SETI's hard steps and how to resolve them
centauri-dreams.org
centauri-dreams.org
It's certainly unclear that photosynthesis is an ironclad requirement for life, and, in any case, it evolved multiple times in parallel on Earth. (Anoxygenic photosynthesis in fact evolved, de novo, more than once.)
I'd add that it's also by no means a given that life, broadly defined, can only evolve around a sunlike star.
Speculation around "hard steps" is pseud naval gazing until we gather more datapoints. It's not even valuable as speculation -- except perhaps as an exercise in countering sloppy thinking.
Not a given, but not unreasonable either. Larger stars die faster so they probably won't be around long enough for life. Smaller stars last longer, but they also emit less energy and so there is less likely to be higher life forms just because there isn't enough energy to support life.
Of course life might not be like we know it, but carbon and water are very common in the universe compared to most of the other options and so they are more likely.
This isn’t really true. The energy density at a planet’s surface is a function of both the star’s luminosity and the planet’s distance. Most star’s (at least in the Milky Way) are small, and most stars have planets in orbits closer than is Earth’s. The so called “Goldilocks zone” is expected to have at least one planet in it for a very large number of stars (again, at least in our galaxy).
I don't think that this is true. IIRC it's an artifact of the techniques we use to detect extrasolar planets -- our techniques are biased to detect planets that are closer to their stars than further away,
I'm convinced the absence of evidence is, itself, valuable evidence. The (apparent) single origin of Earth life is a remarkable statistical observation hiding in plain sight.
That isn't how science works. We have an astounding lack of evidence that there is a Flying Spaghetti Monster roaming around the deepest reaches of space, it doesn't indicate anything but the fact that there's a lack of evidence that there is a Flying Spaghetti Monster roaming around the deepest reaches of space. It may not indicate with absolute certainty whether there is or isn't one, but you cannot say its existence is more likely based on lack of evidence for it - that is superstition.
Of course not. What other evidence of absence do you expect to uncover, other than a lack of evidence for existence? Do you expect to go out into space and have some sort of space god hand you a genuine, bona-fide certificate of nonexistence of something?
It is important not to overestimate the strength of the absence of evidence, but setting the strength to zero is just as wrong.
If we look in many places where we expect to find life, because we have a theory that life is easy and essentially appears the cosmological instant it is possible, and we have a theory that the conditions for life are easy and abundant, yet we look out into the universe and see no evidence of this, yes, we are completely justified in adjusting our understanding of the probabilities involved. The more of the universe we see, and the more it lacks various sorts of life (all the way from the simplest, sparsest bacteria-type life up to galaxy-spanning intelligences, each to their own degree) the more we are justified in coming to conclusions about their probabilities.
Another thing people have problems with in this debate is understanding the difference between justified beliefs and true beliefs. If we hypothetically examined all but one-trillionth of the observable universe and found it bereft of any other life, we would be justified in the conclusion that the most likely scenario is that there isn't any. We could end up being wrong if that remaining one-trillionth just so happened to contain other life, but we would still be justified, having collected extensive "absence of evidence". We can not claim that we have seen enough of the universe to know that it is true that there is no other life, and we have not seen enough of it to be justified in claiming there is no bacterial life, but I do feel at this point we are pretty justified in claiming that at least this galaxy and the nearest ones do not have any galaxy-spanning, multi-million-year civilizations in them, for instance. Maybe they're there and they just don't do anything visible, but that's a pretty precise claim.
Scanned how? What's the operational definition of "life" in that hypothetical scenario? Maybe there's life, but you're looking where isn't, or you're missing something you don't classify as "living". Maybe the universal definition of life is vastly broader than we think.
But for now our definitions of life are good enough.
So if I claim that there is a Flying Spaghetti Monster roaming around our solar system, and it is 1000x larger than our sun, emitting a divine brightness so intense that it would burn the eyes of all who gaze upon it, you're 100% justified to say "no there isn't. We would have seen it."
"Aha!" I say, "but absence of evidence is not evidence of absence! Therefore you can't say anything at all about whether there's a supermassive superluminal Giant Spaghetti Monster!". That is absolute nonsense.
> you cannot say its existence is more likely based on lack of evidence for it
If its existence would imply evidence, then you absolutely can judge its likelihood based on whether that evidence exists or not. And you can also judge its likelihood based on a reasonable extrapolation from prior knowledge, by the way. Yes, it is more likely that there is a 10 kg rock orbiting Neptune than a 10^20 kg Spaghetti Monster, even though we have not directly observed either one. We understand the mechanisms by which a small rock comes to orbit a planet. We see that kind of thing all the time. It may be difficult to formalize some of these priors, but that's a failure of our formalisms, not a mandate that we must not use that knowledge when doing science.
