> With gravity, mass plays the role of charge. It bends space-time around it, and it is always positive. There is no negative mass, so you can always infer the one real arrangement of stuff inside from the warping of space-time at the surface of your box.

This is a simplification to the point of just being wrong. Gauss’s law works for gravity; if you put a shell around Earth you have no way of knowing whether it’s a planet or an Earth-mass black hole inside. Assuming spherical symmetry of density, anyway.

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subtlepseudonym 2 hours ago | parent [-]

> This is a simplification to the point of just being wrong.

Is it? If you project a "3D" singularity onto a surface, it's still a point. If you project a roughly spherical object, it's roughly circular.

Gauss' law in this case would apply to the total force of gravity exerted by the mass within the shell, not its distribution across the surface of the shell.

	▲	Sharlin 16 minutes ago \| parent [-]
		I assumed the analogy meant that just by measuring the gravitational field outside the shell, you could figure out the mass distribution inside. And you can't, not in the general case.