Es el divertido eslogan, What’s Hard to Understand is Classical Mechanics, Not Quantum Mechanics, que se inventa Peter Woit como punto de partida para mostrar el aspecto, para él, realmente confuso de la mecánica cuántica: pasar del formalismo matemático al comportamiento en el mundo macroscópico.
While there’s a simple, beautiful and very deep mathematical structure for fundamental quantum mechanics, things get much more complicated when you try and use it to extract predictions for experiments involving macroscopic components. This is the subject of “measurement theory”, which gives probabilistic predictions about observables, with the basic statement the “Born rule”. This says that what one can observe are eigenvalues of certain operators, with probability of observation proportional to the norm-squared of the eigenvector. How this behavior of a macroscopic experimental apparatus described in classical terms emerges from the fundamental QM formalism is what is hard to understand, not the fundamental formalism itself. This is what the slogan is trying to point to.
When I first started studying quantum mechanics, I spent a lot of time reading about the “philosophy” of QM and about interpretational issues (e.g., what happens to Schrodinger’s famous cat?). After many years of this I finally lost interest, because these discussions never seemed to go anywhere, getting lost in a haze of complex attempts to relate the QM formalism to natural language and our intuitions about everyday physics. To this day, this is an active field, but one that to a large extent has been left by the way-side as a whole new area of physics has emerged that grapples with the real issues in a more concrete way.
Lo mejor es que ofrece una lista de enlaces para profundizar en la cuestión (casi todos ellos sobre Decoherencia). También recomendables los comentarios a la entrada.