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Dr Nathan and his team survive the gravity inversion, they land on the Hollow Earth, which is situated below the Earth's surface:

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How does the Hollow Earth get sunlight?

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    In stories of a hollow Earth (Burroughs' Pellucidar and Mike Grell's Warlord comic, to name two) there is a small sun at the centre of the Earth. This could work if it had negative mass, hence repelling and being repelled by ordinary mass, explaining why there is outward-pointing gravity at the inner surface. (Of course, it doesn't fit in any way with seismic measurements of our planet, but for fantasy, it is an acceptable explanation.) Oct 25 '21 at 7:53
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    Negative mass has a number of problems as an explanation, beginning with probably not existing and only getting worse from there. Another issue is that a star made of negative mass would be unstable: two particles of negative mass would exert a negative force on each other as a function of the distance between them, just like positive-mass particles, but because Newton's 2nd law also has a negative sign when the mass is negative, this force would be repulsive: instead of accelerating toward each other as particles of positive mass would under an attractive force, they would accelerate away.
    – Adamant
    Oct 25 '21 at 14:37
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    To be clear, a star with positive mass (which has the advantage of definitely existing) would not work, either. Although it is less clear, extensive observations and simulations indicate that fusion would not be possible in a star of that size. Its gravitational field would simply not produce sufficient heat or density.
    – Adamant
    Oct 25 '21 at 14:40

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