Search Results for: great filter

Today Is Filter Day

By tracking daily news fluctuations, we can have fun, join in common conversations, and signal our abilities to track events and to quickly compose clever commentary. But for the purpose of forming accurate expectations about the world, we attend too much to such news, and neglect key constant features of our world and knowledge.

So today, let us remember one key somber and neglected fact: the universe looks very dead. Yes, there might be pockets of life hiding in small corners, but for billions of years billions of galaxies full of vast resources have been left almost entirely untouched and unused. While we seem only centuries away making a great visible use of our solar system, and a million years from doing the same to our galaxy, any life out there seems unable, uninterested, or afraid to do the same. What dark fact do they know that we do not?

Yes, it is possible that the extremely difficultly was life’s origin, or some early step, so that, other than here on Earth, all life in the universe is stuck before this early extremely hard step. But even if you find this the most likely outcome, surely given our ignorance you must also place a non-trivial probability on other possibilities. You must see a great filter as lying between initial planets and visibly expanding civilizations, and wonder how far along that filter we are. In particular, you must estimate a substantial chance of “disaster”, i.e., something destroying our ability or inclination to make a visible use of the vast resources we see. (And this disaster can’t be an unfriendly super-AI, because that should be visible.)

Assume that since none of the ~1020 planets we see has yet given rise to a visible expanding civilization, each planet has a less than one in 1020 chance of doing so. If so, what fraction of this 1020+ filter do you estimate still lies ahead of us? If that fraction were only 1/365, then we face at least a 12% chance of disaster. Which should be enough to scare you.

To make sure we take the time to periodically remember this key somber fact, I propose that today, the day before winter solstice, the darkest day of the year, be Filter Day. I pick the day before to mock the wishful optimistic estimate that only 1/365 of the total filter remains ahead of us. Perhaps if you estimate that 1/12 of the filter still lies ahead, a filter we have less than a 2% chance of surviving, you should commemorate Filter Day one month before winter solstice. But then we’d all commemorate on different days, and so may not remember to commemorate at all.

So, to keep it simple, today is Filter Day. Take a minute to look up at the dark night sky, see the vast ancient and unbroken deadlands, and be very afraid.

What other activities makes sense on Filter Day? Visit an ancient ruin? A volcano? A nuclear test site? The CDC? A telescope?

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Filters and bottlenecks

Lots of processes have filters: a certain proportion of the time they fail at that stage. There are filters in the path from dead stars to booming civilizations. There are filters in the path from being a baby to being an old person. There are filters in the path from having an idea to having a thriving business.

Lots of processes also have bottlenecks. These look similar, in that many things fail at that point. For instance the path to becoming a Nobel Prize winner is bottlenecked by there only being so many Nobel Prizes ever year. Rather than a fixed fraction of people getting past that barrier, a fixed number of people do.

It’s worth noticing if something is a filter or a bottleneck, because you should treat them differently often. Either way you can increase the fraction reaching the end by widening the filter or bottleneck to let more past. But for the filter it might be worth doing this at any other stage in the process, whereas for the bottleneck it is pointless at all earlier stages. You can’t get more Nobel Prize winners by improving education, but you might get more thriving businesses.

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At Least Two Filters

Where lies the great filter, i.e., the obstacles that make it extremely unlikely that any one chunk of pre-organic matter originates a visibly expanding interstellar civilization? While it seems unlikely our ancestors passed through much of a filter in the last half billion years, our descendants may face a big filter in the next few thousand years, and there may have been big filters associated with the origin of life, the spread of life, the invention of complex cells, sexual reproduction, or multicellular life.

In many folks eyes, an elegantly simple resolution, which is likely because of its simplicity, is to assume there is just one huge filter: the origin of life. Assuming that first step is enormously hard allows one to think all the other steps are pretty easy. They wouldn’t be sure things of course, but conditional on a big enough origin-of-life filter, one wouldn’t have a strong reason to fear that common analyses underestimate future filters.

