Have you ever wondered about all those famous thought experiments famous scientists come up with? Galileo and the Leaning Tower of Pisa, Einstein and his trains, Schrodinger and his cats. While we can't yet reenact what Einstein and Schrodinger had in mind, we can test Galileo's idea.
Galileo theorized that objects fall at the same rate (regardless of their mass, size, shape) when they are subject to the same gravitational acceleration (on the Earth, Moon, etc.) and air resistance is negligible (i.e., in a vacuum). Essentially, this means that in these conditions, any two objects will fall at the same velocity and land at the same time. Building an experiment to drop pairs of objects is no big deal, but building one to drop them in a vacuum is.