Einstein, Dirac, and Feynman
Dirac and Feynman, courtesy of Caltech Photo Archives. License fee paid. 

Both Dirac and Feynman were intensely interested in combining
quantum mechanics with special relativity. Paul A. M. Dirac
is known to us for his Dirac equation. Richard P. Feynman is
known to us for Feynman diagrams. In fact, these two great
physicists were interested in deeper foundational problems.
 P. A. M. Dirac, Proc. Roy. Soc. (London) A114, 243 (1927).
 P. A. M. Dirac, Proc. Roy. Soc. (London) A183 , 284 (1945).
 P. A. M. Dirac, Rev. Mod. Phys. 21 , 392 (1949).
 P. A. M. Dirac, J. Math. Phys. 4 , 901 (1963).
 R. P. Feynman,
Phys. Rev. Lett. 23, 1415 (1969);
R. P. Feynman, The Behavior of Hadron Collisions at Extreme Energies , in High Energy Collisions, Proceedings of the Third Topical Conference on High Energy Collisions of Hadrons, Stony Brook, New York, edited by C. N. Yang et al., Pages 237249 (Gordon and Breach, New York, 1969).  R. P. Feynman, M. Kislinger, and F. Ravndal, Phys. Rev. D 3, 2706 (1971).
 R. P. Feynman, Statistical Mechanics (Benjamin/Cummings, Reading, MA, 1972).
However, their approaches to physics were quite different. Dirac was interested in mathematical formulas with esthetic appeal. On the other hand, Feynman was interested in observing what is happening in the world and fill in mathematical formulas as needed.
Feynman's parton picture is one of his brilliant observations. Yet, this is valid in the infinitemomentum system and lacks Lorentz covariance. Thus, it is fun to look into the possibility of making it covariant by filling in mathematical formulas from Dirac's papers.
Photo by Bulent Atalay. 
Dirac never liked the present form of quantum mechanics based on probabilistic interpretation. On the other hand, he held the view that,if we accept this interpretation, this form of quantum mechanics should be consistent with Lorentz covariance. Dirac published the following papers on this subject.
photo from the AIP E. Segre Visual Archives. 
In his effort to combine quantum mechanics with relativity, Feynman first considered waves propagating in the Lorentzcovariant world, resulting in Feynman diagrams. How about standing waves? Feynman considered harmonic oscillators. Feynman would have liked to give a covariant description his parton picture, but left this problem as a homework problem for younger physicists. He wrote the following provocative papers on this subject.
 copyright@2014 by Y. S. Kim, unless otherwise specified.
 The photo of Dirac and Feynman is from the Caltech Photo Archive.
This photo was taken by Marek Holzman during the International
Conference on Relativity Theory of Gravitation in Warsaw (Poland)
on July 2531 1962, organized by Leopold Infeld.
Who is this young man? 