I think the anthropic principle is more an argument about why the universe is the way it is, with apparently fine-tuned physical constants that allow it to be stable and allow stars to form, and so on. It’s not really an argument about the prevalence of life in the universe, other than to say that if the universe were built in any other way (as perhaps some are in multiverse hypotheses) there would be no one there to observe it.
Yah…kind of a universe Zeroth Law! (if you know, you know) 
But to the OP it would suggest there is nothing particularly special about the Earth either. Our universe has life in it (proof: you are reading this which circles back to the Anthropic Principle…I forget if strong or weak though). Life could form anywhere in our universe under the right conditions (which are likely to be abundant).
All true.
But note another data point is that the first organisms appeared on earth anywhere from 4.4 to 3.5 billion years ago…really pretty early, the planet was still basically a molten ball.
So the one data point we have implies the first (proto-) cell may not be the big bottleneck.
But of course we don’t know for sure yet.
Also true.
In my case, it’s only a tacit (and now explicit) acknowledgment that I don’t know enough about making either interstellar craft or self-replicating probes to make arguments about their difficulty. I’ll let those who know about what relevant work we’ve done so far in those areas make those arguments.
I agree with you. The problem is: how do we consider it “life”? Think about it: The same single cellular organisms evolved to be things from trees to flies to dogs to this and that! Most people would think there aren’t many similarities (plants don’t have lungs or gills). Who’s to say life in other galaxies necessarily has cells in the same way we do? Scientists even talk about silicon based life! If there was other life and we found it, many scientists might not consider it “life”, depending on its nature. Also, the nearest star (other than our sun) is some 4 light years away. Our best bet to find life is to let it come to us. Most people would picture aliens as the classic large head, human-like being. Even on Earth, most life (that could reasonably do something, let’s leave flora and bacteria-like things out of the equation) has 6 legs. The chances of it also trying (and succeeding) to find alien life are likely low.
TL;DR: There almost certainly is life (in some sense), but we probably won’t ever find it.
It is FULLY on point, since it biases all of our suppositions about what the actual “odds” would be, as this thread has been a shining exemplar of. Yeah, we are here, but our being here almost certainly required the equivalent of thousands of boxcar dice throws in a row, a number which would completely dwarf all the “Oh, there are X “Earthlike” planets in Y region of space!” figures being breathlessly bandied about. Else we wouldn’t be here in the first place to discuss this over tea and crumpets. As I said upthread a few years ago, no matter how big the sample size of potential planets might be, the actual odds of life, much less intelligent technological life, may be many orders of magnitude beyond that.
It has a large moon which has been critical to keep the orbital tilt relatively stable, to name just one factor. It has survived eons of possible life-killing events both intrinsic to the planet as well as from extraterrestrial sources. Jeez people it has elephants and blue whales, mosquitoes, shih tzu’s, sequoia trees, and stinging nettles everywhere. Pink Floyd and Salvador Dali and Mel Brooks and Edgar Allan Poe and Socrates. [Edit: I didn’t see the previous post with a similar list by IridiumIodineSulfur until after I posted, a nice bit of serendipity.] Point is I could be here all day listing shite that is completely unique to this planet. The Earth is IMMENSELY special, sorry; to suggest otherwise, as several people have in this thread (and in older ones*) just continues to completely boggle my mind.
Frankly (back to the thread title) I think it is kooky to think that there are ANY other civs anywhere. Oh there might be a few pools of primitive creatures here and there, sure–maybe.
Rather than thinking about this as a problem, I like to think of this as a core reason to explore the universe. What possible forms can life take, and what other processes might exist which resemble life in some ways, but not in others? Can we count protobiotic replicating molecules as life? What about gels and colloids of biotic molecules that don’t have cells, but which share replicable characteristics? What about replicating entities made only of data, that can exist and replicate in a wide range of suitable substrates, and are not limited by a biological nervous system? Entities made of magnetic fields that persist in the photosphere of a star?
The opportunity to find, study encounter or be found by such entities is probably one of the most enticing goals we can have in the universe - and perhaps not without risks.
Sure; exploring the galaxy will require vast amounts of energy, and won’t happen any time in the next few hundred years - but we shouldn’t think we are limited with respect to available energy. The Sun kicks out a BILLION times as much energy as we will ever need on Earth, so acceleration per se is not a problem. Two problems that are much more limiting are 1/ not boiling the spacecraft during the acceleration phase, and 2/ somehow slowing down when we get there.
I hope so, but it might be a bit optimistic. If one in a thousand stars supports life, that would mean approximately eight worlds with life within 100 light years of Earth. If they exist, these lifebearing stars might just have basic, microbial biospheres, or maybe something we wouldn’t recognise easily. But those worlds would be something to aim for, and a worthy goal.
