"A spacefaring species could easily settle the entire Milky Way given billions of years"
That's somewhat of a wild assumption. What's a "spacefaring species"? Are we one since we've built a few ships and went to the moon?
Have they solved problems that might be intractable, like being able to travel anywhere remotely far in anything else than a "generation ship"?
Have they found the power to deploy those ships, and the materials (and amounts required of them) to withstand the radiation of space, incoming meteors and space debris over decades of flying, while still being able to propel their spaceships out of their gravity?
And why would those settlements even be able to survive in space, going out on some remote systems, and not knowing what they'll find there, and what exactly their Earth-like planets will provide them?
>William Newman and Carl Sagan later wrote a major rebuttal to Hart’s work, in which they argued that the timescales to populate the entire galaxy could be quite long. In particular, they noted that the colonization fronts Hart described through the Milky Way might move much more slowly than the speed of the colonization ships if their population growth rates were so low that they only needed to spread to nearby stars very rarely. They also argued that being a long-lived civilization is inconsistent with being a rapidly-expanding one, so any species bent on settling the galaxy would not last long enough to succeed. In other words, they reasoned that the galaxy could be filled with both short-lived rapidly expanding civilizations that don’t get very far and long-lived slowly expanding civilizations that haven’t gotten very far—either way, it’s not surprising that we have not been visited.
One thing I think often gets ignored is the fact advanced civilizations require complex supply chains that would be extremely hard to bring to another planet. If anything goes wrong everyone is wiped out. Also hopping from one planet to the next would require the next planet to develop over a long period before they could start the next generation ship.
There's no proof that an advanced civilization requires a complex supply chain. In fact one of epochs of technological progression that occurs after an industrial revolution could possibly be an industrial simplification where supply chains and manufacturing processes reach a sort of apex in complexity that leads to absolute simplification.
It may just be that such a developmental landmark is a prerequisite of a civilization reaching interplanetary scale and it may even be a great filter.
What if preoccupation with the material world is a primitive perspective that civilizations simply grow out of eventually? There's an inescapable time limit to the universe and it's all objectively pointless anyway. Why spend valuable resources scouting the cold void of space when you can just turtle up on your planet and live 100 lifetimes at .1x speed in VR?
I don't think we can start wondering about the Fermi Paradox until we actually manage to get humans (or at least a von Neumann probe) to another star system.
We are the only intelligent more-or-less spacefaring species that we are aware of and if we can't do it why would we think other hypothetical intelligent spacefaring species can do it.
Because we went from beating sticks together to manned spaceflight in less than 1x10^4 years. Isn't it fair to extrapolate that to inter-stellar travel in 1x10^6 years?
It's not even clear we'll be around to argue about this in another 1,000 years, let alone another 100,000. It seems like our ability to destroy things is outstripping our ability to fix them.
Part of the Fermi paradox they skipped in this article is the idea of a Great Filter which prevents life from evolving or creating a space faring culture. The assumption is that we're past the filtering event, but it's entirely possible that we haven't reached it and in some unknown future our race will cease to exist.
Imagine a technology which would allow a single insane being to destroy the entire planet, or at least all intelligent life. We're not too far off from that point now if you consider modern political power. If that point of technological advancement is inevitably discovered prior to the technology required to become a space-faring race.
The idea of an individual or small team of scientists creating a world-threatening virus or triggering nuclear war is not that far outside our current state of affairs. Interstellar space travel is likely at least 100 years in the future.
Why should interstellar travel be realizable? If FTL is not possible--and there is no reason to think it is--then interstellar travel requires a craft that is capable of maintaining a colony-sized group of people for centuries or millennia, industrial bootstrap capabilities, repairs for everything in the meanwhile, food and energy for the trip, and the fuel reserves to slow it down at the end and land. The size that such a craft needs to be plausible suggests that the fuel amount in question will probably be best measured in something like "years of total Earth natural gas production."
No, my gut feeling is that we will never reach another star with humans or a self-replicating probe, so until we do I assume nobody else can/will either.
