It was a nice surprise (and a relief) to the early rocket pioneers to realize that we lived on a planet where gravity and chemistry would make orbital rockets possible. The rest was just engineering.

[0] https://library.sciencemadness.org/library/books/ignition.pd...

> "... its density was a little better than that of the other acid, and it was magnificently hypergolic with many fuels. (I used to take advantage of this property when somebody came into my lab looking for a job. At an inconspicuous signal, one of my henchmen would drop the finger of an old rubber glove into a flask containing about 100 cc of mixed acid -and then stand back. The rubber would swell and squirm for a moment, and then a magnificent rocket-like jet of flame would rise from the flask, with appropriate hissing noises. I could usually tell from the candidate's demeanor whether he had the sort of nervous system desirable in a propellant chemist.)"

[0] https://en.m.wikipedia.org/wiki/The_Right_Stuff_(film)

”It is, of course, extremely toxic, but that's the least of the problem. It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured. It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water-with which it reacts explosively. It can be kept in some of the ordinary structural metals-steel, copper, aluminium, etc.-because of the formation of a thin film of insoluble metal fluoride which protects the bulk of the metal, just as the invisible coat of oxide on aluminium keeps it from burning up in the atmosphere. If, however, this coat is melted or scrubbed off, and has no chance to reform, the operator is confronted with the problem of coping with a metal-fluorine fire. For dealing with this situation, I have always recommended a good pair of running shoes.”

https://www.science.org/content/blog-post/sand-won-t-save-yo...

[0] https://www.youtube.com/watch?v=5C9xipCTe8I

[1] https://www.youtube.com/watch?v=lAFMl5bkP5Q

The theoretical aspects are challenging enough. But then you realize just how difficult the practical application of the theory can be. Sure, a mixture of fuming nitric acid and hydrazine will produce enough propulsion, but how do you dump tons of it into an engine without it just exploding?

What if that actually made the exploration of their solar system easier, since once they left the gravity well of their planet getting to other planets with nuclear rockets was comparatively trivial?

Eg. determining location through radio signal triangulation can tell you a location pretty well, but would require placing a lot of signal stations throughout the world. Eg. remember the time-synchronisation mechanisms for watches through AM signals (including in hand watches).

Similarly, we did build a global communications network by placing expensive undersea cables across the world, but systems like StarLink are much cheaper (once you get to economies of scale for launching satellites).

So, like many things, rockets have accellerated discovery and progress, but are ultimately not the be-all solution: they work in tandem with the rest of science and engineering (including cultural development).

With finite time, lifespan and resources, "cheap" is often equivalent to "possible". If you look at the connections between inventions and developments that GP mentioned, it's usually the case that the necessary prerequisites don't involve just knowledge, but something getting cheap enough to be available / worth building.

Putting a dozen satellites in orbit - and out of reach of local authorities - is so much cheaper and more reliable, it’s not just a matter of cost - it’s an entirely different product.

Same with starlink. A big part of its advantage is someone can’t just walk over and cut a cable. And no one needs planning approval to put a cable in.

Line of sight to low orbit is about the only way to accomplish that - maybe some kind of high altitude ballon/plane could (loon?) but they’re so comparatively easy to shoot down that it makes it a very different kind of situation.

https://history.stackexchange.com/questions/58872/did-they-r...

Undersea cables are probably more expensive than satellites today, but we'll still continue to put them in. And nope, someone can't just walk in and cut a cable sitting at 5000m under the surface.

Detecting a StarLink terminal is relatively easy from the ground, and someone can just walk in and demolish it once they locate it.

Basically, all tech has pros and cons.

The cable has to come out on to the surface somewhere, though.

But barring that, dropping IEDs from a fishing boat, with a time fuse and some weight, isn't hard; the trickier part would be knowing where to drop them so they land near the cable. But there are tricks for that too.

All too easy to get bit by the curse of overcomplicating things.

Still possible. But orders of magnitude harder. Nothing says that starlink ground station needs to stay in one place, after all.

Undersea cables get cut all the time, from shipping to nation states.

Trains don’t make cars obsolete, anymore than cars make trains obsolete. Taking out train tracks is much easier and more effective than taking out all possible roads though.

LORAN has also been used near airports in developed areas for a long time.

That isn't the 'base case' though.

The US military initially developed and launched GPS because of the reasons I stated, and it is still widely used as a base case for exactly those reasons.

Planes would be replaced by trains and aquaplanes for sure. Our modern fastest trains (TGV, Maglev) are only half as slow as the fastest commercial planes. Also, you might have rocketry on such a planet, just not for orbit, and for things that right now we use jets for.

The biggest issue with be probably no detailed aerial maps, and in later stages - no space mining, so such civilisation would be limited to resources on their own planet.

Also, I'm imagining that such a civilisation would send out more signals into space to encourage someone to come and visit them, and hopefully dropship resources from orbit :D

Imagine two civilisations living like that in symbiosis - one on the orbit, able to drop things to the one that is lower, but being able to extract only information / art / mental labour / energy from below.

