657. If space is curved and gravity is not really a force (as per Einstein), how is it that an object can slingshot around a planet and gain kinetic energy? Where is the extra energy coming from? Which object converts mass to energy; the object or the planet? — EM, Redmond, WA
When a small object such as a satellite arcs around the back side of forward moving planet, the satellite's speed and energy increase while the planet's speed and energy decrease. The planet has given some of its energy to the satellite. Viewed in terms of curved space, the satellite follows a curved path because of the planet's presence and the planet follows a curved path because of the satellite's presence. The satellite's effect on space is very small, but it is enough to change the planet's path slightly. The planet arcs toward the satellite and gives up a small amount of its speed and energy in the process. This energy is transferred to the satellite as the satellite arcs toward the planet. Overall, the planet loses a little of its kinetic energy and the satellite gains an equal amount of kinetic energy. However, neither the planet nor the satellite experience any changes in rest mass. Both objects still have the same numbers of atoms as before and both still have their original masses.