Searching for an equation: Dirac, Majorana and the others
Abstract
We review the nontrivial issue of the relativistic description of a quantum mechanical system that, contrary to a common belief, kept theoreticians busy from the end of 1920s to (at least) mid 1940s. Starting by the wellknown works by KleinGordon and Dirac, we then give an account of the main results achieved by a variety of different authors, ranging from de Broglie to Proca, Majorana, FierzPauli, Kemmer, RaritaSchwinger and many others. A particular interest comes out for the general problem of the description of particles with arbitrary spin, introduced (and solved) by Majorana as early as 1932, and later reconsidered, within a different approach, by Dirac in 1936 and by FierzPauli in 1939. The final settlement of the problem in 1945 by Bhabha, who came back to the general ideas introduced by Majorana in 1932, is discussed as well, and, by making recourse also to unpublished documents by Majorana, we are able to reconstruct the line of reasoning behind the Majorana and the Bhabha equations, as well as its evolution. Intriguingly enough, such an evolution was identical in the two authors, the difference being just the period of time required for that: probably few weeks in one case (Majorana), while more than ten years in the other one (Bhabha), with the contribution of several intermediate authors. The important unpublished contributions by Majorana anticipated later results obtained, in a more involved way, by de Broglie (1934) and by Duffin and Kemmer (19389), and testify the intermediate steps in the line of reasoning that led to the paper published in 1932 by Majorana, while Bhabha took benefit of the corresponding (later) published literature. Majorana's paper of 1932, in fact, contrary to the more complicated DiracFierzPauli formalism, resulted to be very difficult to fully understand (probably for its pregnant meaning and latent physical and mathematical content): as is clear from his letters, even Pauli (who suggested its reading to Bhabha) took about one year in 19401 to understand it. This just testifies for the difficulty of the problem, and for the depth of Majorana's reasoning and results. The relevance for present day research of the issue here reviewed is outlined as well.
 Publication:

Annals of Physics
 Pub Date:
 June 2012
 DOI:
 10.1016/j.aop.2012.02.016
 arXiv:
 arXiv:1110.6878
 Bibcode:
 2012AnPhy.327.1617E
 Keywords:

 Physics  History and Philosophy of Physics;
 High Energy Physics  Phenomenology;
 High Energy Physics  Theory;
 Mathematical Physics
 EPrint:
 amsart, 34 pages, no figures