SETI's Hard Steps (and How to Resolve Them)

> I'm convinced the absence of evidence is, itself, valuable evidence.

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.

If I read 98% of your comment I might draw a pretty accurate conclusion based on the absence of something in it. But if I read just 2% any conclusion I draw is whatever I want to read in the tea leaves.

Do you think we investigated closer to 98% or to 2% for possible evidence?

I guess the important bit that always gets left out is that these things revolve around life as we know it. Of course you could have a completely different biochemistry resemble the processes that we generally associate with life. But until we either replicate that in a lab or find signs of life on other planets (earth-like or otherwise), the best speculation we can do is based on what we see here. And we see that certain steps on the path towards complex multicellular life took very long compared to others.

> I'd add that it's also by no means a given that life, broadly defined, can only evolve around a sunlike star.

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.

The speed of light is also a factor: most of those stars are so far away that they cannot detect our sun because to them it hasn't even lit up yet. (Our sun is about 4.6 billion years old, the universe is about 93 billion light years across). Stars like our sun are expected to last 10-12 billion years, so most stars that we think can support life in you 10^24 number have already died and we don't know it!. (stars need not be like our sun to support life, but I have to go with something to give numbers)

This is too facile. The Copernican principle assumes that we are not in a spatially or temporally privileged position, and there are no obvious hard steps -- to say nothing of 24. It's frankly bizarre to go from there to "we are unique among 10^22 stars."

Not that I'd call myself a buff, but I don't think my knowledge of larger numbers makes 10^24 a small number in any absolute sense. Cryptographic keyspaces are deliberately designed to be unfathomly large, whereas the number of stars in the universe is simply an observational fact. The number of stars really is huge in a human sense, as much as that's worth anything. There are more stars than there are grains of sand, etc.

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.

I don't think that solves it. The Fermi paradox is that based on the probabilities we estimate, we should see lots of aliens, and we don't. Thus something in our estimates must be wrong, but we don't know what.

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."

>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.

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.

The universe is 93 billion light years across. No alien can reset it within time spans that matter to our solar system which has maybe 8 billion years left before our sun dies)

Novice question here: What use is there in adding a term like that to the Drake equation? It seems like it's value would be completely uncertain (like other terms in the equation) and so you have overwhelming uncertainty in the output of the equation.