Tag Archives: OriginOfLife

Biologists Taboo Artificial Life

Recently I’ve reviewed three new books by academic biologists on the future of life in the universe. All three books have gained high profile and prestigious reviews in major media and academia. (Which is how I heard of them.) And all of these books, and all of these prestigious reviews, seem to share and enforce a taboo against seriously considering the possibility that artificial life will make a big difference to the cosmos.

For example:

Arthur admits the possibility of intelligent life spreading across planets, … and Arthur admits the possibility of artificial life. … But somehow these admissions make little difference to his forecasts, which ignore the possibility of artificial life at places other than planets, or made out of stuff other than carbon. And which ignore the possibility of intelligent artificial life spreading very far and wide, to become even more common than non-artificial life.

Similarly:

I recently reviewed The Zoologist’s Guide to the Galaxy, wherein a [Cambridge] zoologist says that aliens we meet would be much like us, even though they’d be many millions of years more advanced than us, apparently assuming that our descendants will not noticeably change in million of years.

And in a new book The Next 500 Years, a geneticist [and computational biologist] recommends that we take the next few centuries to genetically engineer humans to live in on other planets, apparently unaware that our descendants will most likely be artificial (like ems), who won’t need planets in particular except as a source of raw materials.

I actually just did a written debate with this last author, who wouldn’t even admit that I disagreed with him:

You write a long book mostly on the details of genetic engineering, saying we should use it to slowly change humans and their allied plants and animals, so that in 500 years we could launch them out to the cosmos, to arrive at other stars in a few thousand years.

I say, no, long before then artificial minds and life should have thoroughly replaced biology. A new kind of life, far more robust, able to grow far faster, able to travel out into space much sooner and faster, all made in factories out of stuff dug up in mines, and not at all based on biological cells, so that genetic engineering has little to offer them.

This all suggests more than just a few biologists with a mental block; it suggests an overall taboo within their shared intellectual culture, of biology academics who study astrobiology and our future. A taboo that has likely discouraged and distorted related research and analysis.

Added 30May: This post is discussed at Hacker News.

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The Biological Universe

In his new book The Biological Universe: Life in the Milky Way and Beyond, evolutionary biologist Wallace Arthur predicts the life we will find in the galaxy and universe:

Life forms are to be found across the Milky Way and beyond. They will be thinly spread, to be sure. … we can anticipate what life elsewhere will be like by examining the ecology and evolution of life on Earth.

Arthur defines life broadly:

If an entity is metabolically alive and membrane-bound, and groups of individual entities of this kind are characterized by variation, reproduction, and inheritance, then we describe the situation as ‘life’. … And regarding extraterrestrial life we should try to keep as open a mind as possible (p.13)

He says life is only near the surface of planets:

There are no macromolecules in [interstellar] clouds. There is thus no basis for life even approximately as we know it. So in the end we rule out all of the interstellar medium as a home for life. And that means in spatial terms that we have ruled out more than 99% of the galaxy. … Next we rule out suns. This means all suns and all parts them. No metabolizing, reproducing life, whether simple like bacteria, or more complex, like mammals, could exist in such a hellish environment. … By ruling out suns as possible homes for life, we rule out more than 99% of the matter of the galaxy. … Here’s a selection of other objects that seem likely to be barren. First, dead stars, including white dwarfs, neutron stars, and black holes. Second those entities somewhere in between a small star and a large planet that we call brown dwarfs. … Third, pulsars. (pp.42-44)

Arthur says most life is enclosed, made of carbon, and of long molecules with sequence specificity:

Carbon based life is the most probable, and hence more common, form of life in the Milky Wa, and indeed in the universe. … Life requires a type of macromolecule that exhibits sequence specificity that is that is similar in general, though not necessarily in detail, to the specificity that is found in nucleic acid and proteins. … Membrane-enclosed cellular life is the norm. (p.203)

Life is almost everywhere that it can be:

The fraction of habitable planets that actually become inhabited. My personal view is that it is close to 100%. (p.191)

And here is how many planets of each type:

Number of planets in Milky Way: 1 trillion
Number of planets with microbial life: 1 billion
Number of planets with animal life: 10 million
Number of planets with broadcasting life: between 0 and 1 million

Arthur even predicts more intelligent life is rarer:

Lets define four thresholds levels of intelligence. … animals with a small brain … crossed the first threshold. … Animals that can use tools, and indeed plan their use of tools, … cross the second threshold. … Animals that have begun to investigate the abstract nature of things, and to keep written records of their investigations, have cross the third threshold., … fourth threshold the achieving of a civilization with a technology that includes the use of radio signals and other means of interstellar communication, such as lasers. … It’s hard to believe that the number of planets whose evolutionary processes have crossed these four respective thresholds would go upward rather than downward. (p.328??)

How does Arthur make all these predictions? By assuming that that the distribution of stuff in the universe is much like the distribution of stuff across our solar system and across the history of Earth:

On the basis of Earth’s history to date, the fraction of microbial inhabited planets that also have animals can be estimated by the relative durations of these two types of life here, which is 630 million compared to 4 billion years. (p.200)

The fact that [intelligence] and the physical basis for it – the brain – can be downplayed or even lost altogether in some lineages [in Earth history] should temper our hopes for the discovery of extraterrestrial intelligence. … Natural selection is not on a long-term quest for the ultimate brainy animals. (p.134)

With regard to possible life, the vast majority of the solar system, like the vast majority of the galaxy, is of little interest to us. For the most part, our system looks barren. (p..139)

But doesn’t all this neglect the possibility of that intelligent life on some planet will develop a more robust and powerful artificial life, which then spreads widely across the cosmos? Arthur admits the possibility of intelligent life spreading across planets:

