General Relativity
Einstein’s theory of general relativity explains what happens in a free-fall setting. This theory encompasses a principle called the Strong Equivalence Principle. This principle state that if we were to do a free fall experiment where there was no air resistance, the results would be equivalent to an experiment done without gravity. Similarily, if we were to do an experiment on the moon, where there is no air resistance, in which we launched a human and a golf ball at the same velocity simultaneously into the air, each having the same trajectory, we would find that they follow the path at the same speed and would land at the same time. This is due to gravity having the same acceleration on objects, no matter the size or mass.
Another experiment that would help explain this theory is one using light and a space ship.
Imagine a space ship travelling at an extremely fast speed, near the speed of light disregarding that space shrinks as you near the speed of light, and there is a beam of light intersecting the ship’s window at a right angle. From an outside view, we would observe the light beam travelling across the ship as the ship moves up.
External View
Internal View
If we were observing the light beam from inside the ship, we would find that the light beam would fall to the floor following an arced path.