In 1924 Louis de Broglie published a breakthrough hypothesis: matter has wave properties. Building on Einstein's proposal that the photoelectric effect can be described using quantized energy transfers and by Einstein's separate proposal, from special relativity, that mass at rest is equivalent to energy via E = m 0 c 2 , de Broglie proposed that matter in motion appears to have an associated wave with wavelength λ λ = h / p where p is the matter momentum from the motion. Requiring his wavelength to encircle an atom, he explained quantization of Bohr's orbits. Simultaneously this showed that the wave behavior of light was essentially a quantum effect.