The equation E = h × ν is known since 1899 and E = m × c2 since 1905. It took another twenty years until a young physicist dared to combine these two energy formulae in his dissertation. In the year 1925 Louis de Broglie (PDF-Datei, 146 kB) was the first to write:
h × ν = m × c2
The left side of the equation described a wave, the right side a particle. Generally speaking, this equation defines a correspondence of a particle momentum with its wavelength (or frequency).
At first, his colleagues contemplated this astonishing result skeptically what is evident from a letter of Albert Einstein (PDF-Datei, 176 kB) to Max Born (PDF-Datei, 146 kB) after he had read de Broglie's dissertation:
"You really must read that! Even though it looks crazy, it is also thoroughly solid".
Starting with an equation that combines wavelength λ with frequency ν
λ = c/v
and combining it with a further equation that connects energy E with the particle momentum p:
p = E/c
results in a formula for a wavelength that is called after de Broglie:
λ = h/p
The consequence is that each particle with a momentum p can be assigned with a wavelength λ (de Broglie wavelength), or:
Particles are waves – and vice versa!