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Force of Nature

In case you skip it...

In an environmentalist allegory, this episode establishes that excessive warp travel actually damages the fabric of spacetime. Too much damage in a given area creates a subspace rift, disrupting starship travel and communications and rendering nearby planets uninhabitable. They sure are pretty, though.

A big swirly purple thingy

As a result, the Federation institutes a speed limit of warp factor five. Starfleet vessels may only exceed warp five in case of extreme emergency or with special authorization from Starfleet Command.