An exercise for fun.
Copernicus argued that the earth revolved around the Sun. What’s odd about this statement is that the Sun is presumed to be static. However, the Sun itself resides within the universe and occupies a location, which, oddly enough, is presumed to be an absolute location. However, does the Sun move? And if it does, how? Isn’t the Sun outside the center of the Milky Way Galaxy? Does it revolve around the core of this galaxy? Does the Milky Way itself revolve around a central galaxy?
Einstein argued that it’s all relative. The motion of the Sun and the motion of the Earth are relative to the point of reference. From the perspective of someone at the Sun, the Earth moves relative to the Sun and the backdrop of stars. Or it could be said that the Earth sits still and all the stars move around the Earth and Sun. From the perspective of the Earth, the Sun moves around the Earth relative to the Earth and the backdrop of stars. However, it could also be said that the Sun and Earth sit still and all the stars move around the two of them, and the Earth happens to rotate on an axis roughly perpendicular to the axis between itself and the Sun.
The reason we can say this is because the moving objects are said to be in an “acceleration field”. To Einstein, a force such as gravity merely meant an acceleration field – something that got an object to move. Thus, the alleged “gravity” of Earth is nothing more than an acceleration field experienced by objects that causes them to move towards the Earth. Likewise, the periodic oscillating of the Earth moving close to and farther away from the Sun is just an acceleration field.
However, mathematically, it’s simpler to explain the oscillating phenomenon according to Newton’s First and Second Laws of Motion and say that the smaller objects encircle the larger objects. Notably, due to the gravitational force of the Earth, the Sun also moves in a circular motion.
Why not keep the Sun static? The reason for allowing the Sun to “move” is to eliminate the observed “acceleration field” that cancels the acceleration field we expect to be created from Earth’s gravity. However, this presents a bit of a problem. If the Sun “moves”, then there must be some “static space” in which it “moves”. In other words, there must be some reference frame in which all of the acceleration fields appear to be only Newtonian (where gravity is roughly F=G*m1*m2/r^2).
Now let’s apply this to the Sun and the Milky Way Galaxy. Like the Earth, the motion of the Sun can be easily understand as Newtonian if and only if it revolves around the center of the Milky Way (specifically, the average center of gravity of the stars at the center of the Milky Way). But like the Earth, the Sun creates a gravitational pull on the center, creating an acceleration field. If the center of the Milky Way is to remain static, then a counter-acting, non-Newtonian acceleration field is necessary to keep it still. Again, we would like to find a reference frame in which all of the acceleration fields are only Newtonian, so this implies that the center of the Milky Way must move.
This same argument can be applied again to the Milky Way and the center of gravity of all other galaxies in the universe, thus implying that there is a universal reference frame in which all acceleration fields create only Newtonian forces. This universal reference frame might also thus be considered the “absolute reference frame”, since in this frame of reference, all of “space” itself can be considered “static” (not moving).
Consequently, if we believe forces have some origin, such as in mass (i.e. that mass creates gravity), then there must be a universal reference frame and “static” space. Given the size of gravity, it makes no significant difference in calculating motion. Given that calculations can be done based on relative reference frames, most motion can be calculated to a very high degree of accuracy without taking into account the gravity of distant stars much less assuming a universal reference frame and static space.