Between two and three billion years from now … if new make it that far, we might have the technology to colonize the closest suitable exoplanets. (p.160)

Intelligent life may have colonized nearby planets, as may the the case in the mid-term future wit humans on Mars. (p.315)

Planets on which radio-level intelligence has evolved constitute only a tiny fraction of those on which life in general has evolved. Yet because of the vastness of the universe, and perhaps also because of planetary colonization, there are many planets with such life-forms in the universe right now. (p.328)

And Arthur admits the possibility of artificial life:

But there is a caveat here. What about AI (artificial intelligence)? It’s a moot point whether any of our machines are yet intelligent enough to truly merit that label, though no doubt they will get there eventually. Perhaps the machines associated with ultra-intelligent aliens are already there. In this case, the intelligent universe and the biological universe … are overlapping sets. Having made this point, let’s focus on intelligent living beings across the universe, not intelligent machines. And let’s ignore the advanced organism-machine hybrids of science fiction, even though entities of this type probably exist somewhere. (p.318)

But somehow these admissions make little difference to his forecasts, which ignore the possibility of artificial life at places other planets, or made out of stuff other than carbon. And which ignore the possibility of intelligent artificial life spreading very far and wide, to become even more common than non-artificial life.

Arthur instead assumes that advanced intelligence and artificial life will just not spread much, perhaps due to self-destruction:

Intelligent life may have a tendency to self-exterminate within a few centuries of its inception. (p.221)

Wallace Arthur seems to be yet another biologists who just can’t imagine our descendants being that different from us, or artificial life making much of a difference to the cosmos.

Out of a great many reviews of this book I read, I only found one other reviewer, David Studhalter, a non-academic, making a similar complaint:

Arthur … blithely assumes that humans and their descendants will simply become extinct before advancing to a stage where they are spreading terriform life elsewhere in the Galaxy, and that we will never exceed the bounds of our own Solar system. … Arthur mentions virtually nothing discussed in this last paragraph. But they are crucial to his subject, which does purport to discuss the future of life. (More)

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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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Schulze-Makuch & Bains on The Great Filter

In their 2016 journal article “The Cosmic Zoo: The (Near) Inevitability of the Evolution of Complex, Macroscopic Life“, Dirk Schulze-Makuch and William Bains write:

An important question is … whether there exists what Robin Hanson calls “The Great Filter” somewhere between the formation of planets and the rise of technological civilizations. …

Our argument … is that the evolution of complex life [from simple life] is likely … [because] functions found in complex organisms have evolved multiple times, an argument we will elaborate in the bulk of this paper … [and] life started as a simple organism, close to [a] “wall” of minimum complexity … With time, the most complex life is therefore likely to become more complex. … If the Great Filter is at the origin of life, we live in a relatively empty universe, but if the origin of life is common, we live in a Cosmic Zoo where such complex life is abundant.

Here they seem to say that the great filter must lie at the origin of life, and seem unclear on if it could also lie in our future.

In the introduction to in their longer 2017 book, The Cosmic Zoo: Complex Life on Many Worlds, Schulze-Makuch and Bains write:

We see no examples of intelligent, radio-transmitting, spaceship-making life in the sky. So there must be what Robin Hanson calls ‘The Great Filter’ between the existence of planets and the occurrence of a technological civilisation. That filter could, in principle, be any of the many steps that have led to modern humanity over roughly the last 4 billion years. So which of those major steps or transitions are highly likely and which are unlikely? …

if the origin of life is common and habitable rocky planets are abundant then life is common, and we live in a Cosmic Zoo. … Our hypothesis is that all major transitions or key innovations of life toward higher complexity will be achieved by a sufficient large biosphere in a semi-stable habitat given enough time. There are only two transitions of which we have little insight and much speculation—the origin of life itself, and the origin (or survival) of technological intelligence. Either one of these could explain the Fermi Paradox – why we have not discovered (yet) any sign of technologically advanced life in the Universe.

So now they add that (part of) the filter could lie at the origin of human-level language & tech. In the conclusion of their book they say:

There is strong evidence that most of the key innovations that we discussed in… this book follow the Many Paths model. … There are, however, two prominent exceptions to our assessment. The first exception is the origin of life itself. … The second exception … is the rise of technologically advanced life itself. …The third and least attractive option is that the Great Filter still lies ahead of us. Maybe technological advanced species arise often, but are then almost immediately snuffed out.

So now they make clear that (part of) the filter could also lie in humanity’s future. (Though they don’t make it clear to me if they accept that we know the great filter is huge and must lie somewhere; the only question is where it lies.)

In the conclusion of their paper, Schulze-Makuch and Bains say:

We find that, with the exception of the origin of life and the origin of technological intelligence, we can favour the Critical Path [= fixed time delay] model or the Many Paths [= independent origins] model in most cases. The origin of oxygenesis, may be a Many Paths process, and we favour that interpretation, but may also be Random Walk [= long expected time] events.

So now they seem to also add the ability to use oxygen as a candidate filter step. And earlier in the paper they also say:

We postulate that the evolution of a genome in which the default expression status was “off” was the key, and unique, transition that allowed eukaryotes to evolve the complex systems that they show today, not the evolution of any of those control systems per se. Whether the evolution of a “default off” logic was a uniquely unlikely, Random Walk event or a probable, Many Paths, event is unclear at this point.

(They also discuss this in their book.) Which adds one more candidate: the origin of the eukaryote “default off” gene logic.

