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The Fermi Paradox and the Aurora Effect: Exo-Civilization Settlement (arxiv.org)
92 points by pkrein 29 days ago | hide | past | web | favorite | 96 comments

I’ve always felt that the reason we haven’t heard from aliens is because they know New Physics and they use it to communicate. Given how much of the universe is made up of dark energy, dark matter and other unknown physics, it seems reasonable that there are significant discoveries to be made which could perhaps be exploited.

I mean let’s say we discover a new, non-electromagnetic way to communicate reliably over vast distances using some kind of low power, high bandwidth signalling system. We’d abandon EM in a geological heartbeat. In cosmic terms we would emit a wafer thin EM shell just 150 light-years thick - and then we would fall silent.

So basically I just think we don’t know enough physics yet. We are perhaps a few discoveries away from being able to listen in on alien conversations.

> we discover a new, non-electromagnetic way to communicate reliably over vast distances

I was going to say that of the 4 currently known fundamental forces, only gravitational and electromagnetic interactions lend themselves to long range communication.

However, there seems to be ongoing research about an actual fifth force that might be related to dark matter: https://en.wikipedia.org/wiki/Fifth_force

Sean Carroll argues that all physics, which could matter to us, has been discovered. See for example his talk "From Particles to People" https://www.youtube.com/watch?v=X5Fel1VKEN8

Before I set off watching a 28 minute video (where, worryingly, comments are disabled), could you give an outline that suggests this isn't crazy?

There are obvious things which feel possible and would revolutionise so many things, like room temperature semiconductors, which we don't have yet. Also things we've better understood since 2012 like detecting gravity waves.

I don't understand how anyone could think we've "discovered all the physics which could matter to us", when we know full well there are massive gaps in our understanding.

He basically argues that there are no forces to be discovered which could have a measurable effect on our day-to-day functioning. For example, so far, we have not discovered any dark matter particles. This does not rule out that we will discover them, but for us humans (consisting of protons, neutrons and electrons) it will not have any effect. Because if it would, we already would have discovered them. The existence of other particles, like neutrino's, which zip through our bodies by the billions every day, also have no effect. We are of course effected by photons in some energy spectrums. In view of this, I think it is very unlickly that we discover any new physics which would allow us to communicate with aliens.

That sounds kind of like a caveman arguing that there will never be a practical temperature higher than their campfire, because anything hotter would burn the meat.

But... math! Which can't be wrong, because obviously we already know all the math there is to know. QED.

Carroll says strange things sometimes. Epistemology doesn't appear to be his strong suit.

It's all win-win, for him. If we fail to discover anything, he's right. If we find something, "nobody could have ever predicted that!".

Dark matter interacts gravitationally with normal matter. If it exists and if it can be directed and controlled, it could be used to make stuff lighter or heavier. Big ifs. I'm not holding my breath.

Gravity is the weakest known force, orders of magnitued weaker than the other forces. Dark matter has never been measured on earth due to gravitational interactions. This is probably related to the fact that the particle responsable for the dark matter are spread out rather uniformly and not 'clumbed' together as ordinary matter. That normal matter clums together is caused by all the other forces, not by gravitation.

I doubt if dark matter can be used to make things lighter, possibly dark energy could be used for that (which is even more elusive than dark matter). It could only make thing heavier if there is a method to confine it. But because it seems that dark matter does not interact with normal matter (at least not often), I do not see how it can be confined to ordinary matter. Take notices that neutrinos also cannot used for this purpose, because the simply zap through us by the millions everyday, without us noticing anything. We have not found methods that to confine neutrinos yet. They seem to fly at speeds close to the light speed.

And that’s _exactly_ what they want you to think.

You mean room-temperature superconductors?

Yes, sorry, now too late to edit!

> Sean Carroll argues that all physics, which could matter to us, has been discovered.

This seems to be a case of "not knowing what we don't know", as Donald Rumsfeld would say.

His claim implies that we know what we don't know. The reality is that we most likely don't...

