Tag Archives: Aliens

Hail S. Jay Olson

Over the years I’ve noticed that grad students tend to want to declare their literature search over way too early. If they don’t find something in the first few places they look, they figure it isn’t there. Alas, they implicitly assume that the world of research is better organized than it is; usually a lot more search is needed.

Seems I’ve just made this mistake myself. Having developed a grabby aliens concept and searched around a bit I figured it must be original. But it turns out that in the last five years physicist S. Jay Olson has a whole sequence of seven related papers, most of which are published, and some which got substantial media attention at the time. (We’ll change our paper to cite these soon.)

Olson saw that empirical study of aliens gets easier if you focus on the loud (not quiet) aliens, who expand fast and make visible changes, and also if you focus on simple models with only a few free parameters, to fit to the few key datums that we have. Olson variously called these aliens “aggressively expanding civilizations”, “expanding cosmological civilizations”, “extragalactic civilizations”, and “visible galaxy-spanning civilizations”. In this post, I’ll call them “expansionist”, intended to include both his and my versions.

Olson showed that if we assume that humanity’s current date is a plausible expansionist alien origin date, and if we assume a uniform distribution over our percentile rank among such origin dates, then we can estimate two things from data:

  1. from our current date, an overall appearance rate constant, regarding how frequently expansionist aliens appear, and
  2. from the fact that we do not see grabby controlled volumes in our sky, their expansion speed.

Olson only required one more input to estimate the full distribution of such aliens over space and time, and that is an “appearance rate” function f(t), to multiply by the appearance rate constant, to obtain the rate at which expansionist aliens appear at each time t. Olson tried several different approaches to this function, based on different assumptions about the star formation rate and the rate of local extinction events like supernovae. Different assumptions made only make modest differences to his conclusions.

Our recent analysis of “grabby aliens”, done unaware of Olson’s work, is similar in many ways. We also assume visible long-expanding civilizations, we focus on a very simple model, in our case with three free parameters, and we fit two of them (expansion speed and appearance rate constant) to data in nearly the same way that Olson did.

The key point on which we differ is:

  1. My group uses a simple hard-steps-power-law for the expansionist alien appearance rate function, and estimates the power in that power law from the history of major evolutionary events on Earth.
  2. Using that same power law, we estimate humanity’s current date to be very early, at least if expansionist aliens do not arrive to set an early deadline. Others have estimated modest degrees of earliness, but they have ignored the hard-steps power law. With that included, we are crazy early unless both the power is implausibly low, and the minimum habitable star mass is implausibly large.

So we seem to have something to add to Olson’s thoughtful foundations.

Looking over the coverage by others of Olson’s work, I notice that it all seems to completely ignore his empirical efforts! What they mainly care about seems to be that his having published on the idea of expansionist aliens licensed them to speculate on the theoretical plausibility of such aliens: How physically feasible is it to rapidly expansion in space over millions of years? If physically feasible, is it socially feasible, and if that would any civilization actually choose it?

That is, those who commented on Olson’s work all acted as if the only interesting topic was the theoretical plausibility of his postulates. They showed little interest in the idea that we could confront a simple aliens model with data, to estimate the actual aliens situation out there. They seem stuck assuming that this is a topic on which we essentially have no data, and thus can only speculate using our general priors and theories.

So I guess that should become our central focus now: to get people to see that we may actually have enough data now to get decent estimates on the basic aliens situation out there. And with a bit more work we might make much better estimates. This is not just a topic for theoretical speculation, where everyone gets to say “but have you considered this other scenario that I just made up, isn’t it sorta interesting?”

Here are some comments via email from S. Jay Olson:

It’s been about a week since I learned than Robin Hanson had, in a flash, seen all the basic postulates, crowd-sourced a research team, and smashed through his personal COVID infection to present a paper and multiple public talks on this cosmology. For me, operating from the outskirts of academia, it was a roller coaster ride just to figure out what was happening.

But, what I found most remarkable in the experience was this. Starting from two basic thoughts — 1) some fraction of aliens should be high-speed expansionistic, and 2) their home galaxy is probably not a fundamental barrier to expansion — so many conclusions appear inevitable: “They” are likely a cosmological distance from us. A major fraction of the universe is probably saturated by them already. Sufficiently high tech assumptions (high expansion speed) means they are likely invisible from our vantage point. If we can see an alien domain, it will likely cover a shockingly large angle in the sky. And the key datum for prediction is our cosmic time of arrival. It’s all there (and more), in both lines of research.

