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The 20th-century physics starts with Einstein and ends with Feynman. Einsteinintroduced the Lorentz-covariant world with E = mc^{2}. Feynman observed thatfast-moving hadrons consist of partons which act incoherently with externalsignals. If quarks and partons are the same entities observed in differentLorentz frames, the question then is why partons are incoherent while quarksare coherent. This is the most puzzling question Feynman left for us to solve.In this report, we discuss Wigner's role in settling this question. Einstein'sE = mc^{2}, which takes the form E = \sqrt{m^{2} + p^{2}}, unifies theenergy-momentum relations for massive and massless particles, but it does nottake into account internal space-time structure of relativistic particles. Itis pointed out Wigner's 1939 paper on the inhomogeneous Lorentz group definesparticle spin and gauge degrees of freedom in the Lorentz-covariant world.Within the Wigner framework, it is shown possible to construct the internalspace-time structure for hadrons in the quark model. It is then shown that thequark model and the parton model are two different manifestations of the samecovariant entity. It is shown therefore that the lack of coherence in Feynman'sparton picture is an effect of the Lorentz covariance.Comment: LaTex 15 pages, 1 figure, presented at the Wigner Centennial  Conference held in Pecs, Hungary (July 2002), published in the proceedings  (Acta Physica Hungarica, 2003), minor corrections to the original versio

Einstein, Wigner and Feynman: From E = mc 2 to Feynman's Decoherence via Wigner's Little Groups