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First proposed by Paul Birch, a birch planet is an extremely large shellworld which is built around a supermassive black hole. The largest possible size of a birch planet is one built around a black hole with an event horizon with equal surface gravity to Earth. Such a black hole has a mass 1500 billion solar masses (the mass of an entire galaxy) and a diameter of 1 light year. Beings living on a birch planet experience the pronounced effects of time dilation. However, due to the extreme mass of the object, tidal forces would be extremely small, meaning many thousands of shellworld layers can be built before loss of gravity becomes an issue. Because a black hole takes an incredibly long time to burn out, these could be the last homes of humanity in the universe.

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