The first principle of relativity as presented by Einstein’s 1905 paper ‘On the electrodynamics of moving bodies’ states that all frames of reference are equally valid. The second principle on that same paper is contradictory to the first and their reconciliation gives rise to a multitude of physical impossibilities; contradictions.
Not just paradoxes, because true contradictions are fundamentally fallacious. According to relativity theory it is just as valid for an observer to be moving so that their time is slower than it is that a ‘fixed’ observer is ‘relatively’ moving; hence slowing their time.
In Einstein’s words:
“the same laws of electrodynamics and optics will be valid for all frames of reference for which the equations of mechanics hold good.”
What is interesting is that just a little further down the page, he states the second principle, which is that ‘c’ is the same for all observers:
“and also introduce another postulate, which is only apparently irreconcilable with the former, namely, that light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body. These two postulates suffice for the attainment of a simple and consistent theory of the electrodynamics of moving bodies based on Maxwell’s theory for stationary bodies. The introduction of a “luminiferous ether” will prove to be superfluous inasmuch as the view here to be developed will not require an “absolutely stationary space” provided with special properties, nor assign a velocity-vector to a point of the empty space in which electromagnetic processes take place.” 
So he effectively gets rid of the aether field once and for all, while simultaneously bestowing upon every relativist their own conceptual universe of counter-bending time and matter.
The issue is that, as he states, these two main postulates are not only irreconcilable in and of themselves; they also strongly negate each other. If all reference frames are equally valid, and all observers have each their own measure of time, who’s reality is true?
It is just as valid to say the road is moving under the car in this physical reality. Mutual time dilation is an inherent paradox that arises from relativistic math wherever it is applied; it does not reflect real-world phenomena. The twin paradox, clock paradox, grandfather paradox, car & garage paradox, Ehrenfest/Einstein paradox, Twin frog paradox, and countless others others emerge from its fundamental contradictions.
General relativity has curved space-time and special relativity has flat space-time. In either case, matter is able to bend, wether due to its velocity or its curvature path of least resistance. This was able to explain away why the interferometer arm shrank during mitchelson, Morley, gale, Sagnac,
Special relativity is special in that it does not account for gravity, and the only reason is because without doing away with it, the entire axiom falls apart; becoming riddled with errors and contradictions. General relativity employs gravity as the curvature of space-time; a three- dimensional, and mathematically tangible fabric alleged to constitute the universe.
The clear contradiction exhibited by both theories is that one clock still must work faster than another clock, but there is no distinction between which clock is running faster because, as aforementioned, the first principle dictates that all frames of reference are equally valid.
neither time nor space have any properties, and hence neither can be dilated, contracted, or warped, lest it be in the realm of metaphysical mathematics.
This is where the theory breaks down and Einstein, Dirac, Dyson, Feynman, Sagan and all accredited proponents of relativity could not produced an answer to the simple question of; “Which clock is moving?” Herbert Dingle in Science at the Crossroads  presents a glimpse of the countless correspondences he had with so-called ‘specialists’ on relativity, including members of the Royal Society and publishers in American and British scientific journals. The former of which were often silent or simply condescending and the latter, younger and particularly not adept, met the criticism of the magical spell that ‘relativity’ is with inexhaustible diversions and preposterous responses avoiding the question altogether.
Dingle presents the clear question he posed scientific authorities as it was presented it in his multiple papers, which were all denied publication rights from the highest-ranking journals in which he was deeply acquainted with and esteemed for publishing in. It states: “Two exactly similar clocks, A and B, are in uniform relative motion, Einstein’s special relativity theory (1) that the motion is wholly relative, i.e. belongs no more to one clock than to another; (2) that the clocks work at different rates, i.e. one works faster than the other. My question is: what, consistently with the theory, determines which clock works the faster?” For this question he received no response and when he did, other than baseless, ad- hominem insults of his intellect and diversions, he was met with letters of genuinely confused scientists in the field of relativity who had accepted that understanding it was beyond them. Dingle sets the tone of the times when referencing the case of a genuinely brilliant good friend of his as evidence of the severe psychological effects posed by the unquestionable and authoritative nature of notable scientific bodies.
These organizations are supposed to be servants of the public and instead they continue to go against their foundational principles of empirical truth and the scientific method. The scientific method is, simply, hypothesizing of and finding physical, replicable, observable experimental phenomena from which to subsequently formulate an opinion or theoretical model. Such phenomena must be, either through theory or pragmatic terminology; in logical and objective terms, explained with reference to the natural mechanisms behind it. Relativity employs the opposite; its genesis is in mathematics, and it seeks to only defend itself from all angles instead of engage in an objective juxtaposition to any criticism of it.
That is the attitude of virtually every renowned scientist in the field of theoretical physics. Scientific organizations and their associated scientists base an entire world view on assumptions and contradictions, and still present it to the world and advertise it as absolute truth (of course always staying away from the Newtonian notion of absolute truth in itself) without any regard for the effect that this psychological conditioning and normalization of insanity has had on these poor scientifically (truth-seeking) inclined souls. Arithmetic and equations are a language; a way of expressing ideas. These ideas can be patently false just as much as they can be true.
It is said that the worst errors in science are those based on commonly accepted ideas. Time is a conceptual quantification of a continuum which exists only in the mind that is doing the counting, and yet we have allowed ourselves to accept as truth the idea that time can be bent. Time moves mountains, it turns grasslands into forests, empires into ruins, time is untouchable; immaterial. It certainly cannot be bent.
Edited excerpt from my Dynamic Theory of the Aether
1] Einstein, A. On the Electrodynamics of Moving Bodies. June 30, 1905
2] Run Ze Cao. The Great Mistake: Relativity. June 16, 2015
3] Einstein, A. Does The Inertia Of A Body Depend Upon Its Energy Content? Sep. 27, 1905
4] Paradoxes of Special Relativity. 4-SR. University of Notre Dame.
5] Alfredo Macías, Abel Camacho. On the incompatibility between quantum theory and general relativity. Physics Letters B. 6-[63 99-102. Jan. 2008
6] Wheeler, Ken L. “Definitions of Nature & its Phenomena With A Primer On Cosmology & Ontology.” 12-2020
7] Szabó, Lázló. Lorentz’s Theory and Special Relativity are Completely Identical. Theoretical Research Group of the Hungarian Academy of Sciences. Department of History and Philosophy of Science. Eötvös University, Budapest.
8] Ellman, Roger. The Einstein – Lorentz Dispute Revisited.The-Origin Foundation.
9] Plato Phaedo.
10] Einstein, A. Investigations on the Theory of Brownian Movement. 1905
11] Einstein, A. Concerning an Heuristic Point of View Toward the Emission and Transformation of Light. March 17, 1905.
12] Huygens, Christiaan. Traité de la Lumière.
13] Dingle Herbert. Science at the Crossroads. 1972