Beyond that, Robin has a knack for forcing the issue. If their “hard steps model” for the appearance rate of life is valid (giving f(t) ~ t^n), there aren’t too many ways to solve humanity’s earliness problem. Something would need to make the universe a very different place in the near cosmic future, as far as life is concerned. A phase transition resulting in the “end of the universe” would do it — bad news indeed. But the alternative is that we are, literally, the phase transition.

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What Is At Stake?

In the traditional Christian worldview, God sets the overall path of human history, a history confined to one planet for a few thousand years. Individuals can choose to be on the side of good or evil, and maybe make a modest difference to local human experience, but they can’t change the largest story. That is firmly in God’s hands. Yet an ability to personally choose good or evil, or to make a difference to mere thousands of associates, seemed to be plenty enough to motivate most Christians to action.

In a standard narrative of elites today, the entire future of value in the universe sits our current collective hands. If we make poor choices today, such as about global warming or AI, we may soon kill ourselves and prevent all future civilization, forever destroying all sources of value. Or we might set our descendants down a permanently perverse path, so that even if they never go extinct they also never realize most of the universe’s great potential. And elites today tend to lament that these far grander stakes don’t seem to motivate many to action.

Humans seem to have arrived very early in the history of the universe, a fact that seems best explained by a looming deadline: grabby/aggressive aliens will control all the universe volume within a billion years, and so we had to show up before that deadline if we were to show up at all.

So now we have strong evidence that all future value in the universe does not sit in our hands. What does sit in our collective hands are:
A) the experiences of our descendants for roughly (within a factor of ten around) the next billion years, before they meet aliens, and
B) our influence on the larger mix of alien cultures in the eras after many alien civilizations meet and influence each other.

Now a billion years is in fact a very long time, a duration during which we could have an enormous number of descendants. So even that first part is a big deal. Just not as big a deal as many have been saying lately.

On the longer timescale, the question is not “will there be creatures who find their lives worth living?” We can be pretty assured that the universe will be full of advanced complex creatures who choose to live. The question is instead more “How much will human-style attitudes and approaches influence the hundreds or more alien civilizations with which we may eventually come in contact?”

It is less about whether there will be any civilizations, and more about what sorts of civilizations they will be. Yes, we should try to not go extinct, and yes we should try to find better paths and lifestyles for our descendants. But we should also aspire, and to a similar degree, to become worthy of emulating, when compared to a sea of alien options.

Unless we can offer enough unique and valuable models for emulation, and actually persuade or force such emulation, then it won’t really matter so much if we survive to meet aliens. From that point on, what matters is what difference we make to the mix. Whether we influence the mix, and whether that mix is better off as a result of our influence.

Not an easy goal, and not one we are assured to achieve. But we have maybe a billion years to work on it. And at least we can relax a bit; not all future universe value depends on our actions now. Just an astronomical amount of it. The rest is in “God’s” hands.

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Counter-Signaling On Aliens

For a long time, people who wrote on U.F.O.s have faced extra hurdles. Compared to those who write on other topics, authors on this topic are scrutinized more carefully for credentials and conflicting interests. The evidence they present is scrutinized much more carefully for detail, consistency, and potential bias and contamination, and much less likely alternative explanations are considered sufficient to reject such evidence. And even when they meet these higher standards, such authors still find it hard to gain much media attention.

A week ago Harvard astrophysics department chair Avi Loeb published a book wherein he argues that the object “Oumuamua” that passed quickly through our solar system in 2017 was an artificial alien artifact. The book doesn’t actually go into much detail on data about the object, certainly not enough to allow readers to apply the scrutiny usually expected of U.F.O. claims.

And even though he says he’s nearly alone among astrophysicists in his view, Loeb doesn’t at all help readers to understand why they believe different from Loeb. His story seems to be that they are all just chicken-shit. And his story about what the aliens are doing out there seems to be that they are mostly long dead.