There's also all the stuff we think we know that isn't so.

That brought down Rumsfeld, not that he ever suffered for it. Millions of other people did, instead.

Our physics model is fundamentally broken and this list is the result of the internal discrepancies: https://en.wikipedia.org/wiki/List_of_unsolved_problems_in_p...

A adopted a very fringe, unified physics model. From this perspective we are looking at none of the possible signals that would make sense for long distance communication.

If they are using orbital angular momentum (OAM) coding in normal EM radiation we would not discover it yet because we don't listen for it.

Would we hear it even with simple on/off keyed modulation even? The distances are so vast that even a directional link with anything but our nearest neighborhood seems unfeasible. We should have met a von neuman probe though.

Or possibly advanced civilizations had the same realization that many scifi authors have had about such probes. Something goes wrong and they cause widespread destruction. Or they work fine but still end up causing widespread destruction simply by replicating.

Once it happens once, it's too late for everybody. Naturally, they evolve, and soon invent interstellar travel. So much for that galaxy.

(Cue Fed Saberhagen, *Berserkers".)

Even without using EM to communicate, and assuming we don't need intercontinental radars [1] anymore, a lot of our regular activities do emit EM. What we find weird is that we only see natural EM sources. If a civilization were to reach 2 or even 1 on the Kardashev scale, we ought to see some non-natural emissions.

Of course there are a lot of assumptions there, and lots of hypothesis to explain it a way, but that's the crux of the Fermi paradox: there is something to explain away

[1] https://en.wikipedia.org/wiki/Duga_radar

Maybe all the dark matter is just very efficient alien Dyson swarms and other megastructures doing their own thing ? :-)

It's not dark matter because it blocks light. It's dark matter because its only observable interaction is by gravity.

So aside from being perfect in terms of energy capture so that the underlying stars emit absolutely no EM at any frequency, the proposed Dyson spheres would also need to be completely transparent to any background EM such as visible light picked up by sensors like Hubble.

Good point - if we are able to detect gas clouds, we would be likely able to detect such swarms as well simply by all the blocked light.

The megastructures would have to radiate waste heat if they're doing anything interesting.

Well, there are a lot of brown dwarfs around...

Their spectra look like giant burning balls of gas, though. I guess it's possible to hide one inside a star in theory?

Oh and we are a few discoveries away from receiving huge amount of alien spams...

I think the reasoning of the Fermi paradox is that "at some point in time" a distant civilization must have been using EM wave to communicate. Now, considering the number of planets, it must exists a civilization in the universe that sits at a distance that let us sense this EM dominated past. And this even if in their own local time, they use fancy new tech. Even if they were not trying to communicate with us per se. Their own radiation should be measured.

We would only be able to detect our own everyday transmissions at a distance of a few dozen light years. So the galaxy might be full of EM emitting civilizations, and we wouldn't have detected them.

The power and bandwidth limitations aren’t exactly the problem with interstellar communication, it’s latency.

Perhaps, but if we discovered a mechanism that was orders of magnitude more efficient or higher bandwidth then any rational species would just naturally use it to replace EM communication.

I mean just imagine some alien engineer trying to use EM for a new mobile phone system when everyone else on planet Glerbulon is using DE. The other alien engineers on the team would be like “wtf are you using photons for? Are you nuts? Just use an off-the-shelf unobtainium link. Here I’ve got one in my pocket.”

Well, people do maintain obsolete forms of communication because they can & or have specific circumstances requiring them. Thats why I am skeptical others would completely abandon a communication technology - there would still be the Globulon Technical museum hosting anual morse code transmision relay with steam powered spark gap transmitters between specially selected nodes in the local dyson swarm.

I thought that we already conclusively ruled out alien life by observing that there aren't huge, galaxy-sized smoke signals, generated from their massive candles to heat their galaxy-sized cottages? I mean, if aliens were out there, then surely they'd have sent us a notarized letter in the mail by now, as would any civilized culture. I bet they use really fancy quill pens, too!

