IANA physicist but everywhere I look I see the same mistake being made. The shell theorem does not apply to disks or galaxies.
I see the same simplification in the most advanced writings. Namely 1) matter out to a radius can be treated as a point mass in the center and 2) we can ignore gravity from mass outside a radius because it all cancels.
These simplifications work for spherical shells or solids of uniform density. They do not apply to disks or rings (galaxies). Period.
I see the same simplification in the most advanced writings. Namely 1) matter out to a radius can be treated as a point mass in the center and 2) we can ignore gravity from mass outside a radius because it all cancels.
These simplifications work for spherical shells or solids of uniform density. They do not apply to disks or rings (galaxies). Period.