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.

 

[.Scott]

It seems to me that if life started on earth exactly once in 4.5 billion years, then it is an extremely improbable event that is a very rare if not unique occurrence in the universe.

You're presuming there wasn't another start this morning only to be eaten for lunch.

 

[BWV]

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.

Even then - with a 100LY limit for communication that is 60k stars, so there could be tens of millions of technological civilizations in the galaxy and odds are they are all too far to see or communicate with (that is assuming pan galactic empires are just SF)

 

[PeroK]

Even then - with a 100LY limit for communication that is 60k stars, so there could be tens of millions of technological civilizations in the galaxy and odds are they are all too far to see or communicate with (that is assuming pan galactic empires are just SF)

It's more like tens (or perhaps hundreds) of thousands. With millions of civilizations, randomly spread, there would be a high likelihood or near certainty of one within 100LY.

 

[BWV]

It's more like tens (or perhaps hundreds) of thousands. With millions of civilizations, randomly spread, there would be a high likelihood or near certainty of one within 100LY.

My simple calc was 60k stars within 100ly out of 100 billion potential, but that would have an upper limit of around a million civilizations before it became more than a 50% chance one was within 100ly

 

[phyzguy]

I think the Fermi paradox is different than what has been stated here. I think it goes like this:
(1) We can already envision our species spreading through the galaxy. With technology that we know, we could reach nearby stars in a generation ship in a few hundred years. Once there, we could build a new civilization based on space habitats in a few thousand years. They could then launch more generation ships to nearby stars.
(2) In this way, it is possible for our species to spread through the galaxy in a few million years. There are no technological barriers to this.
(3) Our species may not choose to do this, but it is possible.
(4) If intelligent life like us is common in the galaxy, at least one species would probably choose to do this and spread through the galaxy. Note that not ALL of the intelligent species need to choose to do this, it only takes one.
(5) So Fermi's question was not, "Why haven't we contacted another species?" It was "Why aren't they already here?"

To me, the only logical answer is that intelligent life like us is not common in our galaxy.

 

[PeroK]

My simple calc was 60k stars within 100ly out of 100 billion potential, but that would have an upper limit of around a million civilizations before it became more than a 50% chance one was within 100ly

I guess it depends on how the Sun-like stars are distributed. The current estimate seems to be about 5-10 billion Earth-like planets, with about 20 billion Sun-like stars. The question is how many of these are within a realistic communication distance of Earth?

 

[BWV]

I think the Fermi paradox is different than what has been stated here. I think it goes like this:
(1) We can already envision our species spreading through the galaxy. With technology that we know, we could reach nearby stars in a generation ship in a few hundred years. Once there, we could build a new civilization based on space habitats in a few thousand years. They could then launch more generation ships to nearby stars.
(2) In this way, it is possible for our species to spread through the galaxy in a few million years. There are no technological barriers to this.
(3) Our species may not choose to do this, but it is possible.
(4) If intelligent life like us is common in the galaxy, at least one species would probably choose to do this and spread through the galaxy. Note that not ALL of the intelligent species need to choose to do this, it only takes one.
(5) So Fermi's question was not, "Why haven't we contacted another species?" It was "Why aren't they already here?"

To me, the only logical answer is that intelligent life like us is not common in our galaxy.

Yes, although self-replicating robot probes rather than generation ships is the typical formulation. However generation ships are horribly inhumane and sending self-replicating machines beyond a distance the sender could ever hope to communicate with may not strike anyone as a good use of resources. The only rational reason for interstellar expansion I see given its near impossible difficultly would be to ensure survival beyond the life of the home star- but one or two red dwarfs would be sufficient for that

 

[PeroK]

I think the Fermi paradox is different than what has been stated here. I think it goes like this:
(1) We can already envision our species spreading through the galaxy. With technology that we know, we could reach nearby stars in a generation ship in a few hundred years. Once there, we could build a new civilization based on space habitats in a few thousand years. They could then launch more generation ships to nearby stars.

