howard, Tangents reviews
Perhaps our civilization is not alone in having a shortsighted ruling clade, and none of the others have made the leap off of their home planet in any significant fashion either, because the investment needed could be spent at home buying the support of the populace.
ReplyDeleteAnd of course there would be little support for things like Project Ozma - where's the return on the investment? They need cash now, not promises of improvement later! You can't buy support with promises!
crigh - a weeping sigh, as one might give while contemplating the state of government funding for space travel.
I think it’s a matter of efficiency: perfectly compressed signal is indistinguishable from noise. If we were able to detect alien civilizations based on the leakage from their communications, that would mean they were wasting wireless bandwidth.
ReplyDeleteTwin Beam:
ReplyDeleteWhy no Sleestack artifacts from the long ages of dinosaurs?
But the Sleestak weren't from the past. They were the barbaric DESCENDANTS [emphais mine] of a race that could no longer hold their anger in check. :)
Seriously, I used to watch "Land of the Lost" in its original season (1974) and I STILL remember that revelation verbatim. It was one of the most chilling thoughts I could imagine at the time. This is from memory, but I'd bet good money it's 100% accurate:
I have made a grave error.
I have not moved backward in time, but FORWARD.
The Sleestak are not my ancestors. They are the barbaric DESCENDANTS of a race that could no longer keep its anger in check.
This is not my past. This is my FUTURE.
"FtL is impossible" is a right-side explanation favored by the SETI folk (right side of the Drake Equation.) But it has problems. If you send out self-replicating probes that mine local asteroids to make and send more copies... you can fill the galaxy in just 20 million years, even at 5% of the speed of light as your ship speed.
ReplyDeleteLikewise Gilmoure, you assume a static situation with nobody eagerly expanding - from full colonization all the way down to just sending "howdy probes." And comparing a new sapient race to an ant colony is facile. No matter how many there are, the event is far more rare and noteworthy than ant colonies, by six orders of magnitude.
Next novel goes into all this!
Oh, normal radio dissolves into background, but COHERENT (laser-like) radio beams do not. They can reach Andromeda.
Robert, all Fermi explanations need to explain why some exception hasn't - across 10 billion years... proliferated to fill the galaxy.
Jonathan has a partial-right answer- i believe. 99% of human cultures were conservative oligarchic and quashed progress for the good of the top elites. I figure that may be common among the stars. It certainly seems the way we are now headed.
Explain Sleestack????
"Sleestak" was a bug-eyed-monster, or rather a whole race of them, in the old TV series "land of the lost"
ReplyDeleteTwinBeam's comment was essentially "why didn't any of the dinosaurs evolve intelligence and tool artifacts for us to find today"
LarryHart pointed out that the Sleestaks were to an intelligent dinosaur what H. G. Wells' Morlocks were to humans: brutish evolutionary offshoots.
As for the von neumann probes, that would work, if you only wanted to fill the galaxy with probes. I just have a hard time seeing the motivation. To probe star systems? To leave a calling card? Certainly not to spread life.
May have said this before, why assume a star-faring culture would want to live in a gravity well? One of these days we may see alien mining equipment on a comet.
ReplyDelete"sersholt", part of the vocabulary of inebriation.
Tim Morgan, you left out the best strategy: camouflage civilizations. Can't be detected, can't be found, can't be hurt.
ReplyDeleteIntelligent Design tangent: Could the expanding universe be the cosmic version of the Tower of Babel, separating civilizations so that we can't mess with each other?
If so, the cosmic being started doing that project long before our particular species came along.
ReplyDeleteDr. Brin: There is an exception. Humanity.
ReplyDeleteBy chance we have managed to find the perfect spot in a fairly safe region of the galaxy with a fairly stable star that has allowed us to make it to the point that we can start expanding outward into the galaxy, should we choose to do so.
What's more, it would even make your theory of our universe being a simulation viable: the simulation was designed to have a species step out into the galaxy after the galaxy had already matured, and allow our curiosity as to why we're alone push us as a species to fill this perceived void.
