Earlier today, one of my students asked me this: “So in quantum mechanics an electron can be in different orbits around a positively charged nucleus, and each of these orbits require different levels of kinetic energy for the electron. Electrons can leap from one orbit to another, either emitting or absorbing a photon as they go, but they cannot exist in between orbits. That’s odd. Any theory about such electron-orbit-changes must describe what happens during such a quantum leap. How can it be that at a certain moment an electron leaps from one orbit to a higher orbit without traversing the orbital space in between? Because that’s what quantum theory tells us, right?”

Bohr’s Copenhagen interpretation is famously silent about this. We should only speak, according to Bohr, about what we see, what we observe. In class I was very critical of Bohr, but his ideas are not silly: We never know for certain that an electron is in a certain orbit. We see electrons in certain locations which we might extrapolate with a certain probability to certain orbits. The same goes for leaps from one orbit to a higher orbit: we don’t see the actual leap, but only a sequence of locations which we can extrapolate with

a certain probability to a certain leap. What happens in between observations? Physics is silent about that.

How do the proponents of other interpretations of quantum theory answer the question what happens during a quantum leap? Some would say that an unobserved electron is in a superposition of orbits until it is observed; others would say that the electron just moves from one orbit to another – traversing the in-between-orbits space – we just can’t see that.

Philosopher of physics at Amsterdam University College and Utrecht University, managing editor for Foundations of Physics and international paraclimbing athlete