Unfortunately, the elegantly simple hypothesis that the great filter is mainly a big origin-of-life filter seems at odds with our best evidence. Why? Because if the spread-of-life step had the weakest possible associated filter, then life spreading must be easy. Over billions of years life could have spread to many star systems from its place of origin:

Life could spread across a galaxy via giant molecular clouds reliably collecting life from the stars they drift near, and then passing that life on to a few of the thousands of new stars they create.

If over billions of years life spread to many hundreds, or even billions, of star systems, and no substantial filters stood between arrival of life near a star, and its eventual development of advanced technical civilizations like ours, then why would we now see no any evidence of other civilizations? Yes it is possible that we are the very first, but that hypothesis is of course unlikely by default.

It seems to me that if the great filter is to consist of just one big step, the only plausible possibility is the development of multi-cellular life. All the steps before that one seem able to spread to other star systems via single-celled life hidden in dust, and it seems we haven’t had a big filter step since the multi-cellular innovation.

So if the idea of just one big filter appeals to your sense of elegance, you’ll have to presume that life, including complex life with sexual reproduction etc., is very common in our vast universe, but that Earth is one of the handful of places in all that vastness with multi-cellular life.

If you don’t find that plausible, well then you’ll have to grant there are at least two filters. And if two, why not three? So you must find the possibility of a third filter in our future plausible; beware future filters.

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Brain Size Is Not A Filter

We relate brain size to appearance time for 511 fossil and extant mammalian species to test for temporal changes in relative brain size over time. We show that there is wide variation across groups in encephalization slopes across groups and that encephalization is not universal in mammals. … Encephalization [vs. time] trends are associated with sociality in extant species. These findings … highlight the role sociality may play in driving the evolution of large brains. (more; HT Razib Khan)

The biggest brains have consistently gotten bigger over the last half billion years since multi-cellular life appeared. Big brains seem to be a necessary precondition for human level intelligence and civilization, and human size brains appeared only very recently. These facts strongly suggest that achieving human level intelligence is just not a big component of the great filter.  It appeared quickly after big brains, and big brains seem likely given enough time and sociality, and sociality seems likely.

This unfortunately means that it is very difficult to collect data on all steps of the great filter.  It is big and real and matters enormously, but we can hardly see it.

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Beware Future Filters

Though we can now see over 1020 stars that are billions of years old, none has ever birthed a visible interstellar civilization. So there is a great filter at least that big preventing a simple dead star from giving rise to visible colonization within billions of years. (This filter is even bigger given panspermia.) We aren’t sure where this filter lies, but if even 10% (logarithmically) of it still lies in our star’s future, we have less than a 1% chance of birthing a wave. If so, either we are >99% likely to always forever more try to and succeed in stopping any capable colonists from leaving here to start a visible colonization wave, if given such a choice, or we face poor odds of surviving to have such a choice.

Back in March I noted that Katja Grace had an important if depressing insight:

Back in ‘98 I considered the “doomsday argument” … [but] instead embraced “self-indication analysis”, which blocks the usual doomsday argument.  In ‘08 I even suggested self-indication helps explain time-asymmetry. … Alas, Katja Grace had just shown that, given a great filter, self-indication implies doom!  This is the great filter … Alas I now drastically increase my estimate of our existential risk; I am, for example, now far more eager to improve our refuges.

Katja has just finished her undergrad honors thesis at ANU, which reports that all three of the main ways to pick a prior re indexical uncertainty (on who am I in this universe) imply that future filters are bigger than we’d otherwise think.  And not just by small amounts – the bigger the filters, the bigger the boost to future filters.

