In 1908 H. Minkowski [1] gave a four-dimensional formulation of the special theory of relativity by uniting space and time into a single entity - the four-dimensional spacetime (sometimes called Minkowski space). The essential question arising from here is whether this four-dimensional spacetime is a convenient graphical representation of what we consider to be an evolving three-dimensional world or is a mathematical model of a real four-dimensional world with time as the forth dimension.A consistent analysis of the consequences of special relativity (in terms of dimensionality of the world and in close connection with the problem of existence) shows that if we assume reality to be an evolving three-dimensional world an inevitable contradiction with all kinematic effects of special relativity is reached.

• The relativization of simultaneity can be explained either by assuming that the existence of the three-dimensional world is also relativized (observer-dependent) or by assuming reality to be a four-dimensional world whose existence remains absolute (observer-independent). The analysis of time dilation and Lorentz contraction also leads to the above alternative. C. W. Rietdijk [2] and H. Putnam [3] concluded that relativity of simultaneity directly proves the four-dimensionality of the world.

• The alternative "reality is either an absolutely (observer-independently) existing four-dimensional world or a three-dimensional world whose existence is relativized (observer-dependent)" cannot be resolved by the analysis of the relativistic effects of time dilation and Lorentz contraction because they are based on the relativity of simultaneity which gives rise to that alternative. The resolution of this alternative comes from the analysis of the conventionality of simultaneity and the twin paradox. The fact that the choice of the events considered simultaneous is a matter of convention (revealed by the impossibility to measure the one-way velocity of light) shows that reality is four-dimensional (see R. Weingard [4], V. Petkov [5]). If the world were three-dimensional its existence would be a matter of convention which is obviously unacceptable. Hence, if one continues to insist that reality is three-dimensional whose existence is relativized then its existence proves to be a matter of convention as well. The only options that remains is that the world we live in is four-dimensional. Then the conventionality of simultaneity proves to be trivial: since all events of a four-dimensional world are equally existent it is really a matter of convention which set of them (forming a three-dimensional slice of spacetime) will be considered simultaneous. The analysis of the twin paradox also shows that reality must be considered four-dimensional; if it were three-dimensional (even with relativized existence) this relativistic effect would not be possible [6].

• Once the problem of dimensionality of reality has been resolved the real challenging problem turns out to be the flow of time . It was H. Weyl who first connected the flow of time with consciousness in the context of special relativity:
  "The objective world simply is, it does not happen. Only to the gaze of my consciousness, crawling upward along the life line of my body, does a certain section of this world come to life as a fleeting image in space which continuously changes in time" [7].
  This conclusion seems inevitable: on the one hand, relativity tells us that our body exists equally at all moments of our life but, on the other hand, we realize ourselves only at the moment "now" which shows that it is the hypothesis of our consciousness crawling along our world line that can reconcile relativity with our everyday experience.

• The theory of relativity has profound implications for our understanding of reality, existence, the flow of time, free will , etc. This constitutes the greatest challenge, posed by science, that humankind has ever faced.

The analysis of both the problem of interpretation of the consequences of special relativity and the problem of existence also has a pedagogical value. It helps to overcome a common difficulty in teaching special relativity: the understanding of relativity of space and time and the inseparability of spacetime. This analysis shows that space and time are relative in a sense that spacetime is not objectively divided into space and time which means that: (i) an observer is needed to describe the inseparable spacetime in terms of space and time, and (ii) the observer is free (in certain limits) to choose whether or not time is orthogonal to space.

1. H. Minkowski, "Space and Time" in Lorentz, Hendrik A., Albert Einstein, Hermann Minkowski, and Hermann Weyl, The Principle of Relativity: A Collection of Original Memoirs on the Special and General Theory of Relativity. New York: Dover, (1952) pp. 75-91.
2. C. W. Rietdijk, Philosophy of Science 33 (1966) 341-344; 43 (1976) 598-609.
3. H. Putnam, Journal of Philosophy 64 (1967) 240-247.
4. R. Weingard, British Journal for the Philosophy of Science 23 (1972) 119-121.
5. V. Petkov, British Journal for the Philosophy of Science 40 (1989) 69-76.
6. V. Petkov, (i) "The Flow of Time According to Eleatic Philosophy and the Theory of Relativity", in C. Toegel (ed.), Structur und Dynamik wissenschaftlicher Theorien, Frankfurm am Main, Bern, New York: P. Lang, (1986), pp. 121-149, (ii) "Weyl's view on the objective world", in W. Deppert, K. Huebner, A. Oberschelp, and V. Weidemann (eds.) Exact Sciences and Their Philosophical Foundations. Frankfurm am Main, Bern, New York, Paris: P. Lang (1988), pp. 519-524.
7. H. Weyl, Philosophy of Mathematics and Natural Science, New York: Atheneum, (1963) p. 116.

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