The wave function of quantum theory has always been accepted as an abstract mathematical device – but could this cipher actually be real?
A boat trip on Lake Zurich seemed like the perfect way to unwind after intense discussions about the hot topic of the moment, quantum theory. Appropriate, too, given that all the physicists present were talking about an idea Erwin Schrödinger had put forward a few months earlier. He had suggested that all quantum particles, from atoms to electrons, could be described by intangible entities that spread out through space much like ripples on a lake's surface. He called them wave functions.
When Schrödinger had first published his insights in March 1926, theorists had been thrilled. Symbolised by the Greek letter psi, the wave function gave them a way to apply their much-loved mathematics of waves to the quantum world. And it worked, neatly explaining why electrons in atoms have the energies they do. Yet there was a problem, summed up by a cheeky verse penned on the boat trip:
Erwin with his psi can do
Calculations quite a few
But one thing has not been seen:
Just what does psi really mean?