Can you have a livable environment without gravity?
Since many people have lived in zero gravity for months at a time,
I would have to say yes. If you mean that the "environment" is a self-
sustaining ecosystem, no one has tried it yet, but the "Biosphere" experiment
in Arizona was an attempt to see if a small self-contained ecosystem could
sustain human life for more than two years - it worked. Of course they had
gravity there, so it is not a complete test of your question. You might be
interested to read some of the information about biological experiments
conducted on the Space Shuttle in NASA's "Space Link" bulletin board.
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Update: June 2012