Donatello,
There is a wrong statement in your essay:
"At the beginning of the 19th century experiments suggested that the speed of light was constant. Einstein raised this experimental evidence to a fundamental principle of physics and he derived relativity theory."
No experiments suggested that the speed of light was constant. In 1887 the Michelson-Morley experiment unequivocally confirmed the variable speed of light predicted by Newton's emission theory of light, and refuted the assumption that the speed of light is independent of the speed of the light source:
John Norton: "These efforts were long misled by an exaggeration of the importance of one experiment, the Michelson-Morley experiment, even though Einstein later had trouble recalling if he even knew of the experiment prior to his 1905 paper. This one experiment, in isolation, has little force. Its null result happened to be fully compatible with Newton's own emission theory of light. Located in the context of late 19th century electrodynamics when ether-based, wave theories of light predominated, however, it presented a serious problem that exercised the greatest theoretician of the day."
John Norton: "The Michelson-Morley experiment is fully compatible with an emission theory of light that CONTRADICTS THE LIGHT POSTULATE."
Pentcho Valev