I'm wondering if siege weapons work at all

Wouldn't LTA blimps work BETTER in higher gravity for flying?

So depending on the gravity we're talking about, blimps are out!

Resource extraction from asteroids or moons is a lot easier than carting it out of a big gravity well. Building stations in zero G rather than having to worry about orbital degradation and the like. Atmospheres get in the way of solar energy collection.

Earth is probably only useful as a vacation destination. Unless of course all those UFO reports are actual physics-defying antigrav drives with little green men.

The bad thing about gravity wells is they naturally concentrate things along density gradients.

We are actually on that planet. Spacecraft have what is called delta-v, which is basically a measure of what orbit changes they can perform given the amount of fuel they have onboard. For example getting from the ground to LEO has one measure, and getting from LEO to moon orbit has another.

It varies somewhat by the specific rocket to get into space (due to drag and effects of higher gravity), but once you are there it's basically the same for all spaceships.

It takes around 9.6km/s (no relation to gravity, just a coincidence) of delta-v to get into LEO, however once you are there it's fairly cheap to get around the solar system. To get from Earth LEO to a captured orbit around Mars needs a delta-v of around 5km/s - yes, less than to get into Earth orbit. To get out further to Neptune would need around 12km/s of delta-v.

----

NERVA [1] / Nuclear / 1969 / 246kN thrust / 18,000 kg mass, 841s ISP (seconds of specific impulse - higher is better/more efficient, a little is a lot) / The only completed possibly launch viable nuclear rocket engine, as far as I know.

F-1 [2] / Chemical / 1959 / 7,770kN thrust, 8,400 kg mass, 263s ISP / Powered the Apollo rockets

Merlin [3] / Chemical / 2007 / 981kN thrust, 470 kg mass, 282s ISP / Powers the SpaceX Falcon 9 in a group of 9

Raptor [4] / Chemical / ?? / 2,640kN thrust, 1,600 kg mass / 327s ISP / Powers the SpaceX Starship in a group of 33

----

So what really matters in a rocket, for getting off Earth, is its thrust to weight ratio. NERVA isn't inefficient because it's dated (which was part of the reason I included the F-1), but simply because nuclear itself has an inherently poor thrust to weight ratio. However it just keeps going and going and going, which makes it absolutely awesome for travel once you're already in space.

It's even "fast" in space, because of how travel in space works. You don't just keep thrusting in space; instead you make a limited burn and then coast to where you're going, making a final reversal burn towards the end. So even if it takes hundreds of times as as long to reach a higher cruising velocity, it'll end up getting to the destination long before a chemical rocket, for any sufficiently distant destination.

----

[1] - https://en.wikipedia.org/wiki/NERVA

[2] - https://en.wikipedia.org/wiki/Rocketdyne_F-1

[3] - https://en.wikipedia.org/wiki/SpaceX_Merlin

[4] - https://en.wikipedia.org/wiki/SpaceX_Starship

The dream of course is that you keep thrusting, accelerating until the halfway point, then flip around and burn to decelerate. In that scheme, your thrust doubles as artificial gravity too!

It's kind of insane luck. Bit heavier planet and we wouldn't be able to have a single satellite before building nuclear engines.

A nuclear reactor is a bit like an ion drive: great for long distance space travel, but not great for getting off a planet.

Unless you mean the kind of nuclear engine that consists of detonating atomic bombs behind you? See https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...

Acceleration to 66 km/s is probably a little bit overkill, even.

https://en.m.wikipedia.org/wiki/Operation_Plumbbob

What are you basing this on? NERVA was for getting off the planet. It had a thrust of ~250 kN. In comparison, a SpaceX Merlin engine has a thrust of ~900 kN, while ion drives have

  Additionally, atmospheric density and friction matter a lot in these situations, and getting out of high density atmosphere and ‘up’ as quickly as possible pays large dividends.

*First stage may need to extend well above the atmosphere.

**No, that's for-sure not a Randall Munroe book in my hand.

But I guess you could cheat by having multiple engines along the way that all accelerate in lockstep.

But what if the thing I push on is a quantum particle? Does this same thing happen at the smallest scales? If one end of a quark is pushed on does the other end move instantaneously or is there a small(!) delay?

Probably the answer is just "that's not how quarks work" but I've always been curious.

https://youtu.be/DqhXsEgLMJ0

For subatomic particles, the most intuitive way to think about things is to adopt the "fields are real" mindset. Here fields are the underlying reality, and particles are just a pattern of waves excited in the fields. Disturbances in all fundamental fields we've discovered propagate at the speed of light, and we have pretty solid reasons for believing no future discovery will contradict that, as it would break causality in a fundamental way.

Quarks don't have an "other end." To the best of our knowledge, particles are points.

