Weightlessness is caused by free fall. When everything, including astronauts, fall at the same speed, there is no apparent effects of gravity. Measurements using atomic clocks have even shown that free fall nullifies gravitational time dilation as well. Which means that stable orbits around black holes (outside the event horizon) cannot be used for forward time travel. Weightlessness have health effects on the Human body because muscles and bones do not have to struggle against gravity, and that lack of training causes muscle atrophy and loss of calcium from bones, although those problems can be solved by not justifying things, see Advice of ways to stop justifying, multiple stages of justification poisoning, brain, inheritance of acquired characteristics and self-organization on http://purescience.wikia.com. This page is about zero gravity or microgravity. For cases when the gravity is just lower than Earths, see Lower gravity than on Earth.