Note that the comment you are replying to said nor implied anything of the sort.
That's literally the exact claim being made:
> We have an astounding lack of evidence that there is a Flying Spaghetti Monster roaming around the deepest reaches of space, it doesn't indicate anything but the fact that there's a lack of evidence
A lack of evidence indicates nothing but a lack of evidence. No! That's not true! It's a widely held misconception and it's worth debunking.
> That isn't how science works.
"Science" is a lot of human activities mushed together and there's not "one true way" that it works, except that it is universally done by humans (so far), with all the good and bad that come with that.
> We have an astounding lack of evidence that there is a Flying Spaghetti Monster roaming around the deepest reaches of space
True.
> it doesn't indicate anything but the fact that there's a lack of evidence that there is a Flying Spaghetti Monster roaming around the deepest reaches of space
False.
> It may not indicate with absolute certainty whether there is or isn't one
Nobody has ever cared about "absolute certainty" and science never seriously claims this.
> you cannot say its existence is more likely based on lack of evidence for it
Yes you can! (I charitably assume you meant less likely here, otherwise the entire comment makes no sense, is arguing against a trivial falsehood nobody has ever claimed (certainly not OP), and you had no reason to post it)
> that is superstition
No, it's Bayesian logic; it's the only way humans have ever acquired knowledge.
There, I covered your entire post.
Do you think we investigated closer to 98% or to 2% for possible evidence?
The limits are not just our technology levels. Many of the signs you want to life for of life are things that we know by laws of physics could not possibly reach us no matter how good the detection equipment is.
That is why SETI looks for radio waves - they are one of the few signals we can detect that could signal life. However we would never detect earth technology levels of just 120 years ago (there is some debate about what we could first detect). There are not many stars within 100 lightyears of earth, so the vast majority of our galaxy (must less the universe) still has no idea we are here. Even radio signals degrade with distance, so there is a limit to how far we can detect them or they can detect us.
In short, while there is absence of evidence, there is reason believe that we don't have enough evidence to declare anything. If we find life that will be conclusive, but the we don't before our sun dies there still won't be enough evidence to state there is none.
Glad you expressed uncertainity. Indeed We can see some exoplanets directly. I also only learned this very recently.
https://en.wikipedia.org/wiki/List_of_directly_imaged_exopla...
There are 10^22 to 10^24 stars in the Universe (a big range of uncertainty there but it doesn't matter for this point). That sounds like a lot but anyone with knowledge of combinatronics would immediately say holy shit that's a small number. Cryptography buffs are probably looking at that going "that can't be right, it's a tiny number", you divide it by 10 repeatedly, 24 times in fact and you're down to nothing. 24 not-at-all 'hard' steps and we're probably alone.
You've probably heard "there's soooo many stars in the universe there has to be other life out there". There really isn't that many in terms of probability.
GP may be thinking of this paper, which uses a reasonable probability distribution for each parameter in the Drake equation, and finds a decent chance that there is no other technological civilization in the observable universe:
The fact that we exist to discuss these odds means that whatever the probability distribution, at least one instance of life has occurred. Not only that, but life arose and eventually led to intelligence at our level - something that appears to be rare even on our own planet, but achieved relatively quickly all things concerned (only a few hundred million years).
While the anthropic principle guarantees that we observe intelligence as we're defining it (since we include ourselves), I agree that doesn’t mean intelligence is inevitable or common. A more likely modelling in my opinion is that worlds of microbial life are abundant, worlds with complex multicellular life is rarer, and intelligent civilizations are rarer still. Given the distribution of intelligence levels on Earth, it seems unlikely that we simply passed every constraint while no other planet gets close. Also, if we observed a planet with humans as they were 100,000 years ago, would we even consider them intelligent life? Probably just as intelligent as modern-day humans if raised the same, but literally nowhere near our technological level.
When scientists evaluate whether soil can support certaing thing, they don’t treat each factor (like pH, moisture, nutrients, microbial conditions) as independent hurdles that must be overcome one by one. Instead, they see that multiple factors interact in complex ways. A deficiency in one area (e.g., nutrient content) can be mitigated by another factor (e.g., microbial activity enhancing nutrient cycling). If you extend this to conditions in which life might it arise, it suggests to me that planetary habitability may be more like a network of contributing conditions rather than a checklist -- actually much more difficult to caclulate?