This is a classical picture, but the quantum picture is similar: evolution is generated by a local Hamiltonian constructed out of field operators attached to every point of space.

So, both classically and quantumly, relativity demands the existence of fields filling space to propagate causal influences at finite speed.

The speed of light in a vacuum happens to be the best representation of the maximum speed of causality

Which also makes more sense why you can't do things like travel faster than light (your effect would precede the cause), and why two protons going past each other in opposite directions don't violate this law

Of course, once you’ve managed to build this, the rockets are basically optional. :)

Remember that just getting outside the atmosphere is the almost trivial part of rocketry compared to the problem of having to then accelerate to >= orbital speed fast enough to not fall down!

And anyway you’d have to dismantle the planet to build your launch tower, which I guess would solve your problem, in a fashion. Though – whatever you turned your planet into would just have an annoying tendency to rapidly collapse back into a ball.

We don't talk about ground logistics though.

I'm curious what effect an increase of gravity may have on heavier-than-water displacement craft (canoes and other modern boats). I think probably none, since you're dealing with density, not weight. Except for any increase in density of early building materials and cargo/supercargo. But it's been long enough from physics I'm unsure.

I think atmospheric density is more dependent on magnetic field than gravity.

Absence of comm satellites would also help fragment the world and make the idea of a surprise attacks more enticing.

Orbital space (around modern Earth) is ex-territorial, so killing a spy satellite would be seen as an act of aggression, not legitimate defense. This holds back "kinetic action" in near-Earth space.

That's a historical accident of arrangements on earth (so less useful in the Fermi-paradox / filter debate), and could easily have gone differently. Ie air space could also have been seen as ex-territorial.

My recollection of how it evolved on Earth was the soviet's more asked forgiveness than permission and eisenhower basically shrugged and said "whatever, at least our spy satellites can go over you too"

See also how for us earthlings the moon is considered the common property of all mankind or something like that. And that's all well and good as long as no one can actually reach it well enough to make use of it; but I predict as soon as we have useful moonbases, we will move to a conception that's closer to 'possession is nine tenth of the law', if not outright 'might makes right'.

If not de-jure, then definitely at least de-facto.

Just because we're built for it doesn't mean other species will be.

If evolving in a different environment, they might be built for cooperation. That is, in a certain environment the only species that can evolve enough to go interplanetary might be a species that learned to co-exist internally and externally, otherwise the environment would have kept them down.

> Just because we're built for it doesn't mean other species will be.

You heard of what chimps get up to? Ants? Microbes? They don't just have wars; They have raiding parties, take slaves, serve as battlefield medics, compete in intrafactional and interfactional rivalries that slowly boil over… Hell, even trees actively release toxins to try to kill other nearby plants.

On a long enough timescale, war is almost certainly highly (and lethally) maladaptive.

But in a non-post-scarcity environment with social contact, creatures whose bodies disagree with entropy tend to learn that violence is an effective tactic for taking others' calories/oil and nutrients/minerals.

Maybe there's exceptions. I hope so, anyway.

War works well for them, so they evolved to get better and better at it.

But war is resource-intensive and costs lives. Lives are easily replaced on Earth.

It’s not hard to imagine a planet where going to war would be mutually assured destruction on a species level, even for ants and microbes.

That is very hard to imagine, how do you reckon that would be possible? Does the planet only support a couple of anthills and then all resources are consumed? How would ants even appear on such a planet?

For instance, if two or more extremophiles evolved together but remained separate species. They might even require one another’s contribution to successfully procreate. And successful procreation might be rare.

That sort of life, if it evolved to consciousness, would be averse to any form of damaging competition.

One poorly timed selfish move and the hostile environment wins: everybody dies.

This cooperation imperative would be built into their biochemistry, same as war is built into ours.

You’d probably still find insane or outlier members of their society, who are radically uncooperative or individualistic. But they would be rare and containable, otherwise their species couldn’t exist.

> That sort of life, if it evolved to consciousness, would be averse to any form of damaging competition.

Uh. That sounds like us. Our dependence on our mitochondria and chloroplasts to survive as microbial life, it turns out, did not translate into an aversion to war after we grew up as macroscopic life and everybody around us had their own endosymbionts too.

Honestly I think you're going in the wrong direction with this. A crueler world results in crueler people; scarcity begets conflict. Maybe you could technically create peace by simpy isolating everybody in some kind of desert-like environment, but if you want a Nash equilibrium and selection pressures favouring active prosocial cooperation, then I think what our own history of war, domestication, self-domestication, democratization, etc. shows is that you effectively need (amongst other things) an almost post-scarcity environment, where basic physical resources are no longer a constantly urgent limiting factor on life— A techno-utopia with nuclear weapons and additive manufacturing has both much more to gain from cooperation and much more to lose from war than their less fortunate equivalent struggling just to survive.

But then that goes back to my original reply to you: Anybody who evolves through that initial awkward phase of competition in fear of entropy is probably going to have violence as a part of that history, and part of themselves.

