Quantum mechanics is a mathematical system developed to describe the physics of the atom. Traditional mathematical approaches were unable to cope with aspects of the behavior of the atom. Part of the answer was the quantum theory. This theory developed out of the idea put forward by Max Planck that the atom emits or absorbs the energy of radiation in discrete units, or quanta, whose energy is in direct proportion to its frequency. In May 1925, Werner Heisenberg published a mathematical framework to explain the erratic nature of the atom, which his colleague Max Born recognized as a form of matrix algebra. Heisenberg, Born, and Pascual Jordan worked out the mathematical details over a three-month period in 1925. Because this method corresponded closely to experimental evidence, it soon had its adherents. The early acceptance of this theory by Arnold Sommerfeld at the University of Munich helped establish it in Germany. Most physicists, however, had difficulty understanding quantum mechanics due to the unfamiliar mathematical methods required. Erwin Schrödinger soon made a challenge to quantum mechanics in 1926 by putting forward a wave theory of the atom, which made use of mathematics more familiar to physicists. Soon thereafter it was realized that Heisenberg's matrix mechanics and Schrödinger's wave mechanics were mathematically equivalent expressions of the same theory, which became known collectively as quantum mechanics. Eventually, quantum mechanics was accepted by most nuclear physicists, but Albert Einstein always remained a critic.