...seriously though, I can't fathom why people are so caught up on the Star-Trek-like predictions where hyper-advanced aliens are still essentially humanoid. It's at least as absurd as [protocells](https://en.wikipedia.org/wiki/Protocell) assuming that all advanced life would be essentially protocelluar.

> ...seriously though, I can't fathom why people are so caught up on the Star-Trek-like predictions where hyper-advanced aliens are still essentially humanoid.

I can't fathom why people fail to realise that being bombarded for decades with pictures of hyper-advanced aliens still essentially humanoid somehow shapes the general public imagination regarding "aliens". /s

Some people claim to have ruled out type III civilizations [1], which I think is what you're referring to.

I thought also that some researchers claimed a star had signals consistent with a type II [2].

[1] https://en.wikipedia.org/wiki/Kardashev_scale

[2] https://www.space.com/30832-kepler-telescope-alien-megastruc...

According to recent news, the alien megastructure proposed in [2] may just be a shredded moon [3].

[3] http://www.astronomy.com/news/2019/09/evaporating-exomoon-co...

That's no moon...

No one seriously thought Tabby’s Star was aliens it was just weird.

Indeed. I suspect that the things we know almost nothing about (e.g. the nature of consciousness), and perhaps things we don't even suspect "exist", end up being far more important than physical expansion for intelligent beings over the long term.

Communication itself might be of little interest to such beings.

Fill in time:

> unless the effective probe launch time is greater than 270 million years, the galaxy is old enough for every system to have been settled from an initial single civilization


> When diffusive stellar motions are accounted for,they contribute to the Galaxy becoming fully settled in a time less than, or at very least comparable to its present age, even for slow or infrequent interstellar probes.

>While settlement wave crossing and fill-in times are short, consideration of finite civilization lifetimes in a steady state model allows for conditions in which the settled fraction X is less than 1. Thus the galaxy may be in a steady state in which not every settleable system is currently settled.

Conclusion basically says that if there are expanding civilizations in our galaxy, they spread very slowly, have filled the galaxy sparsely or have limited lifetimes. Assuming they are not avoiding contact.

I wonder whether due to the anthropic principle [1], assuming a (practically) infinite number of different universes exist, the likelhood of being the only sentient species in the whole universe is significant higher than the alternative?

I.e. the number of universes that are incapable of harboring life is many order of magnitudes larger than the number of universes that may accidentally harbor life (i.e. us) which is still many order of magnitudes larger than the number of universes that actually have a more realistic chance of harbouring life.

So us being a singular "accident" may just be a likely consequence of being here and asking questions.

[1] https://en.wikipedia.org/wiki/Anthropic_principle

[update] see also https://en.wikipedia.org/wiki/Fine-tuned_universe

Frankly, I don't understand how this autocentric posture can resist any serious enquiry. To my mind it seems alarmingly full of causal vicious circles, and lake any clear way to be invalidated experimentally.

In particular, taking the "number of possible universes" seems problematic to me, as we can only obverse the one in which we interact. And even taking that for granted, what is so astonishing that the actual one is the one that is compatible with our existence? I mean, if any possible universe could have happened, then the one we live in had actually an equal chance to exist with any other. Since this perspective assume that at least one universe must actualize, then pure randomness by drawing of lots is enough to explain the result. No need for a teleological purpose. Apply Law of Parsimony and you are done: Anthropic principle add needless ad hoc assumptions to reach its predetermined conclusions, especially in its strong version.

Also, the anthropic principle stands on many assumptions, including that the simulated models of tweaked universes do reflect other conceivable world in a fiddle manner would they be actual things.

It also neglects that among an infinity of variation, you surely should be able to come with totally different forms of "life". That is, for example it takes life as a carbon based structure, so you need universes with constant tweaked for enabling star to produce and disseminate carbon before they collapse. But on the counterpart, it can't pretend if you take it seriously, that other universes could not have produced very different form of "life". To my mind, we have no idea if our current actual universe is not also populated with non-carbon based life form.

