Where did the false constancy of the speed of light come from? Einstein "borrowed" it from the ether theory:
Albert Einstein: "...I introduced the principle of the constancy of the velocity of light, which I borrowed from H. A. Lorentz's theory of the stationary luminiferous ether..." https://en.wikipedia.org/wiki/Lorentz_ether_theory
Banesh Hoffmann, Relativity and Its Roots, p.92: "Moreover, if light consists of particles, as Einstein had suggested in his paper submitted just thirteen weeks before this one, the second principle seems absurd: A stone thrown from a speeding train can do far more damage than one thrown from a train at rest; the speed of the particle is not independent of the motion of the object emitting it. And if we take light to consist of particles and assume that these particles obey Newton's laws, they will conform to Newtonian relativity and thus automatically account for the null result of the Michelson-Morley experiment without recourse to contracting lengths, local time, or Lorentz transformations. Yet, as we have seen, Einstein resisted the temptation to account for the null result in terms of particles of light and simple, familiar Newtonian ideas, and introduced as his second postulate something that was more or less obvious when thought of in terms of waves in an ether. If it was so obvious, though, why did he need to state it as a principle? Because, having taken from the idea of light waves in the ether the one aspect that he needed, he declared early in his paper, to quote his own words, that "the introduction of a 'luminiferous ether' will prove to be superfluous."x https://www.amazon.com/Relativity-Its-Roots-Banesh-Hoffmann/dp/0486406768
The introduction of the false postulate was Einstein's original sin. Einstein's second sin was an invalid deduction. In 1905 he derived, from his two postulates, the conclusion "the clock moved from A to B lags behind the other which has remained at B":
Albert Einstein, On the Electrodynamics of Moving Bodies, 1905: "From this there ensues the following peculiar consequence. If at the points A and B of K there are stationary clocks which, viewed in the stationary system, are synchronous; and if the clock at A is moved with the velocity v along the line AB to B, then on its arrival at B the two clocks no longer synchronize, but the clock moved from A to B lags behind the other which has remained at B by tv^2/2c^2 (up to magnitudes of fourth and higher order), t being the time occupied in the journey from A to B." http://www.fourmilab.ch/etexts/einstein/specrel/www/
"the clock moved from A to B lags behind the other which has remained at B"
is non sequitur - it does not follow from Einstein's 1905 postulates. In other words, the argument leading from the postulates to the conclusion is INVALID.
The following two conclusions, in contrast, VALIDLY follow from the postulates:
Conclusion 1: The clock moved from A to B lags behind the other which has remained at B, as judged from the stationary system.
Conclusion 2: The clock which has remained at B lags behind the clock moved from A to B, as judged from the moving system.
Conclusions 1 and 2 (symmetrical time dilation) in their combination give no prediction for the readings of the two clocks as they meet at B. In contrast, the INVALIDLY deduced conclusion provides a straightforward prediction - the moving clock is slow, the stationary one is FAST (asymmetrical time dilation). The famous but idiotic "travel into the future" is a direct implication - the slowness of the moving clock means that its (moving) owner can remain virtually unchanged while sixty million years are passing for the stationary system:
Thibault Damour: "The paradigm of the special relativistic upheaval of the usual concept of time is the twin paradox. Let us emphasize that this striking example of time dilation proves that time travel (towards the future) is possible. As a gedanken experiment (if we neglect practicalities such as the technology needed for reaching velocities comparable to the velocity of light, the cost of the fuel and the capacity of the traveller to sustain high accelerations), it shows that a sentient being can jump, "within a minute" (of his experienced time) arbitrarily far in the future, say sixty million years ahead, and see, and be part of, what (will) happen then on Earth. This is a clear way of realizing that the future "already exists" (as we can experience it "in a minute")." http://www.bourbaphy.fr/damourtemps.pdf
Herbert Dingle tried to expose Einstein's invalid argument in the 1960s and 1970s but it was too late - the gullible world was already irreversibly brainwashed:
Herbert Dingle: "According to the special relativity theory, as expounded by Einstein in his original paper, two similar, regularly-running clocks, A and B, in uniform relative motion, must work at different rates.....How is the slower-working clock distinguished? The supposition that the theory merely requires each clock to APPEAR to work more slowly from the point of view of the other is ruled out not only by its many applications and by the fact that the theory would then be useless in practice, but also by Einstein's own examples, of which it is sufficient to cite the one best known and most often claimed to have been indirectly established by experiment, viz. 'Thence' [i.e. from the theory he had just expounded, which takes no account of possible effects of acceleration, gravitation, or any difference at all between the clocks except their state of uniform motion] 'we conclude that a balance-clock at the equator must go more slowly, by a very small amount, than a precisely similar clock situated at one of the poles under otherwise identical conditions.' Applied to this example, the question is: what entitled Einstein to conclude FROM HIS THEORY that the equatorial, and not the polar, clock worked more slowly?" SCIENCE AT THE CROSSROADS, p.27 http://blog.hasslberger.com/Dingle_SCIENCE_at_the_Crossroads.pdf