The main reason we can't see any Grabby Aliens is because there are very few of them, if any. A;lmost ali alien civilizations are peaceful and non-violent. The reason we don't see them, is because their bodies, their vehicles, their buildings etc are all made of dark matter and so are invisible to us. In fact all planets in the Solar System are inhabited, but all its inhabitants as well as their stuff (houses, vehicles) are made of dark matter. All the planets in the Solar System (except one) are more advanced than us on Earth.

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That was probably a bad way of phrasing it on my part.

My thought is mostly just that any expansion sufficiently close to the speed of light would be invisible until it is upon us. Thus if the model essentially requires such an expansion, it has nothing to recommend it over any other model.

The idea that aliens are moving at the speed of light is doing all the heavy lifting, not any conception of or 13.8GY emergence time being the result of multiple hard try-try steps.

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See figure 8 in our paper, but yeah, still about 1 Gyr.

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What is the conclusion of the simulation about median distance to the grabby aliens? Is it still around 1 Gyr?

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Backward light cones -> backward cones ( only one of the two is a light cone)

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[Not sure whether the below is still relevant given Robin’s preprint, posting in case it is]

The angular size of the Alien domain as seen by us is I think directly determined not by Robin’s horizontal purple ‘How Big They Look’ line, but by a certain line segment Mi normal to the screen, through the midpoint of the red line’s intersection with the Aliens’ forward expansion cone (FEC).

The plane normal to the screen which contains that red line intersects FEC in an ellipse whose minor axis is Mi. The angle subtended by Mi as viewed from the spatial (horizontal) distance between Mi and ‘Us Now’ is the Aliens’ angular size. [1]

That angle depends on only 2 things: the Alien expansion speed s, and how close in time the Alien origin event is to our light cone. We can parameterize the latter by the fraction 0 < f < 1 of the vertical distance between backward light cones is occupied by Robin’s purple line in diagram #4 (in which f ~0.54).

For s=0.77c as in Robin’s diagrams, f < .012 is required to make the Aliens look smaller than the Moon (30’). The values for Venus at its biggest (1’) and for the resolution of the HST (0.04”) are basically linear w/ this: f=.00040 and f = .000027. The corresponding f cutoffs for *very* quick Alien expansion (s=0.99c) are bigger but indeed still small: 0.060, 0.0021, 0.00014.

[1] This uses two approximations which tend to cancel (namely, that small-angle slices of a conical surface are flat and that one sees fully half of an ellipse’s circumference when viewing it edge-on from far away), which I’m pretty sure introduce < 5% error in the preceding small-angle scenarios. FWIW (now going beyond small angles), this approach yields an apparent size of 29 degrees for s=0.77c, f=0.54 as in Robin’s diagrams, which just eyeballing it looks plausible (and is indeed huge).

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Perhaps ʻOumuamua is such a probe?

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While expansion may be selected for initially this is of course the case for most invasive species they soon fill up the virgin lands and with time the expansionary advantage and selection evaporates and they speciate and diversify into a new native ecosystem, this is of course on evolutionary scales millions of years. Grabby aliens will only be able to grab unrestricted when there is nobody there not backwards. Also grabby factions will soon diversify with their own environmentalists / conservatives / meditators on lotus thrones / whatever unless anyone that is not completely devoted to the glorious expansion is immediately eliminated.

This mean there is a front but it may be slow.

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There is also the african honeybee, the scutellata subspecies is significantly more aggresive, swarms much more and is more prodigious foragers than the european bee. When they hybridize with other honeybees being a subspecies these traits dominate and they soon africanize all other honeybees, and took over most of the americas over 60 years when about 26 queens escaped a research station in Brazil. Yet scutellata is not dominant over the globe it is in fact a subspecies. Now that they have took over the americas there seems to be a regression to the mean some colonies is already losing some of their agression. These traits seems to be the result of the dry unique environment of their native african range. These "grabby" bees seems to revert to the main species when they have grabbed al they can.

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Yes indeed, expansion can be selected for. I've added that cite to our draft paper.

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Your results are consistent with our results. Yes, the fact that do we not now see alien volumes in our sky is data that suggests an expansion speed close to the speed of light. I don't see how this is tautologous.

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Hi Robin. Excellent series of posts, look forward to paper.

Re convincing other people of the model, one analogy from real world evolution you can use is dispersal of invasive species. In particular the cane toad in Australia is often cited.