Also this kind of theory emphases greatly on what the world did allowed with these tweaking. But, among an infinity of other combinations, who would seriously consider that nothing better could have been produced than a mankind tormented in wars and questions which are at best extremely hard to solve for its limited cognitive abilities, if not definitely out of its reach?

> And even taking that for granted, what is so astonishing that the actual one is the one that is compatible with our existence?

You've just verbalized the weak anthropic principle. The entire point of it is that this is not astonishing (at least not without additional knowledge), but a necessity for the universe to be one compatible with our existence, irrespective of how rare the conditions are, given we're here to observe it. The prior probability of "us" arising might have been incredibly small (or not, we can't tell), but the posterior probability is 1 - we're here.

> I mean, if any possible universe could have happened, then the one we live in had actually an equal chance to exist with any other.

Yes, but that does not mean that the odds of any given configuration need to have an equal chance.

What GP is saying is if there is an infinite number of universes and if life is extremely unlikely, then the odds of any given universe with life only having one life form might be very high, even if there being even one life form might be extremely unlikely.

We know that the posterior probability of there being one suitable for us is 1, since we're here. But we don't know whether or not life is a rare event or not.

The antrophic principle explains why we "won the lottery" so to speak in being in one of the universes with at least one life form, but life being incredibly scarce might be one explanation for why we might be in a universe with only one life form.

Though of course we can not infer that life is rare across any given set of universes if we determine we're alone in this one, because we don't know how other universes would be configured.

> Though of course we can not infer that life is rare across any given set of universes if we determine we're alone in this one, because we don't know how other universes would be configured.

Not necessarily, you could assume a universal prior distribution over possible universes and count how many of those that have life have more than one.


This might be a sort of a "last tool of science", a way to discover the laws of our universe without observing them.

That presupposes knowledge of more than one universe to even start to enumerate the possible parameters deciding the set, and without a sufficiently complete sequence we would be unable to determine all of the possible parameters, or how they can change.

E.g. even given that we know, say, gravity exists, we do not know if or how its strength varies in a set of infinite universes. Based on a sample of one universe, it could be constant or vary in an infinite number of ways.

> That presupposes knowledge of more than one universe to even start to enumerate the possible parameters deciding the set

I think you entirely missed my point (because you didn't read the link!). You could use the universal prior, that is, you could enumerate all possible computer programs (e.g. Turing machine programs) starting from the shortest and find the shortest one that generates our universe. Now, 2^L where L is the length of such program is the prior probability of our universe.

So to determine e.g. the average value of the gravitational constant across all universes, you would measure the gravitational constant in all universes and calculate a weighted sum where you weight the value of the constant by the prior probability of the corresponding universe. The same for the occurrence of life, etc.

Someone always have to be first, no matter how unlikely.

What if we are the probe and we haven't figured it out yet ? Or what if we somehow didn't get the memo telling everyone to stay hidden because of danger, because we are late to the party ? What if there's a natural/alien occurring culling of some kind and we somehow slipped through and now we are alone ?


> What if there's a natural/alien occurring culling of some kind and we somehow slipped through and now we are alone?

Guess that one of the benefits of living in suburbs[1].

1: https://imagine.gsfc.nasa.gov/science/objects/milkyway1.html

Back to the topic. Let's assume there is a space nomad nazi species out there right now, will that species survive long enough to conquer the entire galaxy?

See also the Zoo Hypothesis [1] :)

[1] https://en.wikipedia.org/wiki/Zoo_hypothesis

> What if there's a natural/alien occurring culling of some kind and we somehow slipped through and now we are alone ?

You are referring to what is known as "The Great Filter". The more troubling question is, What if we are alone because all the more advanced civilizations have been culled?

climate change certainly looks to be a very likely contender for a filter

Hiding does not look like a good countermeassure for a culling, as rapid expansion as possible seems much better & possibly good solution overall as long as the resulting human civilisation does not end up to be the one that does the culling.

> What if there's a natural/alien occurring culling of some kind and we somehow slipped through and now we are alone ?