I don't see any evidence that this is more than sci-fi fantasy.

(2) In this way, it is possible for our species to spread through the galaxy in a few million years. There are no technological barriers to this.

Yes there are (technological barriers). There is no technology that even comes close to doing this.

(3) Our species may not choose to do this, but it is possible.
(4) If intelligent life like us is common in the galaxy, at least one species would probably choose to do this and spread through the galaxy. Note that not ALL of the intelligent species need to choose to do this, it only takes one.

The galaxy is a big place and it's one thing to expand to a few star systems a few lights years away. It's quite another thing to expand a civilization by say 1000 light years. Let alone across the entire galaxy.

(5) So Fermi's question was not, "Why haven't we contacted another species?" It was "Why aren't they already here?"

To me, the only logical answer is that intelligent life like us is not common in our galaxy.

That's far from the only logical answer. That interstellar travel (other than possibly by unmanned probes) is essentially technologically impossible is another possibility.

We simply do not know.

 

[phyzguy]

Yes, although self-replicating robot probes rather than generation ships is the typical formulation. However generation ships are horribly inhumane and sending self-replicating machines beyond a distance the sender could ever hope to communicate with may not strike anyone as a good use of resources. The only rational reason for interstellar expansion I see given its near impossible difficultly would be to ensure survival beyond the life of the home star- but one or two red dwarfs would be sufficient for that

(1) Generation ships need not be "horribly inhumane". A space habitat hundreds of meters in diameter could be quite a pleasant place to live. Add a fusion drive (which we don't know how to build today, but likely will in a century or so) and you could reach Alpha Centauri in a few hundred years. As I said, there are no technological barriers to doing this. It is not of "near impossible difficultly".

(2) Life tends to spread. The motivation that prompted the Polynesians to cross thousands of miles of open ocean to reach Hawaii and Easter Island is still alive in our species. You may not have that motivation, but there are people who do.

 

[DaveC426913]

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.

You're talking about two-way communication.

If we saw unmistakeable signs of life in the Andomeda Galaxy - despite being 2 million years too late to talk to them (OK, 4 million years) - it would still provide us with a wealth of knowledge.

In fact, even one-way comunication could conceivably provide a wealth of knowledge, if they sent us their Encyclopedia Andromeda. Heck, I'd be happy if they just told us how to keep french fries from going soggy in take-out orders.

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.

Two-way communication, yes.

 

[BWV]

(1) Generation ships need not be "horribly inhumane". A space habitat hundreds of meters in diameter could be quite a pleasant place to live. Add a fusion drive (which we don't know how to build today, but likely will in a century or so) and you could reach Alpha Centauri in a few hundred years. As I said, there are no technological barriers to doing this. It is not of "near impossible difficultly".

so at best it would be like condemning yourself and your unborn generations to live out their lives on a cruise ship but with fake food. Would like to see how one designs a cruise ship to maintain its plumbing and electrical systems for 200 years without any outside assistance, let alone maintaining an atmosphere


The ship would have to be nearly a kilometer wide to provide spin gravity that felt approximately normal

 

[javisot]

Civilizations traveling great distances and colonizing new systems, isn't that panspermia?

Panspermia is pretty much ruled out; it's unlikely anything could survive the journey. The same applies to an entire civilization (¿right?)

 

[DaveC426913]

The ship would have to be nearly a kilometer wide to provide spin gravity that felt approximately normal

Why would you need any more than a smidge of gravity? Say, 20%? Just enough to make sure things don't float around in the corridors.

If you're worried about muscle loss, you can always spin the exercise chamber or other functions out on a long tether.


Also, just because the ship spans a kilometer doesn't mean it has to be hugely massive or complex.

A simple way is to have two counter-balanced modules rotating around their barycentre on a long tether.


There's a dozen way to solve such problems.