Well, given this criteria, that they need to be a fully autonomous, physical machine that can seek out and mine raw materials in an unstructured environment and fabricate and produce a complete, identical copy of itself, including a propulsion system, without human intervention, under their own independent power - does not yet exist. Nor is there anything close. We are not even close to making robots that can replicate themselves in that manner - without having to blast themselves off to another star system.
…to name one factor that is unlikely to be that special.
There are hundreds of billions of planets in our galaxy alone*, and if our solar system is any indication, most have moons. Sure, most don’t have proportionally big enough moons, but still a fair estimate of the number of such worlds in our galaxy is likely to be in the tens to hundreds of millions of examples range.
<*> I quite like the youtube channel Epic spaceman for some mindblowing demos of just what billions of planets, and billions of galaxies, actually looks like.
No arguments there, but of course this is a different point. There’s specialness as it relates to conditions for (advanced) life, and there’s the specialness of advanced life and cultures.
Saying that the earth appears typical for the former, does not devalue the latter.
Of course humans today are nowhere near close to such tech.
However time-wise we might be. Remember again that sputnik 1 was within a human lifetime ago, and in that time we’ve landed a probe on an asteroid, sent two probes out of our sun’s gravitational well, and made simple fabrication machines here on earth.
That’s a lot of the steps towards making a replicating drone within the last ~70 years. So I think it’s a fair bet that we could solve the remaining problems in the next few centuries, with an estimate of 1,000 years at the extreme pessimistic end of things.
Let alone the millions of years of technological progress available for a random species in our 13 billion year-old galaxy.
So, where is everybody? 
I disagree with that logic. All of those things are essentially the same thing. All of those craft were designed and carefully hand-crafted on earth by people. Those extremely complex machines still occasionally fail. And all of the parts were machined from materials that were mined or fabricated at great effort by humans at some point. A self-replicating ship would have to (do the equivalent) of mining (with drills?), smelting, casting, polishing, assembling not only another probe that could do all of that, but would have to also fabricate an engine powerful enough to launch that mass into space and accelerate it up to at least escape velocity of that solar system.
Not to mention build its equivalent of electronics - nav systems, etc. And all of that would need to be built within no-fail tolerances for probably hundreds of years.
Yeah, that seems a challenge we won’t be able to meet technologically.. And that’s assuming in a thousand years we will still want to. I’m guessing we will retreat into computers by then. I think the technological leap to download our consciousness into a computer will be a lot less challenging. And that’s a process that’s already started.
Elon Musk’s company, Neuralink, has successfully implanted a brain-computer interface chip into a human for the first time as part of a clinical trial for people with paralysis.
The implant allows the individual to control a computer cursor with their thoughts, with the initial goal being to restore function by enabling them to control external devices like a mouse.
You are correct. We know of two kinds of self-replicating system at the moment; biological organisms and industrial civilisations. Biological self-replication might be the only feasible way to send self-replicating systems out into the universe - an idea suggested by J.D. Bernal in the early 20th century. But maybe a carefully-designed, stripped-down and optimised version of an industrial civilisation would be a faster and more reliable option.
A non-biological interstellar self-replicating system would need to be nearly as complex as an industrial civilisation in order to construct another interstellar spacecraft (or a fleet of them) and send this fleet off to another system. The question is; how simple can we make a civilisation? The self-repping system needs to carry with it all the skills to prospect, mine, process and construct the materials for the replicated fleet, so it needs enough stored data, skills and experience to accomplish all these goals. On the other hand, a self-repping system doesn’t need to be able to create works of art, fine food and wine, and all the other trappings of human culture.
I don’t suppose we’ll be able to create pocket self-repping industrial civilisations any time soon- but in a few hundred years I am confident that we will. Note as well that an autonomous self-repping industrial civilisation would be a very useful thing here on Earth, as well, even though there may be risks involved (once again).
This is a limiting factor, not a complete barrier. The density of dust between the stars in various directions can be measured by observing the stellar extinction, that is to say the amount by which a star’s brightness is diminished in various wavelengths by absorption by dust. This really only gives a reliable estimate for the abundance of the smallest particles, but dust particles are expected to follow a power law, and larger particles would be much less common than smaller ones.
This means the amount of large, dangerous dust particles can be estimated, and shielding can be deigned accordingly. Make your ship long and thin, with a tough shield (or several) at the front, and a ship could travel at 10%c to a nearby star with reasonable expectations of survival, The chance of hitting a pebble in space that would terminate the mission would be small, but not vanishingly so - this sort of enterprise will always be risky. Lose a few starships - who’d notice in thelong run?