Mass Effect Reapers, anyone?

The Fermi Paradox always strikes me as hubris.

As Douglas Adams wrote, “Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.”

We've been looking for alien life for, what, 50 years? And we've made various assumptions about what we should be looking for. Oh, and we've been doing so on a very limited budget, with the technology available to us, in a tiny part of the sky .

Yes, but Fermi's Paradox is based on calculating that it would only take an alien civilization a few million years at the most to colonize our galaxy, which has been around for billions of years. Add to that the likelihood that a slightly more advanced civilization than ours would be able to make self-replicating probes. And it only takes one civilization to do it. So where are they?

Think about it this way. If human civilization persists and continues to advance, what's stopping us from spreading out to nearby stars within the next thousand years? And that's nothing on the cosmic timescale.

Uhhh that assumes alien life has the same intent as current us. Intelligent alien life may just as well have explored a lot, figured "meh..." and build a few Dyson spheres to spend the rest of their existence in a perfectly pleasing simulation.

People already get sucked up by Warcraft and not even 100 years after computers made their breakthrough we fiddle around with haptic dual-4K VR.

It only takes one expansionist species to populate the whole galaxy. Thus, if intelligent life were common, we should detect them. You'd have to argue that no advanced species would ever do this, which is possible, but seems less plausible.

Some people do. Other people climb mountains or work on robotic rovers. There is no evidence that an entire civilization would choose a simulated existence, let alone every civilization. The evidence so far is that there would be ever increasing numbers of things to do, only some of which is simulated. There are plenty of young kids today training to become athletes instead of EA sports champions. Some people prefer to be outside instead in front of a screen.

Maybe they just don't want to.

The Fermi Paradox requires that aliens think and behave like late-20th century humans. But early-21st century humans already exhibit slightly different motivations and behaviours. So it's really hard to imagine that early-201st century humans would be at all familiar, let alone late-736th tier Grabulons.

We have to remember the Fermi Paradox is a Fermi estimation and only that

Another relevant paper: https://arxiv.org/abs/1612.03238

One universal law that these discussions usually fail to consider is opportunity cost. There are a near infinite number of constraints and competing priorities at every stage of civilisional development, and that's why we don't see any Dyson spheres or similar grandiose outputs.

They talk about planets and generation ships. Isn't it obvious that a multimillion-year spacefaring civilization would be mostly autonomous machines that have no need for habitable planets and can sleep for aeons?

They also talk about civilization lifetime and interstellar resettlement. But interstellar resettlement doesn't make sense to me; simpler for machine civilizations to hedge their bets by putting resettlement sleeper ships in highly eccentric orbits with some desired period. Maybe there are civilization-killing effects that disable those resettlers, but they would apply to interstellar resettlement too.

Well, no. That assumption is as good as any.

When one is talking aliens, some number of things might seem obvious but the reality is we're dealing from intense ignorance and the things that seem obvious to us probably shouldn't be taken at face value.

- Where are my socks?

- Did you look in the drawer?

- Why, I've searched everywhere! I can't find them!

- Well, then - you must have no socks. Your socks never existed.

>> Unless the individuals in the species driving the settlement have very long lifetimes (> 100 y) it is dicult to see how a galactic scale culture can arise (i.e. commerce etc. Krugman (2010)).

Why is a mere "more than 100 years" considered a "very long lifetime"? Is there any reason to assume that the majority of intelligent species, capable of technological civilisation, that may inhabit the galaxy, will not have a lifetime measured in thousands, or even hundreds of thousands of years?

The lifetime of the averarge star is a few billion years. We can assume that mos species will not have naturally evolved lifetimes lasting bilions of years, but a) species may evolve lifetimes lasting millions of years, or, b) a technologically advanced species may extend its lifetime indefinitely.

A species whose individuals lived for a few hundred thousand years would have plenty of time to visit the Earth by travelling in sub-relativistic speeds, from a significant portion of the galaxy. A species whose individuals lived for a few million years would have plenty of time to visit the Earth and wait for our own civilisation to die out. A species whose individuals lived for billions of years could pay us an intergalactic visit and still have time for tea.