---

Note that effectively post-scarcity environments do actually appear in nature now and then, and when they do appear, they do sometimes result in apparently utopic, peaceful, and more empathetic societies. E.g. for a particularly stark example, see bonobos versus chimps.

But as with many good things, it seems to usually be highly spacially/socially local, and temporally transient.

Other environments may differ.

But that's not really "mutually assured destruction on a species level", so much as more to gain by working together– Which honestly is better.

Atmospheric density is very much affected by gravity. I'm not sure magnetic field has any appreciable effect at all on the density of the earth's atmosphere. Why would it? The vast majority of the atmosphere isn't charged, so doesn't interact directly with magnetism.

One of the reasons Venus still has a dense atmosphere is because its atmosphere is mostly composed of a relatively heavy compound, CO2, which is harder to lose than lighter gases like H2, N2, and O2.

But magnetic fields don't usually just switch off. If the planet didn't have one to begin with, then it probably doesn't have much of an atmosphere for long enough for advanced life.

As you increase gravity, with fully compressible fluids the buoyancy scales the same as weight, you wouldn't sink lower. But with any incompressibilty you'd need to displace proportionally more (ie sink down more) to counter the increasing weight.

(I think)

The water would also weigh more. Buoyancy is the force of the water around the volume you displaced being pulled down into that space, exerting pressure that pushes you up. So you'd float just as well.

Actually, the compressible fluids would become denser­, and make it easier for you to float (assuming you're relatively incompressible). At the extreme end, you could swim in pressure-liquified air (assuming you survive being crushed, of course).

The hypothesis suggests that larger planets with more mass and gravity than Earth would be more favorable to life. It’s certainly possible that there is a lot more life out there on planets where getting into space is nearly impossible with conventional chemical rockets.

We may be living on a comparatively barren rock, but the tradeoff of that we are actually able to get into orbit.

https://en.wikipedia.org/wiki/Superhabitable_planet

So you just hang around and talk with radiowaves, sending them pictures of their world from above they could never see otherwise.

But I do like the idea that you wouldn't be able to.

So instead you slingshot your orbital craft past the planet, using its gravity well itself to build up speed— And you release a cable ahead of you, that swings down through the atmosphere to zero surface velocity at the point of your perigee, so your away team and their new friends can attach it to a glass elevator and be smoothly hoisted into space.

Different problems entirely. You don't need a lot of thrust to get to high velocities. But you need a lot of thrust to leave a planets gravity well. On a planet, your thrust needs to win not only gravity, but also any atmospheric losses. E.g. on a 3g planet, you'd need thurst in excess of 3g's to leave.

But to reach say 0.25c, a tiny ion engine over a long enough time would suffice. an engine that wouldn't even get you off of earth.

Tsiolkovsky says otherwise, by a factor of over 10^663 (not even counting relativity):

https://www.wolframalpha.com/input?i=1%2Fe%5E%2874900km%2Fs%...

…Seriously. I tried to figure out just how much xenon you'd need to make that work. But you'd need to be able to store it in something like 15-dimensional space to even fit it within the diameter of the observable universe. And even if your ion engine and Hubble-scale fuel tank weighed less than the mass of the lightest quarks, the amount of propellant you'd need for it to reach 0.25c is still well over 10^600 times the combined mass of the entire observable universe, and would also collapse somewhere around 10^600 times the diameter of the observable universe into a single black hole:

https://www.wolframalpha.com/input?i=2G%282.2MeV%2Fc%C2%B2%2...

Of course, this also shows the intent of my original comment: Energy density matters, and somebody packing enough to casually cross interstellar densities isn't going to struggle with a planetary gravity well unless Idk they're doing like a low-tech off-grid trend or something.

Also, skyhooks!

Not being multiplanetary seems like the least of our existential problems here on Earth, and will continue to be that way for awhile.

At the same time, chemical rocket efficiency becomes totally irrelevant for a slightly more advanced civilization than us.

A jet engine capable of leaving a deep gravitational well must have a big ratio of thrust to weight. If a chemical rocket is too weak, a nuclear jet engine is the only remaining option. Would you be comfortable running it in the thick atmosphere of a densely inhabited planet?

What I'm saying is that whatever engineering and environmental limitations we currently perceive are probably irrelevant.

That might be true, especially if your 'for awhile' talks about millennia at most.

But it's an extremely relevant concern in the context of the Fermi paradox.

So maybe they'd figure it out.

If throwing things well had been much harder, perhaps no animal would have ever bothered?

It is interesting how we got nuclear technology that would allow for way more capable rockets at the same time we perfected chemical rockets enough to get to orbit. So much that we could have been able to escape a 10g planet almost as soon as we have escaped Earth.

More conventional nuclear propulsion has similar trade-offs to an ion drive: great for long distance travel when you are already in space, but useless to get off a planet.

