Alien life and probability

[DaveC426913]

Who are you to say what has to do with definitions of life? There hundreds of them.

Yes. Many of which do not apply to this topic of discussion. This is about exo-biology.

Competent scientists are not looking at possible life on other planets and wondering if that muloid is alive because it is infertile. They don't bother with naive lay-person definitions.

C'mon.

I did not say there are no contentions about life versus non-life. All I said was mules and ants are terrible examples. A much better example of contention is a virus.

 

[BWV]

The Fermi paradox is specifically about advanced life able to make contact with us. This is different from the origin of life, but life originating (once at least) is a condition that enables any higher life forms.

This is how I think about this group of issues:

1. origin of life (an emergent event, based on a special set of geochemical conditions (I have my favorite scenarios)). This creates simple life forms, like prokaryotes - no nucleus, limited chemical power and genetics). This has happened at least once on earth, possibly more.
2. Complex single celled life (another emergent event based on two different cells combining to form a single cell like a eukaryotic cell with a nucleus, mitochondria and bunch of other useful stuff). This has happened several times on earth (mitochondria, chloroplasts, and at least two for other inclusions that provide the ability to process other chemical energy sources (I have previously made some posts on these). However, it took ~2 billion years for that to happen (mitochondria and chloroplasts). It required to two different original cell types and the right environment to favor the results of such a combination.
3. Multicellular life made of complex (eukaryote-like) cells. (Emergent event based on having the single cells). Multicellularity has happened many times on earth. There are even multicellular prokaryotes. ITs an easy quick thing.
4. Intelligence, tool use and developing socially accumulated knowledge and space travel or whatever Fermi was talking about. Also an emergent event. Likelihood and the effect of other other factors (like possibly self-nuking) that could affect it arising.

With intelligence goes a ladder of exploitable energy resources to get to a technological civilization. This would include plants and animals (or some food source) that could be selectively bred and domesticated and sources of fuel from biomass through fossil fuels - and it’s hard to imagine how industrialization could have happened without coal, which was created in some unique conditions in the Carboniferous period. Then the technological civilization has to survive long enough to be detected by their neighbors, who also have to survive. I think our ability to detect a signal not directly sent from a planet at our technological level is around 10 light years - so not much of a sample.

Also each step requires a more stable climate - for example agriculture was not possible during the last ice age. Climate change and mass extinction events fall harder on more complex organisms and societal structures

 

[Borek]

Recently "assembly theory" has been proposed as a way of identifying living things at a distance.

Interesting approach, and definitely good candidate for something that can be turned into a detection technique.

I feel like it is kind of a measure of "how thermodynamically impossible is to see the molecule produced by a natural, abiotic process" (yes, very handwavy, but I am sure it is the thermodynamics that has the final say here ).

 

[BillTre]

I feel like it is kind of a measure of "how thermodynamically impossible is to see the molecule produced by a natural, abiotic process" (yes, very handwavy, but I am sure it is the thermodynamics that has the final say here ).

I have come to feel that thermodynamics rules in many of these evolutionary events.

 

[PeroK]

Another point I would make is that saying that the observable universe is "teeming with life" is not a very useful conclusion. If, say, there is (on average) an advanced civilization in one galaxy in ten, then that means we are probably alone in the Milky Way. It's questionable whether communication with an extra-galactic civilization would ever be possible, so for all practical purposes we are alone - or, we may as well be.

That's why I think the critical question is how many civilizations there are in the Milky Way. That being our only realistic hope of extra-terrestrial communication.