[lubujackson]

The third option is that everything is simply too far apart and any detectible transmissions are either too faint or buried behind all those massive fusion furnaces.

For instance, if our best Earth tech was pointed at Earth from a distant star, how far away would we be able to detect our own traces of life? We know that life is "a little rare" at least, but there is a huge range of rarity where life is everywhere across the vast, vast universe but sparse across the dinky number of light years we could see each other. Maybe aliens have tech that is 10,000x better than ours but the scale of distance is just too big of an obstacle.

There is an assumption that alien races must have figured out things well beyond our understanding and be doing things like harnessing stars in some dramatic way, but maybe that just isn't a reasonable thing for life forms to do? Or if they do have that ability, they may operate on a much longer timeline, simply nudging things along in a way that escapes notice.

[thrashh]

Another possibility is that energy is really hard to come by. Humans absolutely struggle to amass much energy. Think of the insane amount of energy required to lift things into space, and how many millions more amounts of energy we’ll need just to build something in space. Heck if you go drive off grid on Earth, you can barely bring any energy with you and you won’t really be able to generate an appreciative amount of energy to do anything major. Climate change is ultimately caused by our energy needs.

One day we might be able to harness the output of a whole star but the amount of energy we’d need to get to that point seems out of reach right now.

I wouldn’t be surprised if the lack of energy is a universal issue across the stars.

A huge game changer would be if we discovered some physical phenomenon to allow us to safely extract energy easier from mass. It would do to our progress like what the telephones and the Internet did to communication.

[pfdietz]

It actually doesn't require THAT much energy to get into orbit. The energy of a kilogram of mass in low earth orbit is about 9 kWh, although rockets will use a bit more than that (not enormously more, though). Fuel cost remains a minor fraction of launch cost, even with Falcon 9.

In comparison, the best aluminum plants in the world use 13 kWh to make a kilogram of aluminum. Yet no one says aluminum is out of reach because it takes too much energy to make it.

The per capita primary energy consumption in the US is about 10 kw; your share of this energy over your lifespan would be enough to accelerate 1 kg to 2% of the speed of light.

[shagie]

An interesting problem is "how much energy would it take to launch from a super-earth?"

https://www.space.com/40375-super-earth-exoplanets-hard-alie...

> To launch the equivalent of an Apollo moon mission, a rocket on a super-Earth would need to have a mass of about 440,000 tons (400,000 metric tons), due to fuel requirements, the study said. That's on the order of the mass of the Great Pyramid of Giza in Egypt.

We may be in a Goldilocks zone not only for distance from the primary star (lots of systems are binary which gets problematic), but also in terms of size of the planet (not to light to loose atmosphere, not to heavy to trap everything), and we've got a nice big moon to keep things sloshing around and stabilize the tilt.

I am of the opinion that life is rare because we live in a surprisingly boring solar system with a terrestrial planet in the right range, not to large and not too small, and with a massive moon.

[pfdietz]

A super earth would require exponentially more energy with chemical rockets, due to that darn rocket equation. Fortunately, Earth isn't that bad.

[foobarian]

> A super earth would require exponentially more

Polynomially, right?

[pfdietz]

No, exponentially. I used that word precisely, because the mass ratio in the rocket equation is exponential in (delta V)/(exhaust velocity).

[saiya-jin]

I agree with rare earth theory too, but even with it, the observable universe is so huge that even those tiny fractions of various probabilities should at the end materialize on a massive scale.

And its not like with higher G or some other singular aspect moving into more challenging territory, everything becomes suddenly impossible. Think about how many obstacles mankind could conquer in a relatively short amount of time, say a million years.

[GoblinSlayer]

We see only super earthes because they are easier to see. Small planets can be more numerous.

[shagie]

They can... but a challenge with a small planet in the habitable zone of a star is that it also has a weaker magnetic field and is more prone to having its atmosphere stripped away.

Consider that the moon had an atmosphere - https://en.wikipedia.org/wiki/Atmosphere_of_the_Moon#Ancient...