So, the known quantities that term refers to tend to be steps more like planetary habitability and abiogenesis, which might prevent complex life from getting established in the first place. But it sounds like you mean some kind of cataclysmic event which wipes out an already existing industrialized civilization.

What, specifically, are the "Great Filter" scenarios which being multiplanetary is actually supposed to help with?

Supernovas? GRBs? Simple asteroid impacts? You can usually see those coming from millions of years in advance. And surely building a couple layers of solar sail material to shield the planet, stockpiling ozone generators to repair the damage quickly, gently nudging the asteroid, or simply digging some holes/eating a gas giant and weathering the storm, would be easier and save vastly more people than establishing a sizable population in another star system.

The other "Great Filter" idea which seems to be memetically adapted for proliferating in modern discourse is the idea of a locust-like swarm of technologically advanced aliens that kill any industrial civilizations which do emerge. But in that case, presumably settling multiple star systems is the opposite of what you'd want to do; You'd be better off quieting your emissions to shrink your footprint than spreading even more biomarkers around at high blueshift.

Frankly, I think this entire idea of needing to "become multiplanetary and survive great filters" is more mainstreamed now largely due to one specific individual fancying himself a savior of humanity. SpaceX builds interesting machines, but I liked it better when it was people like Sagan, Aldrin, and Zubrin getting excited about Mars.

But even then, I'm not sure if the idea of colonizing more planets in order to survive planet-scale catastrophes really jives with how people think— Plenty of us already live within splash radius of the Pacific Ring of Fire, Yellowstone Caldera, tornadoes, tropical cyclones, land below sea level… and yet there's no billion-dollar emergency backup cities in Antarctica to "make San Francisco into a multicontinental city and survive great quakings".

The assumption is that we have a problem getting anything off the planet. All these would require some good rocket engineering.

I agree with everything else here a lot.

Very depressing to me to think about how vanishingly rare smart, spacefaring life might be. But on the flipside of that, there may be a little corner of the universe where multiple spacefarers contemporaneously live within a few light years of each other. That might be cool from a space opera point of view but it'd probably end up being dominated by a space fascist enslaving everyone.

Rockets are most convenient for Earth's variables so engineers optimized for them.

There's a lot more variables that just gravity.

Fascism already barely works on earth, and gets out-competed. See https://tvtropes.org/pmwiki/pmwiki.php/Main/FascistButIneffi... Similarly with slavery. (See https://www.econlib.org/library/Columns/LevyPeartdismal.html to go off an slight tangent.)

In space, slavery is even less useful. That's mostly because humans are even less useful: we are already doing pretty much all of our useful space exploration with robots, and sending humans is just for bragging rights. Keeping space slaves alive costs you more than they ever could conceivably do for you.

Of course, aliens might have biologies that are much better adapted to surviving in space, maybe?

Keep in mind that human technology is pretty close to being good enough to detect not just foreign civilisations (via eg radio waves), but signs of life itself: studying the spectra of light reflected by exoplanets can tell you what chemical elements are in their atmosphere, so you can detect atmospheres that are far from chemical equilibrium, like earth's oxygen rich one.

We emitted radio waves for only a few decades. But earth had oxygen for billions of years. So that widens the window of time of development that we could detect.

It's really not that hard to explain. The last time I looked into this it was the case that we wouldn't be able to detect ourselves more than a light year out. The closest star is the red dwarf flare star Proxima Centauri at 4.2 ly. So, the question really isn't "where is everyone?" but "why is no one doing active SETI right now in a way that we can detect?".

That's exactly OPs point, we are heaping assumptions upon assumptions and most people discussing this topic don't even know how mind bogglingly huge those assumptions really are.

The first part:

See how fast technology is developing, and especially the decreases in the cost of rocket launches.

Within the next few hundred years humanity (or our drones) will have spread to countless habitats around the solar system. Within a few thousand years, we will have started building a Dyson swarm that will obscure a measurable fraction of the light of our sun.

We don't need any new science for this, nor radically new engineering.

For this confident prediction, we only need a few key ingredients:

* Humanity doesn't blow herself up completely. (Civilisations blowing themselves up is one way to resolve the Fermi paradox.)

* At least some humans are interested in space exploration. (The proportion of the total population can even go down compared to today.)

A thousand years is nothing on cosmic time scales. It's not even much in terms of geological time scales.

Even today's technology could detect the humanity of 3024 from countless light years away: just point a spectroscope at the star and notice a vast excess of infrared (the waste heat of our solar collectors has to go somewhere) and an corresponding deficit of shorter wavelengths.

So we might not be able to detect ourselves at the moment, but we would be able to detect our 20-minutes-into-the-future selves already.

The second part:

As I already mentioned, our sensor are pretty close to good enough to measure the chemical make-up of the atmospheres of exoplanets via spectroscopy. Atmospheres that harbour life look very, very different from those of life-less planets.

