Is There a Great Filter?
She then points out that the SIA indicates we are almost certainly not in a universe in which there was much of a filter in the past. If there is a great filter that means it is in the future.
The great filter, as described by Robin Hanson:
Humanity seems to have a bright future, i.e., a non-trivial chance of expanding to fill the universe with lasting life. But the fact that space near us seems dead now tells us that any given piece of dead matter faces an astronomically low chance of begating such a future. There thus exists a great filter between death and expanding lasting life, and humanity faces the ominous question: how far along this filter are we?
I will argue that we are not far along at all. Even if the steps of the filter we have already passed look about as hard as those ahead of us, most of the filter is probably ahead. Our bright future is an illusion; we await filtering. This is the implication of applying the self indication assumption (SIA) to the great filter scenario, so before I explain the argument, let me briefly explain SIA.
SIA says that if you are wondering which world you are in, rather than just wondering which world exists, you should update on your own existence by weighting possible worlds as more likely the more observers they contain. For instance if you were born of an experiment where the flip of a fair coin determined whether one (tails) or two (heads) people were created, and all you know is that and that you exist, SIA says heads was twice as likely as tails. This is contentious; many people think in such a situation you should think heads and tails equally likely. A popular result of SIA is that it perfectly protects us from the doomsday argument. So now I’ll show you we are doomed anyway with SIA.
On the other hand, the theory that there is a great filter rests on the apparent absence of extraterrestrial civilizations. A theory that I saw on Usenet (yes, there are or were occasional glimpses of rationality there) some years ago from Kaehler, Oh, and Krummenacker gives us good reason to think there are extraterrestrial civilizations in the observable universe:
(I was originally planning to give a link to Google groups, but the above message doesn't seem to be there.)
From: firstname.lastname@example.org (Markus XXX Krummenacker)
Subject: Re: Drake Equation & Fermi Paradox / KOK hypothesis
Date: 10 Jul 1995 03:40:15 -0700
Organization: The Portal System (TM)
First I need to admit that I am probably going out on a limb with what follows, and that I do not really know much about cosmology and astrophysics.
However, I would like to ask: How can we know that the universe is indeed not yet infested with Dyson spheres ? Maybe the universe is already 90% full, instead of still empty.
In a very crowded universe, there is a desire to squeeze out as much utility and energy efficiency from scarce starlight as possible, so one would expect Dyson spheres to be constructed which emit "waste radiation" at the lowest photon-energy that is physically possible, "milking" the higher-quality photons for useful work to the fullest extent. Such Dyson spheres would be designed to emit "waste-photons" at a temperature which is just above the thermal background radiation of the universe, i.e. essentially in the region of 3K. It would likely be extremely difficult to detect them, because they will be black objects before a black background.
In general, one can probably assume that even if the universe is populated by many advanced civilizations, we would not be able to recognize and/or comprehend them at all anyway. The current SETI efforts look more like a joke, because I tend to think that such civilizations would not have much interest in sending beacons, which are designed to be easily understood, to us little human critters (much as we do not care to send signals to ant-colonies, to make them recognize that we exist). However, if we live in a densely populated universe, we probably are staring at their artifacts and/or signals already, and are just too stupid to recognize them as such. Advanced civilizations will leave a strong mark on the universe, which can not possibly be overlooked. Presumably, if such evidence can be found in the sky, humans have photographed and recorded it already many times. In some sense, money for SETI projects is not really needed, just quite a bit more imagination for how to re-interpret what we already have in our archives. How about a thorough data-mining session ? :-)
In the above picture of an already densely populated universe, the question would not be whether we can find other civilizations in our galaxy. Very likely, entire galaxies will be taken over completely by civilizations on a short time scale, so finding "partially consumed" galaxies in the sky will be extremely rare. Either entire galaxies are completely black, or still totally pristine. Maybe 90% of all galaxies in the universe are already fully populated, which would be an interesting explanation for the "missing mass" phenomenon. This mass is black, which is why we do not see it. If the universe is so crowded by advanced civilizations, it also becomes plausible why maybe about 10% of the universe was left untouched: they agreed to leave a "natural reservation". In this view, we happen to live in this reservation. Once we will start to expand outwards, we will at some point be made aware of the rules that the other civilizations around us have established regarding resource consumption. Isn't the large scale structure of the visible universe reminiscent of multiple intersecting bubbles ? Would it not make sense for advanced civilizations to establish these reservations at the interface boundaries between large empires, as a buffer zone, so to speak ? (It would be clear to them that expanding further and consuming the tiny buffer zone would not do them much good, because they would not gain all that much more real estate, and they might also risk an intergalactic war by intruding on their neighbor's territory.)
BTW, the hypothesis above was cooked up sometime in fall '93 after a nanotechnology discussion meeting called "The Assembler Multitude", in Palo Alto, CA, and has been tentatively labelled "KOK Hypothesis". KOK stands for Kaehler-Oh-Krummenacker, the names of the three hackers who have conceived this theory, which will force all cosmology text books to be rewritten. :-) :-) (To be taken with a grain of salt, at least until the aliens have been contacted for confirmation of the hypothesis.)
Now I would like to have all the real physicists and cosmologists out there jump on it, and tear apart this hypothesis by pointing out all the fatal flaws. :-)
Come to think of it, the SIA indicates we are more likely to be in a universe with lots of extraterrestrials.
Addendum: On the other hand…