Although the initial problems were solved by making space and time relative to the chosen observer, this solution (of course) was not consistent with 3D space (= Euclidean space) and constant time of Newton Mechanics. This problem, however, was tackled by the renowned Jewish-German mathematician Hermann Minkowski (1864 - 1909). In 1907 he showed that the 'relativity of space and time' can be mathematically accepted in a so-called Euclidean spacetime: Three spatial dimensions to which time has been added as a fourth dimension. So in fact we are dealing with a flat, i.e. non-curved, four-dimensional spacetime.
In terms of the Sphere Observer, however, the space time is by definition curved because of its shell structure. A 'non-curved space time' can at most mean that its curvature is negligibly small because the afterworld of that space time contains absolutely no matter. Minkowski's spacetime is therefore a spacelike spacetime and as such it fits only with intergalactic space.
The fact that Minkowski's space-time is called Euclidean, therefore, is not transparent. The name 'Euclidean' indeed suggests that this is a 3D space as we people know it. The impression is being conveyed that this four-dimensional (4D) spacetime differs so little from the usual 3D space with constant time, that today's (modified) western physics is coming away unharmed. By mixing up the concepts space and spacetime as well, this effect is obviously increased. In favor of Newton mechanics, the illusion is thus maintained that 3D space with constant time does really exist.
In this way, modified Western physics seems to be able to ignore the abstract two-dimensional spacetime or the earlier world of the Sphere Observer, as assumed earlier because of the sensitivity-hypothesis, and, in fact, confirmed by Einstein with his absurd discovery of the cause of gravity as we know it. But appearances are deceptive ...
Space and spacetime are clearly totally different physical quantities.
4D Minkowski spacetime, for example, only has a mathematical significance, not a physical one. So it’s absolutely not an ordinary-like 3D space, as has been thought frequently, but an abstract spacetime.
Note 35 According to Paul Ehrenfest, a colleague and very good friend of Einstein's, who considered multi-dimensionality as his favorite subject, only a 3D space can have a physical meaning. With complex integral calculations he was able to demonstrate that 4D spacetime, and in fact every spacetime with a different set of dimensions, should be regarded as an assumption. Just as you can project a 3D Earth on a 2D plane, 3D space is in fact an image of the points in 4D spacetime, that occur simultaneously from our perspective. If physical space would have more than three dimensions, this projection from 4D spacetime could not take place. Thereby, our visible universe would no longer have a right to exist. Ergo: physical space must have three dimensions.*
So in order to be able to utilize Minkowski spacetime in actual practice, events defined in the usual classical way, i.e. in normal Euclidean space (3D) with constant time, can easily be converted to 4D Minkowski spacetime using Pythagoras’ theorem. This so-called coordinate transformation is executed as follows:
An observer can roughly estimate the spatial distance between two random events A and B, but the "interval in time", he can only measure with his own (atomic) clock, not as a "distance in time".
However, the distance in time can easily be calculated by multiplying the time interval as measured by the clock, by the speed of light, about 300,000 km/s (in vacuum 299,792,458 meters).
Suppose that the interval of time between events A and B, as measured by the observer, is ten minutes, then the distance light travels in space would roughly equal 10 x 60 x 300,000 km.The interval in time between the two events has so become a spatial distance. {Note The velocity of light must therefore be considered as a constant (c).}

The interval in spacetime is now obtained using Pythagoras’ theorem (see Fig.). "Take the square of the distance between two events (AB) and the square of the distance that light would travel in the period of time between the two events (AC): subtract the smaller of these squares from the larger one and obtain the square of spacetime distance between the two events (BC) '.
C is the intersection point on a circle with radius AC, which is determined by the perpendicular line on line AB through point B. The radius AC is the measured time (10x60 seconds) multiplied by the speed of light.
It soon became apparent, conspicuously, that classical calculations of spatial intervals were more accurate when converted to intervals of spacetime using Pythagoras' theorem.* Thanks to a 'calculation tool' added by Minkowsky - I will come back to this later - it even turned out that this also applies to areas where there is material. So abstract 4D Minkowski spacetime turns out to be more reliable than the concrete 3D Euclidean space with its constant time we humans can perceive. Minkowski spacetime can therefore be interpreted as an abstract four-dimensional reality.
But note: The latter is only partially true, as will be shown later.
Continue to: 5.3.1. Visualization of spacetime