Also, mostly as an aside, we also have the advantage of knowing that life and then intelligence arose relatively quickly once conditions stabilized - only a few hundred million years. n=1 but I think this is a promising indication on where any variables might lie.
My absolute favorite Fermi's Paradox theory is that life on Earth was evolving too slowly to survive, so aliens sped up the process while hiding themselves, but that came with a variety of problems that humanity bumps into, and the aliens are trying to clean up the mess without destroying humanity's potential for a unique identity.
The sci-fi novel practically writes itself.
You need to watch "Jigureul jikyeora!": https://m.imdb.com/pt/title/tt0354668/
We don't even know what goes on inside our own Sun (a place with immense energy flux ripe for sustaining lifeforms, whatever they might be) and these people are looking for a second Earth with little green men. I don't think it's as much scientific curiosity as some deep-seated psychological issue.
Life is sustained fire, I don't think it much matters what is burning, just that there is infinite fuel available and a remotest possibility of a spark.
I call it "Fragile Universe", based off of the notion of the "Fragile" or "Vulnerable World Hypothesis" [1].
f_c is the parameter for the fraction of "civilizations that reach the technological level whereby detectable signals may be dispatched", so that handles the "Fragile World Hypothesis". Alien civilizations can wipe themselves out, exhaust resources, etc. before we get to see their detectable signals [2].
What the equation doesn't take into consideration is the possibility that an advanced species can trigger the destruction or total reset of the entire universe, eg. by nucleating the vacuum collapse. The first advanced species to reach that point could kill every species in the universe and start the whole thing over from scratch.
This is an extreme version of the anthropic principle [3]. We exist because the universe hasn't been reset yet. By us or otherwise. Presumably we might be the first to get there.
Universe fragility could be the reason we don't see aliens. We could be the very first, and we could wind up hitting the reset button.
[1] https://nickbostrom.com/papers/vulnerable.pdf ; tldr: advanced species can trivially wipe themselves out and will tend to do so. Not just nukes, but in the extreme, your average citizen can create grey goo at home with the press of a button that will turn the entire planet into paper clips.
[2] It could be that the time frames in which advanced civilizations emit detectable signals are so geologically small as they shift into non-detectable modes (eg. dark forest, disinterest in expansion, shift into a higher plane of existence, etc.), but that's orthogonal to the discussion.
How do you imagine the Universe resetting? Assuming the false vacuum decay occurs, I don't think any species (no matter how advanced) could reset the vacuum state back to a metastable state post-collapse.
It does just appear to be the anthropic principle, but with some extra steps.
Unfortunately, Drake equation now is just estimate, without any real data, as humanity just don't know example of any extraterrestrial life at all.
And what even more sad, our empty knowledge is not because non-existence of life in Universe, but because our science is very young and we just don't have powerful enough instruments to detect et life.
We could only be sure, life was not exist on Moon equatorial stripe on surface, but for example we cannot be sure about deep under-surface and about polar regions.
Because we only could understand et with similar to our level of sci/tech development, but what if we encounter civilization, even one magnitude older than our?
For example, now our science have evidences about human civilizations up to somewhere 10k..50k years, and our current civilization oldest evidences are about 5k years. Imagine, how could look our civilization in 500k years?
And if we use adequate considerations, human life was appear on Earth ~ 500 thousands years ago, and before was possible reptile civilization, for about 10 millions years, but our Earth become life planet ~ billion years, and exist planets in other star systems, which are tens billions years older than Earth.
So what I mean, in our 5k years our civilization decided to re-circling resources and probably in ~ 100 years we could implement this decision and will become invisible to ets of our level.
Imagine, what technologies would be available for magnitude older civilization. Could someone be sure about possibility of detection so much newer technologies?
You may hear classification of seti scientists as Cygnus, Cancer and Pike. What I want to say, civilizations younger than our are detectable for us, but older are detectable only if them decided to contact us.
This fragility idea just shifts it from "we should see lots of aliens and we don't" to "the universe should have been destroyed by now and it hasn't been."
> When we take account of realistic uncertainty, replacing point estimates by probability distributions that reflect current scientific understanding, we find no reason to be highly confident that the galaxy (or observable universe) contains other civilizations, and thus no longer find our observations in conflict with our prior probabilities....When we update this prior in light of the Fermi observation, we find a substantial probability that we are alone in our galaxy, and perhaps even in our observable universe (53%–99.6% and 39%–85% respectively). ’