Now existential risk is important even if its odds are low – so much is at stake in whether our descendants die out or colonize a big chuck of the visible universe. But the bigger the odds, the more important it gets. Let’s review the main ways to estimate existential risk:

  1. Inside Model – using an internal model of how a particular risk process works, use your best guesses on likely model parameters to estimate the chance this process happens.
  2. Outside Scaling – Use prior rates of smaller events similar to a particular risk, and how such rates scale with size, to estimate the chance of events so big as to be a filter.
  3. Doomsday Argument – Assuming self-sampling and a reference class, estimate the chance of doom soon based on our time order in the reference class.
  4. Great Filter – Using estimates of total filter size and the chances of prior filters of various sizes, to estimate distributions over the total future filter size.
  5. Indexical Filter Boost – Redo the great filter analysis given all the main ways to get indexical priors, and weigh answers accordingly.

Now while many folks use approach #1 to estimate big chances of particular dooms, most such “models” have little formal structure; they are mostly vague intuitions.  So this approach usually influences my opinions rather weakly. Approach #2 is pretty solid, but usually leads to pretty low estimates. Using this approach, war and pandemics seem most likely to destroy half of humanity, but not very likely, and the odds of destroying us all see much lower. Approach #3 gets some weight, but less for me as I find self-sampling pretty implausible relative to self-indication.

This leaves #4, #5 as the main reasons I worry about existential risk. So having to take #5 seriously in addition to #4 is quite a blow. There is some tension between this and the results of #2, so I must wonder: what big things future could go wrong where analogous smaller past things can’t go wrong? Many of you will say “unfriendly AI” but as Katja points out a powerful unfriendly AI that would make a visible mark on the universe can’t be part a future filter; we’d see the paperclips out there.  Neither would the risk that our descendants’ values diverge from ours, nor  the risk of a rapidly expanding wave of (nanotech) grey goo – only slowly spreading grey goo could count in the future filter.

Browsing Nick Bostrom’s survey, that leaves us with: weak grey goo, engineered pandemics, sudden extreme climate change, nuclear war, totalitarianism ends growth, and unfriendly aliens. While these all risks seem apriori unlikely, either the entire great filter is in our past, or one of these (or something not listed) is far worse than it seems. But which?

Also, how likely is it really that such events would destroy all advanced life on Earth, to prevent other primates or mammals from recreating intelligence? After all the fact that human level intelligence arose so soon after human size brains appeared suggests that it was not a past filter of ours. The most likely resolution of all this still seems to me that almost all the filter is in our past, perhaps at the origin of life. But I’m not willing to bet our future on that.

The good news is that refuges seem effective against most these risks.  While unfriendly aliens mights dig us out of any holes, and prevent other Earth life from re-evolving intelligence, the other risks aren’t intelligent enough for that.  So: let’s make more and better refuges, and for #$@&* sake please stop broadcasting to aliens!

Added 10a: Refuges would also not protect much against totalitarian world culture and/or government that stops growth. So let’s also try extra hard to avoid that too.

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To Innovate, Unify or Fragment?

In the world around us, innovation seems to increase with the size of an integrated region of activity. For example, human and computer languages with more users acquire more words and tools at a faster rate. Tech ecosystems, such as those collected around Microsoft, Apple, or Google operating systems, innovate faster when they have more participating suppliers and users. And there is more innovation per capita in larger cities, firms, and economies. (All else equal, of course.)

We have decent theories to explain all this: larger communities try more things, and each trial has more previous things to combine and build on. The obvious implication is that innovation will increase as our world gets larger, more integrated, and adopts more wider-shared standards and tech ecosystems. More unification will induce more innovation.

Simple theory also predicts that species evolve faster when they have larger populations. And this seems to have applied across human history. But if this were generally true across species, then we should expect most biological innovation to happen in the largest species, which would live in the largest most integrated environmental niches. Like big common ocean areas. And most other species to have descended from these big ones.

But in fact, more biological innovation happens where the species are the smallest, which happens where mobility is higher and environments are more fragmented and changing. For example, over the last half billion years, we’ve seen a lot more innovation on land than in the sea, more on the coasts than on the interiors of land or sea, and more closer to rivers. All more mobile and fragmented places. How can that be?