What happens is a selection effect where the leading edge of the invasion has longer legged toads. Why? Not because long legs have an edge against other toads in the conquered areas. Not because the toads are consciously trying to go in any particular direction (they don't know where the unconquered lands are). But because at the edge, the toads that move fastest outrun the slower ones in *all* directions. Including go backwards into conquered lands. But....of the random toads that decide to go outward on the invading edge, the fastest long legged ones form the edge of the wave.

A quick search finds this paper:https://www.ncbi.nlm.nih.go...

Let me quote the key bit:"Dispersal rate is a key feature of invasion biology, and an extensive literature suggests that dispersal rate typically evolves upwards during an invasion [6–10]. As a result, individuals in the invasion vanguard tend to exhibit dispersal-enhancing features (such as seeds that drift further on the wind, larger feet, wings or flight muscles [3,11–15]) relative to conspecifics in the range-core. Alleles that code for fast-dispersal morphological traits may accumulate in the invasion vanguard because of spatial sorting (successive generations of interbreeding between the fastest dispersers [2]) or natural selection (reflecting fitness benefits to unusually fast-moving individuals [16])."

No course, grabby aliens are conscious, so know quite well that grabbing unconquered lands is the way to go. All I'm saying is that even if there is no conscious decision to do this, evolution strongly selects for it anyway. Put in context, if the alien civilization consists of what humans would call conservative, environmentalists, tech enthusiasts, whatever, unless the aliens coercively coordinate and genocidally stop the grabby faction, the grabby faction will expand out regardless, and that expansion speed will ramp up over time towards close to the speed of light.

So there's no need for an alien civilization to decide to expand and become grabby. Like longer legs in cane toads, this is selected for automatically. So the argument is flipped. To avoid the grabby alien scenario, one must posit that *all* seperate creations of alien civilizations that have ever occurred decide to coordinate and *not* be grabby.

Very long legged alien cane toads are coming our way!

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I'm afraid I'm not convinced.

I've been playing around with my own implementation of the model, and I'm finding that having a sky empty of visible aliens is fairly unlikely over the plausible range of parameters. For instance, at 5 hard steps and an expansion speed of c/4, the median civilization sees 20 other "grabby" civilizations in their sky that combined cover 17% of the heavens. Pushing the expansion speed to c/2 kicks the median number of visible civs to 2, filling 2% of the sky (and of course at expansion speeds very close to the speed of light, have empty skies like ours).

Reducing the number of hard steps does decrease the visibility of other civs, but 2 hard steps at c/4 is still 9 visible civs over 6% of the sky. And even at 1 hard step, I still get 2 visible civs, though the fraction of the sky is about 2/3rds of a percent.

So, as far as I can see, this model would say we are either 1) very early (i.e. destroying the motivation for the model), 2) the expansion speed is very close to the speed of light (which has a flavor of tautology), or 3) we haven't searched the sky well enough to see multiple civilizations that each span a cone of several degrees (which isn't the impression I get from SETI, but since these civilizations would likely cover a group of galaxies hundreds of millions to billions of light years away, that could be possible).

Though if you're getting wildly different results when calculating visibilities-at-birth in your simulation, rather than just speculating why they might not be visible, I'd be happy to know so I could go bug hunting in mine.

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According to these guys (“Eternity in six hours: Intergalactic spreading of intelligent life and sharpening the Fermi paradox”), if you can colonize the galaxy with self-replicating probes, it’s not really much harder to colonize the visible universe.


This suggests that grabby aliens might go through two phases, extensive and intensive. They could start the extensive phase when they got to roughly a Kardashev 2 level. In this phase, rather than expanding locally, they would put their main colonizing effort into sending self-replicating probes to the farthest corners of the universe at near light speed. Eventually they and their probes would switch to more intensive exploitation, filling in empty spaces and more fully exploiting available resources. The originating civilization would start out in the yellow area of your chart, and would initially be barely visible to us. Depending on how close to light speed their probes travelled, and how long the founders took before switching from extensive to intensive, there might be a lot of their probes in the Milky Way, replicating away in inconspicuous spots, keeping hidden to avoid spooking the locals, before the original founders around their home star(s) went from inconspicuous to conspicuous (from yellow to green in the chart).

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I appreciate you taking the time to respond. Maybe I am having trouble intuitively 'getting it' so I'm going to re-read and give it another try.

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As I said in the post, we can show that the volumes they control are huge, or not visible. So if they had any different appearance to them they'd be obvious. But most of our grabby alien analysis doesn't need the concept of "visible". The only place that shows up is in assuming we are not now in a volume controlled by grabby aliens, and in inferring that their speed must be high, because else we'd see them.

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