Is it considered a false dilemma because the chance theory appears to present only two possibilities, when in reality there could be many?

Now that you bring this up, I think the chance theory (or whatever it is called) forces us to make a choice/decision between extremes.

As it happens I’m writing a novel on the subject of the propagation of life. — Wayfarer

Wow! This is very intriguing!

I will keep the Forum posted, but only if I find a publisher.

If life were not only carbon based, I do think we would be right to expect more aliens. That said, if it is carbon based and only forms on planets similar to Earth, most of those planets are either still forming or are young compared to Earth, meaning we would not expect there to be ETI, or at least not that many ETIs; so I agree that some pessimism is warranted in that regard, but not about the possibility of ETI. — NotAristotle

I don't think anything warrants that level of certainty. I have not seen any analysis that we can say that the earth is exceptional in how early it formed.
The only requirements I am aware of are a third-generation star and icy comet impacts. Even assuming these are mandatory requirements (which is debatable), it still leaves a window billions of years wide, including billions of years before the Earth formed.

The timescale on when an ETI would be expected to send out a radio signal will consider 1. the odds of abiogenesis, and as ↪Wayfarer pointed out, 2. the times at which those planets formed. — NotAristotle

I wouldn't get too focused on radio signals. Yes, SETI and other organizations look for radio signals because what else can we do? But in the context of the Fermi paradox I think it is wrong to see it as requiring ETIs to be broadcasting a radio signal at just the right time for us to look.
For the Fermi paradox, we're talking about anything detectable, so that would include things like replicating probes spreading to our star system, and megastructures like Dyson spheres, both of which could persist in some form long after a species has gone.

And, importantly, it includes technology that we can't understand. What I mean by that is, we don't have to suggest that advanced species will make Dyson spheres. Maybe the most important technology for a million years+ advanced species is a giant helix, that radiates X-rays intensely, for reasons we couldn't possibly understand yet.
Well, we don't see that either. We don't see anyone absorbing, emitting or refracting EM radiation on a large scale anywhere.

Of course, it doesn't prove anything. But, given that there's no reason that we yet know of that the sky couldn't have been lit up with the lights of a million species, it's not a positive data point.

We do not know how life formed or whether other Earthlike planets have the conditions to enable life to form. The odds seem to be: what are the chances of all the right ingredients being in the right place at the right time? In another sense, there may not be any "odds" involved, it is either going to happen on another planet or it will not as a result of the atomic, chemical, (whatever else), forces involved. When I say the "odds" I am referring to the credence we attribute to the result of life arising on another planet that is like Earth. So it is not so much that the process of life forming is random, but that our knowledge of whether another planet will or will not give rise to life is chancy. Or, to try to narrow it down a bit further, maybe the question of "odds" is really a question of just how similar other planets are to Earth, the closer the similarity, the more likely the chances of life on that planet.

Most Earthlike planets are estimated to have yet to be born -- https://science.nasa.gov/universe/exoplanets/most-earth-like-worlds-have-yet-to-be-born-according-to-theoretical-study/ . <a href="https://assets.science.nasa.gov/content/dam/science/missions/hubble/releases/2015/10/STScI-01EVSR5F1P8JVARK199WAZPNBQ.pdf" target="_blank" rel="nofollow">https://assets.science.nasa.gov/content/dam/science/missions/hubble/releases/2015/10/STScI-01EVSR5F1P8JVARK199WAZPNBQ.pdf</a>

If this estimate is correct, and if it is also correct that life is carbon-based only, and if life only arises on some Earthlike planets but not all, then the fact that most Earthlike planets have not formed yet suggests that, as you said initially, we are one of the very first intelligent species. But I see no reason to reject the hypothesis that abiogenesis can happen on other planets. You are right that the timeline of discovery might be more like billions of years; that is a matter of when the other Earthlike planets form. (So my initial lower bound estimate of at least a 20,000 years waiting period for alien contact might have been an underestimate).

I see your point about detecting (or not detecting) other alien technologies. Given that the chances of ETI are low in my opinion, I think it unlikely that Dyson spheres or anything like that will be detected; at least, not in our lifetime.

Or, to try to narrow it down a bit further, maybe the question of "odds" is really a question of just how similar other planets are to Earth, the closer the similarity, the more likely the chances of life on that planet. — NotAristotle

I agree.

Now that we got to this point, I think it is worth asking ourselves: would they (the civilisations of an Earth-like planet) show themselves to us if they were capable of doing it?

Furthermore, what if it is actually better for us and for them that our paths haven't crossed yet?

haha, yeah I was thinking that myself, and I think it is a good question. It would seem to have to do with the evolution of that species and how risk adverse they are, or perhaps, with how communicative they are. If they are less communicative and more cautious like (I don't know turtles hiding in a shell?) then maybe they will refuse to broadcast any signals. Of course, if nobody broadcasts then the chance of any communication plummets.