Thing that's interesting to me is that we have a number of proposals for how we might reach another star with essentially present-day technology. So even us dumb humans think we could get a robot probe out there. A self-replicating one is much further off of course.
Are you just questioning the Von Neumann machine aspect or the entire feasibility of sending a basic dumb probe out to another star for a fly-by?
A fly-by doesn't count. Even the Voyager probes will someday fly-by some star. Going to a star and setting up shop (whether a human colony or self-replicating probes) is the only thing that counts in my book.
>Even the Voyager probes will someday fly-by some star.
Not necessarily. I think you underestimate how much of space is space and how little of it is anything else... and that most of that "anything else" is dark matter. Only ~4% of the mass energy of the entire universe is matter.
It seems more likely than not to me that any arbitrary path through interstellar space would never pass near anything larger than a grain of dust.
Oh yeah that's way harder. I'd be OK arguing that we could fling something at Alpha Centauri and have it stop using tech with a very low-risk path from today's state of the art, but even just the 3D printing needed for self-replicating probes (such a printer has to be able to print something exactly as complicated as itself from asteroid material, which is really going to be incredibly hard to do, not to mention energy intensive) is a long, long way off and there's no way I'm going to be able to convince you of the viability of that!
The problem with Von Neumann probes is they couldn't land on a planet like Earth and use it as a launching off point since the escape velocity is too high for a simple probe to produce off of whatever it can extract from the soil. If any landed here it would have eroded away by now, and for all we know it could be all around our solar system and we wouldn't know.
How about a few Voyager-sized probes with a little decision making ability and some DNA? We aim them at the nearest stars which have at least one planet in the habitable zone. It sounds like something which might be possible soon to me.
What makes you think it sounds possible? Unless you're one of a very small number of people in the world who actually work on this kind of thing, "sounds possible" doesn't mean a whole lot. I don't work on the spacecraft, but I do work on the rovers they carry. Based on my limited expertise as someone in an adjacent field, the ability to send any spacecraft to another star on human timescales is not currently possible in any sense whatsoever. The ability to send a "spacecraft" weighing tens of grams to our nearest neighbor might be possible within my lifetime. The ability to get a voyager-sized probe to another star within my lifetime is as far outside the realm of possibility as anything can be.
Maybe I’m misunderstanding it, but the Fermi Paradox always seemed hopelessly naive and myopic to me. We understand such a small percentage of reality, but more importantly, everything we learn and observe is through our human perspective.
It seems more logical to assume that the universe appears empty to us because we are the only organisms like ourselves, and that other life forms exist in ways that are simply incomprehensible to us at the current level of scientific understanding.
Edit: To use a metaphor I’ve just thought of (and think works): many people assume that meeting extraterrestrial life will be akin to Europeans landing in the New World. It’s more likely that it will be akin to the discovery of the atom.
Even if we assume that the vast majority of life forms are undetectable/incomprehensible, there is still no good explanation for why Earth would be the only place suitable for life forms similar to us.
Among the many steps necessary to evolve life as we know it, which one could only happen here?
There are billions of stars in a galaxy, and billions of galaxies in the known universe. There could be more worlds with life similar to ours than there are grains of sand on Earth's beaches, and drops of water in its oceans, and we could still never even be aware of them simply because the universe is that vast.
Precisely the opposite. There are so many possible world that match all the criteria (that we know of) for life just within our galaxy, that we should detect some of them.
Our solar system is pretty young compared to the age of the galaxy (4B years old vs 13B years old). Imagine a civilization identical to ours, but that would have appeared a mere 1 billion years before us, not only radio signals should be detectable (because the galaxy is just 100k light years across), but a civilization similar to ours, with 1B more year to develop technologically should have easily been able to send probes, at the very least, all over the galaxy (see von neuman probes) so we should also see that, and yet we don't.
And this just assuming a single civilization, while as you said: there could be more worlds with life similar to ours that there are grains of sand on Earth. So where is everyone?
