Final answer:
The idea that free fall is indistinguishable from zero gravity is known as the equivalence principle in the field of Physics.
Step-by-step explanation:
The concept that free fall is indistinguishable from zero gravity is a fundamental idea in physics, known as the equivalence principle. Albert Einstein proposed that there is no way to differentiate between the sensation of free fall in a gravitational field and the sensation of weightlessness as experienced far from any significant sources of gravity. For instance, astronauts inside the International Space Station (ISS) feel weightless not because there is no gravity, but because the ISS is continuously free-falling towards the Earth, thus creating the same effects as being in deep space, free from gravitational forces.
In practice, if two people were to jump into a bottomless chasm, ignoring air resistance, they would experience this principle firsthand. They would accelerate at the same rate due to gravity and would be able to throw a ball to each other as if there was no gravity at all. The ball would maintain a straight trajectory between them.