5.2   A French Prince Intervenes


As you, Nefertiti and Cormorant are enjoying a quick drink, to fortify you for your journey into quantum mechanics, you hear a rather distinguished French voice: “Before you begin, mes amis, allow me to point out that you are treading in the footsteps of royalty.”  Nefertiti looks unimpressed. The speaker is Louis Victor Pierre Raymond, 7th duc de Broglie who, indeed, was the first to suggest that wave-particle duality might be a feature not just of photons, but of electrons and, indeed, all particles. He came up with the idea when he was just a graduate student and it was so revolutionary that his university couldn’t decide whether his thesis was genius or garbage. So they sent a copy to Uncle Albert:  “And he was immediately impressed by the quality and, indeed, the momentous importance of my work” insists Louis. In fact Einstein said that the work “sheds a first, feeble light on this, the worst of the problems of physics”, thereby confirming his own genius by demonstrating the ability praise someone’s work with the aid of the word “feeble”.

As every physicist longs to do, Louis expressed his central idea in the form of a memorably simple equation. In fact we have already seen this equation, when we derived an expression for the momentum of a photon. Recall that in Chapter 3 we used the energy-momentum relation to deduce that even though a photon has no mass, it does carry momentum, given by the relationship:

 

p  =  h / λ                                                                               (V-1)

 

Louis proposed, essentially, that this relationship should apply not just to photons but to any particle. Thus, in effect, his hypothesis is that all particles have wavy properties, and that this simple equation can be used to calculate the wavelength of this “matter wave”.


Of course it doesn’t immediately follow that what the French nobility say is necessarily the truth. Robespierre and his ghostly cronies - having already decapitated Louis’ great-great-great-grandad in the aftermath of the French revolution – are scenting blood, enjoying the scepticism that surrounds Louis’ proposition, thinking it might turn ugly. What Louis needs is some evidence …….