It occurs to me that any force which acts on all of your atoms individually (as gravity does) would have the same feature that one would not "feel" accelerated. We only detect acceleration because parts of the system are acted on by different amounts right? When we accelerate in a car, the seat pushes on our back and we "feel" that because the car only accelerates our back at first, creating increased pressure and causing a chain-of-pushing that we feel (if we had a spring with a mass or a pendulum, we could see the acceleration too). But if a car somehow applied a force evenly on every atom in our body, it could accelerate us arbitrarily fast, and we would not be able to detect it. And if we had springs and pendulums with us, they would be uniformly accelerated too and we would not be able to detect the acceleration even with instruments right? So I have a hard time seeing how this thought experiment was at all surprising, and how it differs from any other force which may act on each individual atom/particle through a field. If you were a charged particle in an electric field, you would not "feel" accelerated either (assuming the charge was uniformly distributed) right? As I ask this question it also occurs to me that I suppose the charge on each atom would have to be proportional to the mass of the atom or else some atoms would be accelerated quicker than others and thus the acceleration would be detected. So is it really just the fact that inertial mass is the same thing as "gravitational charge" ensures this is always true? Is this the insight of this thought experiment? That the same stuff that causes acceleration is the very same stuff that resists it? So all matter is subject to force which is balanced by its own resistance to force, and thus all matter always accelerates by the same amount as all other matter within a uniform gravitational field?