Dirac replied: “I am trying to obtain a relativistic theory of the electron.”
“But Klein has already solved this problem.” (Bohr was referring to the Klein-Gordon equation)
Dirac did not agree. In fact, it is well known that he was the protagonist with a fundamental work in 1928 in which he derived his relativistic equation for spin ½ particles and which also imposed the existence of antimatter.
But he was not the only one.
Another theory was presented by two young theoretical physicists: Dmitri Ivanenko and Lev Landau. Unlike Dirac who used the Hamiltonian, the two used the Lagrangian to build their equations and therefore were able to use variational methods. However, the two works presented many analogies and the results were comparable. Ivanenko Landau were keen to state that:
“Both theories, apart from the complete difference in methods and equations, appear to be equivalent, even if their detailed connections are not so clear to us.”
Despite this, the Ivanenko-Landau theory immediately fell into oblivion while the Dirac equation managed to become famous (even incorrectly) outside the field of physics. Remaining in the scientific merit, one explanation is that Dirac’s version is much more elegant, sober and simple compared to the cumbersome and “aesthetically unpleasant” version of his colleagues.