(Or to steelman that argument against nitpicks: there might be some forms of life that do not push the atmosphere of their planet out of chemical equilibrium. But for that to resolve the Fermi Paradox, that would need to be the vast, vast majority of biospheres.

And I'm not talking about an oxygen-rich atmosphere necessarily. Just any sign of chemical disequilibrium.)

So, yes, humanity couldn't detect ourselves right now. But humanity in a only few years could already detect signs of life hundreds of light years away while still at the equivalent of the Cambrian explosion.

Strictly speaking my part two is not a problem right now in 2024, but I'm fairly confident in predicting it will become acute in the next ten years as our telescopes get better. Astronomers have already done some basic spectroscopy on some exoplanets. Their skills are only improving.

---

You are right to warn about making too many assumptions about alien life. However, not all assumptions are born equal.

For example, life is almost by definition associated with being far outside thermodynamic equilibrium. Being outside of chemical equilibrium isn't much of a stretch.

Similarly, my first part assumes that humanity (or our alien equivalents) will keep multiplying and expanding. And again, that's not much of a stretch: yes, at any given time only some portion of life might be interested in these activities, but future generations will be predominately made up of those that showed the greatest interested and skill in multiplying.

The past century of rapid progress is anomalous in human history. We made do with stone tools for 100k years. In the middle ages basically nothing technological happened for a 1000 years. Consequently, this view of eternally ongoing progress is an artifact of the specific time and place we find ourselves in and not something that really has to happen. While we had some good results from semi-conductors I see our progress as already slowing down. Most of our science doesn't reproduce. Our cosmology is in shambles (more on this later) and our engineering is surfacing issues around putting the correct number of bolts on airplanes.

> Within the next few hundred years humanity (or our drones) will have spread to countless habitats around the solar system.

Why? What is there beyond the gravity well that isn't on earth? Maybe there are some substances, such as deuterium, that we can exploit remotely but this idea of "space habitats" is just romantic science fiction. Maybe someone will make it happen but only because we humans are in love with space, not because it makes rational sense.

> Within a few thousand years, we will have started building a Dyson swarm that will obscure a measurable fraction of the light of our sun.

Again, why? Fusion and fission can supply all the energy we want. If we want to do mega engineering for more energy we could just bore down to the earth's core. I'd bet that's what intelligent aliens on Enceladus would do. Regardless, it's completely useless to predict the future a thousand years out. We simply don't know and we have no data to anchor our fantasies to reality. From this initial starting point, nobody ever made and predictions worth anything.

> We don't need any new science for this, nor radically new engineering.

So why don't we do it then?

> For this confident prediction, we only need a few key ingredients: > > * Humanity doesn't blow herself up completely. (Civilisations blowing > themselves up is one way to resolve the Fermi paradox.)

Like I showed, you are baking in more and more assumption. Basically every sentence of yours is one more huge assumption.

> Even today's technology could detect the humanity of 3024 from countless light years away: just point a spectroscope at the star and notice a vast excess of infrared (the waste heat of our solar collectors has to go somewhere) and an corresponding deficit of shorter wavelengths.

Some SETI and Fermilab people looked already back in 2006 and found 17 candidates. There is this flawed idea that we already have detected and interpreted everything that there is. But JWST is showing us right now that we don't know anything. Every piece of our cosmology is currently under reexamination because wherever we point JWST we see stuff we can't explain. Even just the question of cosmic distance measurement isn't simple and clean cut.

All our space science is based on the premise that we are doing natural science where natural things exhibit regular patterns that we can describe with laws. If the universe were to contain ubiquitous mega engineering we would, by definition of the science we are doing, not recognize it. For instance, your Dyson spheres detection assumes that we see an abnormal spectrum. But, abnormal compared to what? If the universe was full of Dyson spheres we would say that there are "infrared stars" and then we would hypothesize how these come about naturally until someone can crowbar in an explanation that sort-of-kinda could work. Cosmology is full of these kinds of after-the-fact explanations btw, just look at Tabby's star or Oumuamua.

So again, we end up at a place where the Fermi paradox isn't very impressive. If the aren't building (arguably pointless) mega structures, we won't see them. If they all are doing mega engineering we wouldn't recognize it. For us to see them the aliens must be doing things that are very 20th century western hemisphere human things.

That we see nothing implies intellgent life is rare, short lived, or we're early in the age of the universe. For example, red dwarfs will last trillions of years compared to the sun's 5B lifespan.

Or it implies that not everybody's first instinct on seeing a vast galaxy is to try to take it over ASAP.

If you want resources for quality of life— Gas giants are a thing. If you're an explorer driven by curiosity, then take only samples, leave only memories, right.

If you want money— Century-long shipping times with civilization-scaled fuel costs tend to eat into profit margins.

If you're worried about survival— genuinely worried about survival, on a level personal enough to motivate action, not just academically or for fun— then the focus is on people you know and care about; all of those are here.

In fact, I suspect the ones that see other stars and immediately think "Mine mine all mine!" probably have a higher chance of nuking themselves before they even get out of their star system.