Maybe big things tend to be older, and old things rot. Maybe the simple theory mentioned above focuses on many small innovations, but doesn’t apply as well to the few biggest innovations, that require coordinating many supporting innovations. Or maybe phenomena like sexual selection, as illustrated by the peacock’s tail, show how conformity and related collective traps can bedevil species, as well as larger more unified tech ecosystems. It seems to require selection between species to overcome such traps; individual species can’t fix them on their own.

If so, why hasn’t the human species fallen into such traps yet? Maybe the current fertility decline is evidence of such a trap, or maybe such problems just take a long time to arise. Humans fragmenting into competing cultures may have saved us for a while. Individual cultures do seem to have often fallen into such traps. Relatively isolated empires consistently rise and then fall. So maybe cultural competition is mostly what has saved us from cultures falling into traps.

While one might guess that collective traps are a rare problem for species and cultures, the consistent collapse of human empires and our huge dataset on bio innovation suggest that such problems are in fact quite common. So common that we really need larger scale competition, such as between cultures or species, to weed it out. To innovate, the key to growth, we need to fragment, not unify.

Which seems a big red loud warning sign about our current trend toward an integrated world culture, prey to integrated world collective traps, such as via world mobs. They might take some time to reveal themselves, but then be quite hard to eradicate. This seems to me the most likely future great filter step that we face.

Added 10Jan: There are papers on how to design a population structure to maximize the rate of biological evolution.

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Seeing ANYTHING Other Than Huge-Civ Is Bad News

The great filter is whatever obstacles prevent simple dead matter from evolving into a civilization big and visible on astronomical scales. The fact that we see nothing big and visible in a huge universe says this filter must be large, and a key question is the size of the future filter: how much have we passed and how much remains ahead of us?

I’ve suggested that evidence of life elsewhere below our level makes the past filter look smaller, and thus our future filter larger. From which you might conclude that evidence of a civilization above our level is good news. That seems to be what  says here at Vox:

If (and I must stress that this is a quite unlikely “if”) UFO sightings on earth are actually evidence that an advanced alien civilization has developed a system of long-distance probes that it is using to monitor or contact humanity, then that would be an immensely hopeful sign in Great Filter terms. It would mean that at least one civilization has far surpassed humanity without encountering any insurmountable hurdles preventing its survival. (more)

But I don’t think that’s right. This would move the filter more to above their level, but below the level of becoming big and visible, without changing the size of the total filter. Which implies a larger future filter for us. In addition, any UFO aliens are likely here to actively impose a filter on us, i.e., to stop us from getting big and visible (or “grabby“).

So if UFOs as aliens is not good news, what would be good news re our future filter? Aside from detailed engineering and social calculations showing that we are in fact very close to becoming irreversibly grabby, the only good news I can imagine is actual concrete evidence of big visible aliens civilizations out there. Maybe we’ve misread their signatures somehow.

Looking out further and in more detail at the universe and still finding it dead suggests the total filter is larger, which is bad news. And finding any evidence of anything other than death suggests the filter is smaller up to the level of that finding, but doesn’t revise our estimate of the total filter. Which is bad news re our future. Thus a perhaps surprising conclusion: finding anything other than a big visible civilization out there is bad news re our future prospects for becoming big and visible.

Remember also: the SIA indexical prior (IMHO the reasonable choice) favors larger future filters. Beware the future filter!

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On UFOs-As-Aliens Priors

A careful analysis of UFOs should consider lots of data, and consider it in much detail. I oft hear skeptics seek shortcuts, such as by declaring all testimony invalid, or insisting that only some long conjunction of encounter features could be sufficient. But consider a legal accusation of attempted murder. Even though the prior odds that a random  X attempts to kill Y during hour Z is terribly low (<10-12), we are still willing to entertain such claims, and we accept personal testimony as an important part of supporting evidence.

Yes, advocates of things like UFOs seem willing to put more time into such details, and it may seem unfair to expect skeptics to put in as much work. But jurors and lawyers must put in a lot of effort in legal trials. This is the great problem of how to divide intellectual labor; as with most topics, we do best if we task a few with going into great detail on each topic, so the rest of us can defer to their analysis. If you aren’t willing to go into sufficient detail, then admit this isn’t one of your topics, and defer to others on it.