 

[BWV]

Why would you need any more than a smidge of gravity? Say, 20%? Just enough to make sure things don't laot around in the corridors. If you need to worry about muscle loss, you can always spin the exercise chamber or other functions out on a long tether.

For one, wounds won’t heal properly in microgravity and we have no idea what the impact would be on a developing fetus (and never will as it’s unethical to try). We don’t know the impact of microgravity on healthy adults for extended periods

 

[russ_watters]

I guess it depends on how the Sun-like stars are distributed. The current estimate seems to be about 5-10 billion Earth-like planets, with about 20 billion Sun-like stars. The question is how many of these are within a realistic communication distance of Earth?

An unstated assumption in the line of discussion is that they exist simultaneously/now. Given that humans have only had the ability to communicate off-world for a hundred years vs 4.5B for the planet, it suggests odds might not be good for an advanced species being nearby even if there's one in every candidate solar system at some point in time.

...unless they manage to survive for a billion years without destroying themselves.

 

[BillTre]

The scenario of the slow spread of humanity through the galaxy on generation ships has some interesting possibilities. Generation ships of non-frozen populations would lead to the divergent evolution of each if the separate groups of people went to different places.

1. Small populations isolated from breeding with other groups would limit genetic diversity and lead to evolutionary divergence. This might be off-set to some degree by taking a very large sperm bank with to add to the expedition's genetic diversity. If frozen eggs become more technologically feasible (not a stretch) then that way to increase genetic diversity could be extended to mitochondrial sequences (which sperm does not provide).
2. If the people on the generation ships were wake and breeding, then there would be a whole new set of adaptive pressures that would direct their evolution, probably furthering their divergence.

This could lead to an ever diverging set of derivative human-like people going in different directions. Any adaptations could made they better suited to space travel, allowing them to better sweep across the galaxy at a very sub-light speed. Similarly, changes in societies would be expected to occur.


WRT breeding in zero or low gravity:
Animals and other things have probably already been bred in zero gravity space (probably mice and zebrafish). This kind of data should not be hard to come by. I expect people will be breeding in space regardless of some of these concerns (people are horny people because its a behavior that's been selected for).
Any pregnant women would probably be a high priority to protect with specialized environments (like living in a centrifuge for a while. Especially if any bad effects only had important effects during certain critical developmental periods.

 

[DaveC426913]

If the people on the generation ships were wake and breeding, then there would be a whole new set of adaptive pressures that would direct their evolution, probably furthering their divergence.

Possibly, although I think natural evolution would be drastically reduced.

1. Our medical technology would correct or compensate for most defects that might otherwise interfere with competition in a natural environment.
2. It's not like we would just let nature take its course as far as breeding went. Breeding pairs and frequency might actually be stringently controlled or at least monitored. Mutations would not be allowed to run rampant through the population.
3. There would perhaps be regulations about pedigree - just like dogs and horses. Mutations or divergences might be discouraged.

Of course, all this assumes they would want to keep their human pedigree. That's by no means guaranteed. A few generations down the road, the ship denizens might decide to set their own course for their genetic legacy.

 

[PeroK]

(1) Generation ships need not be "horribly inhumane". A space habitat hundreds of meters in diameter could be quite a pleasant place to live.

What's the longest you have ever lived like that?

Add a fusion drive (which we don't know how to build today, but likely will in a century or so)

Even if we do build a "fusion drive", there is the problem of the rocket equation. The ship would be mostly fuel. S;lowing down and landing on a planet at the other end would also be highly problematic.

and you could reach Alpha Centauri in a few hundred years. As I said, there are no technological barriers to doing this. It is not of "near impossible difficultly".

There are technological barriers. Just saying the magic words "fusion drive" or "warp drive" or "generation ship" does not overcome the near impossibility of the task of colonizing a distant planet.

(2) Life tends to spread. The motivation that prompted the Polynesians to cross thousands of miles of open ocean to reach Hawaii and Easter Island is still alive in our species. You may not have that motivation, but there are people who do.

