1 Light Year Diameter Planet
Suppose a species has limitless resources and the ability to construct megastructures of an astronomical scale.
Would it be possible, given antigravitational technology (hypothetical), to create a planet with the diameter of a light year? Suppose there is an internal structure inside the planet that enables the gravity of the planet to be cancelled at correct points to ensure stability. This would stop the planet from collapsing in on itself?
Perhaps without antigravitational technology the planet would turn into a blackhole due to the massive mass (assuming relative composition to that of the earth)?
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1 answer
As the others have said, without anti-gravity it won't work, with anti-gravity you can pretty much do whatever you want.
Just a few things to think about
With a diameter of 1 light year ($5.879 \cdot 10^{12}$ miles) you get a circumference of $1.85 \cdot 10^{13}$ miles, and a surface area of $1.09 \cdot 10^{26}$ square miles.
The Earth only has a surface area of $1.969 \cdot 10^{8}$ square miles, so this thing is going to be really really big. If you took a big bag of Earths and skinned them like oranges, you would need $5.535805 \cdot 10^{17}$ Earths to cover this monster.
The point is that heating and lighting the place is going to be hard. You'd essentially want fleets of Sun-sized stars orbiting the planet in order to keep things warm and to provide enough light to grow things.
If you only put them around the equator, one AU away from the surface and spaced them 2 AUs apart, you'd need $99,505$ suns to circle the planet. However you'd need more than one band to keep the place warm, so maybe 2 bands at the 45th parallels too...
Just to put the numbers in normal notation instead of scientific notation:
surface area of $1.09 \cdot 10^{26} = 109,000,000,000,000,000,000,000,000$ square miles
$5.535805 \cdot 10^{17} = 553,580,500,000,000,000$ times the surface area of Earth
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