I keep hearing how motion only exists relative to something else. But what if there is some object in space, and it is the only object in the universe. It has some kind of propulsion system, and it activates it, goes to a certain speed, then stops accelerating. Is that object really moving, or since there is nothing to compare it to, then is it technically not moving? If it is not moving, where did its kinetic energy go? Another situation: What if you had only two objects in the universe. They are moving at the same speed in the exact same direction so are parallel to each other. Are they moving at all either? And now here raises another point. We all know that the earth is revolving around the sun, not the other way around. How can we know that as a fact since due to inertial frames, the sun and all the stars may just as well be rotating around the earth instead? Thanks.