It seems scientifically implausible that we are alone. I base this on a few things:
Life started on Earth almost as soon as the surface was cool enough to support it. The surface seems to have been stable enough for life by 3.8 billion years ago, and we have some weaker evidence of life at 3.7 and stronger at 3.5. At any rate, based on what we can observe, life began relatively early and so it seems that as soon as the basic building blocks and preconditions are present, life evolves.
We now know that exoplanets are extremely common. Based on observations we can estimate that about 40% of stars have planets, but that’s just what we’ve been able to observe.
There are a lot of stars in our galaxy, 100-400 billion.
And a lot of galaxies in the observable universe, 200 billion to 2 trillion.
And that’s just the observable universe based on our local light cone. For all we know, there could be an infinite number of galaxies.
So we have n=1 where life started right away, and perhaps 10^25 planets in the observable universe. That’s a lot of rolls of the dice!
Hence the need for a proposed great filter that explains why we don’t get a visitor ever other week. Complex multicellular life seems like a possibility, that took quite awhile, a few billion years before the Cambrian explosion. Complex intelligence also took awhile, resulting in only us after 3.5 billion years of life. And yeah, given that we haven’t been around all that long and seem well on our way to destroying ourselves, seems like complex society could be a great filter too. But space is vast, EM signals attenuate quickly and the speed of light is a harsh mistress. They are probably out there, but we’ll probably never meet them.
Uh, we don’t necessarily need a great filter to not be constantly visited - we just need a lack of ‘cheat codes’ (like warp drives, wormholes, dirt cheap energy from magical fusion or zero point or whatever), combined with relatively short lived complex civilizations.
If no one can afford the trip (due to what we currently know of physics), or survive as a complex civilization long enough (100k+ years), then… you see what you see right now.
Something like a fleet of von Neumann probes could expand across the entirety of the galaxy in a million years or so, and wouldn't care if the original civilization had long since flamed out. Which is long time by our standards, but not too long on the cosmological scale. In theory there should have been lots of habitable places where life could have arisen in our galaxy long enough in the past that the whole place could be "colonized" in this manner.
Of course, there could be plenty of these floating around our solar system and we wouldn't know. They could be quite small and the solar system is quite big, and most of the proposed use cases for a civilization wanting to do this would not require it be in constant contact, so they might not even be broadcasting on anything resembling a frequent basis.
But in general there are a lot of theoretical technosignatures that we think we should be able to see if there was technological advanced life in the galaxy. (Should be able to see in that it is possible with our current technology - not that we would conclusively have seen it by this point.)
It’s a nice idea in theory, but we’ve seen no evidence Von Neumann probes are even possible, or that any civilization would consider them worth investing in.
If you do the math, unless there are the aforementioned ‘cheat codes’, they frankly just don’t pencil out as worthwhile. No FTL/subspace? No ‘infinite cheap energy’ hack? No ‘infinite self replicating yet controllable builder of anything’ molecule?
No way to propagate, no way to energize, and no way to communicate at any real scale over the distances involved. Even gigawatt power, high efficiency, high precision aimed lasers aren’t going to have much luck at light year distances, and nothing is going to be useful for two way communication.
It’s easy to speculate of course, but like ‘grey goo’ it runs up against real issues with thermodynamics and physics if we stop and think about it much.
Worth investigating options of course, or talking about, but if physics is what it seems to be so far, that would perfectly explain the lack of us seeing them - they’re not useful/practical/worth the expense for anyone.
Worthwhile to invest in I think is the bigger question - from a practical 'can we build a self replicating probe fleet that would eventually cover the galaxy', there's at least evidence that it's possible in the next hundred years or so.
In general I do think it makes sense to not just contemplate interstellar visitors as living things making the trek or biological civilizations expanding, but there's a wide variety of artificial ~things~ that could end up in our solar system for whatever reason.
Uh, that isn't evidence? That's speculation on potential future technical advances.
The only things we see self replicate are currently biological systems, and they can only do it under very specific circumstances and usually with a LOT of external help.
Circumstances which do not exist in space, let alone under the variety of environments on just the handful of planets we see in our solar system - except one, of course.
It's a very hard problem, and it isn't clear there is an actual answer that is economic to stick on the end of a rocket, even just to get to the moon. Something that survives 10k+ year journeys to unknown environments, and then does something useful? Even less clear.
Gets there AND then has the level of success necessary to transmit any sort of signal, or propagate itself further? Even less so.