In their detailed analyses, Schulze-Makuch and Bains look at two key indicators: whether a step was plausibly essential for the eventual rise of advanced tech, and whether we can find multiple independent origins of that step in Earth’s fossil record. These seem to me to both be excellent criteria, and Schulze-Makuch and Bains seem to expertly apply them in their detailed discussion. They are a great read and I recommend them.

My complaint is with Schulze-Makuch and Bains’ titles, abstracts, and other summaries, which seem to arbitrarily drop many viable options. By their analysis criteria, Schulze-Makuch and Bains find five plausible candidates for great filter steps along our timeline: (1) life origin ~3.7Gya, (2) oxygen processing ~3.1Gya (3) Eukaryote default-off genetic control ~1.8Gya, (4) human-level language/tech ~0.01Gya, and (5) future obstacles to our becoming grabby. With five plausible hard steps, it seems unreasonable to claim that “if the origin of life is common, we live in a Cosmic Zoo where such complex life is abundant”.

Schulze-Makuch and Bains seem to justify dropping some of these options because they don’t “favour” them. But I can find no explicit arguments or analysis in their article or book for why these are less viable candidates. Yes, a step being essential and only having been seen once in our history only suggests, but hardly assures, that this is a hard step. Maybe other independent origins happened, but have not yet been seen in our fossil record. Or maybe this did only happen once, but that was just random luck and they could easily have happened a bit later. But these caveats are just as true of all of Schulze-Makuch and Bains’ candidate steps.

I thus conclude that we know of four plausible and concrete candidates for great filter steps before our current state. Now I’m not entirely comfortable with postulating a step very recently, given the consistent trend in increasing brain sizes over the last half billion years. But Schulze-Makuch and Bains do offer plausible arguments for why this might in fact have been an unlikely step. So I accept that they have found four plausible hard great filter steps in our past.

The total number of hard steps in the great filter sets the power in our power law model for the origin of grabby aliens. This number includes not only the hard filter steps that we’ve found in the fossil record of Earth until now, but also any future steps that we may yet encounter, any steps on Earth that we haven’t yet noticed in our fossil record, and any steps that may have occurred on a prior “Eden” which seeded Earth via panspermia. Six steps isn’t a crazy middle estimate, given all these considerations.

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Humans Are Early

Imagine that advanced life like us is terribly rare in the universe. So damn rare that if we had not shown up, then our region of the universe would almost surely have forever remained dead, for eons and eons. In this case, we should still be able to predict when we humans showed up, which happens to be now at 13.8 billion years after the universe began. Because we showed up on a planet near a star, and we know the rate at which our universe has and will make stars, how long those stars will last, and which stars where lived far enough away from frequent sterilizing explosions to have at least a chance at birthing advanced life.

However, this chart (taken from our new paper) calculates the percentile rank of our current date within this larger distribution. And it finds that we are surprisingly early, unless you assume both that there are very few hard steps in the evolution of advanced life (the “power n”), and also that the cutoff in lifetime above which planets simply cannot birth advanced life is very low. While most stars have much longer lives, none of those have any chance whatsoever to birth advanced life. (The x-axis shown extends from Earth’s lifetime up to the max known star lifetime.)

In the paper (in figures 2,17), we also show how this percentile varies with three other parameters: the timescale on which star formation decays, the peak date for habitable star formation, and a “mass favoring power” which says bu how much more are larger mass stars favored in habitability. We find that these parameters mostly make only modest differences; the key puzzle of humans earliness remains.

Yes, whether a planet gives rise to advanced life might depend on a great many other parameters not included in our calculations. But as we are only trying to estimate the date of arrival, not many other details, we only need to include factors that correlate greatly with arrival date.

Why have others not reported the puzzle previously? Because they neglected to include the key hard-steps power law effect in how chances vary with time. This effect is not at all controversial, though it often seems counter-intuitive to those who have not worked through its derivation (and who are unwilling to accept a well-established literature they have not worked out for themselves).

This key fact that humans look early is one that seems best explained by a grabby aliens model. If grabby aliens come and take all the volume, that sets a deadline for when we could arrive, if we were to have a chance of becoming grabby. We are not early relative to that deadline.

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Try-Menu-Combo Filter Steps

A great filter stands between simple dead matter and a visible expanding lasting civilization. Many hard steps (and also easy ones) must be passed to make it through this filter. But what kind of steps are these hard steps?

The first kind of steps that most people imagine are try-try steps. The local system must keep trying random variations at a constant rate until a successful one is found. Here the chance per unit time is a constant, and the chance of success by a time is linear, at least for small times. When a system must go through many hard steps by time t, but that success is quite unlikely, then for n hard steps the chance of that unlikely success by time t goes as tn.

I recently pointed out that there’s another kind of hard step: try-once. Here the local system has only one chance; if it fails then, it fails forever. For these sort of steps, the chance of success doesn’t increase with time trying.

In this post, I want to point out that there are worse kinds of steps than try-try steps. Such as try-menu-combo steps.

Imagine that to pass some important step, evolution needed to create a species with a particular combination of eyes, hands, feet, stomach, ears, etc. Except that the available menu for each of these parts increased linearly with time.

For example, at first there is only one kind of stomach available. All species must use that kind of stomach. Then there are two kinds, and then three. Which kind of stomach is the next to be added to the stomach menu is pretty random. But there is zero chance of achieving his menu-combo next step until the right kind of stomach is added to the menu.

In this scenario, having the right kind of stomach on the menu is far from enough. The system also needs to add the right kind of eyes to the eye menu, and so on. Once all of the right kinds of items are on the right menus, then the last thing needed is a try-try step, to create a specific species that includes all the right parts via randomly combining menu items.

