Fascinating. This may weigh down the Drake equation, particularly in reducing the average time civilizations survive on planets with high gravity because their ability to become multiplanetary and survive great filters is limited.
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.
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).
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.
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.
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?
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.
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.
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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.
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.
So you just hang around and talk with radiowaves, sending them pictures of their world from above they could never see otherwise.
In fact, if they are anything like humans, they have probably already realized that species that live on a lower-gravity planet could escape that planet using the same chemical reactions that are available to them also.
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!
https://en.wikipedia.org/wiki/Breakthrough_Starshot
Power density still matters though. Diffraction would get in the way at long-duration low-thrust.
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.