The de Broglie hypothesis states that particles of matter also had wavelengths and could behave as waves, just as photons did. String theory will later say that both types of particles — matter and energy — are manifestations of vibrating strings, but that’s about 50 years down the road from de Broglie’s time. He proposed his theory in 1923.Here was de Broglie’s line of reasoning for the bold new theory. Under special relativity, matter and energy were different manifestations of the same thing. The photon, a particle of energy, had a wavelength associated with it. Therefore, particles of matter, such as electrons, should also have wavelengths. His PhD dissertation set out to calculate what that wavelength (and other wave properties) should be.