In that spirit, instead of expressing opinions on many UFO topics, let me instead focus on the area where I have the most relative expertise: the priors to associate with the some-UFOs-are-aliens hypothesis. As far as I can tell, the main reason that most give for skepticism that aliens visit Earth in the UFO style is that this theory seems a priori crazy unlikely. But that estimate seems wrong to me. Let me explain.

A full Bayesian analysis of the four main UFO theory categories (error, hoax, secret Earth orgs, aliens) needs eight numbers: one prior and likelihood for each theory. In this post I try only to estimate one of these eight numbers: the prior for the aliens theory. Here goes.

Life exists here on Earth, and our standard best theories say that this was not a miracle, nor was Earth the only place such things could happen. Furthermore, our universe also seems very large (perhaps infinite). Thus our standard best theories predict that advanced life has appeared and will appear many times out there.

These standard best theories also predict a wide range of dates when this could happen. As a result, two independent alien origins are likely to be millions to trillions of years apart in time. Which gives aliens a lot of time to travel to visit other aliens.

So we can break down doubts on prior expectations about UFO as aliens into three parts:

  • What is the chance that advanced aliens appear often enough in space and time for some of them to have been born early and close enough to travel to Earth to be here now?
  • What is the chance that aliens (or, more likely their robot descendants) who can travel actually do travel to Earth by now, but do not visibly remake the local universe?
  • Given that aliens exist, and travel to here, but don’t remake the local universe, what is the chance that they would act the way that UFOs seem to act, i.e., being somewhat evasive, but not completely hiding nor announcing themselves?

First, how close might aliens be? As my co-authors and I discuss here, humans seem to have arrived quite early in history, at least if one assumes that the universe would remain empty and wait indefinitely for advanced life like us to appear. This is the main reason we offer for postulating a grabby aliens deadline, to explain human earliness. And our grabby aliens model implies that aliens do appear often enough for maybe some of them to have come here by now.

Now grabby aliens arriving by here now would also be quite visible to us now. But our basic model is quite consistent with variations wherein there are many, perhaps thousands or millions, of non-grabby alien civs per grabby civ, all born at the same sort of places and times. These non-grabby civs do not remake their local universe. So either they die fast, life long but do not expand, or they expand long but do little to remake their universe.

In my view, the most likely scenario that puts long-expanding-but-not-remaking aliens here now is panspermia siblings. Life arose long ago on some very rare Eden, which then seeded our Sun’s stellar nursery, with life quickly spreading to most stars in that nursery. At least two of these stars eventually developed advanced life, but Earth was not the first. Aliens at the first star looked for their panspermia siblings, noticed simple life on Earth here long ago, and then long ago traveled to near here to await the arrival of advanced life. Where they now do their weird UFO encounter things.

So to explain UFOs as aliens, we must postulate that these first star sibling aliens had preferences and coordination abilities sufficient to do the following:

  • prevent any parts of their own civ from expanding and visibly remaking the local universe,
  • travel to sibling stars that might birth civs, to stand ready to prevent them from also expanding, but also not kill them, and
  • while waiting here they allow or induce the sort of UFO encounters we see, but prevent any clearer more direct interactions.

I estimate a chance of at least 10% for each of the following events, given the prior events:

  1. Earth was seeded by panspermia in its nursery
  2. A sibling star gave rise to a long-lived advanced civ long before now
  3. That civ prevents itself from expanding, tries to prevent siblings from expanding, and long ago traveled to here to wait to enforce this preference,
  4. They induce or allow UFO-style encounters while they wait here.

Note that #1 requires a high enough rate of rock transfer between star systems, #2 requires that most of the great filter happened on Eden, #3 is more likely when civs adopt strong “world” governments, and #4 is relatively likely because we shouldn’t really expect to be able to predict detailed behaviors of strange alien civilizations.

