[obastani]

A sibling post talked about special relativity, but you don't need special relativity to answer this question, just Galilean relativity. The answer is that you can measure velocity in any inertial reference frame you want. If you are in an inertial reference frame, you can say your own speed is 0 km/hr, or you can consider an inertial reference frame of the galaxy, which is some ungodly number, or you can measure your velocity relative to the cosmic microwave background. It's all the same -- only the relative velocity between you and another object has a real physical meaning.

[ingenuous2]

That is to say, everything is in an inertial reference frame, so there is no absolute center.

Just like we can only mark a distance traveled as from a starting point.

[plaguuuuuu]

Easiest way to think about it: Picture two satellites crashing into each other. It looks exactly the same if both satellites are doing 100 m/s, or one satellite is stationary and the other is doing 200 m/s.

>1. When does this stop? What is the most supreme center in the universe?

Is one of the above satellites the centre? The other? It doesn't change anything about the impact force. There's no physical difference. So we can choose either satellite to be the centre (0 km/h) or neither (100 + 100, 50 + 150, whatever)

>2. Just as the child could roll down his window, and have instant access to the outside, is there a way we can do the same?

The problem with the earth is that it's really big/heavy and gravity prevents stuff just sort of "hanging in the air" at human-height, relative to the rotation of the earth... speaking in practical terms, I mean.. so it's harder to visualise. But at the same time, sure. If you're on the earth, sitting in your chair, doing 0 m/s relative to the ground, and a satellite hanging around at head height that ISN'T rotating along with the earth hits you in the face, you just successfully accessed an intertial reference frame that's outside the earth :D