If there were just one kind of part needed, the chance of success by some date would increase linearly with time, making this an ordinary try-try step. But if there were two kinds of parts needed, chosen from two menus, then the chance would go as t2. With three menus, it is t3. And so on.

So now we can see that the tn rule for the chance of many hard steps by time t can be generalized. Now instead of n being the number of hard steps, n becomes the sum of powers m for each of the hard steps. Step power m is zero for a try-once step, is near one for a try-try step, and is greater than one for try-menu-combo steps.

In terms of its contribution to the tn power law for completing all the hard steps, a try-menu-combo step is the equivalent of several try-try steps all happening at the same time. That is, great filter hard steps can in some sense happen in parallel, as well as in sequence.

With ordinary try-try steps, one only sees progress in the history record when steps are passed. So looking at the many forms of progress we’ve seen in the past half billion years through the lens of try-try steps, one concludes that these were many easy try-try steps, and so contained no hard steps.

But what if some sort of combo step has been happening instead? During a menu-combo step, one should see the progress of increasingly long menus for each of the parts. And yet it could still be a very hard step, the equivalent of many hard try-try steps happening in parallel. Maybe something about humans was a hard step after all?

Can anyone think of other plausible mechanisms by which hard steps could have a tm dependence, for m > 1?

Added 10a: I expect that an m power step will be completed on average in m/(n+1) of the available window for life on Earth, where n is the total power of the steps done on Earth. So that’s still a problem for having a lot happen in the last half billion years.

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How Far To Grabby Aliens? Part 2.

In my last post, I recommended these assumptions:

  1. It is worth knowing how far to grabby alien civs (GCs), even if that doesn’t tell about other alien types.
  2. Try-try parts of the great filter alone make it unlikely for any one small volume to birth an GC in 14 billion years.
  3. We can roughly estimate GC expansion speed, and the number of hard try-try steps in the great filter.
  4. Earth is not now within the sphere of control of an GC.
  5. Earth is at risk of birthing an GC soon, making today’s date a sample from GC time origin distribution.

I tried to explain how these assumptions can allow us to estimate how far away are GC. And I promised to give more math details in my next post post. This is that next post.

First, I promised to elaborate on how well tn works as the chance that a small volume will birth a GC at time t. The simplest model is that eternal oases like Earth are all born at some t=0, and last forever. Each oasis must pass through a great filter, i.e., a sequence of hard steps, from simple dead matter to simple life to complex life, etc., ending at a GC birth. For each hard step, there’s a (different) constant chance per unit time to make it to the next step, a chance so low that the expected time for each step is much less than t.

In this case, the chance of GC birth per unit time in a small volume is tn, with n = h-1, where h is the number of hard steps. If there are many oases in a small volume with varying difficulty, their chances still add up to the same tn dependence as long as they all have the same number of hard steps between dead matter an a GC.

If there are try-once steps in the great filter, steps where an oasis can fail but which don’t take much time, that just reduces the constant in front of tn, without changing the tdependence. If there are also easy steps in this filter, steps that take expected time much less than t, these just add a constant delay, moving the t=0 point in time. We can accommodate other fixed delays in the same way.

We have so far assumed that, one the prior steps have happened, the chance of each step happening is constant per unit time. But we can also generalize to the case where this step chance per time is a power law tm , with t the time since the last step was achieved, and with a different mi for each step i. In this case, h = Σi (1+mi). These step powers m can be negative, or fractional.

Instead of having the oases all turn on at some t=0, oases like Earth with a chance tn can instead be born at a constant rate per unit time after some t=0. It turns out that the integrated chance across all such oases of birthing a GC at time t is again proportional to tn, with again n = h-1.

A more elaborate model would consider the actually distribution of star masses, which have a CDF that goes as m-1.5, and the actual distribution of stellar lifetime L per mass m, which has a CDF that goes as m-3. Assuming that stars of all masses are created at the same constant rate, but that each star drops out of the distribution when it reaches its lifetime, we still get that the chance of GC birth per unit time goes as tn, except that now n = h-1.5.

Thus the tn time dependence seems a decent approximation in more complex cases, even if the exact value of n varies with details. Okay, now lets get back to this diagram I showed in my last post:

If the GC expansion speed is constant in conformal time (a reasonable approximation for small civ spatial separations), and if the civ origin time x that shapes the diagram has rank r in this civ origin time distribution, then x,r should satisfy:

((1-r)/r) ∫0x tn dt = ∫x1 tn (1 – ((t-x)D/(1-x))) dt.
Here D is the space dimension. D = 3 is appropriate on the largest and the small many-star scales, but D = 2 across galaxy disks, and D = 1 in filaments of galaxies. This equation can be solved numerically. The ratio of the time from an GC origin til that GC directly meets aliens, relative to universe age at civ origin, is (1-x)/x, and is shown in this table:

The x-axis here is the power n in tn, and the y-axis is shown logarithmically. As you can see, aliens can be close in the sense that the time to reach aliens is much smaller than is the time it takes to birth the GC. This time til meet is also smaller for higher powers and for more spatial dimensions.

Note that these meet-to-origin time ratios don’t depend on the GC expansion speed. As I discussed in my last post, this model suggests that spatial distances between GC origins double if either the median GC origin time doubles, or if the expansion speed doubles. The lower is the expansion speed relative to the speed of light, the better a chance a civ has of seeing an approaching GC before meeting them directly. (Note that we only need a GC expansion speed estimate to get distributions over how many GCs each can see at its origin, and how easy they are to see. We don’t need speeds to estimate how long til meet aliens.)

