In his original 1905 paper on special relativity titled "On the Electrodynamics of Moving Bodies," Albert Einstein introduced "spatially relative" terms such as "inertial frame" and "events" when discussing time dilation to emphasize the fundamental idea that all motion is relative and that there is no absolute standard for measuring time and space.
Here are the reasons why Einstein used these terms:
Inertial Frames: Einstein introduced the concept of inertial frames of reference, which are frames in which an object not subject to any forces moves with a constant velocity or remains at rest. The laws of physics, including the principles of special relativity, hold true in all inertial frames. Einstein used this concept to show that observers in different inertial frames may measure different intervals of time and space due to time dilation and length contraction. The use of inertial frames reinforces the principle of relativity, where no frame is considered more privileged than any other.
Events: In special relativity, "events" refer to specific occurrences in spacetime, represented by a combination of a location in space and a moment in time. Einstein used the concept of events to describe the positions and times of different observations made by different observers in relative motion. The analysis of events allows one to understand how measurements of time and space vary between observers in different inertial frames, leading to the discovery of time dilation.
By employing the terms "inertial frames" and "events," Einstein highlighted the notion that the observed properties of time and space depend on the observer's reference frame. This was a revolutionary departure from the traditional Newtonian worldview, where time and space were assumed to be absolute and invariant. Special relativity's emphasis on the relativity of motion and its use of spacetime events laid the groundwork for a new understanding of the universe's fundamental nature and led to significant advancements in modern physics.