If Loeb doesn’t talk much about the technical details and evidence, what does he talk about? Mostly his childhood, philosophy, other projects, bigshots he knows, etc. (Though he does also mention me.) And the media have overall been very kind to him, giving him lots of coverage and little criticism.

You might think that Loeb’s claim about this space object and common U.F.O. claims would seem to support each other. But in a few places, Loeb is very dismissive of ordinary U.F.O. evidence. (here and here). He’s clearly trying to say that what he says is nothing like what they say.

All of which seems to me a pretty clear example of countersignaling. Just like you are often nice to acquaintances to distinguish them from strangers, but mean to friends to distinguish them from mere acquaintances, we often do the opposite of the usual signal to show we are special. Loeb doesn’t have to follow the usual rules that would apply to most folks offering data on aliens, because (as he repeatedly reminds us) he is a Harvard astrophysics department chair.

All of which may help you understand why people often don’t follow the usual epistemic rules. Because the usual rules are for little people, and you aren’t little, are you?

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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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An Adventure

Back on Dec. 16, I began a great intellectual adventure, one which reaches a climax today as we post our preprint:

A Simple Model of Grabby Aliens

Robin Hanson, Daniel Martin, Calvin McCarter, Jonathan Paulson

According to a hard-steps model of advanced life timing, humans seem puzzlingly early. We offer an explanation: an early deadline is set by “grabby” civilizations (GC), who expand rapidly, never die alone, change the appearance of the volumes they control, and who are not born within other GC volumes. If we might soon become grabby, then today is near a sample origin date of such a GC. A selection effect explains why we don’t see them even though they probably control over a third of the universe now. Each parameter in our three parameter model can be estimated to within roughly a factor of four, allowing principled predictions of GC origins, spacing, appearance, and durations till we see or meet them. (more)

Back on Dec. 16, I envisioned the basic “grabby aliens” model, and found a simple math model to express its key symmetries. At which point I went into “manic mode”, forsaking most else in a push to “do this”, cutting way back on blogging, tweeting, reading emails, etc. (My few blog posts were mostly on this, two on Dec. 21, another Dec. 23, then one Jan. 3 and Jan. 8.)

On Dec. 28, I asked random strangers to help:

I quickly took on three collaborators (Daniel Martin, Calvin McCarter, Jonathan Paulson) who I knew little about other than that they had each quickly done something helpful to the project, and they were each willing to work for just coauthorship. I then rejected all others, and we began from a plan I’d sketched out. You might think this strategy quite risky, but it has in fact worked out.

Jan. 3-24 I visited Texas, and caught covid that last day Jan. 24, which has slowed me down a bit. Even so, we have pushed to get to this key milestone: a working paper to share with the world. So now I can catch my wind, get well, read old emails, etc. I’m quite proud of what we’ve done in such a short time. And I expect to be rejoining the world soon. though slowly. But I also expect to long remember this latest great adventure.

Added: More posts of mine, and one by Scott Aaronson, and discussion of that.

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Why We Can’t See Grabby Aliens

In two posts, I recently explained how a simple 3 parameter model of grabby aliens can explain our apparent early arrival in the universe, via a selection effect: we might give rise to a grabby civ, but that had to happen before other grabby civs took over all the volume.

With some collaborators, I’ve been exploring computer sims of this model, and found one striking statistic: at the origin time of a grabby civ, on average ~40% of universe volume is controlled by grabby aliens. A stat which seems obviously contradicted by what we see, namely nothing. In the volumes we see, they can’t be controlling much, at least if control would make it look much different. What gives?

In this post I want to show how this apparent emptiness can be explained by a parameter choice and a selection effect. First, let’s get oriented. Here is a spacetime diagram showing us now, and all the events that we can see from here, as our red backward light-cone.

Next, consider the fact that if we extend a yellow cone back in time from where we are at the grabby civ expansion speed, no grabby civ could have had their origin in that excluded volume, because if so then they would have prevented us, to prevent us from becoming grabby.

Because that’s the definition of grabby: they expand and prevent the origin of other grabby civs within the volumes they control. We could only see grabby civs who have their origin in the green volume, as their expansion would not have reached us yet.

