Hollow Worlds

There seems to be a group of UFOlogists, or ancient astronaut theorists, or whatever they call themselves, that takes seriously the notion that the Earth and the Moon are hollow. Honest to God. They say the Moon is a gigantic interstellar spaceship put in orbit around Earth to allow its crew to monitor Earth, and they further hold that there are likely alien bases, living dinosaurs, and all manner of stuff inside our planet.

Of course, Newton's Laws of Gravity allow us to "weigh" worlds by observing their interactions with other bodies and plugging the data we get from that observation into Newton's equations. That approach has allowed astronomers to determine the masses of stars and galaxies across the universe, and it has allowed humans to navigate space probes across their home solar system with exquisite precision. It works, and it is an extraordinary testament to the power of the human mind.

Those same laws tell us the concept of a hollow Earth and Moon is nonsense. We have directly measured the density of the outer shells of both worlds, and we know precisely how they interact gravitationally. If they were hollow, that interaction would be different because they would be less massive. Hollows lack the mass of solid, compressed rock.

Alien visitation of Earth early in the human epoch is not impossible. Continued visitation to this day seems to stretch reality almost-- but not quite-- to the breaking point. Making such a case, therefore, is extremely difficult for the most serious, rational investigator. Allowing such fantasies as exotic, abundant life filling huge open spaces in a hollow Earth to go unchallenged only makes the fundamental task virtually impossible.