Why we would do it, is also unclear.
Hopefully we'll figure out something that makes it less fantastical at some point - all the things that make it realistic would also help us in many other ways too!
We've got a pretty good track record in predicting near-ish-future scientific/technological advances, which is what we would need for self replicating machines. To be clear, I'm not talking nanobots or similar - much more macro sized stuff. All the constituent pieces to make a self replicating mining robot or whatever are there today, and we already automate a lot of the processes required. There are some important gaps to fill, for sure, but none that we have any real reason to believe are insurmountable.
We're in agreement that the 'why' is less clear, at least for probes exploring the galaxy. But there are some very clear reasons why this sort of tech in general is useful - mining the asteroid belt, building reflectors to better harness the energy of the sun, building all sorts of other infrastructure in space without having to use a rocket to get it there. At that point once we have the tech and are doing it - and I would be shocked if the human race is still around in 500 years and NOT doing that sort of thing - it's relatively cheap to kick off a bunch of probes doing similar things.
I'm still not sure if it's worth it in that scenario, but it's conceivable that it could be. And that puts us again on the question of why we don't see it. We think there should be a lot of intelligent life out there. There's a lot of very important 'if's in all of this, but if we can think of it and and conceive of some potential reasons, and if other intelligent life has these capabilities that we believe will be possible, and if it becomes as resource/energy/time efficient to do as it should be, then it seems like someone, somewhere, should have been able to convince their government or corporation or whatever to do it, even if the results aren't necessarily going to be super apparent. The Pioneer plaque is obviously much cheaper, but we knew beforehand that the utility there is basically 0%.
Like I said, I'm not necessarily arguing that these probes are out there. I'm guessing the answer is more likely "There's no compelling reason to colonize or even explore large swathes of the galaxy" and concerns around interstellar travel are on such large timescales that civilizations capable of interstellar travel would only care about their nearest neighbors. Maybe that changes as star formation spins down and the galaxy starts to slowly go dark and your local galactic resources are less and less abundant. I'm just saying that there are technosignatures that would indicate the presence of other intelligent life in the galaxy that have nothing to do with biological beings visiting us, and we would expect that some of these are possible for us to see now, if they're out there.
You just proposed a great filter, short lived civilizations.
If civilizations lasted for billions of years or their AGIs did the same, we’d have a lot higher chance of them expanding to fill the galaxy and converting the surface of all 8 planets and major asteroids into AGI probes and compute.
That’s not what I’d consider a great filter in the usual usage. We’ve only had ~10k years of recorded history. If we can’t make a meaningful dent somewhere outside the solar system in another 10x that amount of time, I doubt we (or anyone) ever would.
A civilization doesn’t have to survive a billion years (do you know how long that actually is?) to not be ‘short lived’.
I would define short lived as the difference between dying out in a local star cluster or continuing to expand till the heat death of the universe. A long-lived and expansionary civilization will eventually reach us, even if it takes 1k years to hop from star to star. A short lived civilization will never reach us.
None of the situations you’re giving are based entirely off longevity - they use longevity as a factor of another equation.
A billion year old civilization that isn’t expansionary will never reach us either.
Or one that doesn’t want to spend the energy ($$$) if other cheap energy forms we’ve speculated aren’t possible or as inexpensive/portable as would be needed.
They could just as easily be happy being billion year old zen masters, and we’d never know - even if they had warp drives.
They’d still exist though, and if we ever wanted to find them I guess we’d be able to do so. But we’d never know about them proactively.
If you're talking about sending live people, absolutely
If you're talking about sending alien drones, the light mass and lack of having to keep them alive makes energy requirements a lot lower, but they'd probably just be scouting us (if not trying to kill us) so we wouldn't be able to detect them
Considering no human object has yet to leave the solar system (meaningfully), and we’ve nearly wiped ourselves out several times just in recent memory - that’s still in the realm of pure fantasy.
There is no evidence yet that anything we can construct (or a near peer) could make it to another system intact enough to know it had arrived, let alone DO anything regardless of the form any intelligence takes or how it is packaged.
Hell, as a species we still struggle to land probes on mars and Venus. And they don’t have to do anything but send data back.
Getting a toehold on a random solar system, or even arriving and taking pictures and sending them back is so many orders of magnitudes harder to do based on what we know now it’s essentially impossible. Even if we had a ‘fast forward’ button.