Rob H.
ell:
ReplyDeleteYup I left out others as well: Sleepers who go quiet until the threat passes, Betrayers who find other civilizations and then attract the Predators to them, Defensives who try to survive inevitable attacks, Dispersers who try to spread their civilization wide enough/fast enough so that the Predators can't wipe them all out, Seeders who leave hidden civilization bootstrapping "seeds" to recreate successor civilizations, and so on.
My earlier list wasn't intended to be exhaustive. Natural Selection has an odd way of filling niches you never thought about.
Your Intelligent Design/Expanding Universe/Tower of Babel idea is very interesting. I like it.
Seems to me the flip-side of the argument poses an interesting advancement technique: a particularly bellicose (and ballsy) species could blare out their existence, in hopes of attracting someone to come visit - then spring the trap: obliterate the new arrivals and reap their technology. A cosmic anemone, if you will.
ReplyDeleteSociotard & others: see:
ReplyDelete"lungfish"
at
http://www.davidbrin.com/shortstories.htm
David:
ReplyDeleteThanks for the link to "Lungfish". I enjoyed rereading it.
An article about privacy/transparency with regard to the "smart grid"
ReplyDeletehttp://www.readwriteweb.com/archives/5_steps_to_ensure_your_power_meter_doesnt_betray_y.php
sorry for forgetting my e-manners. That's
ReplyDeletehttp://www.readwriteweb.com/archives/5_steps_to_ensure_your_power_meter_doesnt_betray_y.php
I maintain that mass extinctions, whether geological or cosmological, happen often, killing off sophont or potential sophont species before they manage to colonize other planets. For all we know dinosaurs developed some manner of civilization.
ReplyDeleteSo yeah, we need to get off this planet as soon as possible.
The debate over "active SETI" is a false dichotomy. As if anyone who argues against broadcasting our "Yoohoo beasties, come and eat us" signal right now must be advocating sitting and cowering forever.
ReplyDeleteBut we've only been listening for 50+ years. And most of that on very limited gear. We don't have a single radio-telescope off of Earth. We don't have a single telescope (optical or radio) at the solar-focal-points at the edge of the solar system.
Surely we can quietly listen for a thousand years or so before we start shouting into the void. On the scale of the life-span of species, on the scale of our galaxy, a thousand years is just a moment.
If there's anything out there to hear us, then according to the Drake Equation, the lifespan of technological civilisation must be long.
ReplyDeleteEven assuming no interstellar travel, within a few centuries (certainly within a few millennia) anything remotely like us would have optical and radio telescopes out at the gravitational focal-points of their sun, one facility for each neighbouring star-system.
Even if everyone decided to just listen, they'd pick up even the random radio emissions from radar/wi-fi/etc. (Unlike background noise, planetary emissions are a point-source, they never truly dissolve into the background if you have a big enough lens.)
Upon detecting radio-emitting neighbours, they can sit and study for as long as they want. And obviously, if you are listening to them talking about listening to you... why not send them a tight-focused "hello".
After a few hundred millennia, a slowly expanding Sagan Network would form. Once it's a few tens of thousands of lightyears wide, then no matter how timid they started, all of those species would by now be comfortable listening for, and announcing themselves to, young civilisation like ours.
And we hear nothing.
Two explanations:
1) They aren't there. In which case, "Active SETI" is a waste of time and energy.
2) They are all staying very quiet. In which case, maybe we should take the hint.
Weird, I wasn't expecting Blogger to let that one go through. I wish I knew (even in general terms) what the criteria are.
ReplyDelete(glypork: Mmmm, it's people!)
Paul you were right on in almost every respect. (That makes me kinda nervous.)
ReplyDeleteExcept we're not even asking for 1,000 years! Just a moratorium long enough for extensive discussions to engage the worlds best minds, from all fields (not a narrow cult of radio astronomer-mystics) to debate it openly in front of the whole world.
David,
ReplyDelete"(That makes me kinda nervous.)"
Hey, cheek!
"Except we're not even asking for 1,000 years! Just a moratorium long enough for extensive discussions"
But that still implies we're going to make a final decision (send/don't-send) in a year or two.
Except without the big deep-space facilities we need to directly observe neighbouring planets, we are deaf and blind. We don't have anything to base the debate on.
