Say I have two masses connected by an unstretched massless spring at some height above a planet with a strong gravitational field. Once I let go, both masses would follow geodesics in spacetime towards the center of the planet. Because the masses travel radially, the spring would compress and store spring potential energy. Where does the spring energy come from? It doesn't come from a force since gravity isn't a force in GR. Classically, we can say that the cosine of the gravitational force vectors compresses the spring. Classically, it alines with F=-dU/dt. But in GR, it seems that the compression just happens out of nowhere, simply because space gets narrower. I thought about the potential energy due to height being converted into kinetic and spring energy as an explanation. But this sounds too classical for me. I also thought of the energy coming from spacetime itself. But I'm not so sure. If this were the case, then the surrounding spacetime would lose energy. This might make sense, as the mass falls, the spacetime it leaves behind becomes slightly less warped due to the two masses simply not being there. But then again, it enters a new region, and the masses simply warps that region as well. This reminds me somewhat of the rocket propulsion problem but instead of trading chemical PE for motion, it trades potential energy due to position in height for spring compression energy and kinetic energy. This reasoning is still seems a bit classical, so I'm not certain.