Response- Caiti

I like Eric's suggestion that the ultimate frame of reference is from the center of our expanding universe. I hadn't even thought about that. I suppose Einstein's theories of relativity regarding motion are based on motion in empty space, but if our universe has a center then we are not in empty space, are we? This would suggest that there is absolute motion, at least in our universe, as believed by Newton.

If we measure all motion from the universe's center (which is stationary) using the speed of light(which is the same for all observers) for distance and time, we can determine absolute motion. Afterall, the entire purpose of finding an ulitmate frame of reference is so we can know absolute motion, right?

Of course if we do not wish to know absolute motion, if we instead need to compute motion relative to the Earth, then we can use different frames of reference as tools, as suggested by Caroline. These view points are not ultimate, nor is the calculated motion absolute, but relative frames of reference can be more practical than using the center of the universe, especially for situations on Earth.

So, conclusively, there are two types of motion. Absolute motion (Newton) and relative motion (Einstein). Absolute motion must be determined from the center of the universe because that is the only stationary point in the universe. (I don't think the possibility of something beyond our universe has any importance since space and time only apply specifically to the universe we are in.) The second type of motion, relative motion, can be determined from any non-stationary point of view.