Twenty-five years ago, if you were making a list of potential Great Filters that could explain why aliens have not said hello to us yet, you could have started the list with: No planets. Back then, it was a viable concept that most stars did not have planets, and our sun was the lucky one. Remember that Great Filters are things that prevent aliens from either existing, being intelligent enough to build a starship that wouldn’t break down on a trip to Earth, and interested in coming here for one reason or another. The No Planets Great Filter was perhaps the most basic one. Without a planet to get started on, there was no need to say anything further about the construction of starships or the boringness of Earth or the difficulty of big brains evolving or any other topic that amused you.
But over the last quarter century, that concept has dissolved in a puff of smoke. For some reason, humankind had chosen to spend many billions of dollars on exploring space, and one portion of that funding went to telescopes of increasingly clever kinds and ever increasing sizes. We paid a lot to resolve the very first Great Filter, and our funding paid off. We now know there are planets around almost all stars, and many of them get the right amount of thermal flux from their star so that the surface temperature might be in the range of liquid water. This is in the so-called habitable zone, which is a name that astronomers use to confuse laymen. It means a range of distances from each class of star, chosen so that if the albedo of the planet were as assumed, water could exist there.
An ocean at 90 degrees Celsius may not be your idea of a habitable zone, but it’s a start. It would be possible to come up with a much better definition, probably a much narrower habitable zone, but it’s on the To-Do list. Perhaps another post?
The real point of this ephemeral Great Filter is that if funding were made available for other Great Filters, we might see them evaporate as fast as water on the inner edge of the habitable zone. What are the other ones?
We need to know what does the origin of life need. What are the conditions in which it can happen, and what are the times required for the processes to come to fruition. Timing is important, as if the processes are random in character, and take a billion years to happen, seeing a planet with the right conditions doesn’t answer the story. We need to wait the billion years to see if it keeps these conditions. If we assume the first step is self-replicating chemicals, we need to ask what are the conditions for an initial formation of one, and what are the conditions for replication. Does it need a substrate of a given kind? Does it happen only under pressure, as in a deep ocean? Does it happen only with a certain pH? Does it need a narrow range of some component, such as sulfate in the water? What is the source of energy that will form it, as the self-replicating chemical is likely to need some more energetic molecules to power the process itself? There does not seem to be any momentum in the scientific areas that would bring in funding comparable to that which astronomy gets. But why? Knowing about life in the universe will help us mature our own philosophy and may have a calming effect on our society. Why not fund it?
Perhaps the answer is that film is to blame. We have all seen movies about far off planets, or aliens, or exploring space as a dangerous place, or similar things. This has spun up the interest in astronomy to the level it deserves, but there has been no comparable set of videos about the remaining Great Filters, even the next ones in line, the origin of life ones.
It is hard to conceive of some film plot that would make the science of finding self-replicating chemicals as interesting as the discovery of new Earths. What would it have – two hours of some white-coated chemists mixing vat after vat of trial solutions, and putting them in a temperature and pressure controlled environment, where they were destined to sit for a very long time? How do you make a molecule as interesting to movie-goers as an alien who shoots at you with laser somethings and probably wants to eat you? We may have to face the fact that the next Great Filter set is going to be around for a long time.
The line of Great Filter concepts is quite long, and most of them fall in the bin of ‘really uninteresting stuff’. A chemist might be exhilarated to see how a primitive cell wall could form around some other self-replicating chemical, but who else would be? We hardly have enough interest in these areas to list the various steps that have to occur for primitive life to exist, to say nothing of land animals or smart ones. After you get cell walls, you need to have the wall develop various means of allowing useful chemicals to get in, waste ones to get out, and some way to use them inside the cell. It may be that most of the genes in the average creature's genome relate to things on the cellular level, and the visible stuff is only some percentage of it. That means that there are a slew of Great Filter concepts relating to cell development, and we don’t really understand if any of them are the very low probability events that sort out Great Filter concepts into real Great Filters, assuming there is one or more.
In another post, a word was created for a scientific field that advanced alien societies would have, archeogenetics, which is the reverse investigation of genetics, so they could find out the history of the genes of existing life. What we have been discussing is a way to solve that problem from the opposite end, trying to figure out how life started. Maybe this could be called proto-genetics for lack of anything better. At some point, proto-genetics and archeogenetics might meet, and a complete understanding of the genetic basis of evolution would be understood.
There are pre-proto-genetics Great Filter concepts, those which go from pure chemistry to the first self-replicating chemical, then onward to more complex chemicals and eventually a group of chemicals that self-replicate, eventually allowing one to become a coding mechanism. By this time, there may be a membrane around the group of chemicals, which is either connected to them or one of the things that gets replicated. This batch of steps is as likely to contain a Great Filter as the genetics batch.
A different post pointed out that there could be Great Filters after intelligent life formed a civilization, and several posts discussed what some could be. We have not yet pushed science far enough to understand these, or even to envision a set of steps where they could be resolved. The science is simply not yet defined, and chaos reigns.
Perhaps the best thing that could be done to resolve the Great Filter question is to think through the various batches of questions that could lead to Great Filter concepts, and then try to envision how they might be resolved. The exo-planet former Great Filter was resolved by astronomers who could envision how to make the very ingenious measurements, observations and calculations that led to the discovery of thousands of example exoplanets. Vision seems to be the missing key to this puzzle.
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