<< A scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it>> [Max Plank 1949].

Great scientists are often reluctant revolutionaries (1) but arguably this is how progress can be achieved: by questioning the new theories and asking for good explanations. The story of Quantum Theory starts with Max Planck, who was the first to introduce into physics the notion of a quantum of energy, and was made by several great scientists whose creative efforts led to our current understanding of nature end how the physical world works. In his book “The beginning of infinity”, David Deutsch poses the question: can this creativity continue indefinitely? Although progress has no necessary end, it does have a necessary beginning. For physics this can be dated back to December 14, 1900, when at the meeting of the German Physical Society, Max Planck stated that radiant energy can exist only in the form of discrete packages or light quanta. Soon after Albert Einstein successfully applied this idea to explain the empirical law of photoelectric effect, while the Danish physicist Niels Bohr extended Planck’s idea of quantization to the description of the energy of electrons within an atom. The following decades saw the born of Schroedinger’s Wave Mechanics and an equivalent treatment of quantum problems by means of non-commutative algebra simultaneously developed by Werner Heisenberg. The newly born quantum theory was immediately object of a lively debate whose main actors were Bohr, who sustained the so-called Copenhagen interpretation, and his opponent Einstein who couldn’t accept such an interpretation for it implied that events were merely the probability of their occurrence thereby denying the existence of an objective reality. In the years, Einsteins’ influence waned while Bohr’s grew, and the Copenhagen interpretation became synonymous of quantum mechanics. Conceptual difficulties remain unresolved though, such as the measurement problem and the inability to say exactly where the quantum world ends and the classical world of the everyday begins. The measurement problem stated in the large poses the question of having an observer outside the universe to observe it: without one, according to the Copenhagen interpretation, the universe should never come into existence but remain forever in a superposition of many possibilities. This led to the rehabilitation of a theory of quantum mechanics introduced in 1957 by Hugh Everett under the name of `the many worlds interpretation’ but sadly never taken seriously until after Everett’s death in 1982. This theory could circumvent the problems related to the quantum measurement to which the Copenhagen interpretation had no answers; it has gained today the favour of the majority of physicists. While the great debate about the nature of reality is still far from any conclusion or solution, the consequences it has delivered to fields other than pure physics are astonishing in regards to the progress they provoked. The Einstein-Bohr challenge was thought-provoking; it inspired Bell’s theorem, and the testing of Bell’s inequality directly or indirectly influenced the development of quantum cryptography, quantum information theory and quantum computing. It was indeed another beginning of infinity…
NOTES(1) This is how Manjit Kumar defines Max Planck in his book “Quantum: Einstein, Bohr and the Great Debate About the Nature of Reality” published in 2008 by Icon Books Ltd, Cambridge, UK, and one of the main sources of our account of the historical quantum debate.