Whether they constitute a formidable filter is the thing that would need to be demonstrated. Strictly speaking about sending a craft interstellar differences, what things are you suggesting are on the high end of hurdles?
Firstly, it’s speculation at every end of this, so yes obviously we don’t know that any ETIs exist. The point is simply that if an ETI could have appeared any time in the last ~5 billion years, then it is extremely unlikely that their level of progress will be within mere thousands of years of ours.
As Arthur C Clarke put it, “…we will find apes or angels, but not humans”.
Secondly, if we’re speculating that species will not reach the point of being millions of years more advanced than humans thenthat’s the great filter.
Whatever is the thing preventing species getting much more advanced than Homo sapiens 2025 is the problem; this line of reasoning tacitly concedes that engineering challenges in themselves, are unlikely to be the significant hurdle given the ludicrous timescales involved.
Seems to me that we only need to speculate that species don’t succeed in advancing additional millions of years in basically the same direction as humans have recently been advancing in.
I have another question: what, exactly, do people (ok researchers) mean when they say “Earth-like”?
If an alien were to have looked at our planet at various points in its history, they could have, instead of observing it as it is now with pretty white clouds and deep blue oceans, alternately seen it as a magma planet, a wasteland planet with a noxious atmosphere, and as an ice planet (depending of course on when they observed it). Would they have then been justified in thinking that said miserable little rock was a potential haven for advanced multicellular life, given upwards of a few measly billion years or so, so they’ll return and check in every so often to see if it has indeed progressed towards such?
Would Venus and Mars have been classified as “Earthlike” 4-3 billion years ago, when they may have had liquid water? Or does that designation necessarily require an oxygen/nitrogen atmosphere with liquid water on the surface? The kicker is of course that such an atmosphere is the product of life and its processes, a classic catch-22/circular definition. Would they (us) then simply designate any rocky body of reasonable size that is within the habitable zone of its star as fitting the definition, or does it require a much more specific set of criteria? Would Venus and Mars still be said to have potential, or noted as now being dead end worlds?
Good questions. I’ll add another: Would aliens necessarily have the same criteria as we do? Or do they look at us and see a wasteland planet with a noxious atmosphere?
It is entirely opossible that an alien species would have significantly environmental needs to humans. This would especially be true if they are mostly, or entirely artificial, robotic entities. Would such different entities look at Earth and see a ‘wasteland’?
Or would they look at our complex biosphere and human infrastructure and recognise it as (at the very least) ‘interesting’? The Earth is probably on the far end of a scale of ‘interesting’ worlds in the galaxy, even if we don’t breathe the same atmospheric gases as them.
On the other hand, if the Earth really is a boring wasteland compared to the other worlds in the universe, that would be a wonderful thing, and we should be out there looking for these non-boring worlds.
If the speculation is that their form / type of technological progress is such that they are 1,000 times as slow at figuring out propulsion tech than humans – that a problem that would take humans 1 year to solve would take them 1,000 years, then there’s been enough time for them to figure out interstellar craft thousands of times over.
This is the kind of deep time that we’re talking about.
If on the other hand, the meaning is that they won’t want to make tech like interstellar craft or the like, then I dont think this works as a fermi paradox solution, for reasons that I gave upthread.
I’m happy to restate my reasoning here (and indeed have my mind changed), but before that, let’s acknowledge that jumping to a different fermi paradox solution once again tacitly concedes that “engineering challenges” doesnt, in itself, work as a convincing paradox solution.
I was thinking mostly that they might think of free oxygen as a toxic gas, and simply assume that there couldn’t be any advanced life here. For all we know, planets that developed oxygen producers are relatively common, but the first few million that the intergalactics looked at had only microbes, so they stopped looking.
It might if combined with a filter: that the societies which keep developing in our general direction aren’t the ones that survive long term. Maybe it’s the ‘let’s take over the universe’ attitude that’s deadly; or something else about the technology that would be required to do so.
The idea that there is a Great Filter is a bit of a misnomer. There can be thousands of reasons we haven’t seen or heard from ETIs. Each one equally valid. And one doesn’t cancel out the others. For example, here is Robert Hanson’s original (incomplete) list.
