Monthly Archives: January 2021

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.

Added 7Mar: Here is the likelihood ratio for seeing our data of no big alien volumes in the sky, as a function of power n and speed s/c:

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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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