Detect with what? We've had radio signals for about a century or so and our signals have barely made out of the solar system at which point it's just background noise.
We just barely can detect planets and only in optimal conditions. Our tech for all extra terrestrial purposes is laughably primitive.
You greatly overestimate the size of the solar system.
Our solar system is a mere ~20 light hours in radius. We even have two man made objects out of the solar system (Voyager 1 and 2).
In a century, our signals already made it past ~15 thousands star systems.
So now imagine a civilization similar to us but slightly older, not only would this civilization's signals have reached all parts of our galaxy, but those signals would likely be much stronger than we have produced in the last 100 years.
It would also be possible for a civilization a few hundred millions years older than us to send probes to every star of the galaxy by now.
"there is still no good explanation for why Earth would be the only place suitable for life forms similar to us"
1. There is still no good explanation _that we know of_. We don't even know if we are the only life forms in existence or not: none of the observations made so far support the latter, but do not disprove it either. So we can't prove or disprove _any_ hypothesis about extraterrestrial life.
2. "Suitable for life similar to us" and "suitable for life to spontaneously appear" are two very different things. And we still don't know enough about either.
Maybe because worlds are continuously created and destroyed, so they have no time to create life and then develop technological civilization. We are just lucky to fit tight limit.
That is pretty much the 'great filter' solution to Fermi paradox: there exist some kind of filter that prevents most life forms from being galaxy-spanning where they would be easily detectable.
The biggest issue I've had with the Fermi Paradox is why should we expect to have seen evidence of alien life by now? At our current level of technology, we can only say with confidence that there is no alien life of about our complexity in our solar system since the last glacial maximum or so. We can't detect life as close to us as the nearest star, let alone anywhere else in the galaxy.
I mean sorta. But think about the incredible variety of life on Earth. Read some science fiction. I'm not saying our search space / our expectations are perfectly broad enough.
But I've never seen any sign the people involved in SETI are doing this with blinders trying to find bipedal creatures with a mouth that breathes oxygen and are about 1-2 meters tall.
But is it actually that varied when compared to the infinity of the universe? All life on Earth is carbon-based, to start with.
The assumption always seems to be that experience as we perceive it now is some sort of default, objective one. It’s far more likely that we are simply locked into a very narrow viewpoint. The scales are simply so large that we are unable to comprehend them in any meaningful way.
Carl Sagan, who coined the term, has admitted that he is a carbon chauvinist. And I suspect most biochemists are as well; there are decent chemical reasons to disfavor any other system.
I agree if you're speaking about "life", but if we're talking about self-aware intelligence then I believe that once they discover their analog to the Second Law, they would have to (eventually) realize that their survival depends on extra-planetary expansion.
Agreed. Assuming we avoid extinction, it's not obvious we'll be able to recognise ourselves a few hundred years from now, never mind a few thousand. And a gulf of a few million years would be utterly incomprehensible.
In fact I'd be very suspicious if recognisable alien life appeared and knocked on our door, because it would literally be so astronomically unlikely I'd be skeptical about taking it at face value.
Well, yes and no - plenty of terrestrial organisms that haven't shared a common ancestor with us for hundreds of millions of years can recognize us (like, say, crows). But I agree that easily recognizable sentient aliens visiting us in person is very unlikely, due to the scale of the universe.
My gut feeling says panspermia is likely occurring regardless of whether Earth life is descended from stuff from other worlds or vice-versa. I don’t expect it to be a deliberate colonial mechanism, thought perhaps it might work as a deliberate terraforming mechanism, as one step in a plan in the vein of Stuart Armstrong’s talk on disassembling Mercury and using it to directly colonise the entire supercluster essentially simultaneously?
Maybe our perspective is just wrong. If you consider that DNA itself is the life on earth, and not "us" in particular and that maybe DNA is the only life possible in this universe then this view makes more sense. Humans could be viewed as just an appendage of DNA.