> > Or it implies that not everybody's first instinct on seeing a vast galaxy is to try to take it over ASAP.

Or it may simply imply that intelligent life is good at hiding and does not want to be seen e.g. the dark forest hypothesis.

I don't see how you could rationally justify spending this stupefying amount of resources on sending people into the void where, even if all goes well, you can barely talk to them, have no trade and no social contact with them at all.`

Basically you are paying to split off a one-way branch of the species. Just consider that if we could colonize Alpha Centauri that means that each message has a 8 year or 1/10 of a human life span roundtrip. What would you even talk about with that kind of latency?

Let’s assume super tech they can build that somehow allows vastly faster speeds than we can today ~120 km/s worth of DeltaV. Half that is spent slowing down so we’re talking 0.02% c.

Now let’s assume half the time is spent in flight and half the time is spent colonizing stars before launching ships. So now we’re down to 0.01% C. Suddenly 1 Billion years is a more reasonable estimate and even that takes super tech we don’t have any idea how to build and assumes nothing fails.

Several more advanced civilizations could be colonizing the galaxy today that are still 10 billion years from finishing.

Colonization using a massive habitat capable of extreme redundancy and asteroid mining could be a completely viable solution to colonization. But such a structure wouldn’t be light.

Hitting 1% c could very well take megastructures that a civilization would rather spend on redundant craft etc. We can dream, but we’re nowhere close to being able to say what’s actually viable.

This word is wild. Very interesting.

It basically means "evil bad guy".

You know, exactly the kind of ideology you don't want a neighboring nation to have, no matter how you judge their actions morally

National Socialism and Italian Fascism were all about the strong taking responsibility for the nation, in a philosophical sense.

These were social welfare states that provided for the "people" far more than modern American liberalism (the standard operating produre for free economies of scale in the modern world), for example.

Now, obviously I'm not saying this was a good system, but you're so far off base that it's ridiculous.

The discourse around this complex historical movement is profoundly anti-intellectual. I know this because I grew up in the same society you did, the one where I also learned and used fascist as a pejorative without any further information.

Now, as someone interested in having sophisticated ideas about systems, I'm not really in a mental state where I'm going to take appeals or emotion or shortcuts that shut down thought seriously. Regardless of the context, I still want to try and reflect reality as closely as possible in my minds eye.

How many liberal, or even communist thinkers have you read? Personally, countless. As for fascists, almost none. It's a taboo, the works aren't translated, etc -- but it's there, and has intellectual underpinning that is more complex than mindlessly calling people you don't like "fascists".

This can't be conscionable to any earnest intellectual. Imagine sitting here and tolerating people pejoratively calling people "communists". It's so stupid.

Edit: I get in trouble here because there's something really interesting going on with controversial topics: all you need to do is make a choice and your model of reality is much more accurate than the presented model of reality. It's the easiest way to take Ws out of discourse, nobody has good ideas when the id has the reins.

Also tolerated - by calling them "Russians".

The logic goes, if there's nothing innately wrong with Communism then Russians (and, to extent, Chinese) must be up to blame.

Soviet Union had socialized assigned housing, affirmative action policy and equalized wages right from the start. Still, it is largely ignored by socialist LARPers of today.

I wouldn't call any of these groups "fascist" except in the loose pejorative sense

The closest governments to actual Fascism that I can think of are governments like modern China or even Singapore, and I don't mean that in a pejorative sense. They're just very fascist in character, i.e. national interest, social welfare, strong-arming of capital, etc.

But you can absolutely use an aircraft to gain height and speed, and then launch a much smaller rocket from that aircraft (where the speed is the primary advantage, and is what rockets use most of their fuel for). This setup is used by Virgin Galactic's SpaceShipTwo. There is also Virgin Orbit's LauncherOne, which is a small rocket that launches from a modified Boeing 747. On Earth it's just about not worth the additional complexity, but on planets with stronger gravity but comparable access to powered flight this might be the preferred method of reaching space.

One important factor might be the speed of sound. Subsonic flight is much easier for aircraft than supersonic flight. In an atmosphere with a much higher speed of sound, like say hydrogen, aircraft could reach much higher speeds and thus would be a much more advantageous launch platform for rockets. Assuming you already solved the issue of powering those planes of course.

This all adds up to a plane needing to carry many times less mass to gain the same altitude and speed as a rocket, at least within relatively dense atmosphere.

However, past Jupiter size the mass keeps increasing while the radius doesn’t, so even from a floating platform you’re contending with multiple G’s.

Yes, you could use a balloon filled with vacuum, but lifting something the size of an orbital rocket in a hydrogren atmosphere would require a vacuum chamber at least the size of a city, possibly the size of a small state. It would probably be easier to build a tower.

The atmosphere gets denser further down. You just need a negative pressure vessel, or to heat the hydrogen, like a hot air balloon. At 1 (Earth) atmospheric pressure the gravity of most Gas giants is quite low.

