A birch planet is a theoretical far future Giga-structure where a shell world is built around a supermassive blackhole. I have been playing around with different astrophysical calculators to find out the gravitational force, time dilation and escape velocity for different radius structures around ultra massive blackholes but I am having trouble understanding how a low gravity surface with a very high escape velocity can be survivable for humans.

As the black holes mass increases so does its event horizon radius which means the gravitational force lowers, so when its mass is 1.5 trillion solar masses its gravitational force becomes 1g, which sounds perfect for human life but the escape velocity is near the speed of light and time dilation is very high but how would this high escape velocity effect conditions there? could things lift off the ground with the same ease as on Earth, could planes fly in the sky or people jump off the ground?

I have not decided on the mass or radius of the Birch planet so its escape velocity may not be as extreme as the earlier example but it will still have a similar issue of a very high escape velocity but low gravity, other than the increased amount of energy needed for spacecrafts to escape the gravity of the Birch planet and black hole, what will be the other effects of a high escape velocity on structure which contains current earth-like life and features?


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could things lift off the ground with the same ease as on Earth, could planes fly in the sky or people jump off the ground?

Of course! a 1g gravitational field is still just a 1g gravitational field. Assuming atmospheric pressure at the surface is similar to Earth (but do think about how much gas that requires, and where you might get it from) you'll probably have the same atmospheric scale height, too.

What you won't be able to do is to orbit the planet, because orbital velocities will be ridiculously high. Suborbital hops and boost-glide trajectories would still work for long distance travel, but you won't have satellites that stay up there.

The other major issue is that objects from space falling into the black hole's gravitational field are going to be travelling Quite Fast by the time they reach the surface... they'll hit the atmosphere with velocities approaching the world's own escape velocity, and that means they're basically indistinguishable from relativistic kill vehicles.

Presumably all of the matter in the black hole's accretion disk has been removed (and possibly used to build the shellworld) but getting rid of everything might be impossible. If there's still stuff out there, a new disk may begin to form, which will produce plenty of relativistic debris and radiation, which won't be much fun on the surface.

  • $\begingroup$ The problem of infalling material applies to all engineered super-worlds with appreciable surface gravity, whether or not the mass is a black hole. $\endgroup$ Mar 27, 2022 at 2:57
  • $\begingroup$ @LorenPechtel I feel that "everything is an RKKV now" does represent an upper limit to the problem. $\endgroup$ Mar 27, 2022 at 7:28
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    $\begingroup$ Yeah, I think engineered super-worlds are simply not viable because of this. You can clean out the system, but Oumuamua exists. Even vaporizing it won't save the world. $\endgroup$ Mar 27, 2022 at 19:38

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