Extremey long lifetimes are not impossible and they are not even particularly improbably. Here we are wondering whether there are other technological civilisations among the stars. Why should we assume that they are anything like we are?

Long _natural_ lifetimes are a fundamentally at odds with evolutionary progress. The lifespan of a species is directly related to how quickly that species can iterate generationally. Fewer generations mean less natural selection, which means a less evolved life form. Also, if the organisms don't die off fast enough, they end up competing with their own offspring for resources.

That said, I would imagine that once most life forms get to the point that they are able to extend their lifespans with technology, they probably always do. They just don't evolve anymore...

>> Long _natural_ lifetimes are a fundamentally at odds with evolutionary progress.

I'm not sure I see that. For example, humans live many orders of magnitude longer than bacteria and yet we have both evolved just fine.

Anyway, I don't see why a species is pressed to evolve quickly in absolute terms. If all species on a given world evolve lifetimes many orders of magnitude longer than on the Earth, then there's no reason for any particular species to hurry.

I think how long or short a species' lifetime is can only be understood in the context of the environment in which it evolves. In any case, we don't know anything about life on other worlds, and my point is that we can't really make such big assumptions based on the single example we're aware of.

All of these calculations are predicated on us existing in a physical universe and not a simulation (simulverse?) it could be we are the only life in this specific simulated existence, even though the mathematical rules of the place would make it appear there should be others.

The assumption is that there is a set of laws of physics which are consistent with our observations, that are the same across space, that do not vary with time, and that all phenomena in the universe are ultimately caused by these laws.

Whether these are the laws of nature, the laws of a simulation, or the laws of some Gods playing around, it makes no difference.

Unfortunately, without these assumptions it is impossible to have any kind of scientific knowledge - if we can't assume that the sun rising for the last hundred thousand years means it will rise again tomorrow, or if we don't assume that the universe extends in the past as well as the future, there is nothing meaningful to study.

Note: the laws of physics I am talking about above could be very different from what we know today - just as QM and Relativity are to Newtonian physics - but that would not change the assumptions I'm talking about.

Even if my half baked thought experiment was true I agree in that it shouldn't make any difference in our goals or ideals. I was merely positing a possibility for why if statistically our spacetime could be packed with neighbors we haven't noticed anybody. Another reason could be there are visitors they just exist on dimensions perpendicular to the 3d space we perceive, maybe even our concept of spatial dimensions is a oversimplification or a small slice of a full spectrum.

there's so little evidence of it being a simulation just for us. Even if all of this is just a simulation, I'd bet that we're just forgetten children of the simulgods and the true focus is something more obvious.

See also Omphalos by Ted Chiang.

The idea of simulation is unfalsifiable and therefore shouldn’t be taken into account anywhere unless somebody proposes a proper experiment.

It's ridiculous that people who ought to know better are so fixated on the simulation hypothesis. It's a cool idea, but it holds no more water than "What if God created the universe just for us?" There's no reason to even bring it up in any halfway-serious conversation.

Here's the thing— we're assuming advanced civilizations want to expand in the first place. Why?

Life in OUR planet was shaped by competitive evolution and we have reason to believe life in other planets has been as well— it's how natural selection works after all, and at least in our planet it's proven to be an effective way to go from primordial soup to complex, intelligent life that can out-smart its competition hoard all the resources.

But how do we know that this is not a behaviour that civilizations, well, outgrow once they've hoarded all resources?

Unbound growth is an effective strategy in a competitive scenario like the evolutive stage of species, in large part because it's also one of simple incentives. When a species becomes dominant though, it is very dumb, because the species will just end up reaching the limit of available resources and burning through them, thus killing themselves, like bacteria in a petri-dish multiplying until all the food has been eaten and they all starve to death.

The argument goes, that's when a civilization would expand. Those bacteria would try to find another petri-dish. The species finds more resources so it can keep growing, but, can this incentive of "we have to keep growing" stand the test of time?