Hopefully we’ll find something out that we’re currently missing, but as of yet we have no reason to believe it exists.
Not sure what "meaningfully" is supposed to mean here, but both Voyager probes have gone passed the heliopause and are outside our solar system from that perspective.
"However, if we define our solar system as the Sun and everything that primarily orbits the Sun, Voyager 1 will remain within the confines of the solar system until it emerges from the Oort cloud in another 14,000 to 28,000 years."
"The twin Voyagers are the only man-made objects to have entered interstellar space. However, while they have left the heliosphere, they have not yet left the boundary of the Solar System which is considered to be the outer edge of the Oort Cloud."
It's still in the area of things that primarily orbit the sun, even if it is no longer in the 'atmosphere' or 'shade' of the Sun (aka the heliopause, which is what was being used for PR purposes).
And it's nowhere near anything which isn't orbiting the sun, and nowhere near 'not being in' the area of things orbiting the sun.
So meaningfully, it's still in the solar system, and will be for a very long time. It's just out of the nice part.
Distance wise, Voyager 1 (the furthest) is ~ 22 light hours away, or .02% of a light year - after travelling for 45 years. Proxima Centauri, the closest known star to us is 4.25 light years away.
It would take ~ 73,000 years at Voyagers blistering pace (38,000 mph) to reach it, and there is essentially zero chance we could keep contact with it or it would be functional even 1/10th of that time - it's RTG is already nearly dead, and with a ~ 87 year half life, it would be a miracle if it even lasted another 50-100 years.
Voyager was meant to take pictures of objects in our celestial sphere, if instead we were in a race to, for instance, yeet our stuff faster than the soviets towards Proxima Centauri, everything would have been different.
Care to do the napkin math on what it would take energy wise for anything to make it from here to Proxima Centauri in 50 years or less? (Approx. rounded up time of the entire Cold War)
Hint: even with instant construction, even nutty ideas like Project Orion would be incapable of doing it, let alone anything else. We’re talking > 10% the speed of light for the entire trip.
as soon as the basic building blocks and preconditions are present, life evolves
If not for that one mutation under specific conditions and abundance of easily freeable oxygen, we’d still be goo in mud today. We were for 1.4 billion years until GOE happened.
It doesn’t mean that it cannot happen elsewhere. But there’s no clear linear route from complex acids to democracy. And maybe complex enough life requires a set of very special catastrophes. We can’t write off that we managed to evolve, but we can’t write off the exact procedure either.
All it takes is for a Magnetar to have a hiccup and it could sterilize an entire galaxy.
When we take numbers and probabilities in consideration it's good to also note how many hostile to life events can occur on a cosmic scale. You have supernovas, gamma ray bursts, black holes, neutron stars, magnetars, solar flares and coronal mass ejections, and a host of other shit we don’t even now about that could destroy life at any stage.
> All it takes is for a Magnetar to have a hiccup and it could sterilize an entire galaxy.
There are tens of millions of magnetars in the milky way and we can and do observe their "hiccups" - we've traded a multitude of GRBs back to stars we believe to be magnetars.
They do not have anywhere near enough energy to sterilize an entire galaxy.
Life started on Earth almost as soon as the surface was cool enough to support it. The surface seems to have been stable enough for life by 3.8 billion years ago, and we have some weaker evidence of life at 3.7 and stronger at 3.5. At any rate, based on what we can observe, life began relatively early and so it seems that as soon as the basic building blocks and preconditions are present, life evolves.
We now know that exoplanets are extremely common. Based on observations we can estimate that about 40% of stars have planets, but that’s just what we’ve been able to observe.
There are a lot of stars in our galaxy, 100-400 billion.
And a lot of galaxies in the observable universe, 200 billion to 2 trillion.
And that’s just the observable universe based on our local light cone. For all we know, there could be an infinite number of galaxies.
So we have n=1 where life started right away, and perhaps 10^25 planets in the observable universe. That’s a lot of rolls of the dice!
Hence the need for a proposed great filter that explains why we don’t get a visitor ever other week. Complex multicellular life seems like a possibility, that took quite awhile, a few billion years before the Cambrian explosion. Complex intelligence also took awhile, resulting in only us after 3.5 billion years of life. And yeah, given that we haven’t been around all that long and seem well on our way to destroying ourselves, seems like complex society could be a great filter too. But space is vast, EM signals attenuate quickly and the speed of light is a harsh mistress. They are probably out there, but we’ll probably never meet them.