In the context of the Fermi paradox, we’re talking about species the equivalent of millions of years more advanced than us. That’s because the galaxy is 13 billion years old, so even applying contraints like needing a third generation star, still leaves a window for ETIs to appear of at least a 3-4 billion years. The chance of them being technologically within mere centuries or decades of human progress is incredibly low.
So again, you’re listing engineering challenges, and they just aren’t relevant in this context. Humans went from sharpened stones to landing probes on asteroids within a few thousand years. It’s “physically impossible”, or nothing.
As far as self-replication is concerned (including building an entire massive industrial plant which would churn the things out en masse), a probe is much better off landing on an asteroid where the needed metals would be much more accessible and the escape velocity minimal.
Not original, I’m sure, but: If there was massive amounts of intelligent life in this galaxy, at least one civilization out of the likely hundreds would have lots of avid colonizers.
It could be that avid colonizers are blocked by some super-powerful anti-colonization federation. Could be.
Re the probability of every advanced civilization killing themselves off: There may be, pace Churchill, a new dark ages made more sinister and prolonged by perverted science. But dark ages do not last forever.
So it is not kooky to think we are the first in the Milky Way. My best guess is that we are.
The escape velocity of that body would certainly be less but to colonize the galaxy wouldn’t that craft need to attain escape velocity of that local solar system?
That’s assuming we can extrapolate current technology by millions just because there will be a future then. I’m not at all convinced we can. I’m guessing there will be a practical upper limit. That is of course also assuming humankind (or ALI) will in some form hold it together for (the equivalent of) millions of years. Certainly also not at all a given.
This discussion sort of loses its teeth if we don’t keep it grounded in some sort of reality. Otherwise one might well say we will have Star Trek technology in the future.
I’ve run across the argument many times, not just here on the SDMB, and I just don’t see how it holds. I don’t get how waving away engineering challenges by stating that they won’t be an obstacle due to the large time scales involved solves anything. We don’t have any good reason to believe that interstellar travel is possible (barring a binary / trinary star system with distances that involve small fractions of a light year) no matter how much time an intelligent species is given. The distances involved and the harshness of the environment are likely just to great to overcome, even for an intelligent species that is somehow magically guaranteed to be cooperative with itself (maybe something like sapient ants or bees with a real life equivalent of a Borg Queen) and pursue technology as their primary goal.
I made it very clear, I explicitly stated, that I am not saying that an advanced ETI will be able to just break the laws of physics. What I am saying is that engineering challenges are just not sufficient as great filters; they have no real bearing on the fermi paradox.
Let’s take a problem like, say, radiation shielding.
Humans today could not make a large vehicle that could travel interstellar distances without being damaged significantly by radiation. We know various ways to protect craft in principle: strong magnetic fields, coupled with buffer material like water to absorb neutrons, a machine that is capable of repairing its own circuits thanks to redundancy etc etc. But all these ways add significant bulk and/or vast energy requirements.
So does radiation shielding work as a great filter? Does it work as something that would make self-replicating probes impossible?
No. Not within the bounds of credulity anyway.
Because, to give context again, humans were still fighting and hunting with sharpened stones a few thousand years ago. An advanced ETI has typically thousands of times the difference between our stone age and space age tech. The vast majority of their engineering challenges will be things we’re not aware of, and almost everything that we’re aware of, will be in their “completely trivial” bucket. Much more primitive to them than a sharpened bone is to us.
I sense at this point that the response again will be that they cannot do what is physically impossible. But we already know that radiation shielding is not physically impossible, as I originally outlined. So this objection would be irrelevant.
There’s nothing stopping aETIs from finding practical solutions over the millions of years available, to a problem that humans only became aware of about a century ago.