But we don't have to decide now. We can wait until we can directly observe our neighbours (if they exist), and then decide if they are safe to talk to.
What if the WOW signal was an actual transmission of some sort? Does that change the parameters of the equation?
ReplyDeleteWhat if transmission happens on some sort of frequency that we haven't properly analyzed yet?
A short version of the post Blogger just ate.
ReplyDelete1. A really cautious approach surely wouldn't be "nbusiness as usual" it'd be to reduce oru current EM profile as much as possible.
2, Maybe we just advanced enough yet to recognize the signals lighting up the galaxy like a Christmas Tree and maybe we're so unremarkable that no-one thinks it's worth contacting us.
Small note: one other reason we may not have noticed other civilizations out there is that we might be in the wrong space-time region to see these signals. Our own radio signals are only visible at under 100 light years around us. If a civilization rose and fell within the last thousand years but was within the 1000 light year zone, we might have missed seeing signals from them. Likewise, civilizations that are tens of thousands of years old but who exist over tens of thousands of light years away will be invisible to us because their signals haven't reached us yet.
ReplyDeleteLight isn't instantaneous. Neither are radio waves. Finding an alien signal is a matter of being in the right place at the right time.
Rob H.
Now I'm thinking of that other explanation, as found in the short story "They're made of meat."
ReplyDeleteDr. Brin, do you have a cite for your assertion that the oil and coal companies spend money that dwarfs that of the environmentally concious lobbies?
ReplyDeleteI ask because of this article, which asserts that:
Environmental groups and their supporters spend more money on climate-change and clean-energy activities and campaigns than sceptical right-wing groups and their industry supporters, according to a report by a US social scientist, who questions some of the most common reasons given for US political inaction on global warming.
I mean, you obviously seem right. It'd just be nice to see your data.
Robert,
ReplyDeleteI don't think that changes the Drake Equation. The number of civilisations currently in the galaxy should equal the number of civilisations we can detect, even if they aren't the same civilisations.
And if they are too far away for their radio-bubble to reach us, then they are too far away for Active SETI to be worthwhile.
Similarly alien technology. Active SETI is using the same dishes, same frequencies, that passive SETI uses.
If we can't hear them because of $EXPLANATION then they can't hear us because of $EXPLANATION. So Active SETI is a waste of time.
And if they can hear us, but aren't transmitting because of $PROBLEM, then maybe we should respect the opinion of much older and wiser races about the severity of $PROBLEM.
Vote Thursday for the "best sci-fi film never made" -- the novel you loved that you think would make a great movie... if only.
ReplyDelete(BTW guess who made the prelim nomination list four times... surpassed only by Heinlein?)
Go back to the site Thursday to vote!
http://www.theregister.co.uk/2011/04/19/sci_fi_poll/
http://www.theregister.co.uk/2011/04/
19/sci_fi_poll/
The "there are only a few widely-spaced civilizations" excuse is a solid one... but it begs the question. WHY are they so sparse and few? And why hasn't ONE of them expanded outward... even at 1% of light speed their wave would fill the galaxy.
Watch till the end...
ReplyDeletehttp://www.facebook.com/video/video.php?v=10150171311922668
http://www.facebook.com/video/video.
php?v=10150171311922668
I cry foul!
ReplyDeleteWho Goes There has been made into a movie twice, as The Thing.
Many other worthy entries, and not a few I a shamefully admit to not knowing.
My first vote to Out of the Silent Planet. But it would be a difficult story to really translate to film honestly.
One day....Startide? The CGI tech for dolphins should be there.
Tacitus
My vote would go to "The Postman". I realize there WAS a film, but with all due respect, it didn't look like a film OF the actual novel (the same could be said for "Dune", come to think of it). 'Course, it's probably too late to cast Michael York as Gordon, which for some reason is how I mentally envision the character.
ReplyDeleteIf they can do a film of "Atlas Shrugged", they could certainly do "Out of the Silent Planet". Actually, having given it about a minute's thought, I'm not sure I see why you think that novel is unfilmable. "Perelandra" would seem to me to be harder because so much of the action/conflict takes place internally in the protagonist's mind.
I'd love to see the "Foundation" trilogy in film if it were done right. Just the original trilogy, though.
