Even in the light of the Compton Effect, critics of the early single-photon interference experiments dismissed the importance of the observation by noting that a photon doesn’t have mass. Through some fancy hand-waving, they argued that the low-light interference could be caused through splitting and recombining the light quanta’s wavefront. Decisive proof would come when particles with mass would show interference.
In 1924, French physicist Prince Louis-Victor de Broglie (pronounced “de Broy,” and by the way, he did belong to the French royalty) proposed that maybe it’s not just light that has this dual personality, maybe it’s everything! He reasoned that if the quanta of light could be both a wave and a particle, then maybe the same could be true of electrons.
Remember that although a photon doesn’t have mass, it does have momentum p given by:
De Broglie’s hypothesis was that matter—which is commonly described by our perception as “solid”—can also behave as a wave. He took the concept used to find the momentum of a photon, and applied it to particles, proposing that the wavelength associated with a particle is:
where the momentum p of a particle equals the product of its mass m, and velocity v. In the same way, he used Planck’s relationship to propose that the frequency f of the matter wave is related to the energy E of the particle by:
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