I thought I made it clear that my objections aren’t related to any requirements to break the fundamental laws of physics. I’m not objecting on the basis of requiring technology based on faster than light travel, or temperatures closer than absolute zero, or using singularities, or things of that sort. But I don’t see how time, no matter how much of it is available, can overcome practical problems. I’m simply in the camp that, to take your example of radiation shielding, that the difficulty in engineering any solution that can withstand the rigors of interstellar travel over distances involving light years and time scales measured in mega years / kilo millennia, that such things are just too impractical to ever be developed. There are limits to how far we can advance our tech, and those limits aren’t going to be due to the barriers like the speed of light or absolute zero or some other fundamental barrier imposed by the laws of the universe. They just aren’t practical. It’s not just radiation shielding either. It’s acceleration, both the positive to get to even 0.01c and negative to slow down at the destination. It’s the question of being able to build self replicating probes that will be able to not just replicate themselves, but build another entire ship, with all the tech such a thing would require, once it arrives. Keep in mind at that point they would no longer be dealing with interstellar space, they would be dealing with (presumably) something like an asteroid, and all the different challenges that would entail compared to being designed for acceleration and the travel between solar systems. Then building all the stuff that would be needed for identifying an Earth type planet and a ship able to travel between some initial asteroid landing and an actual planet or moon of interest (because starting off by heading directly for an Earth type planet would present even more difficulties). What if there are no naturally occurring types of asteroids with all the required materials? Are we talking about something that could survive inside a high pressure gas giant planet like Jupiter? Or maybe a really hot planet like Venus? Where is the energy to do all the needed tasks going to come from? Obviously not fossil fuels. Getting to the point where uranium, plutonium, neptunium, or such can be harvested to use fission would also be unimaginably complex. So would carrying all the stuff needed to even get started with something like a tokamak reactor or whatever we end up developing (assuming we ever find anything that works) for nuclear fusion. And it’s just a guess, but I imaging Star Trek style matter - antimatter engines would be even more difficult to use. Maybe it’s just that my imagination is limited, but I don’t think such things problems can be solved to make interstellar travel a reality, no matter how many millions of years we have available.
Radiation shielding alone might not be enough to to be a great filter, but there are hundreds if not thousands of other technological impediments that would need to be overcome for a generation ship to be viable, and when taken in concert present quite a formidable filter. And frankly, radiation shielding is kind of on the low end of hurdles.
A hypothetical advanced ET. We zero evidence that such a civilization exists or that any civilization could function that long without falling prey to self destruction.
So because the relatively minor problem of radiation shielding can be confidently addressed, then we can hand-wave away all the other (more serious) concerns away?
Sorry, that doesn’t track.
ETA: Radiation shielding won’t prevent debris damage. Going at a speed of .02% speed of light, collisions with very small objects like space dust or tiny pebbles would be highly dangerous. The kinetic energy released would be similar to a high-speed meteor strike. The energy would be enough to punch holes through the ship and cause significant damage to systems
If we’re talking about practical problems with interstellar colonization, this makes sense.
What about interstellar communications? That will go at the speed of light and would be seen as a fun and safe activity, especially if the impossibility of sending significant amounts of complex intact matter to another star system becomes evident.
Suppose that there are a few advanced civilizations, tens of thousands of years ahead of us, with intelligent beings having a high level of curiosity, located within, say, a couple thousand light years of here. Shouldn’t they have the ability to suspect that earth has a high probability of advanced civilization presence, and then currently target us with radio or television broadcasts using highly directional transmitting antennas?
Maybe multiple civilizations are already sending this to our solar system, but earth is a bad place for reception. Perhaps we will get the signals as soon as we put a good antenna array on the far side of the moon. Since I think we are alone, I doubt it, but I’d be willing to pay slightly higher taxes to find out.
Or maybe the amount of energy needed to send out complex and interesting targeted audio/visual signals, that could be picked up, say, 100 light years away, will be impractically high even with fusion energy. Is that it? Are there any estimates of the energy requirements?
Then I don’t know what to tell you, and I guess we just have to agree to disagree at that point.
Human beings find practical solutions to problems. Sometimes quickly – when there’s significant money invested and governments competing – sometimes slowly. But even the “slow” lane is lightning fast when compared to the deep time that would separate us and a random ETI.
If you take the view that problems that humans just heard about a human lifetime ago will leave an intelligent species scratching their heads for millions of years, then fine. I can’t prove that view wrong. But it does seem ludicrous to me.
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.