Simply from a practical standpoint, Alpha Centauri is 104691 times further than Mars. The technological gap between sending a colony to a planet in your system, versus one in another star is gargantuan.

First of all let's consider that planets are all ballpark similar in size and resources, therefore value. Is the investment even worth it? Is traveling 104691 times more for the same resources a stronger incentive that simply learning to find equilibrium with the resources easily available in your system?

Even if for the sake of argument the civilization decides it is, could they even achieve it before running out their available resources? It'd be a race against the clock.

There is reason believe the civilization would be forced to find stability and equilibrium within their existing system before developing the technology to be able to feasibly colonize a neighboring star system for the purpose of extracting resources. And if not to extract resources, why put the effort to continue expanding?

Fear? Of what? A nonexistent galactic empire?

Pride? If their history is anything similar to ours (which, since we're talking about similar motivations, is perhaps a feasible assumption to make), they would've learned colonizing far and wide does anything but promote a stable, unified, timeless nation.

Therefore, my guess is one of two things might happen:

1. They achieve stability because it's the only way to survive, and the motivation to expand to another system dies off. Probe and explore, perhaps. But if in a stable system, there's no incentive to introduce instability once again by adding another habitat. On the contrary, the prospect of instability is incentive to not.

2. They don't achieve stability in their system, and so collapse.

In a nutshell, I don't think there's incentive for unbound expansion once stability has been achieved, and I don't think unbound expansion can be achieved before stability. Of course, this is all just my opinion, but it's worth thinking perhaps the entire premise of extrapolating primitive evolutive behaviors to an advanced civilization is out of frame.

Consider the future world of Brave New World by Huxley, or even the Time Machine by H. G. Wells. Neither seem to have galactic empires; instead opting for (very distinct) forms of stability (and decay).

IMHO, human nature means that people will settle other planets or simply space if they have the technological capabilities.

Just look at SpaceX and co. and all the people interested in that. If the technology exists some people will go. This does not have to be a large number compared to the overall population. That's enough to kick-start the process in a chaotic and unplanned manner...

> Using our steady-state model we constrain the probabilities for an Earth visit by a settling civilization before a given time horizon. These results break the link between Hart's famous "Fact A" (no interstellar visitors on Earth now) and the conclusion that humans must, therefore, be the only technological civilization in the Galaxy. Explicitly, our solutions admit situations where our current circumstances are consistent with an otherwise settled, steady-state galaxy.

I mean if there are no other intelligent civilizations (yet) in the universe — which seems highly unlikely based on the numbers, then we have an obligation to seed the universe with ourselves and propagate other species through genetic enhancement and grooming.

We might be the progenitors of other intelligent species who might look back at us fondly and remember our shepherding.

Again highly unlikely, but a good point that we should not kill ourselves beforehand.

I find the assumption that a space-faring civilization would need ideal planets to settle on somewhat questionable.

I mean a lot of this all turns on what happens to us in the next 200 years. Which... That's slightly less time then federated America has existed.

Are we at the peak of what people achieve on temperate world's? Maybe technology doesn't meaningfully advance much beyond what we already know - you get a computing boom, Moore's law ends and then a couple decades of discovering no substantial alternatives to the silicon chip.

I've been homing in on this conclusion, or a neighbour of it for a while. Take the periodic table. That's it people, we already know all the stable elements. Nobody's going to discover unobtanium. Aside from some high energy physics and some details of quantum mechanics, we probably know most of the useful practical physic there is. The open frontiers are biology and chemistry, but those are all about combinging things we mostly already know about in new and interesting ways. Eventually we'll figure all of those out as well. Then what?

I'm not saying it's impossible we'll discover some amazing, paradigm shaking new physics that will upend everything. Maybe. But we can't bank on it and it's entirely possible no such civilisation shaping new physics exists.