To get more realistic estimates, I also made a quick Excel-based sim for a one dimensional universe. (And I am happy to get help making better sims, such as in higher dimensions.) I randomly picked 1000 candidate GC origins (x,t), with x drawn uniformly in [0,1], and t drawn proportional to tn in [0,1]. I then deleted any origin from this list if, before its slated origin time, it could be colonized from some other origin in the list at speed 1/4. What remained were the actual GC origin points.

Here is a table with key stats for 4 different powers n:

I also did a version with 4000 candidate GCs, speed 1/8, and power n = 10, in which there were 75 C origins. This diagram shows the resulting space-time history (time vertical, space horizontal):

In the lower part, we see Vs where an GC starts and grows outward to the left and right. In the upper part, we see Λs where two adjacent GC meet. As you can see, for high powers GC origins have a relatively narrow range of times, but a pretty wide range of spatial separations from adjacent GC.

Scaling these results to our 13.8 billion year origin date, we get a median time to meet aliens of  roughly 1.0 billion years, though the tenth percentile is about 250 million years. If the results of our prior math model are a guide, average times to meet aliens in D=3 would be about a factor two smaller. But the variance of these meet times should also be smaller, so I’m not sure which way the tenth percentile might change.

A more general way to sim this model is to:

  • A) set a power n in tn and estimate 1) a density in space-time of origins of oases which might birth GCs, 2) a distribution over oasis durations, and 3) a distribution over GC expansion speeds,
  • B) randomly sample 1) oasis spacetime origins, 2) durations to produce a candidate GC origin after its oasis origin times, using tn , and 3) expansion speed for each candidate GC,
  • C) delete candidate GCs if their birth happens after its oasis ends or after a colony from another GC colony could reach there before then at its expansion speed.
  • D) The GC origins that remain give a distribution over space-time of such GC origins. Projecting the expansion speed forward in time gives the later spheres of control of each GC until they meet.

I’ll put an added to this post if I ever make or find more elaborate sims of this model.

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How Far To Grabby Aliens? Part 1.

Many have tried to estimate how far away are aliens. For example, some apply the Drake equation, which is the product of 7 parameters, some of which can vary over quite wide ranges. Resulting estimates tend to be quite uncertain and disputable.

In this post, I introduce a more precise and definitive answer, at least for one especially important kind of alien. My median estimate is that, if we survive, we will meet this kind of alien in roughly a half billion years. In this post, I’ll try to give key intuitions. In my next post, I’ll give more math details.

We are now quite early in the history of the universe. Some of the stars around us will last a thousand times longer than our Sun. This key fact makes it hard to believe that, if Earth did not exist, no other civ (civilization) would ever colonize this area. If civs were that hard to make, then our civ shouldn’t be so early.

We should instead guess that eventually the universe will be mostly filled with civs, and thus one of the key constraints on the origin of any one civ is a need to pass a local great filter, going from no life to simple life to complex life to intelligence, etc., before some other civ arrives to colonize that area, and prevent new pics there.

That is, one key kind of alien is a “grabby civilization” (GC), which rapidly expands its sphere of control, and within that sphere a GC prevents the origin of any other GC. (Though it may allow the origin or continued existence of other kinds of aliens. And this “grabby” label says little about what happens when it directly meets another civ.)

It looks like there is a non-trivial chance that we here on Earth will give birth to such an GC near here. And soon. (Say within a million years.) I’m not here claiming (nor disputing) that this would be a good idea, or even that this chance is especially large. But this chance does seem real enough to justify treating our date now, 13.8 billion years after the Big Bang, as a data point drawn from the distribution of GC origin dates. Allowing us to draw inferences about that distribution.

My strategy will be to describe a mathematical model of this distribution that is both well-grounded theoretically, and also simple enough to allow concrete analysis and inference. One result of which is concrete estimates on how far away are the nearest aliens.

My mathematical model has just three parameters, two of which are already known to within roughly an order of magnitude, and the third of which we can infer about that well from our one timing data point. The first parameter is the speed at which an GC expands to colonize the space around it. At least until it directly meets another GC. This speed must be less than the speed of light, and grabbiness would tend to push an GC to higher speeds, but it isn’t clear just how much less than light speed an GC will have to accept.

The second parameter is the number of hard try-try steps in each local great filter. The fact that we now see no alien civilizations anywhere strongly suggests that any one oasis (e.g., planet) has a very low chance to start from simple dead matter and then give rise to a clearly visible civilization. Assume that this dead-matter-to-visibility filter has a similar size to the filter for dead matter giving rise to an GC. Assume also that even if there are also other try-once steps in this GC filter, the try-try steps are by themselves sufficiently hard that any one oasis (like Earth) is quite unlikely to, by itself, get through its great filter by today’s 13.8 billion year date. (Easy steps just create time delays, and any steps near the border between easy and hard give nearly mixed effects.)

These assumptions imply that the chance that any one small volume actually gives birth to an GC by a particular time t since the Big Bang is (after a time delay) proportional to tn, where n is near the number of hard try-try steps. (I’ll elaborate on this relation in my next post.)

The third parameter sets a constant in front of tn, an overall filter strength. This gives an absolute chance that the great filter is passed in one of the oases in a small standard volume by a particular date t. Our key datum of our being near ready to start an GC at 13.8 billion years after the Big Bang lets us estimate this filter constant. Given it, and also estimates on the other two parameters of speed and number of hard steps, we can infer our distance to the nearest aliens.

If that claim surprises you, consider the following diagram:

Assume that potential GC origins are uniformly distributed in space. If we integrate the probability density tn-1 over the yellow region, and then renormalize, that renormalization in effect sets the value of the overall filter strength, relative to the origin time of that one civ in the diagram.