Four factors of 10% gives a minimum prior chance of 10-4, but as most of the probability weight should above these minimums, I estimate the total chance to be at least 10-3. As I’ve said before, combining all the relevant priors and posteriors I judge the hoax and aliens theories to be most likely for the hardest-to-explain UFO cases. But I don’t claim as much expertise on all the other numbers required to judge that, as I do for the one number I estimate here:

The prior chance of the aliens theory of the hardest-to-explain UFO cases is at least 10-3, relative to the other three theory categories of error, hoax, and secret Earth orgs.

This prior is actually pretty high compared to the usual priors in most legal cases. So the types and amounts of evidence on particular cases that is sufficient to convict in legal cases seems sufficient to judge UFOs-are-aliens as more or less likely than not. But again, I have no special expertise to offer you for judging the details of UFO encounters. I can just say that you need to look at such details; you can’t just dismiss UFOs-as-aliens theory with a wave of your philosophical hands.

Added 10June: Many take issue with my estimating 1/10 for the chance that aliens waiting here would be somewhat evasive, but not completely hide nor announce themselves. They don’t see this as a good plan for any goals they can think of.

But we are talking about an entire alien civilization here! Human societies often do things, like fight wars or stop having kids, that seem counter-productive from the point of view of that society as a whole. In addition, individual humans often do things that seem counter-productive until you consider their signaling incentives. I wrote a whole book on this.

If we often have trouble explaining the behaviors of human societies and individuals, I don’t think we should feel very confident in predicting detailed behaviors of a completely alien civilization. After all, many have reasonably doubted if we could even communicate with aliens, or recognize them when we saw them. Having outlined some possible signaling motives for alien UFO behavior, I can see that there are many possible explanations for aliens-as-UFO behavior. Thus a 1/10 prior seems reasonable to me.

Added 13Jun: I did 6 Twitter polls to elicit relative priors and likelihoods for the four main theory categories:

Added 14Jun: Thinking through the consequences of the show-but-don’t-talk strategy suggests that it will work out pretty well for the aliens.

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Panspermia Siblings

The UFOs as aliens hypothesis is only as believable as the most a priori believable story for how it could be true. When I tried to find a story like that, I ended up relying heavily on the idea of panspermia siblings. And now that I’ve given that idea a bit more thought, I’ve realized that it is somewhat harder to arrange than I’d realized, and thus somewhat less believable. Making UFOs as aliens less likely, though still quite possible.

The scenario, if you recall, is that there are aliens visiting Earth today who have not expanded much to colonize and remake the universe, aliens who were born at a planet around a star that is a sibling to our sun. That is, this alien’s star was born in the same stellar nursery as our sun. This scenario requires three key elements:

Old Non-Expansionist Aliens – A substantial fraction of advanced civilizations choose not to expand and visibly remake the universe, but do choose to go visit their sibling stars that develop advanced life, and these civilizations last for longer than the typical differences between when advanced life would appear when grown from simpler life at the same level four billion years before. Thus a substantial fraction of alien civilizations must last for several hundred million years. (Oh and they choose do all these apparently-useless glow-buzzings of our treetops.)

Easy Earth Filter – In order for there to be at least two advanced civilizations both born from the same stellar nursery, it can’t be too hard to evolve advanced life from the sort of life that Earth starts with. The time of the origin of life on Earth and the time now remaining suggest 3-9 hard steps happened on Earth, if this whole time was take up by hard try-try steps. So we need some combination of a large nursery, fewer such hard steps, much of Earth history being taken up with delay steps instead of hard try-try steps, and the “hard” try-try steps not being that hard. So, for example, in a nursery of ten thousand stars, there might be just three try-try steps each only a factor of ten hard, and perhaps half of Earth history was taken up with delay steps.

