So I understand that in classical physics, movement only makes sense with reference to another body. So my car moves relative to the surface of the road, for example. But it seems that with acceleration, things appear to be more complicated than that. Let's say we have two objects in space. One is accelerating relative to the other, and as an outside observer, I can't tell which unless I have some kind of objective frame of reference. But the objects can tell, because one of them experiences the acceleration and the other does not. Put in another way, if I spin myself around and around, how can I tell that I'm spinning, and the universe is not? Of course, I can feel centrifugal force acting on my body, so I know it's me; but who is to say that the universe shouldn't be spinning instead? Is there some kind of objective frame of reference that says that something is fixed? FYI - my physics is limited to college PHYS101, so please keep it simple if you can.