If we then assume that this civ origin time is at the median of the renormalized distribution that we’ve calculated, we get a self-consistent model that gives an exact answer for the spacing between such civs! Yes, this model is only in one dimension, and doesn’t fully allow for variation in GC origin locations and timings. But it shows how it is possible to get a spacing between civs from only an expansion speed, a number of hard steps, and a sample origin time.

Note two key symmetries of this simple model. First, we get exactly the same model if we both double the duration from time start to this GC origin, and also the spatial distance between GC origins. Second, we get exactly the same model if we double both the expansion speed and the spatial distance between GC origins. Thus given a power n, an expansion speed, and a median GC origin time, the model is fully determined, setting a complete space-time distribution over GC origins and spheres of control.

In sum, it is possible to estimate how far away in space and time are the nearest aliens, if one is willing to make these assumptions:

  1. It is worth knowing how far to grabby aliens (GCs), even if that doesn’t tell about other alien types.
  2. Try-try parts of the great filter alone make it hard for any one oasis to birth an GC in 14 billion years.
  3. We can roughly estimate the speed at which GCs expand, and the number of hard try-try steps.
  4. Earth is not now within the sphere of control of a GC.
  5. Earth is at risk of birthing a GC soon, making today’s date a sample from GC time origin distribution.

In my next post I’ll give more math details, and discuss what concrete estimates they suggest about aliens.

Added: Here is a 2 hour interview I did with Adam Ford on this topic.

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Searching For Eden

In my last post, I reviewed the standard theory that life on Earth got very lucky to complete a series of hard try-try steps to get to our human level before its window for life closes. I said that this theory has had only mixed success in predicting Earth history timings, and does noticeably badly in predicting that Earth should score well on the key figure of merit of (V*M*W)N, for number of hard try-try steps N, volume V, metabolism M, and time window W not used for easy steps. This seems a pretty big deal, as this is a pretty basic theory on a pretty important process. If we are very wrong about it, that’s important to know. (Just as its important to get to the bottom of credible UFO sightings.)

This evidence conflict might be tolerably weak if one only estimated N=1, one very hard step. But, I said, life designs look so complex and well-integrated that I estimate at least ten hard steps, much more than the few perhaps seen in Earth’s fossil record. This graph conveys a similar intuition, suggesting that those added hard steps happened either as many try-once steps very early in Earth history, or as many try-try steps before Earth.

In my last post, I suggested that this conflict could at least be cut by positing many hard try-once steps, instead of the usual hard try-try steps, early in Earth’s history. But now I’ll admit I don’t think that is enough. I find it hard to believe that more than half of (the integrated magnitude of) hard steps are of the try-once sort, and yet even N=5 gives a quite strong evidence conflict. So I’m forced to take seriously panspermia, the hypothesis that life had another oasis before Earth, and was transferred from that oasis to Earth very early in Earth’s history. Call that prior oasis “Eden”.

Yes, interstellar panspermia seems hard and risky, in effect adding another try-once great filter step which life must compete. Here’s a recent estimate:

But if this scenario allows an Eden with a volume times metabolism as large as Earth’s, and if there are R times as may oases that could support the more robust early sort of life, relative to the more fragile multicellular sort that comes later, then the relative chance of this Eden scenario, compared to just-Earth, is a factor of R*2N times the chance that Eden could successfully transfer life to another (any other) suitable oasis. If Eden’s volume times metabolism were larger that Earth’s, this factor gets larger. So even a low chance to transfer to another oasis might be more than compensated by such a factor.

If there really was an Eden, then finding it is likely to become one of the great historical quests of our descendants. So what can we say about Eden now, to advise this quest? Here are some clues:

  1. Until we replace our usual theory, we still probably want to use the usual figure of merit to predict Eden. Except N becomes the number of hard try-try steps that happen at Eden, and the time deadline for delivering life to Earth says that the time window W can only be extended by having an oasis that starts earlier. So Eden likely meets the usual constraints (like temperature), has a large volume V and metabolism M, and started early.
  2. A set of planets or moons that are close to each other in the same solar system may have a high enough rate of life traveling between them to count as one larger planet, thereby gaining a big advantage. The same may apply to stars that are close and have much moving stuff nearby to induce high rates of transfers. For example, if seeding one star in a stellar nursery effectively seeds S starts in that nursery, then the panspermia theory gets a factor of S boost relative to alternate theories.
  3. The more try-try hard steps that happened on Eden, as opposed to on Earth, the weaker is the evidence conflict re Earth. So maybe most such steps happened on Eden.
  4. Eden mainly needs to give rise to single-cell extremophiles that could travel well enough. So it needn’t support fragile multicellular life, and may work better if it has high (but not overly high) variability to encourage the evolution of robustness. So there seems to be a substantial R factor, and Eden may have been far from a gentle protective “garden”.
  5. Eventually, Eden would have been home to the sort of life that gave rise to Earth’s sort of life. With carbon, water, and DNA. So that rejects exotic hypothesized life such as made of silicon, or in plasmas or neutron stars.
  6. This robust single-celled life seems much less vulnerable to the gamma ray bursts, supernova, and asteroids that tend to kill off fragile life like us close to the galactic center. Or close to the large solar flares common near red dwarf stars. So while Earth was not allowed to be in such places, Eden is allowed there.
  7. Panspermia gets easier in places where stars are spaced closer together, as toward the galactic center. So all else equal, expect Eden in such places.
  8. However, if dormant cells only survive between stars for a million years, then if its dust or rock travel host moved at a typical relative velocity of 30km/s, it could only travel 100 light years in that time, which doesn’t get you far in the galaxy. Thus either Eden was quite near to Earth when Earth acquired life, or dormant life can last much longer, or hosts could fly much faster.
  9. The usual analysis of interstellar panspermia gets pretty low rates. But the chance of panspermia should be increased by the density of stuff flying around near the travel origin and destination locations. Such stuff can kick up life from Eden, and help grab stuff traveling past Earth. Earth had lots of stuff flying about when its solar system formed, and that was embedded in larger complex turbulent dynamic molecular clouds which had more stuff flying about. So if Eden was close to Earth then, Eden was plausibly in a similar cloud area then, which helped induce the travel origin. Seems worth analyzing how molecular clouds change panspermia rates.
  10. Life could have continued on Eden long after it sent life to Earth, but the selection effect of seeing our existence doesn’t enhance the chance of that. The higher is our estimate of the number of oases to which Eden life would have spread, the easier it will be to find such life out there. But unless that chance is enormous, or R is enormous, we expect Eden and any of its other descendants to be quite hard to find. Stars that are siblings of our Sun, born in the same nursery, seem good candidates.
  11. The hypothesis of two prior oases in sequence, instead of one, would also be penalized by a low chance of transfer between them, but might allow a larger total time window W, and a boost in R via more possible oases. Furthermore, this scenario might allow a split wherein try-try steps happen in the oasis with a large figure of merit, but try-once steps happen in multiple parallel small oases, giving them a larger chance of success.
  12. Life in the atmosphere of a brown drawf seems an interesting possibility, but it seems harder for passing stuff to kick out or grab life from such a reservoir. Those things seem easier for life on a planet near a red dwarf, but those may suffer too much variability.
  13. Life may have been possible in a few places 10-17 million years after the Big Bang, from heavy elements formed by supernovae in rare star-forming fluctuation regions that constitute ~10-17 of all matter.
  14. (I’ll add more here as I or others suggest them.)