If that's the case, then the future will all be about engineering based on parameters we already pretty much know. I often see the Daedalus project held up as 'proof' interstellar travel and even interstellar colonization are possible. That was a designs for a very modest space probe payload, but even so the resources required to build them would pauper our whole civilization. Even if, or when, we master the resources of our whole solar system, those probes would be a significant cost and it seems likely they're way too puny to deliver a robust self-replicating bootstrap infrastructure. There's nothing inevitable or necessary about interstellar colonisation. The universe doesn't owe us Star Trek.

Why would technological advance be entirely limited by the end of Moore's law? There's still advances to be made in biology, nanotech, materials science, and making full use of the computing we already have. And there are plenty of proposals for future engineering on and off world, if we have the motivation for those projects.

Agree. A space-faring civilization would have space ships perfectly adapted to their need for sustainability, and they can move to avoid any danger. We can also assume that well advanced civilization have mastered transmutation and that they might need access to planets only to get matter they can transform in whatever they need.

The motivation for traveling in space can only be to decrease probability of extinction and to increase their knowledge. The later may include studying other life forms and what they have learned. As a consequence, I assume they would specifically target solar systems with a high probability to host well advanced life forms. I don't give much credit to a random expansion in the milky way.

Regarding the Fermi paradox, the only paradox is excluding the UFO phenomenon as possible extraterrestrial manifestation.

You're not wrong, but, you have to admit that even if one is a highly advanced civilization, some planets are a lot easier to live and thrive on than others. I mean sure, we can TECHNICALLY settle Mars and live on there, but it will cost a lot of resources, the inhabitants will be constrained to enclosed spaces, etc. Even Antarctica is easier to live in despite its harsh conditions because you can step outside and not die from asphyxiation. (I'm mentioning that because the camps / settlements on Antarctica are probably the closest thing we have right now to a remote / off-world settlement).

If you're a spacefaring civilization, you probably don't live on planets to begin with. Terraforming is hugely energy inefficient, planets that fit the conditions to be comfortable for you biologically are probably rare, etc. Even ideal planets are incredibly inefficient when it comes to use of material.

You can support many times more people in things like O'Neil cylinders, bishop rings, etc. than you can on planets, using less resources. If you're the kind of civilization that can settle a galaxy, you're also advanced enough that you're going to care about that sort of thing, because you're already working on huge timelines, and you probably want to stretch your existence out for as long as possible. Being efficient with resources is going to be important there - the universe isn't making any more of them.

Given the scale and vastness of the universe, if there was/is other life we have a good change of finding some remnants of them, most likely spaceships?

Actually I'd argue that the scale / vastness of the universe is the main reason we HAVEN'T seen anything yet. I mean we can barely make out the planets in our own solar system, let alone asteroids - even assuming there's derelict spaceships out there several kilometers across, we can't really tell the difference between them and asteroids. Take Oumuamua, the asteroid (?) that passed through our solar system - even when all of our equipment was trained on it, we still could only make vague guesswork about its nature.

But anyway, that already assumes there's tons of spaceships out there, enough to actually show up on our equipment. Even if there's a galaxy-spanning civilization out there, the chances of running into them are low. I mean for a close analog, look at the game Elite: Dangerous, that can give you an appreciation of the scale of the universe - and in that game it's basically cheating because you can jump from one solar system to the next in seconds. And even at those speeds it takes forever to get anywhere.

Given the vastness of the Universe, I would say there is low chance finding some spaceships wandering out there even if they are present.

This is how I feel as well. Some will say it would be so obvious by now we would have seen some sort of indication but then I think of all the planes that crashed without a trace and we have not been able to find even with an idea of where they were flying. The universe is huge. We have no idea how they will try and communicate maybe it's not something as obvious as radio transmission but maybe they have figured out quantum entanglement communication or something beyond our means but super obvious and useful to species who have figured it out. I grew up watching all the Star Trek shows. I have to believe the universe is filled with other civilisations waitin to talk with us.

Interesting premise. Pity I can't read the article.

Maybe the link was updated? It now points to arxiv, from where you can download the paper (or does arxiv implement some geoblocking?).

yep, was just abstract earlier

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