Sailing a few thousand miles is nothing compared to interstellar travel. People have rowed across the world's oceans.

No one knows the future. We may spend the 21st Century desperately fighting runaway climate change and fighting wars against each other. We may gradually let AI and machines take over and become moribund. There may be a global rebellion against technology and we may regress to a state of religious fundamentalism. Or, we may follow a utopian technological path and have colonized Mars in 20 years, Alpha Centauri in 100 years and eventually have a human presence on a billion habitable planets around the Galaxy. Heck, we may even colonize the Andromeda galaxy eventually!

 

[BillTre]

There are certainly a lot of possible paths.

I don't know what exactly would happen but one of the largest things affecting rapidity of change is the population size (really the effective population size), thus sperm storage etc.. After they leave and they are on their own, each group will be taking its own diverging path. It might rule out interbreeding among these groups, who knows with the possibility of future medicine.

The smaller the population sizes, the the more likely it is to be affected by random genetic drift. These kind of changes might affect traits under less selective pressures to ot change.

 

[Borek]

If, say, there is (on average) an advanced civilization in one galaxy in ten

I would add "at any given point".

Our technological civilization grew in just about 3k years (give or take, add a zero, divide by ten, it won't change the weight of the argument by much), there is no way to say how long it will survive but I am rather pessimistic and I don't think it will last (I am not saying we will all die, rather we are already at the peak and our capabilities will go down with diminishing resources). That can mean the observational window for any neighboring civilization trying to detect us is rather limited - make it 50k years, compare it with the age of the Earth, this is nothing.

 

[javisot]

I would add "at any given point".

Our technological civilization grew in just about 3k years (give or take, add a zero, divide by ten, it won't change the weight of the argument by much), there is no way to say how long it will survive but I am rather pessimistic and I don't think it will last (I am not saying we will all die, rather we are already at the peak and our capabilities will go down with diminishing resources). That can mean the observational window for any neighboring civilization trying to detect us is rather limited - make it 50k years, compare it with the age of the Earth, this is nothing.

Perhaps artificial intelligence will become interstellar before human intelligence; I mean, for a simple mission of reconnaissance and spreading of microorganisms, we don't need to bring an entire civilization on the ship.

 

[evansscott271]

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

And Within ach galaxy so many chances for a large am ok unt of them systems would have habitable worlds with wide ranges of possible exobiologic forms of life or precursors such as having the right chemicals required for life. But for complex multicellular life that list beco.es more strict then to get us its harder, its a bit like playing chess for the long game!

 

[Hornbein]

It is a common fallacy that any event one observes must have a probability greater than zero. That is true with finite sets but not infinite sets.

I think it best to think of it like this. If the Universe is infinite then all events have measure zero. All finite sets of events have measure zero. Infinite sets of events may or may not have a measure greater than zero. This is however a slippery topic as it is not easy to rigorously define criteria for set membership here in the real world. "World with intelligent life" is a good example. If you can't define precisely the rule for membership in a set then you can't figure out what the measure of that set might be, even if you somehow were somehow able get enough data.

The upshot is that observing one event tells you nothing about the measure of any set in which it is a member.

 

[evansscott271]

It seems to me that if life started on earth exactly once in 4.5 billion years, then it is an extremely improbable event that is a very rare if not unique occurrence in the universe. I’m aware that the number of planets in the universe, including interstellar and intergalactic planets, is an absurdly high number. However, is it possible that abiogenesis is even more unlikely than the amount of places where it could occur? Could the answer to the Fermi paradox be that life is confined to earth?

Oh im sorry this is my first time online! online online anyway i havent been very tech Savoy since windows 95 came out so im a bit out of date!

 

[evansscott271]

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.

Yes! But it may be childish to point this quoate by agent smith on the matrix that we as in "Humans" are not ujnlike another lifeform on this planet "A Virus" "You infest consume and destroy" as morphus fights to hang on .