Added 17Dec: Note that a prediction of the Eden scenario is that the earliest and simplest form of life on Earth is likely a form that enabled panspermia, staying alive but dormant deep in rock for long periods. So life now deep in rock on Earth is predicted to be early and simple, instead of being variations on surface life that migrated down and colonized deep rock.

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Try-Try or Try-Once Great Filter?

Here’s a simple and pretty standard theory of the origin and history of life and intelligence. Life can exist in a supporting oasis (e.g., Earth’s surface) that has a volume V and metabolism M per unit volume, and which lasts for a time window W between forming and then later ending. This oasis makes discrete “advances” between levels over time, and at any one time the entire oasis is at the same level. For example, an oasis may start at the level of simple dead chemical activity, may later rise to a level that counts as “life”, then rise to a level that includes “intelligence”, and finally to a level where civilization makes a big loud noises that are visible as clearly artificial from far away in the universe.

There can be different kinds of levels, each with a different process for stepping to the next level. For example, at a “delay” level, the oasis takes a fixed time delay D to move to the next level. At a “try once” level, the oasis has a particular probability of immediately stepping to the next level, and if it fails at that it stays forever “stuck”, which is equivalent to a level with an infinite delay. And at a “try try” level, the oasis stays at a level while it searches for an “innovation” to allow it to step to the next level. This search produces a constant rate per unit time of jumping. As an oasis exists for only a limited window W, it may never reach high levels, and in fact may never get beyond its first try-try level.

If we consider a high level above many hard try-try levels, and with small enough values of V,M,W, then any one oasis may have a very small chance of “succeeding” at reaching that high level before its window ends. In this case, there is a “great filter” that stands between the initial state of the oasis and a final success state. Such a success would then only tend to happen somewhere if there are enough similar oases going through this process, to overcome these small odds at each oasis. And if we know that very few of many similar such oases actually succeed, then we know that each must face a great filter. For example, knowing that we humans now can see no big loud artificial activity for a very long distance from us tells us that planets out there face a great filter between their starting level and that big loud level.

Each try-try type level has an expected time E to step to the next level, a time that goes inversely as V*M. After all, the more volume there is of stuff that tries, and faster its local activity, the more chances it has to find an innovation. A key division between such random levels is between ones in which this expected time E is much less than, or much greater than, the oasis window W. When E << W, these jumps are fast and “easy”, and so levels change relatively steadily over time, at a rate proportional to V*M. And when E >> W, then these jumps are so “hard” that most oases never succeed at them.

Let us focus for now on oases that face a great filter, have no try-once steps, and yet succeed against the odds. There are some useful patterns to note here. First, let’s set aside S, the sum of the delays D for delay steps, and of the expected times E for easy try-try steps, for all such steps between the initial level and the success level. Such an oasis then really only has a time duration of about W-S to do all its required hard try-try steps.

The first pattern to note is that the chance that an oasis does all these hard steps within its window W is proportional to (V*M*(W-S))N, where N is the number of these hard steps needed to reach its success level. So if we are trying to predict which of many differing oases is mostly likely to succeed, this is the formula to use.

The second pattern to note is that if an oasis succeeds in doing all its required hard steps within its W-S duration, then the time durations required to do each of the hard steps are all drawn from the same (roughly exponential) distribution, regardless of the value of E for those steps! Also, the time remaining in the oasis after the success level has been reached is also drawn from this same distribution. This makes concrete predictions about the pattern of times in the historical record of a successful oasis.

Now let’s try to compare this theory to the history of life on Earth. The first known fossils of cells seems to be from 0.1-0.5 Ga (billion years) after life would be possible on Earth, which happened about 4.2 Gya (billion years ago), which was about 9.6 Ga after the universe formed. The window remaining for (eukaryotic) life to remain on Earth seems 0.8-1.5 Ga. The relatively steady growth in max brain sizes since multi-cellular life arose 0.5 Gya suggests that during this period there were many easy, but no hard, try-try steps. Multi-celluar life seems to require sufficient oxygen in the atmosphere, but the process of collecting enough oxygen seems to have started about 2.4 Gya, implying a long 1.9 Ga delay step. Prokaryotes started exchanging genes about 2.0 Gya, eukaryotes appeared about 1.7 Gya, and modern sex appeared about 1.2 Gya. These events may or may not have been the result of successful try-try steps.