The right star system (including organics)
Reproductive something (e.g. RNA)
Simple (prokaryotic) single-cell life
Complex (archaeatic & eukaryotic) single-cell life
Sexual reproduction
Multi-cell life
Tool-using animals with big brains
Where we are now
Colonization explosion
Maybe life never initiates on a planet. Maybe it does but never evolves. Maybe it does but the planet is covered in water and they can’t see the sky or make fire. There are thousands of maybes, none of which invalidates the other. There is not just one great filter.
Just like the people in 1932 who had no idea an incredible new and dangerous weapon was about to be created, maybe there is another even more dangerous leap ahead. Maybe it’s uncontainable and that’s the great filter.
Or maybe they go the way we did and the “engineering challenges” of building an interstellar generation ship that goes a percentage of the speed of light proves to be too much. For us, I believe this is true and I see no reason it wouldn’t apply to other ETIs.
Here’s some very real reasons it might not work:
Generation ships are considered impractical due to immense technological, logistical, and social challenges that span centuries, including creating a perfectly sealed, self-sustaining environment, maintaining a propulsion system for millennia, and overcoming the psychological toll on generations who live and die without seeing the destination. Major problems include the failure of life support systems, loss of materials from wear and tear, the necessity of artificial gravity, the high probability of social breakdown and conflict, and the risk of forgetting the mission’s purpose over time.
Technological and engineering challenges
Propulsion and power: A ship would need an extremely long-lasting and powerful energy source to maintain a high enough speed for interstellar travel and to generate artificial gravity.
Material and system degradation: Over thousands of years, materials would degrade and wear out from constant use and radiation exposure. While some parts can be repaired, others will eventually require replacement, and there is no external source for materials on a long voyage.
Life support: Creating a perfectly sealed, self-sustaining ecosystem is incredibly difficult. A small loss of air or water can be catastrophic, and generating enough fresh water for a large population over millennia is a massive challenge.
Artificial gravity: To prevent health problems from long-term zero-gravity, a generation ship would need to be large enough to rotate to create artificial gravity. Any failure in this system would have devastating effects.
Logistical challenges
No external resources: A generation ship is a closed system, meaning it cannot rely on external sources for resupply. Any material or resource lost is gone forever, which limits the ship’s ability to grow or make repairs.
Population dynamics: The ship would need a large initial population to maintain genetic diversity and a sufficient labor force. Over generations, population control becomes a major issue, and social problems linked to population decline are likely to arise.
Skills and expertise: The ship’s population would need the knowledge to maintain its complex systems for thousands of years. This includes skills like medicine, engineering, and agriculture, which could be lost over time.
Social and psychological challenges
Social breakdown: Generations of people would be born and die on the ship, unable to see their destination or experience a life beyond the vessel. This can lead to social unrest, conflict, and a loss of motivation for the mission.
Psychological toll: The isolation of living in a confined space for generations would be immense. The inhabitants might lose the original purpose of the mission or even resent the voyage, with some generations wishing to return to Earth.
“Lord of the Flies” scenarios: Without a clear external authority or a stable social structure, there is a risk that the ship’s society could collapse into violence or chaos, particularly in the face of a crisis or leadership failure.
Question: If you think generation ships are not an impediment to stellar colonization, what do you think is?
Sure.
Of course multiple small hurdles combining to make the number of ETIs in our galaxy very small is of course a totally valid possibility, given the many unknowns.
However, in general I would lean away from hypotheses that require (virtually) all civilizations everywhere to be not-X, when the one example of a technological species that we have is X.
We were discussing the hypothesis that the challenges of making interstellar craft or making specifically making self-replicating probes would be the primary reason we don’t see such evidence. And I was explaining why it doesn’t seem a very convincing hurdle in itself.
If the response is to shift to speculating that it may be one hurdle among many and not necessarily the biggest, then this is a tacit acknowledgement of not being able to make this a convincing primary hurdle because otherwise we would have stayed on debating the merits of this specific hypothesis.
Anyway, if the point is being put to me that the engineering challenges are just part of a much wider picture, then of course I agree: the ludicrous distances and hostility of outer space are known problems, and the fact that it’s a heck of a lot harder to get to alpha centauri than climb an oak tree is part of the premise. As long as we aren’t claiming that this is the primary reason for radiosilence then I have nothing to disagree with.