Panspermia or Huge Try-Once Step – In order for life to spread across a large fraction of a stellar nursery, that life would have to appear within roughly a hundred million years after that nursery formed. So either life appeared from nothing very fast, mainly via some very hard try-once steps, or our nursery was seeded by life from an Eden at some other passing star, either just as our nursery was forming, or via a prior seeding of the molecular cloud which collapsed to form our nursery. (Which requires life to survive a long time in a molecular cloud.) On average stars pass within 5 parsecs of  such clouds every 50-100Myr.

While this prior Eden would have had a similar number of hard steps as Earth, those steps would on average be much harder, so that most of the total great filter would have happened at Eden. Very hard steps might include the very first life, and the transfer from Eden to a stellar nursery.

A 2012 paper in Astrobiology works out details of this scenario for life moving between star systems in a stellar nursery, where many stars are crammed together and many rocks are flying between them.

We don’t know when life first appear on Earth, but current best guess is 400Myr, with a range 200-800Myr, after the Earth and Sun formed together. They were formed together with ~1K-10K other stars, all packed close together.

Earth had water to support life within ~160–290 Myr, while our cluster took ~135–535 Myr for sibling stars to drift away from each other (the largest value is for the largest star clusters). During this early period there were a lot of rocks smacking into Earth kicking up a lot more rocks. Maybe the top kilometer of rock across Earth was kicked up.

About ~1% of these rocks were ejected from Earth with a weak enough impact shock to let life survive, and rocks of >10 kg seem like they could protect life from radiation and impact over the 3-5 million years it would take to drift to the closest star system in this cluster during this period. Some kinds of life could last that long.

About 2 * 10^11 such rocks would escape our solar system at a slow enough velocity to be captured by a neighboring star. Given such assumptions, if the nearest star were also Sun-like, then the number of such rocks ejected from Earth in this period that would land on an Earth-like planet around that nearest star is about 3*10^4. If that star had half the sun’s mass, this number falls to just 10^4.

Thus if our Sun’s stellar nursery were big enough, and if life appeared early enough in this cluster, then life might have spread to many stars in this cluster. And thus aliens could have evolved before us at one of those stars, and then came here to be the UFOs we see. But this is a lot of ifs, and so the a priori unlikeliness of this scenario has to be weighed against the a priori unlikeliness of: secret Earth orgs with really advanced tech, a vast conspiracy to create the false appearance of UFO encounters,  or mass delusions widespread enough to create the same.

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SETI Optimism is Human Future Pessimism

If one takes the hard steps model of evolution seriously, humans seem to be early in the history of the universe. We can explain this by postulating that grabby aliens set an early deadline; humans couldn’t show up after aliens had filled the universe. As our grabby aliens model has three free parameters, each of which can be estimated from data, we are forced to conclude that such aliens are quite rare; if we are lucky enough to survive that long we should meet them in roughly a billion years.

This next diagram shows distributions over how many galaxies each one controls when they meet each other. The distributions shown are for expansion speed s=c; more generally this goes as (s/c)3. (The likelihood ratio for not seeing big alien volumes today is only one above s/c ~ 3/4.)

As you can see, for the best estimate power of n=6, each one typically comes to control millions of galaxies. (We avoid making assumptions about what happens after GCs meet. All our distributions depend on hard steps power n. All were made with help of my coauthors Daniel Martin, Calvin McCarter, and Johnathan Paulson.)

Assume that each grabby civilization (GC) arises “soon” (within 10Myr) from a non-grabby civilization (NGC). As GCs by definition keep expanding fast and change the appearance of their volumes, NGCs that don’t become GCs don’t expand much or long, or don’t change their volume appearances. As NGCs are much harder to see, and don’t much block GC behavior, there could be far more of them than there are GCs. 

Thus a key question about aliens is: what is the ratio R between NGCs and GCs? And this ratio R is at the heart of a key conflict: you need to expect a high ratio R to be optimistic about SETI success anytime soon, but you need to expect a low ratio R to be optimistic about the future prospects of our descendants. (I described this conflict abstractly in my original great filter paper; here I discuss specific numbers.) Continue reading "SETI Optimism is Human Future Pessimism" »

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