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Not exactly. Time does run more slowly near a strong gravitational source, and if you are near the event horizon of a stellar–mass black hole this effect can be extreme. However, the larger the black hole is, the flatter the space around it. Furthermore, “near” is relative. A Birch world would be built around a black hole which is approximately a light–year in diameter. The structure would be “near” the black hole’s event horizon in relative terms but in absolute terms it would still be pretty far away, perhaps a quarter of a light year. Expect a time dilation of just 2:1, meaning that for every year on the Birch world two years pass for the rest of the universe.

It might seem like this costs you a lot, since it halves the amount of time you can live near your black hole. However, the lifetime of that black hole will be somewhere between 10¹⁰⁰ and 10¹⁰⁶ years, which is pretty insane even if you only get to use half of them. Furthermore, this is many orders of magnitude longer than the lifetime of a galaxy, so your civilization could potentially outlive everything else in the universe. Large stars burn out the quickest, but with black holes it is the other way around: small black holes evaporate the soonest. You might think that storing hydrogen in brown dwarf planets for use in fusion reactors would power a civilization for a long time, but fusion reactors are surprisingly inefficient. A civilization built around a rotating supermassive black hole can take advantage of the Penrose process to extract more usable energy from the same mass than the fusion reactors would.




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