From everyday life, we’re used to the Earth’s gravity pulling every body down to the ground, and the strength of that force is called its weight. If no such force is present, then bodies placed at a certain location in space simply stay where they are, even without any support. Whenever that is the case, we are in a situation of weightlessness.

There are two types of situation in which weightlessness occurs. First of all, one could move far, far away into space, distancing oneself from all massive bodies so far that their gravitational influence becomes negligible. This would be a truly gravity-free situation. The second type of situation is more common. In free fall – be it an elevator plunging to earth, be it a space-station like the ISS in orbit around the Earth – bodies are weightless. The fact that, at least locally, there is no way to distinguish between these two types of weightlessness is embodied in the equivalence principle, one of the fundamental building blocks of the general theory of relativity.