> In October 2017, NASA scientists at the Marshall Space Flight Center and the Lunar and Planetary Institute in Houston announced their finding, based on studies of Moon magma samples retrieved by the Apollo missions, that the Moon had once possessed a relatively thick atmosphere for a period of 70 million years between 3 and 4 billion years ago. This atmosphere, sourced from gases ejected from lunar volcanic eruptions, was twice the thickness of that of present-day Mars.

http://time.com/4974580/nasa-moon-had-atmosphere-volcanoes/

Which brings us to

https://worldbuilding.stackexchange.com/questions/13583/what...

While smaller planets are certainly more numerous - many of them likely lack the mass and active magnetic field to retain an atmosphere in the habitable zone.

[hoseja]

Aluminium has been out of reach for much of history.

[pfdietz]

But not because it needed too much energy.

[zabzonk]

Then for what other reasons?

[pfdietz]

Well, it needs electricity. For most of history, we didn't have electricity. And, we didn't have cheap electricity until electromagnetism was understood and electric power could be made from rotating machinery. After that, there needed to be found a chemistry that would allow aluminum to be made directly, rather than from sodium or potassium reducing aluminum compounds.

The actual physical amount of energy needed to make aluminum would have been available since forever; it's the form of that energy that wasn't there.

[vbezhenar]

That's why it makes no sense to build something big by lifting heavy things from the Earth. Just mine asteroids, extract iron, melt it into chunks, then transport those chunks very slowly but with huge throughput from asteroids to the necessary building site in space and use them to build space ship. You can melt iron just by focusing enough sun light and you can build ships closer to sun for cheaper sun light. It's imaginable with current technologies, we just need industry to move into space.

Imagine that with all your knowledge you suddenly moved to the ancient times. You know how to make CPU but you can't even make a shovel because iron production is not there. You need to spend enormous amount of time and efforts to build all the industries required to build a factory to produce simple resistor. That's where we at when it comes to space right now. We need to spend lots of time and effort to build space industry. But once it's there, space ships will not be a big deal.

[pdfernhout]

Thinking through those interrelations for manufacturing processes was my hope for this: https://www.kurtz-fernhout.com/oscomak/ https://www.oscomak.net/ https://openvirgle.net/

And earlier, this: https://pdfernhout.net/princeton-graduate-school-plans.html https://pdfernhout.net/sunrise-sustainable-technology-ventur...

Still think it is all a good idea. Glad the "maker" movement is making some progress in that area. As I noted on the LibrePlanet mailing list a few weeks ago: https://lists.gnu.org/archive/html/libreplanet-discuss/2022-...

Someday after I "retire" I hope to have more time to work on all that.

Tangentially, on a super earth with more gravity, they would have an easier time going into orbit energetically using mass drivers or laser launch systems instead of rockets. And in any case, how big a seed do you really need to make a space infrastructure if you are patient? And the ground can communicate to space with radio or lasers (like for telepresence) which are not affected much by planetary gravity even for super earths.

[Animats]

Everything is simply too far apart.

That's very likely. There are only two known systems with possibly habitable planets within 40 light years.

[brabel]

Not to mention that when we talk about other galaxies, then distances are on a whole different scale again. While travelling to other stars takes "only" a few thousand years, to go to another galaxy you might need several hundred million years with any technology we can currently even imagine.

Even if intelligent life is rare enough that it only appears once on a single galaxy every several billion years, we're still talking probably millions of possible civilizations, though each almost completely out-of-reach of each other.

[pfdietz]

We can achieve relativistic speeds with technologies we can imagine (specifically, laser light sails). This would require just millions of years to get to M-31, for example (which is 2 million light years away).

[ncmncm]

Arriving not in atomically finely tattered shreds is left as an exercise for the descendants. And, stopping.

[pfdietz]

They'd presumably do this starting from the fringes of the galaxy where the gas is thinner, and with a sufficiently big vehicle (perhaps assembled out of parts that are accelerated separately) that it could withstand some erosion. The sail itself would be discarded after acceleration.

Drag on the interstellar medium using magnetic fields could be used for deceleration.

[ncmncm]

Standing up to millions of years of relativistic erosion seems difficult.