"We are survival machines – robot vehicles blindly programmed to preserve the selfish molecules known as genes. This is a truth which still fills me with astonishment"
- Richard Dawkins, The Selfish Gene, 1976
Dawkins talking about the first sentence, about 1980:
"...that was no metaphor. I believe it is the literal truth, provided certain key words are defined in the particular ways favoured by biologists. Of course it is a hard truth to swallow at first gulp. As Dr Christopher Evans has remarked, "This horrendous concept - the total prostitution of all animal life, including Man and all his airs and graces, to the blind purposiveness of these minute virus-like substances - is so desperately at odds with almost every other view that Man has of himself, that Dawkins’ book has received a bleak reception in many quarters. Nevertheless his argument is virtually irrefutable" ...
Presuming we're not in a simulation then I think it is unlikely that we're alone in the Milky Way.
There are too many stars and too many close enough galaxies that could contain life. If they've been spacefaring for even ten thousand years longer than us then they're almost certainly able to monitor us without our ability to detect it.
But if we're doomed to a great filter in the near future, like nuclear war, then it still doesn't mean we're alone. It just means that out there in the sky there are others kinda like us. Searching for meaning before hitting a technological explosion of one kind or another that wipes us out. But just because we don't have contact doesn't mean we're alone, even if we're only present in each others respective imaginations.
It seems like you're assuming the difficult part about creating self-assembling replicators capable of sustained, error-free exponential growth across interstellar space and billions of years is merely coming up with the idea.
It's entirely conceivable that every space faring civilization which has tried has failed simply because the odds are so stacked against success, even once, at every step along that process. Just because the math is simple, doesn't mean the application is simple.
Hell, we don't even know what the odds are of becoming a space faring civilization to being with, and it doesn't seem axiomatic that every such civilization would even want to seed the galaxy with self-assembling replicators to begin with.
I've been mulling this over: given the constraints of information travelling at lightspeed and physical objects travelling at a lot less, what if it simply doesn't make financial sense for any actor in a market economy to colonize the galaxy?
What if the ROI of space colonization even at multi-decade or multi-century timescales is negative? Then why would anyone do it?
But what if that happens once every, say, 2 billion years on average? What is the likelihood we exist in a time close enough to that happening for us to be able to find ruins of it? That is the point the article made. There is an assumption that, if that happens, the colonization is permanent. Maybe that's not a good assumption.
Either of the following suffices to account for the evidence:
1. Expansionist civilizations that encounter one another reliably obliterate one another. Non-expansionists don't spread out.
2. Advanced civilizations have no need for goldielocks-zone planets. Where mass is needed, Kuiper-belt objects are more convenient sources. The notion of Dyson spheres seems especially silly: even extreme primitives would have confined fusion power, and have no need to rely on primitive, diffuse stellar radiation.
That's somewhat of a wild assumption. What's a "spacefaring species"? Are we one since we've built a few ships and went to the moon?
Have they solved problems that might be intractable, like being able to travel anywhere remotely far in anything else than a "generation ship"?
Have they found the power to deploy those ships, and the materials (and amounts required of them) to withstand the radiation of space, incoming meteors and space debris over decades of flying, while still being able to propel their spaceships out of their gravity?
And why would those settlements even be able to survive in space, going out on some remote systems, and not knowing what they'll find there, and what exactly their Earth-like planets will provide them?
>William Newman and Carl Sagan later wrote a major rebuttal to Hart’s work, in which they argued that the timescales to populate the entire galaxy could be quite long. In particular, they noted that the colonization fronts Hart described through the Milky Way might move much more slowly than the speed of the colonization ships if their population growth rates were so low that they only needed to spread to nearby stars very rarely. They also argued that being a long-lived civilization is inconsistent with being a rapidly-expanding one, so any species bent on settling the galaxy would not last long enough to succeed. In other words, they reasoned that the galaxy could be filled with both short-lived rapidly expanding civilizations that don’t get very far and long-lived slowly expanding civilizations that haven’t gotten very far—either way, it’s not surprising that we have not been visited.
That sounds much more logical!