Illustration: Colourbox

Key moments in the birth of quantum mechanics

Tuesday 03 Oct 17


  J.J. Thomson discovers the electron.


  German physicist Max Planck proposes that light consists of small ‘packets’ of electromagnetic energy (called quanta).


  Albert Einstein uses Planck’s quanta to propose that light can be perceived as particles (later called photons) and to explain the photoelectric effect—the phenomenon that electrons are emitted from a surface (typically metal) when exposed to light.


  British physicist Ernest Rutherford has discovered the atomic nucleus and describes the atom as a heavy core with electrons around it.


  Danish physicist Niels Bohr proposes a quantum theory for the structure of the atom based on the hydrogen atom. Bohr has a theory that the electrons have fixed orbits around the nucleus.


  German physicist Werner Karl Heisenberg rejects Bohr’s theory of fixed electron orbits. Instead, Heisenberg talks about atoms having stationary energy levels. He also sets forth matrix mechanics, which describes quantum systems as particles.


  Austrian physicist Erwin Schrödinger formulates the Schrödinger equation, which describes an atomic system’s energy state.


  Erwin Schrödinger develops a new theory of the atom—the theory of wave mechanics, which describes quantum systems as waves.


  German physicist Max Born sets forth the probability interpretation of wave mechanics, which states that the square of a particle’s wave function at a given location indicates the probability of finding the particle there. He thereby helped pave the way for what became known a few years later—spearheaded by Niels Bohr—as the Copenhagen interpretation of quantum mechanics.


  British physicist Paul Dirac shows that the Heisenberg and Schrödinger formulations of quantum mechanics are equivalent (i.e. either equally true or equally false), and formulates the mathematical description (formalism) that is most commonly used today. In other words, Heisenberg’s matrix mechanics and Schrödinger’s wave mechanics together comprise the common perception of quantum mechanics.


  Large differences between how physicists perceive quantum mechanics led to what is known as the Copenhagen interpretation, with reference to Niels Bohr—based in Copenhagen—who suggests that a physical phenomenon can be observed in two different ways depending on the nature of the experiment. For example, light can sometimes behave as waves and sometimes as particles. Einstein disagrees with this perception.


  Paul Dirac develops a wave equation for electrons that describes their spin. One consequence of the equation was prediction of the electron anti-particle–the positron—which was not discovered experimentally until several years later.


  Erwin Schrödinger presents his ‘Schrödinger’s cat’ thought experiment to illustrate the cognitive problems of quantum physics. It is a thought experiment involving a cat in a closed container, that can be either dead or alive depending on a quantum event. The paradox is that the two universes—one with a dead cat and one with a living cat—appear to exist in parallel until the box is opened and investigated.