Einstein's statement that "motion is relative" refers to the concept of relativity in physics, specifically the theory of special relativity that he formulated. In special relativity, Einstein proposed that the laws of physics should be the same in all inertial reference frames, regardless of their relative motion. This principle challenges the classical notion of absolute space and time and introduces the idea that the observed physical phenomena can depend on the observer's reference frame.
In the framework of special relativity, both space and time are combined into a four-dimensional continuum known as space-time. Objects and events are not solely described in terms of spatial coordinates (position) but are also described by their temporal coordinates (time). The concept of motion in this context involves changes in both spatial and temporal coordinates.
According to special relativity, there is no preferred or absolute reference frame in which all motion can be measured objectively. Instead, the physical laws and observations are relative to the observer's reference frame. Different observers in relative motion will have different perceptions of space and time intervals, lengths, and even the order of events.
For example, the famous "twin paradox" illustrates the relativity of motion. Suppose one twin travels at a high speed in a spaceship while the other twin remains on Earth. Due to the relative motion, time will appear to pass more slowly for the traveling twin compared to the stationary twin on Earth. When the traveling twin returns, they will have experienced less time than the stationary twin, resulting in an apparent discrepancy in their ages.
Einstein's assertion that "motion is relative" reflects this fundamental shift in our understanding of the physical world, recognizing that observations and measurements are intrinsically tied to the observer's frame of reference. It highlights the idea that the perception of motion and the relationships between objects depend on the observer's relative motion, as described within the framework of special relativity.