Walk me through better to not have crossed paths; why would that be so?

Of course, if nobody broadcasts then the chance of any communication plummets. — NotAristotle

This is ture. Indeed, there is a positive aspect to that assertion.

Walk me through better to not have crossed paths; why would that be so? — NotAristotle

Understandable. Perhaps the rest of the civilisations on the Earth-like planets are thinking the same thing right now. We always tend to select the cautious choice.

If this estimate is correct, and if it is also correct that life is carbon-based only, and if life only arises on some Earthlike planets but not all, then the fact that most Earthlike planets have not formed yet suggests that, as you said initially, we are one of the very first intelligent species — NotAristotle

The cite doesn't really support the conclusion you're drawing though.
Yes, in a relative sense we might be "early" but even that tentative estimate still suggests around a billion rocky worlds before ours. And that's just in our galaxy.

Yes, in a relative sense we might be "early" but even that tentative estimate still suggests around a billion rocky worlds before ours. And that's just in our galaxy. — Mijin

we are one of the very first intelligent species. — NotAristotle

I also think we are not the very first intelligent species. As pointed out, the estimate suggests around a billion rocky worlds before ours, so it is difficult to believe that we are actually the only intelligent species in the universe. However, given that we accept this point, perhaps we should start to wonder why they would want to communicate with us, or perhaps they have already been here but we never noticed it. It appears that we are in the middle of this paradox—whether they would rather not communicate or they already did.

NOTE: When I say "they," I am referring to the possible intelligent lives of Earth-like planets.

Of course, if aliens are not just carbon-based, that should make alien life more abundant and increase the likelihood that we hear from aliens in a less massive time frame.

Final thought: each day that we do not detect aliens strengthens the case that aliens are carbon-based lifeforms only, like us. — NotAristotle

Fair. My own view is that if there’s intelligent life out there, distance may not matter given technologies that would look like magic to us. We can imagine that the laws of physics we currently cherish might have 'workarounds' we simply don’t yet understand. And how would we determine that they aren’t visiting, or even aren’t here now? The usual assumption is that we’d be able to detect them and that they would announce themselves, but I don’t see why that follows.

or perhaps they have already been here but we never noticed it — javi2541997

Exactly, see above.

That said, I have no good reason at present to believe they’re here, or even that they exist.

I agree, Tom.

This is what I like the most about this topic – it is open to many interpretations, and I consider them all valid. Furthermore, I am learning a lot precisely from having different perspectives. :smile:

My own view is that if there’s intelligent life out there, distance may not matter given technologies that would look like magic to us. We can imagine that the laws of physics we currently cherish might have 'workarounds' we simply don’t yet understand. — Tom Storm

Yes, this. I'm in a fermi paradox debate on another site (the straight dope) and there I am running into the problem that people don't appreciate how big a deal it is for an intelligent species to persist over deep time. When I talk about an advanced ETI making self-replicating probes, the responses are as though I am a hyper-optimist, talking about what humans will achieve in the next ~50 years. I'm not.

A technological species living for a million years or more is hundreds to thousands of times the duration of all of human history. Our stupendously hard engineering problems will be their prehistoric utterly trivial ones. More simple to them than a sharpened stone is to us.
When we're speculating what they could achieve, engineering challenges (like, say, shielding from micrometeorites) won't cut it as barriers to progress. We need to propose that certain things are physically impossible. And indeed, even many things we think are physically impossible will likely not look that way to an advanced ETI, but that's the level we need to at least start from for such problems to potentially explain the Fermi paradox.

However, given that we accept this point, perhaps we should start to wonder why they would want to communicate with us, or perhaps they have already been here but we never noticed it. — javi2541997

1. Arguments like "they wouldn't want to" are not very convincing as primary Fermi filters.
Because, the problem with "behavioral" explanations for the paradox is that we are making a claim about all species, always. Every faction of every civilization has to always come to the same conclusion. If even 1% of species 1% of the time choose to be noisy, then it doesn't work as our main filter.
And note that humans have already attempted to be noisy: we've beamed out signals. These signals are feeble of course, but the point is, a hypothesis that requires all aliens to behave a way that the only technological species that we know of has not, doesn't seem promising.

2. The objection to aliens silently visiting the solar system is basically the same. If there are one or two advanced ETIs in our galaxy, then OK, maybe both came here, or are here, in a way that was/is silent.
It doesn't work as a primary filter though.

For me the Fermi paradox loses a lot of it's argumentation, when one takes into account that the first radio signals we have ever sent to space have reach only a tiny spec even in our own galaxy. Add then the fact that radio signal get weaker when the ranges get longer.

