In his original paper on special relativity titled "On the Electrodynamics of Moving Bodies" published in 1905, Einstein introduced the concept of time dilation by using spatially relative terms such as "inertial frame" and "events" to emphasize the fundamental nature of the phenomenon.
Einstein's approach was rooted in the idea that space and time are intimately connected, forming a unified spacetime framework. He aimed to develop a theory that treated space and time on an equal footing and was consistent with the principles of relativity. In this context, he needed to redefine our understanding of time.
By using the term "inertial frame," Einstein referred to a reference frame in which an observer experiences no acceleration or gravitational forces. In such a frame, the laws of physics take on a simpler form, and the observer can define a standard set of coordinates to describe events in spacetime.
The concept of "events" is crucial in understanding time dilation. In this context, an event refers to a specific occurrence with well-defined spatial and temporal coordinates. For example, the emission and absorption of light by two synchronized clocks located at different positions would constitute two separate events.
Einstein's use of spatially relative terms aimed to highlight that the measurement of time is not absolute but depends on the observer's relative motion and position. In his paper, he showed that the measurement of time intervals between events could vary for observers in different inertial frames. This led to the discovery of time dilation, where time appears to run slower for moving objects relative to stationary ones.
By introducing the concepts of inertial frames and events, Einstein emphasized the importance of considering the relative nature of time in the context of special relativity. It laid the foundation for a profound paradigm shift in our understanding of space, time, and their interplay, leading to the development of the theory of spacetime in general relativity.