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Why do we say spacetime, as though stressing that this is some kind of unity: space together with time?

I understand a little that mathematically we have 4 variables in the same (Einstein) equation(s) (3 for space, 1 for time), but can it be put in other words?

P.S. This question was inspired buy reading quantum origin for spacetime review (idea that entanglement produces space-time) and especially the original paper but it is another story - matter sources curvature (at macro level) and matter can be in a superposition of different positions (at quantum level), allowing superpositions of different curvatures. In other words, "quantum gravity" allows superpositions of different geometries.

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    $\begingroup$ IMHO, if you don’t yet understand how space and time are united in Minkowski spacetime through the invariant spacetime interval and the Lorentz transformations, you should not be complicating your understanding by reading about quantum spacetime. Basics first! $\endgroup$
    – G. Smith
    Aug 3, 2019 at 17:26
  • $\begingroup$ That said, your question is a good start toward learning the basics. $\endgroup$
    – G. Smith
    Aug 3, 2019 at 17:32
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    $\begingroup$ The idea is that one observer’s space coordinate will be mapped in the reference frame of another observer to a mixture of the second observer’s space and time coordinates. So the distinction between space and time is frame-dependent and in a sense artificial. This is similar to how in ordinary spatial geometry we recognize that x-, y- and z- coordinates are tied to an arbitrary choice of axes and so it doesn’t make much sense to consider them separately from each other. Instead, the axes of those coordinates are merely components of a more fundamental thing we call “space”. $\endgroup$
    – GenlyAi
    Aug 3, 2019 at 20:08
  • $\begingroup$ To understand the technical details better, see any book about relativity. $\endgroup$
    – GenlyAi
    Aug 3, 2019 at 20:09
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    $\begingroup$ It might be good to read the source of the notion of “space-time”... Minkowski’s en.m.wikisource.org/wiki/Space_and_Time $\endgroup$
    – robphy
    Aug 3, 2019 at 22:34

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Why do we say spacetime, as though stressing that this is some kind of unity: space together with time?

Special Theory of Relativity (STR) teaches that time is a coordinate rather than a universal parameter.

That is, when we transform (using the Lorentz transformation ) from one inertial coordinate system (ICS) to another, relatively moving ICS, the time coordinate is involved in this transformation so that, for example, a spatial coordinate in one ICS is a mixture of one or more spatial coordinates and the time coordinate in the other ICS.

That is, space and time are now 'woven' together into a spacetime where a purely spatial or temporal translation in one ICS is a mix of both temporal and spatial translations in a relatively moving ICS.

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