This is probably a huge one, at least for humans. The time scales involved are something like hundreds to thousands of times the total amount of time that human civilization has existed so far. Maybe a hive mind type species like sapient bees or ants could overcome this particular challenge, but probably not humans. And then there’s all the other ones you mention, and many others as well. As you say, the practical challenges are likely too large, no matter the species or how much time they have to work on a solution.
ETA: Maybe it was possible in the old days, back when the universe was a lot smaller and the distances involved not so great. But then again there would have been a lot less raw materials to work with, especially elements heavier than iron, back in those days.
My point is not that there must be a great filter. I was simply pushing back against the idea that the engineering challenges would be the primary obstacle – would be the primary reason we see no evidence of ETIs. Because this doesn’t stand up to scrutiny. Humans are right on the cusp – within mere centuries – of being able to start being noisy, and start spreading bric-a-brac around the galaxy. By some definitions, we’ve already started.
In terms of the rest of your post, mostly I don’t dispute it. I have already stated that there are many, many unknowns in terms of the probability of life starting, becoming multicellular and having sexual reproduction, evolving intelligence and developing technology. Many of the points along this developmental path remain as plausible hurdles in a way that technological limitations do not.
But to push back on one or two of the points:
You don’t need any energy to maintain speed.
Again, radiation shielding is physically possible and we aren’t talking about a machine of gaskets and cogs.
This assumes that we’re talking about manned craft.
And that no attempt has been made at bioengineering or cybernetics say.
It is a bit frustrating that we’re still talking about problems at this level despite the millions of years in play.
I myself am an engineer. And I’m usually quite pessimistic (although I would say realistic) about what can be achieved. But an intelligent species operating over deep time is a very extreme situation. We’re not talking about a bit more funding, or someone a bit smarter than some other guy. We’re talking about taking everything humans have ever achieved and multiplying it by thousands of times.
Yes, they cannot achieve what is physically impossible. But they will be basically at the line of what can be done.
Why would we do that? Why would we commit massive amounts of resources to be noisy and send bric a brac around the galaxy that we couldn’t hope to see a response to for 1,000 generations?
I don’t think sending or receiving physical stuff around the galaxy is how we are going to find out about ETIs. I think we continue to improve our ability to see the galaxy from our solar system, and will discover signs of ETI somewhere else, where we can never hope to go.
An advanced civilisation that only examines a few million planets when looking for life would not be a particularly capable civilisation. A truly competent intelligent species would construct a catalog of as many different types of biosphere and variations of biochemistry, even if they only use telescopic observations to do so.
Why would they do that? Well, any life-bearing world has the potential to develop a civilisation, which could be a collaborator, an ally, or a rival.
Quite true; our ability to observe distant worlds is going to improve significantly, especially using gigantic interferometry arrays. I think this will only inspire the urge to send ‘bric a brac’ around the galaxy - if and when we start discovering life-bearing worlds, we will have somewhere interesting to send these devices.
I would think it is very likely there is intelligent* life in the universe. The YouTube channel Veritasium recently posted a video on evolution (see below). It shows how evolution works. Unless you think it ONLY worked here, on our planet, chances are really, really, really good it has happened somewhere else as well. That does not mean it produced a species more advanced than an ant but still likely to have happened. It is an interesting video. Worth a watch.
Well, as I say; we already have.
We’ve leaked radio signals, directly beamed signals at other star systems and there’s nothing stopping the voyagers travelling interstellar distances.
Now, an obvious retort is that these are pretty pathetic signals, and incredibly slow littering of the galaxy. That’s true, but misses the point. The point is that humans have been “noisy” to the maximum extent that our technology has allowed. So the hypothesis that no species ever chooses to be noisy looks like a bad bet.
This hits the inverse of the deep time to solve engineering problems hypothesis. Even a civilization that is noisy for thousands (millions?) of years, is still only noisy for the briefest of moments on a galactic scale. They will generate an expanding shell of noise, but if the detecting civilization is too early or late, they will completely miss it.