Can we test this history against the predictions that try-try hard step durations, and the window time remaining, should all be drawn from the same roughly exponential distribution? Prokaryote sex, eukaryotes, and modern sex all appeared within 0.8 Ga, which seems rather close together, and leaving a long uneventful period of almost ~2 Ga before them. The clearest hard step duration candidates are before the first life, which took 0.0-0.5 Ga, and the window remaining of 0.8-1.5 Ga, which could be pretty different durations. Overall I’d say that while this data isn’t a clear refutation of the same hard step distribution hypothesis, it also isn’t that much of a confirmation.

What about the prediction that the chance of oasis success is proportional to (V*M*(W-S))N? The prediction about Earth is that it will tend to score high on this metric, as Earth is the only example of success that we know.

Let’s consider some predictions in turn, starting with metabolism M. Life of the sort that we know seems to allow only a limited range of temperatures, and near a star that requires a limited range of distances from the star, which then implies a limited range of metabolisms M. As a result of this limited range of possible M, our prediction that oases with larger M will have higher chances of success doesn’t have much room to show itself. But for what its worth, Earth seems to be nearer to the inner than outer edge of the Sun’s allowable zone, giving it a higher value of M. So that’s a weak confirmation of the theory, though it would be stronger if the allowed zone range were larger than most authors now estimate.

What about volume V? The radii of non-gas-giant planets seems to be lognormally distributed, with Earth at the low end of the distribution (at a value of 1 on this axis):

So there are many planets out there (at r=4) with 16 times Earth’s surface area, and with 64 times the volume, ratios that must be raised to the power of N to give their advantage over Earth. And these larger planets are made much more of water than is Earth. This seems to be a substantial, if perhaps not overwhelming, disconfirmation of the prediction that Earth would score high on VN. The higher is the number of hard steps N, the stronger is this disconfirmation.

Regarding the time window W, I see three relevant parameters: when a planet’s star formed, how long that star lasts, and how often there are supernova nearby that destroy all life on the planet. Regarding star lifetimes, main sequence star luminosity goes as mass to the ~3.5-4.0 power, which implies that star lifetimes go inversely as mass to the ~2.5-3.0 power. And as the smallest viable stars have 0.08 of our sun’s mass, that implies that there are stars with ~500-2000 times the Sun’s lifetime, an advantage that must again be raised to the power N. And there are actually a lot more such stars, 10-100 times more than of the Sun’s size:

However, the higher metabolism of larger mass stars gives them a spatially wider habitable zone for planets nearby, and planets near small stars are said to face other problems; how much does that compensate? And double stars should also offer wider habitable zones; so why is our Sun single?

Now what if life that appears near small long-lived stars would appear too late, as life that appeared earlier would spread and take over? In this case, we are talking about a race to see which oases can achieve intelligence or big loud civilizations before others. In which case, the prediction is that winning oases are the ones that appeared first in time, as well has having good metrics of V,M,W.

Regarding that, here are estimates of where the habitable stars appear in time and galactic radii, taking into account both star formation rates and local supernovae rates (with the Sun’s position shown via a yellow star):

As you can see, our Sun is far from the earliest, and its quite a bit closer to galactic center than is ideal for its time. And if the game isn’t a race to be first, our Sun seems much earlier than is ideal (these estimates are arbitrarily stopped at 10Ga).

Taken together, all this seems to me to give a substantial disconfirmation of the theory that chance of oasis success is proportional to (V*M*(W-S))N, a disconfirmation that gets stronger the larger is N. So depending on N, maybe not an overwhelming disconfirmation, but at least substantial and worrisome. Yes, we might yet discover more constraints on habitability to explain all these, but until we find them, we must worry about the implications of our analysis of the situation as we best understand it.

So what alternative theories do we have to consider? In this post, I’d like to suggest replacing try-try steps with try-once steps in the great filter. These might, for example, be due to evolution’s choices of key standards, such as the genetic code, choices that tend to lock in and get entrenched, preventing competing standards from being tried. The overall chance of success with try-once steps goes as the number of oases, and is independent of oasis lifetime, volume, or metabolism, favoring many small oases relative to a few big ones. With more try-once steps, we need fewer try-try steps in the great filter, and thus N gets slower, weakening our prediction conflicts. In addition, many try-once steps could unproblematically happen close to each other in time.

This seems attractive to me because I estimate there to be in fact a great many rather hard steps. Say at least ten. This is because the design of even “simple” single cell organisms seems to me amazingly complex and well-integrated. (Just look at it.) “Recent” life innovations like eukaryotes, different kinds of sex, and multicellular organisms do involved substantial complexity, but the total complexity of life seems to me far larger than these. And while incremental evolution is capable of generating a lot of complexity and integration, I expect that what we see in even the simplest cells must have involved a lot of hard steps, of either the try-once or the try-try type. And if they are all try-try steps, that makes for a huge N, which makes the prediction conflicts above very difficult to overcome.

Well that’s enough for this post, but I expect to have more to say on the subject soon.

Added 19Jan: Turns out we also seem to be in the wrong kind of galaxy; each giant elliptical with a low star formation rate hosts 100-10K times more habitable Earth-like planets, and a million times as many habitable gas giants, than does our Milky Way.

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