A depiction of quantum entanglement Tangled theories Bohr and Einstein's famous debate on the very nature of physical reality, in the light of quantum discoveries, still remains unresolved (Courtesy: iStock/Pitris) Next year sees the centenary of the summer in which German theoretical physicist Werner Heisenberg sought refuge from hay fever on the North Sea island of Helgoland. There, he figured out how to express the perplexing spectroscopic observations of atoms – whereby they absorbed and emitted light at well-defined, characteristic frequencies – in mathematical form. Heisenberg’s mentor, Danish physicist Niels Bohr, had proposed that the spectra could be understood on the assumption that an atom’s electrons may possess only specific energies, switching from one energy level to another by emitting or absorbing a single “quantum” of light with an energy proportional to its frequency. That quantum hypothesis for light had been proposed by Albert Einstein in 1905, and Bohr had developed it into a new theory of the atom – albeit one that made no sense in classical terms.