And does it have to be a NOVEL? "Thor vs Captain America" would make a good film. It couldn't go by that title, though, and would now probably look like a knock-off of Marvel's "Thor" and "Captian America" movies.
If short stories are allowed, I might have to nominate Charles Harness's 1964 (I think) "Probable Cause". It's got everything from the psychic phenomena to the Lincoln assassination to the US Supreme Court.
Oh, I see. There's already a set list of books to choose from.
ReplyDeleteNever mind.
Yeah, I have a much better script for The Postman sitting in a drawer. But the DUNE movie was very faithful. TOO faithful.
ReplyDeleteHuh! Anyone speak Italian? I think I recall these guys interviewing me...
http://www.youtube.com/watch?v=lXo5ic7ZpV4
Onward to next post.
Some of those Drake variables could be scrutinized. Augmented, or a couple new variables added.
ReplyDeleteForms of life which act out of our normal duration concepts; life which is so slow we don't recognize it as life. Amino acid cycles, huge, but which appear uncomplex. Other analogs of this in different chemistries. Cold life. And hot life! Reaches singularity in several years, not millennia.
If an advanced alien is looking for us I suggest there is only a small probability that they fit in the space where they can easily find our randomly sent intentional transmissions but take a long time to find our unintentional ones.
ReplyDeleteAnd the minimal cost of looking would imply pretty much every civilization would be looking passively.
But more than that I doubt that hiding is a god strategy - because you can build self replicating probes to search the entire galaxy in 20 million years (your numbers) so you cant hide from a even mildly determined searcher any longer than that. And if they have the resources of 99.99% of a galaxy and you have 1 planet you cant possibly stand against them in a war (I am also assuming they develop tech faster with their larger empire).
If nothing else they could drop a some kind of black hole bomb on your solar system - a billion times.
I suggest - in galactic warfare - the best defense will always be a good offense.
GNZ
The lifespan of a star is approximately inversely proportional to the cube of it's mass. So if our sun were only a little heavier, it wouldn't have lasted this long. Perhaps we are unusual after all.
ReplyDeleteIn his "Revelation Space" novels Alastair Reynolds develops this idea in epic detail -- and much more originality. Not only an ancient mechanism to suppress space-faring races, but a pretty interesting rationale for doing so. (I don't want to give too much away.) It's not my favorite solution to the Paradox, but it makes for good drama.
ReplyDeletesex
ReplyDeleteporn
escort bayan
Humans are an arrogant, belief riddled species whose very nature seems to be to believe that we are the epitome of everything including evolution.
ReplyDeleteIt was only last year (to my shock) that humans were considering looking for "aliens" that might not be based on life as we know it. Hardly a shining endorsement of our intellect!
While empathy appears to be winning out on earth (mostly); that doesn't mean it has everywhere else. Personally I don't think you can have a space fairing species without it (or an alien equivalent) but that's just my opinion.
Other than a first contact, would we want to make contact with a civilization that was older than ours? Even if by only a short time? Imagine some alien looking at our planet and it isn't too hard a stretch to understand why we would be "quarantined" from any other civilizations. If it were us, somehow I doubt we would want to get involved in alien religious or political squabbles or debates on whether to use some deadly force like nuclear weapons. I think we would stay away and warn others!
There are so many reasons as to why we haven't detected or been allowed to detect aliens and until we look long and hard at ourselves, I think it will always be that way.
We have a LONG way to go with no guarantees that we won't destroy ourselves in the meantime and lets not forget the varied and numerous ways we can and ARE doing this to ourselves today.
I have a few additional terms to the drake equation.
ReplyDeleteFs - fraction where sexual reproduction develops
Fd - fraction where dreaming develops (compare brain sizes of spiny anteater to human)
And a few ideas on lowering a few of the terms.
Earth's atmosphere is quite thin due its very large moon. Perhaps the zone between Venus and Mars is much narrower than expected.
Our level of intelligence may be the result of sexual selection and not natural selection.
Environmental and cultural preservation. Maybe no interstellar civilization can arise without being concerned for disrupting the growth of others. Or the first one is still in existence and enforces protection on younger races.