Accelerating ISM ions near your destination to slow down would necessarily consume as much energy as it took to get up to speed, and without assist from folks at home (who are anyway a different species by now). You will have plowed through untold tons of relativistic IGM on the way that I suppose you could collect on the way and then fuse to power your EM brakes.

Retaining all that IGM seems mainly a matter of dissipating heat, provided you have discovered how to keep it from instead blasting bits off of your bumper. It would, also, all need to be accelerated to match your speed. A substantial fraction of it would be neutral, so purely ballistic.

Maybe you have a million (or ten; hundred?) km of very diffuse spray foam for your front bumper, that you will wring out for protons when time comes to stop?

Presumably you want to stop where whatever you made the trip for is, assuming it is still there ("lo!") those millions of years later. It must be something you couldn't find anywhere among the half-trillion stars back home. (You did look, right?) Or maybe you just wanted not to be there -- neighborhood gone downhill, galaxy too small for both you and the ex?

Given that you can weather a million years of relativistic battering, and actually collect industrial feedstocks from it, maybe you don't really need a galaxy or anything in one anymore, and can just keep going. It ought to be safer out there, without an ever-present threat of magnetars around always threatening to belch slightly and sterilize everything for much farther than can be seen without a telescope.

[Tepix]

We have yet to discover an earth-like planet with an earth-like orbital period. They are unlikely to be rare, they are merely hard to detect with our current technology.

[arunix]

What are those two systems?

[makerofspoons]

Not OP but I imagine they mean TRAPPIST-1 and perhaps Proxima Centauri.

[rewgs]

Yeah, I think that the Fermi Paradox can be pretty easily explained: the universe is fucking huge.

Those who talk about the feasibility of alien life visiting us very simply haven't thought it through, or are ignorant of the reality of space.

It's just too staggeringly big.

[omnicognate]

Yep, the Fermi "paradox" is no paradox at all. We have basically no evidence that life (including intelligent life) isn't common yet.

There are so many ways to think of how big our galaxy is but two are particularly relevant to how difficult it would be to detect other life, or be detected:

1. The first radio signals the human race produced have so far travelled something like a thousandth of the distance to the other side of our galaxy. Light travels really slowly on astronomical scales.

2. A supernova goes off roughly once every 50 years in the milky way. That's an energy release that is utterly incomprehensible to a human being. A system-incinerating catastrophe briefly producing an energy output comparable to that of all the stars in the galaxy combined. But to us they appear as just brighter-than-usual dots in the sky. We (humans) have only noticed a handful of them. It's not that we're shielded from them. These events happen in direct line of sight. They're just insanely far way and light intensity falls off with the square of distance. The kinds of energy outputs produced by a species of comparable technology to our own are simply not visible to us over these kinds of distances, even with the very latest technology.

And that's just our galaxy. If you consider the entire universe the things happening in direct line of sight include billion-solar-mass supermassive black hole mergers, and you need a gravitational wave detector to even tell they're happening.

We've only just recently started to be able to detect the presence of planets around other stars. The Fermi Paradox, Great Filter, "Where's the intelligent life?" stuff is absurdly premature.

[ncmncm]

I wonder whether any organic life in a galaxy could survive merger of a pair of 1e9-solar-mass black holes. Would 25,000 light years be far enough out? 5,000? 500 light years is supposed to be the sterilization range of a minor magnetar hiccup, 5 light years for a workaday supernova, IIRC. We will soon find out how much energy is released in such a merger; one is about to happen in a year or three, about a billion light years out. Sadly, we will not have deployed a gravity-wave detector big enough to measure it with, by then, and will have to wait for the next one.

If life sorts more or less neatly into expansionist and sedentary cultures, the sedentary species will generally be much older and, typically, much more technologically advanced than the expansionists that try to encompass them. Aggressive expansionists will tend to be destroyed by the first, second, or anyway tenth sedentary culture they try to strong-arm. So, the galaxy is full of very old, sedentary cultures not interested in visiting us, and young expansionists popping up all the time and being obliterated before they reach us.

[WillPostForFood]

is absurdly premature.