Now if the expansion speed were small, that green area would encompass most of the volume in our past light-cone, and we’d still have a puzzle: why don’t we see them? But as their expansion speed approaches the speed of light, the green volume gets small, making for a low chance of seeing any grabby aliens. (The chance of not seeing one goes as roughly the fraction of their expansion speed to the speed of light.)

Now let’s look at one of those grabby civs we could see:

Since its origin is in the green volume, its forward expanding cone of control (in orange) intersects our backward light-cone. At the closest intersection point, the spatial extent of that civ is given by the horizontal purple line, which is large compared to its distance away. (Imagine space were 2D, fixing one end of the purple line at the origin axis, and rotating the other end out of the diagram.) So it would be absolutely huge in the sky. This diagram also shows our forward expansion cone intersecting its forward cone relatively soon in the future; we meet them soon.

Now look at the vertical purple line in this next diagram. Holding constant the spatial location of this alien origin, consider the other possible times at which this civ could have originated at that location and still be visible to us.

The higher is that origin point in the diagram, and the closer is that origin to our red backward light cone, then the smaller is that vertical purple line. And since geometrically the two purple lines must move in proportion, the smaller of an appearance that civ would make in the sky.

As civ origin times should be roughly uniformly distributed over that vertical range, there is thus only a tiny chance of seeing aliens that take up a tiny fraction of our sky. Either we see them huge, or not at all. So there’s little point in building bigger SETI telescopes or deeper surveys to try to see very tiny grabby aliens very far away.

Thus our grabby aliens model can use selection effects to explain not only why we have appeared so early in the history of the universe, but also why we don’t see them even though they should on average take up (and modify) ~40% of universe volume at the moment. At least if we postulate that their expansion speed is a substantial fraction of the speed of light. Which we already had reason to believe, just based on the idea that “grabby” civs try to grab as fast as they can.

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The Long Term Future of History

Assuming that dark energy continues to make the universe expand at an accelerating rate, in about 150 billion years all galaxies outside the Local Supercluster will pass behind the cosmological horizon. It will then be impossible for events in the Local Group to affect other galaxies. Similarly it will be impossible for events after 150 billion years, as seen by observers in distant galaxies, to affect events in the Local Group. (More)

My last two posts suggested that the average spacing between independently-originating aggressive alien civilizations is roughly 1-4 billion light years. If we can eventually get light signals from galaxies that are roughly 100 billion light years away today, this suggests that we’ll be able to dimly see the first billion or so years of the history of a few tens to hundreds of such alien civilizations. But just seeing them dimly won’t really tell us that much about them, and we may be terribly curious to know much more.

Aliens who aspire to win in the great universal meme evolution contest should seek to take advantage of this curiosity, by sending out messages about themselves and their memes. There are two obvious ways to do this. They can either sent data in light (or other fast particle) signal messages, or they can send physical emissaries that carry lots of data with them.

Over these distances, data sent by physical emissaries goes slower, and is sent directly to fewer locations, but its quantity can be far more. However, it will be harder to believe that the emissary data you receive is actually the data that was originally sent. Especially if it is passed on via several intermediary civilizations. In contrast, while less data can be sent in light signals, not only does it go faster, but one can have stronger confidence that the signal received was actually the signal sent.

The possibility that history may be rewritten is a problem not only for emissaries, but also for ourselves. In fact, the most trustworthy data on our own history might be the signals that we sent out long ago to others, which they then simply reflect back to us. By mixing up the signals that you send out with the signals you reflect back to others, you give them a modestly stronger incentive to read what you send.

To believe our reflected signals, we’d need to encrypt what we send our outgoing signals in some way to make it very hard for them to change them without corrupting them. However, if there are cryptographic hash scheme that can’t be cracked over billions of years by civilizations eager to change history, we could use this not only to trust our reflected signals, but also to let distant aliens verify that the large data they get via emissaries was actually the data that we sent out long long ago.

As with all cosmic beacons, there’s be an advantage to coordinating on where to look when to see them. Such as sending a signal right after seeing a gamma ray burst, and in the exact opposite direction so your signal follows the burst. Then listeners look for your message right after seeing a burst, and in that same direction.

