Mecha Talk

That may be true, however even the biggest colonies have negligible gravitational pulls of their own it would be impossible for a person to notice there being gravity on a colony that wasn't spinning to create gravity inside it. If the colony did generate its own gravitational field, then we wouldn't see people working on the outside with tethers to their suits.

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The Gundam wiki

"Reality makes a crappy special effects crew." - Adam Savage

Yep, since colonies are too small to produce a useful amount of gravity, they spin to simulate the effect of gravity.

Not to mention that, no matter how massive a colony is, it won't produce much gravity inside the colony because all the mass is around you and pulling you in different outward directions. I recall that, mathematically speaking, the gravitational forces inside a perfect hollow sphere cancel each other out and leave you with zero gravity at any point within the sphere's interior. (So much for all those "hollow Earth" fantasy epics.) A space colony isn't a perfect sphere, but it'll still have the same problem, so spinning it is your only real option.

-- Mark