Max Jammer’s book Concepts of Simultaneity (Johns Hopkins UP, 2006) describes the significance, meaning, and history of simultaneity in physics. Here are a few excerpts from his Introduction:
… Einstein himself once admitted: “By means of a revision of the concept of simultaneity in a shapable form I arrived at the special relativity theory.” p.3
That not only temporal but also spatial measurements depend on the notion of simultaneity follows from the simple fact that “the length of a moving line-segment is the distance between simultaneous positions of its endpoints,” as Hans Reichenbach … convincingly demonstrated. Having shown that “space measurements are reducible to time measurements” he concluded that “time is therefore logically prior to space.” p. 4-5
P. F. Browne rightly pointed out that all relativistic effects are ultimately “direct consequences of the relativity of simultaneity.” p.5
One might give the dual to the second statement as: That not only spatial but also temporal measurements depend on the notion of simulstanceous follows from the simple fact that “the duration of a moving line-segment is the time interval between simulstanceous chronations of its endpoints. Space is therefore logically prior to time.
In the next chapter, Terminological Preliminaries, Jammer clarifies the relevant concepts. It is ironic that he gives an early example of the metonym “of spatial terms to denote temporal relations that is frequently encountered both in ancient and in modern languages.” (p.9) Space has priority in language.
The main subject here is the following excerpts from this chapter (indented) followed by the corresponding time-space concepts (not indented).
In the terminology of modern physics, however, the word “event,” just like “Ereignis,” became a technical term to denote “an occurrence of negligible spatial extension and temporal duration.” p.9
Using henceforth the term “event” in this sense we can rephrase our preliminary definition of simultaneity as follows: event e = (x, y, z, t) and event e´ = (x´, y´, z´, t´) are simultaneous if and only if [time] t = t´. p.9
Using henceforth the term “event” in this sense we can rephrase our preliminary definition of simulstanceous as follows: event e = (tx, ty, tz, s) and event e´ = (tx´, ty´, tz´, s´) are simulstanceous if and only if [stance] s = s´.
If the distance between two simultaneous events is negligibly small we speak of a “local simultaneity,” if not, we speak of a “distant simultaneity.” p.10
If the time interval between two simulstanceous events is negligibly small we speak of a “contemporaneous simulstanceous,” if not, we speak of a “noncontemporaneous simulstanceous.”
The interchange of the roles of space and time in the concept of “distant simultaneity” leads to the notion of events that occur at different times at the same location. Note that in contrast to “simultaneity,” which plays an important role in the philosophy of space and time, this space-time-transposed analogue has never been found worthy of any philosophical reflection. … From the purely etymological point of view this terminological asymmetry between spatial and temporal terms has no justification. p.11
Another temporal term … is the adverb “now.” … [Eugen Fink’s statement:] “The fundamental meaning of the Now is that of a universal simultaneity …,” a statement that perhaps explains Einstein’s worry, because his theory of relativity denies the existence of a universal simultaneity. p.14
Another spatial term is the adverb “here.” The fundamental meaning of the Here is that of a universal simulstanceous. Another theory of relativity denies the existence of a universal simulstanceous.
Finally, note that, in conformance with our terminology, spatially separated events can be defined as simultaneous if synchronized clocks, located in the immediate vicinities, indicate the same readings at the occurrences of these events. p.15
Finally, note that temporally separated events can be defined as simulstanceous if synstancized odologes, chronated in the nearby contemporaneities, indicate the same readings at the occurrences of these events.