Interesting argument. Yes, it is true that one of the flaws of this paradox is that we are making a claim about all the species, always. However, I think it is worth beaming signals to the vast universe. Perhaps we will reach more conclusions in the future; perhaps not. Better this than waiting to receive the broadcast from the other "neighbors." :rofl:

Add then the fact that radio signal get weaker when the ranges get longer. — ssu

This is true, ssu.

Yet the topic gets intriguing when we fantasize about the possibility that another intelligent life may be able to send a better broadcast.

For me the Fermi paradox loses a lot of it's argumentation, when one takes into account that the first radio signals we have ever sent to space have reach only a tiny spec even in our own galaxy. Add then the fact that radio signal get weaker when the ranges get longer. — ssu

As ludicrously big as the universe is*, it's also billions of years old. When we put the two together, there has been ample time for thousands of species to have made noise detectable to us.

And I use terms like "noise" because in the context of the Fermi paradox we are speaking much more broadly than just radio signals:

1. Megastructures like dyson spheres
2. Self-replicating probes, that could "quickly" flood a galaxy
3. Anything else detectable, even if we have no idea what it is for**

* My favorite demonstration of this is: if the sun were the size of a golf ball, then the next golf ball, our nearest neighbor, would be 800 miles away. FTR the earth is the size of a grain of sand on this scale.

** I mentioned this upthread, but it bears repeating: things like Dyson spheres are just examples of detectable tech that would have a useful function that we can conceive of. People often make the error of thinking we are claiming to know that this is what advanced ETIs will do. We don't know. We just know that the set of useful, detectable tech is non-zero.

So I think a key parameter in making a probability calculation is the similarity of these other billion worlds to Earth. My understanding is: what is used is the ESI (Earth Similarity Index). https://en.wikipedia.org/wiki/Earth_Similarity_Index . This index looks at features like the size of the planet. But it really does not tell us the planet composition nor whether it has a magnetosphere; a lot of information appears to be missing to say if it is really "like" Earth. So if we cut out a large percentage of those sort-of-Earth planets then there are perhaps far fewer than a billion planets that could, (or that we would expect) to host life.

That is not to mention that our planet was hit by an asteroid (or meteor?) that wiped-out the dinosaurs and arguably paved the way for the emergence of intelligent life. So while life may be a more likely occurrence on a planet that is similar to Earth in key respects, intelligent life may be far more rare.

And how would we determine that they aren’t visiting, or even aren’t here now? — Tom Storm

You may be interested to read Sagan's article on interstellar diffusion/propogation: https://ntrs.nasa.gov/api/citations/19790011801/downloads/19790011801.pdf . There are more recent articles too that have narrowed down the estimate for galactic colonization based such factors as the rotation of star systems in the galaxy.

For me the Fermi paradox loses a lot of it's argumentation, when one takes into account that the first radio signals we have ever sent to space have reach only a tiny spec even in our own galaxy. Add then the fact that radio signal get weaker when the ranges get longer. — ssu

Agree. Any ETI that formed around the time we did and that sent out radio signals within the past century or so would take a long time to even reach us (10s of thousands of years).

there has been ample time for thousands of species to have made noise detectable to us. — Mijin

I think this claim may be contested. We really just may not know how rare planets similar to Earth are, life is, or ETI is.

That is not to mention that our planet was hit by an asteroid (or meteor?) that wiped-out the dinosaurs and arguably paved the way for the emergence of intelligent life. So while life may be a more likely occurrence on a planet that is similar to Earth in key respects, intelligent life may be far more rare. — NotAristotle

Yes, I almost forgot this important feature. Our planet was hit by a rocky object but fortunately was not very destructive. Therefore, there is a big possibility that the same happened to the Earth-like planets. This makes the chances of finding intelligent life even lower. :confused:

I think this claim may be contested. We really just may not know how rare planets similar to Earth are, life is, or ETI is. — NotAristotle

Quite right.
The issue is that it gets repetitive typing out "based only on what we know right now" but that was the intended meaning.

The point is, when we're talking about filters like whether aliens will want to stay hidden etc, it needs to be borne in mind that, based only on what we know right now, there could be a million advanced ETIs in our galaxy alone. So there needs to be a pretty big filter(s) before things like behavioral filters are going to be relevant.

That filter could indeed be that small, rocky worlds, with liquid water and plate tectonics turn out to be astonishingly rare; we can't rule that out yet.
Pretty much all we can rule out is that planets in general, in the habitable zone of their parent star, are rare. From how many we've detected given our methods only allow detection in very specific circumstances, it's a pretty safe inference now that they are common. But beyond that, all bets are off.