Added 24Dec: As I’ve discussed before, humans cultures had separated diversity for ~1Myr, and now have much stronger  integration, but will again diversify in 1Kyr+ as we spread out among the stars. It seems a similar pattern will play out among alien civs later. They go from separated diversity for 1st ~1Byr, to much stronger integration at ~1-100Byr, but then they diversify again as they lose contact w/ each other after that.

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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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If Aliens Are Near

Most of us have core beliefs about which we feel pretty confident, but to which we get emotionally attached. One useful exercise to help overcome such attachment is to think explicitly about how you would change other beliefs if you became convinced that a central belief were wrong.

I suggest this mostly as a private exercise, as I worry it won’t go well if critics can selectively demand: “You think you’re so rational; tell us what you’d believe if X were wrong” for any X they like. Similarly to how it wouldn’t go well if critics could selectively demand that rivals reveal nude or other severely unflattering pictures of themselves. Such tests might go better if applied uniformly applied to all, but that’s harder to arrange.

Even so, I’m inspired today to try one version of this exercise: what else would I think if I thought aliens were actually near?

My best guess is that the universe is vastly larger in space than the distance we can see. And so in all that vast volume, there are probably aliens. Even intelligent civilized aliens. But my estimate is that the nearest such are very far away, outside the visible universe. (Low intelligence alien life may be closer.) So if you offered evidence purporting to convince me otherwise, I’d be initially skeptical. If I were willing to give you the benefit of the doubt, I’d guess that you’d made an analysis mistake somewhere.

If you somehow managed to convince me of your evidence, my guess is that it would be regarding aliens who are very far away, but just not quite as far as I’d thought. And if you convinced me that no, aliens have frequently been visiting us here on Earth lately, I’d be a lot more surprised. But what if you did in fact convince me?

My guess is that the most likely scenario consistent with this assumption is that these aliens are from one of the Sun’s sibling stars, born in the same stellar nursery that likely birthed 100 to 10,000 stars in the same ten million year period. There was only one Eden in the visible universe which managed to seed one star nursery with life. That Eden was in our galaxy, and that nursery was our Sun’s. Eden wasn’t well suited to support fragile multi-cellar life, but against great odds it created robust extremophiles that could travel far.

These sibling stars drifted far from their nursery over the last four billion years. (They can be identified from far away via their spectra.) Some were not seeded with life, and most of the rest remain far from creating intelligent civilizations. But some, like our Sun, have already done so. Many of those killed themselves, or locked themselves down to stay on their planet or in their star system. But one managed, many millions of years ago, to create a very stable civilization that could travel to other stars.

For some unknown reason, this one successful civilization has strongly limited its internal variation, to prevent any of its parts, or later sibling civilizations, from mass colonization of the universe. Many stable civilizations will develop a ruling body with strong central control, and it seems hard to predict in general what such bodies will want or choose, other than that their choices must allow them to maintain control. So its not crazy to think that this first civilization might decide to prevent mass colonization, even if it allows limited development of a few key resources that we can’t now see.

Part of such prevention would be keeping tabs on, and limiting the growth of, life around sibling stars. Sterilization might be hard, and it is plausible that they’d be curious about and entertained by how life evolves around sibling stars.  So its not crazy to think they might make frequent if limited visits to Earth. And its further not crazy to think they might be sloppy about hiding their visits; maybe they feel very secure that we can’t threaten them, and maybe they get a kick out of being noticed.

Yes, I don’t like having to resort to multiple “not crazy” assumptions in my most likely scenario, but I am being forced to explain what I see as an unlikely scenario.

If these aliens have a policy of preventing mass colonization, they will have to step in at some point to limit Earth’s expansion. But they will have been preparing to do that for many millions of years, and may have already done this several times at other sibling stars. So our chances to defy their plans and expand anyway can’t be great.

Perhaps we have a greater chance to persuade them to change their policies. They may limit what those internal to their civilization are allowed to say on the subject, but it seems they’ve been more hands off with us, and they may allow many within their civilization to see and hear us. In which case we have a chance to persuade. Though we should expect that the more likely scenario is that they persuade us, fairly or unfairly, to endorse their policy.

If you ask me to tell the most realistic story I can wherein we see or meet aliens today, this is it. Not terribly likely, but at least not crazy. Which is actually an unusually high standard in science fiction.

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