The statement that linear motion is relative while rotation is absolute is not entirely accurate. Both linear motion and rotation can be described relative to a reference frame, and their relativity depends on the choice of that reference frame.
In classical mechanics, the laws of physics are formulated in terms of inertial reference frames. An inertial reference frame is a frame of reference in which an object not subject to external forces moves with a constant velocity (including zero velocity). In such a frame, linear motion can be described relative to that frame, and the laws of physics are consistent.
Linear motion is considered relative because the measurement of an object's velocity or position depends on the choice of reference frame. For example, if you are riding in a moving train, your velocity and position measurements will be different depending on whether you consider the train as your reference frame or an observer standing on the ground. However, the laws of physics, such as Newton's laws, remain the same in both reference frames.
On the other hand, rotation can also be described relative to a reference frame. If you consider an object rotating, its angular velocity and position can be defined relative to a chosen axis or point. However, there is a sense in which rotation can be considered absolute because the laws of physics related to rotation, such as conservation of angular momentum, hold true in all inertial reference frames. This means that the total angular momentum of a system remains constant regardless of the choice of reference frame.
It's important to note that with the advent of modern physics, particularly with the development of general relativity, our understanding of motion and reference frames has become more nuanced. In general relativity, the concept of inertial frames is extended to include frames in gravitational fields, and the laws of physics are formulated in a way that incorporates gravity as the curvature of spacetime. In this framework, both linear motion and rotation are described relative to the curvature of spacetime and are affected by gravitational fields.