During his early period, Bohm made a number of significant contributions to physics, particularly quantum mechanics and relativity theory. As a postgraduate at Berkeley, he developed a theory of plasmas, discovering the electron phenomenon now known as Bohm diffusion. His first book, "Quantum Theory", published in 1951, was well received by Einstein, among others. But Bohm became dissatisfied with the orthodox interpretation of quantum theory he wrote about in that book. Starting from the realization that the WKB approximation of quantum mechanics leads to deterministic equations and convinced that a mere approximation could not turn a probabilistic theory into a deterministic theory, he doubted the inevitability of the conventional approach to quantum mechanics.