It's hydrogen inside an oxygen atmosphere that's the problem.

(So on Jupiter, you wouldn't want to ride in an oxygen balloon, ie you wouldn't want to ride in a hot air balloon there.)

https://en.wikipedia.org/wiki/Nuclear_thermal_rocket

The practical designs we have for NTRs are solid core, which after long effort got up to a thrust to weight ratio of 7:1, meaning they could in principle carry up to 6 times their weight and accelerate up in Earth's gravity rather than down. Chemical rockets can get 70:1. No one ever had plans to use NTRs in lift platforms- instead they could serve as more efficient upper stage engines, for orbit-orbit transfer burns and the like. In principle there are engines which are technically NTR and offer much better performance, but no one's ever gotten a working prototype. Also you probably wouldn't want to launch with an open cycle rocket, since the open part describes how the radioactive fuel is ejected out the rear. Unfortunately, with the technology we have, we have to make tradeoffs between efficiency and thrust. For the lift stages chemical rockets are, for now, unrivaled.

(Unless of course your nuclear propulsion is of the more, shall we say, entertaining variety. Project Orion has its proponents...)

If the background radiation of earth was 100x higher, would we care about an Orion launch? Or a small nuclear exchange…

Using the chart in the accepted answer, launching with chemical engines takes 50 thousand tons at 3x gravity and 3 million tons at 4x gravity.

Now consider a theoretical engine that has a 7:1 thrust to weight ratio at 1G but sips fuel. Take a 25 ton engine, strap 10 tons of fuel to it and 1 ton of payload. Watch it go to orbit on a single stage.

A real NTR doesn't save nearly as much fuel, but it can still be useful in certain ranges.

And maybe I'm taking Terra Invicta too seriously but maybe they would wait until they figure out nuclear fusion and have more options.

- he thought it could be made to work

- all big engineering projects (dams, skyscrapers etc) kill people

- putting all that radiation into the earth's atmosphere couldn't be justified

A true nuclear rocket. Just like a chemical rocket is a controlled explosion, NSWR is a controlled (cough) nuclear explosion.

From the article:

Early publications were doubtful of space applications for nuclear engines. In 1947, a complete nuclear reactor was so heavy that solid core nuclear thermal engines would be entirely unable[23] to achieve a thrust-to-weight ratio of 1:1, which is needed to overcome the gravity of the Earth at launch. Over the next twenty-five years, U.S. nuclear thermal rocket designs eventually reached thrust-to-weight ratios of approximately 7:1. This is still a much lower thrust-to-weight ratio than what is achievable with chemical rockets, which have thrust-to-weight ratios on the order of 70:1.

What it mean, shockwave from supersonic engine exhaust creates literally powerful pressure on construction, so on mentioned scale, nothing will withstand it long enough.

If it is possible to create much stronger materials, as I know at the moment, is unknown and we cannot forecast.

Sea level is important, because, at the moment I only remember TWO space rockets, which started from much different position, and high altitude (air) launch have very different atmosphere properties, which could be solution to shockwave problem (but have other limitations).

https://en.wikipedia.org/wiki/Northrop_Grumman_Pegasus https://en.wikipedia.org/wiki/LauncherOne

and even if it did exist, I have no idea how that thing would be put in place

if I remember, in the mars trilogy, it's assembled in high altitude, low gravity, and then put in place?

but gravity is lower on mars so rockets work better?

anyway, for earth, assembling a space elevator in space, meaning putting tough cable in orbit, would require so many launches and would emit a lot of CO2 in the process.

also the cable might be progressively thicker starting maybe at 1/3 of the distance, to bear the entire weight of the lower cable that is the most affected by gravity, while the rest of the cable would have a progressively centrifugal force away from earth to compensate, so maybe the cable would not need to be thick everywhere.

maybe that question was already asked

You put a platform in geosynchronous orbit and then lower a cable while raising a counterweight. The orbit of the entire structure is then balanced. The tricky part is lowering the bottom part through the atmosphere and securing the base.

I want to mention that this would only be for heavier than air based airborne shipping. Liquid based shipping is unaffected by gravity. Archimedes' principle has the buoyancy force as the weight of the displaced liquid. The gravitational effects cancel out. Also, dirigibles would be possibly more useful here as, again, gravity cancels out.

Something neat I remembered, great comment all the same, thank you.

Though I don’t suppose we’ll be visiting any aliens with chemical rockets regardless. We don’t have that kind of patience.

1. Saturn V first stage

2. Saturn V second stage

3. Saturn V third stage

4. Lunar module descent stage

5. Lunar module ascent stage

6. Service module for Earth return.

Five stage rockets are a lot more exotic. There is the Minotaur V, which was launched exactly once, and India's ASLV, which they abandoned after a couple launches due to budget issues.

https://www.reddit.com/r/ProjectHailMary/comments/s5n7j4/eri...