Only relative to our own existence. If the universe is ~14 billion years old, and life was plentiful, you might expect life to have emerged elsewhere hundreds of millions or a few billion years before us. That allows for enough time for signals to have reached earth, even at slow light speeds.

[omnicognate]

That part was more about us being detected by others, really. The portion of the galaxy within which aliens could theoretically have noticed us since we developed radio technology and responded in a way we could have noticed by now is tiny.

As far as us noticing other civilisations is concerned the second point is the major one: the signals involved would just be really, really tiny. The "prematureness" is relative to our development of technology that could detect them.

You can of course posit that really advanced civilisations would produce massive energy signatures but I'm not sure that makes sense. The visible night time glow our planet produces due to our presence is a signature of inefficiency and light pollution. I'd hope that with better technology and politics we could make our planet look dark again from space. Efficiency, lack of wasted energy seems at least as likely to be a hallmark of a very advanced civilisation as huge, catastrophic energy releases. And even if those energy releases exist and really are spectacularly huge they'd still be massively hard to spot.

[WillPostForFood]

It is plausible that our visible light signature might be reduced (though I don't agree it is necessarily wasteful - like lighting up evening outdoor events). But we should consider the entire RF spectrum. You'd have to go to an extreme to not have a modern civilization without notable RF emissions at some level. (e.g. Dyson sphere).

Section 2 of this paper has some fun info on the ability to detect our radio signature at great distances.

https://arxiv.org/pdf/1207.5540.pdf

For example, the Arecibo Planetary Radar typically transmits at a power of 0.8 MW and a frequency of 2380 MHz, with a gain of ~108. This means that low bandwidth transmissions from Arecibo, with B ~ 0.1 Hz, would be detectable by a watcher with a 1 km2 receiving antenna at distances up to 200,000 light years, while high bandwidth signals, with B ~107 Hz, would be detectable out to about 5 light years by the same watcher.

By comparison, television carrier waves have similar power but gain ~10, B ~ 1 Hz, and frequencies in the range of 100 to 2700 MHz; such signals could be detected with a square kilometer array out to a distance of about 50 light years.

RIP Arecibo

[softjobs]

To drive home the enormous scale of the local solar system, and by extension the universe, I'm fond of this: https://joshworth.com/dev/pixelspace/pixelspace_solarsystem....

[Arelius]

What I don't see being brought up often.. So, the universe could be infinite right? Or at least many orders of magnitude larger than the "visible universe"?

If that's the case, the occurance rate of life could be very very small, and we are still practically gaurenteed to exist, along with many (infinite) more life.

But it's entirely possible that they are all over a "visible universe"'s distance away. Which with an expanding universe, would mean that we could never meet or even observe them.

In some ways, that's maybe more lonely, or at least sad, than being entirely alone.

[dcvz234]

Maybe we give ourself too much indulgency about how further we are in the technological evolution. Maybe we are just at caveman level +1 and looking for radio signals is like trying to find smoke based communications in NY.

Maybe we are still missing something fundamental about the universe and we don't know, like we ignored electricity or nuclear forces existed just decades or 150 years ago.

That UFO the US navy has in footage, from 2019, making a diving nose maybe at 200 mph into the ocean without making a single wave or splash, is telling you something: you don't know everything about physics, just yet.

[wruza]

But we can’t deny a natural barrier. Modern sci-tech if used in 18 century would dramatically explode into fundamental discoveries. Einstein Bohr et al would jump for joy having these “toys”, cause all they’ve had was basically pen and paper. But now sci-tech mostly supports existing theories for many decades. Not saying there is nothing new, but there is nothing groundbreaking either in fundamental parts of nature.

[Moodles]

This is what I find most plausible as well. If we just look at facts we currently believe to be true, then we know there are enormous distances between planets and no fast way of travelling between them. Then for a lot of probabilities you want to assign to the likelihood of intelligent life emerging on a Goldilocks planet, the universe would look exactly like it does today. I.e. we’re all on isolated islands on a fast ocean, and it doesn’t matter how advanced our tech gets: we’re still all limited by the laws of physics so we all think we’re alone.

[GoblinSlayer]

Also light is very slow. The Arecibo Message was sent in 1974, it's now 48 light years away, which is tiny.