Of Filters, Great and Small
7 October 2015
In my recent post Is encephalization the Great Filter? I quoted Robin Hansen’s paper that gave the original formulation of the Great Filter. Again, Hanson wrote:
“Consider our best-guess evolutionary path to an explosion which leads to visible colonization of most of the visible universe… The Great Silence implies that one or more of these steps are very improbable; there is a ‘Great Filter’ along the path between simple dead stuff and explosive life. The vast vast majority of stuff that starts along this path never makes it. In fact, so far nothing among the billion trillion stars in our whole past universe has made it all the way along this path. (There may of course be such explosions outside our past light cone [Wesson 90].)”
Robin Hanson, The Great Filter — Are We Almost Past It? 15 Sept. 1998
In filtration technology, the “steps” between the input and the output of a filter are called “elements,” “layers,” or “media.” I will here speak of “elements” of the Great Filter, and I will here take seriously the idea that, “…one or more of these [elements] are very improbable.” In other words, the Great Filter may be one or many, and we do not yet know which one of these alternatives is the case. Most formulations of the Great Filter reduce it to a single factor, but I want to here explicitly consider the Great Filter as many.
What is the Great Filter filtering? Presumably, the higher forms of complexity that are represented by the successive terms of the Drake equation, and which Big History recognizes (according to a slightly different schema) as levels of emergent complexity. The highest forms of complexity of which we are aware seem to be very rare in the universe, whereas the relatively low level of complexity — like hydrogen atoms — seems to be very common in the universe. Somewhere between plentiful hydrogen atoms and scarce civilizations the Great Filter interposes. And there may yet be forms of complexity not yet emergent, and therefore a filter through which we have not yet passed.
Hanson mentions visible colonization of the visible universe — this is a different and a much stronger standard to overcome than that of mere intelligence or civilization. Our own civilization does not constitute visible colonization of the universe, in so far as visible colonization means the consequences of intelligent colonization of the universe are obvious in the visible spectrum, but there is a sense in which we are highly visible in the EM spectrum. Thus the scope of the “visibility” of a civilization can be construed narrowly or broadly.
Construed broadly, the “visible” colonization of the universe would mean that the effects of colonization of the universe would be somewhere obvious along some portion of the EM spectrum. We can imagine several such scenarios. It might have been that, as soon as human beings put up the first radio telescope, we would have immediately detected a universe crowded with intelligent radio signals. We might have rapidly come to a science of analyzing the classifying the variety of signals and signatures of exocivilizations in the way that we now routinely classify kinds of stars and galaxies and now, increasingly, exoplanets. Or it might have been that, as soon as we thought to look for the infrared signatures of Dyson civilizations, we would have found many of these signatures. Neither of these things did, in fact, happen, but we can entertain them as counterfactuals and we easily visualize how either could have been the case.
The difference between a universe that is visibly colonized and one that is not is like the difference between coming over the ridge of hill and seeing a vast forest spread out below — i.e., a natural landscape that came about without the intervention of intelligence — and coming over the ridge of a hill and seeing an equally vast landscape of a city spread out below, with roads and building and lights and so on — i.e., an obvious built environment that did not come about naturally — out of reach from a distance, but no less obvious for being out of reach. At present, when we look out into the cosmos we see the cosmological equivalent of the forest primeval — call it the cosmos primeval, if you will (with a nod to Longfellow’s Evangeline).
In the illustration below the Great Filter is everything that stands between an empty universe and a universe filled with visible colonization by intelligent agents and their civilization. The Great Filter is then broken down into seven (7) diminutive filters, each a filter “element” of the Great Filter, which correspond to the terms of the Drake Equation. We could choose other elements for the Great Filter than the terms of the Drake equation, but this is a familiar and accessible formalism so I will employ it without insisting that it is exhaustive or even the best breakdown of the elements of the Great Filter. The reader is free to substitute any other appropriate formalism as an expression of the Great Filter, with any number of elements.
In this illustration the lower case letters along the left margin that correspond to arrows each stopped by an element of the Great Filter are to be understood as follows:
a – failure of stars to form
b – failure of planets to form
c – failure of planets to be consistent with the emergence of a biosphere
d – failure of planets consistent with the emergence of a biosphere to produce a biosphere
e – failure of a biosphere to produce intelligent life and civilization
f – failure of a civilization to produce technically detectable signatures
g – failure of a technologically detectable civilization to survive a period of time sufficient to communicate
h – a civilization on a trajectory toward visible colonization of the universe
Given a Great Filter constructed from a series of lesser filters, relations between the elements of the Great Filter (the individual lesser filters) describe possible permutations in the overall structure of the Great Filter, as I have attempted to illustrate in the image below.
In this illustration the pathways marked by arrows are to be understood as curves, the X axis of which is the difficulty of passing through an element of the Great Filter, and the Y axis of which marks the gradual emergence of complexity strung out in time, as follows:
A – An inverse logarithmic Great Filter in which successive elements of the filter are easier to pass through by an order of magnitude with each element
B – An inverse linear gradient Great Filter in which successive elements of the filter are easier to pass through by degrees defined by the gradient
C – A constant Great Filter in which each element is equally easy, or equally difficult, to pass
D – A linear gradient Great Filter in which successive elements of the filter are progressively more difficult to pass through, with the change in the degree of difficulty between any two elements defined by the gradient (call it Δe, for change in difficulty of passage through an element)
E – A logarithmic Great Filter in which successive elements of the filter are each progressively more difficult to pass through by an order of magnitude for each element (my drawings are, or course, inexact, so I appeal to the leniency of the reader to get my general drift).
In the case of a Great Filter of an inverse logarithmic scale, the first filter element is by far the most difficult to pass through, and every subsequent element is an order of magnitude easier to pass. Once given the universe, then, intelligence and civilization are nearly inevitable. While such a filter seems counter-intuitive (most filters begin with coarse filtration elements and proceed in steps to finer filtration elements), something like may be unconsciously in mind in the accounts of the universe as a place teaming not only with life, but with civilizations — what I have elsewhere called an intelligence-rich galactic habitable zone (IRGHZ) — and I note that such visions of an IRGHZ often invoke the idea of inevitability in relation to life and intelligence.
However, this is not the problem that the universe presents to us. We do not find ourselves in the position of having to explain the prolixity of civilization in the universe; rather, we find ourselves in the predicament of having to explain the silentium universi.
The above analysis ought to make it clear that, not only do we not know what the Great Filter is — i.e., we do not know if there is one factor, one element among others, that is the stumbling block to the broadly-based emergence of higher complexity — but also that we do not know the overall structure of the Great Filter. Even if I am right that encephalization could be singled out at the Great Filter (as I postulated in Is encephalization the Great Filter?), and the one especially difficult element of the Great Filter to pass beyond, there are still further filters that could prevent our civilization from developing into the kind of civilization that Hanson describes as visibly colonizing the universe, that is to say, a cosmologically visible civilization.
We can easily project a universe with a spacefaring civilization so pervasive that the stars in their courses are diverted from any trajectory that would be based on natural forces, that the constellations would have an obviously artificial character, and that use of energy on a cosmological scale leaves unambiguous infrared traces due to waste heat. A universe that was home to such a civilization would have passed beyond a filtration element that we have not yet passed beyond.
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