Is it a particle, or is it a wave? The nature of light has haunted physicists for centuries and is the subject of Anil Ananthaswamy’s Through Two Doors at Once.
At the very core of this question is the double-slit experiment, developed by English polymath Thomas Young in 1801. It turns out that the reason this experiment has puzzled physicists for so long is that it shows that light is both a particle and a wave at once.
Danish physicist Niels Bohr was a supporter of the particle-wave duality of light. His interpretation helped formed what is known as the Copenhagen interpretation, where light is described by a wave function. This wave function expresses the probability of a photon of light being found at any one place. Any measurements, i.e. a photon making a dot on a screen will cause this wave function to collapse.
But other physicists were not so believing. This had led to many more increasingly sophisticated versions of the double-slit experiment, including the quantum eraser, weak measurement, and delayed experiments. And of course, the famous thought experiment concerning the interference of large molecules -- Schrodinger’s cat.
Ultimately, this probabilistic wave function has been the subject of arguments amongst quantum physicists and philosophers for decades. One of the most outrageous out-there interpretations is that every time a quantum event happens, the universe splits into multiple, slightly different copies of itself. This has come to be known as the “many-worlds interpretation” and its extravagance is a true testament to the complexity of light.
To describe the essence of the book, Ananthasamy eloquently poses the question -- is the wavefunction ontic, or is it epistemic? In other words, is the wave function real and tangible, or is it simply a tool that represents our incomplete understanding?
The nature of physics is such that we are not given any closure. This question remains an open book, and I suspect it will remain that way for many years to come. But for those looking to understand the basics of quantum physics, and the philosophical conundrums it poses, this book is a masterful marriage of history and theory.