I notice I’ve mentioned many possible risks in recent posts - the risk of hitting a dust pebble, the risk of encountering a dangerous, paranoid alien civilisation, the risk of creating interstellar rivals by over-enthusiastic expansion; maybe every alien civilisation out there is so risk-averse that they decide that expansion is not worth the candle, even if it is practically possible. Another possible explanation for the Fermi Paradox.
Yes: we don’t have self-replicating probes today. That’s not a great revelation.
We’re talking about reasonable extrapolations of where technology is heading. In fact, we’re not even talking reasonable extrapolations, I’m pointing out that even the most incredibly pessimistic projections (but within the context of human civilization still going*), will achieve this technology many orders of magnitude quicker than the time window that would be relevant to the Fermi paradox.
<*> Yes, it’s a (likely) possibility that humans will nuke ourselves back to the stone age at some point in the near future. But again, if that’s what we’re positing happens to sentient species everywhere, then that’s the Fermi paradox solution. This position is basically saying that technology is not the significant hurdle, it’s simply a hurdle that no species ever reaches.
Again, that’s a different point. There’s whether we could make a particular technology and whether we (or any other species) would want to.
I think the speculation about whether we would want to is a deflection, whether intentional or not, from the fact that there isn’t any particular technological barrier that you or anyone else can think of. Listing problems to be solved does not constitute evidence that even centuries (let alone millions of years) will be insufficient to solve such problems.
And in terms of a Fermi paradox solution, again, I don’t think positing that all species will exhibit the same behaviour is a promising direction to go. The one example of a sentient species that we know of has many competing motivations, and societies, and changes its views on many things over relatively short amounts of time.
So even positing that one species decides to do X, for all time, is a bit of a stretch, let alone that they all do.
Industrial civilizations don’t replicate without the biological organisms which create them. So I don’t see how they’re self replicating.
And in our quite short term experience with them, one of the things which seems apparent is that they mutate like crazy. You’re positing something that will stay on the same mission for millions of years; which seems massively unlikely to me for any civilization, biological or otherwise: or at least for any civilization created by any creature capable of enough development and change to get into a position to try to do such a thing in the first place.
Exactly.
And the species itself, depending on how you define it, has been around for maybe a third of a million years.
Combine all that with the extraordinary distances involved, and I don’t think anybody’s going to get all that relatively far from home while they’re still the same people, let alone the same civilization.
When we talk about a self-replicating starship, we have to imagine something that can operate like an industrial civilisation; one which can find, process and construct materials and energy sources, to form a completed vessel (or preferably a fleet of them). Individual humans can’t do that by themselves. So a hypothetical self-repping star probe would need to have all the skills and capacities possessed by an industrial civilisation, minus the other, irrelevant cultural activities that accompany them in our civilisation. A self-repping starship wouldn’t need to write poetry, for example, or direct horror movies.
Here’s Robet Freitas talking about his concept of a self-replicating interstellar probe; A Self-Reproducing Interstellar Probe … his concept includes the bare minimum of elements, but it includes just about everything needed to run an industrial civilisation in miniature.
Maybe this ‘stripped down’ industrial civilisation concept does need the capacity for writing poetry - otherwise you might get something as monopathic as Fred Saberhagen’s Berserkers.
I’m not following the logic of why a self-replicating probe would need to be as complex as all civilization. A self-replicating probe would be complex sure, and again, far beyond what modern humans could create. But the set of science and engineering not relevant to a self-replicating probe is already vast.
It is a matter of degree. The only system we now know of that can build interplanetary rockets at all is an industrial civilisation; to build an interstellar spacecraft, or a series of them, that industrial civilisation must be even more complex or competent than our own civilisation.
Now to pack all that competence and technical ability into a self-replicating probe, we would need to distill the capabilities of our civilisation down to a bare basic level - to achieve self-replication in a distant solar system, the probe must be competent in astronomy, geology and prospecting, refining and processing materials, logistics. Many of the resources required to build and fuel a new interstellar probe would be far apart - quite possibly in different parts of the solar system. All these processes then need to be tied together into a spacecraft more complicated than anything we can build today, and then launched successfully - all without any real-time input from the home system.
If we could build a system as competent and as autonomous as that, we would basically be building a miniature industrial civilisation in a distant location. It seems to me that such a complex and competent system could then be used to build other things as well, or instead. Could we do it? I think that eventually we will be able to do something like this, but not for many centuries, and not until we really have a handle on how to build competent machines and comprehensive databases that can inform these machines.
What role would good old meathead, flesh-and-blood humans play in such a process? Probably not much, to be honest. But once you’ve got a suitably stripped-down, optimised, and distilled version of an industrial civilisation established in a nearby solar system, then maybe humans of some kind might be able to go there and thrive.
Assuming the competent machines allow it.