Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [127]
Complementarity neatly sidestepped the difficulties that arose from having to use two disparate classical descriptions, waves and particles, to describe a non-classical world. Both particles and waves were, according to Bohr, indispensable for a complete description of quantum reality. Either description by itself is only partially true. Photons paint one picture of light, waves another. Both hang side by side. But to avoid contradictions, there were limitations. The observer can look at only one of them at any given time. No experiment would ever reveal a particle and a wave at the same time. Bohr argued that 'evidence obtained under different conditions cannot be comprehended within a single picture, but must be regarded as complementary in the sense that only the totality of the phenomena exhausts the possible information about the objects'.62
Bohr found support for his emerging ideas when he saw something in the uncertainty relations, pqh/2 and Eth/2, that Heisenberg, blinded by his intense dislike of waves and continuity, did not. The Planck-Einstein equation E=hv and de Broglie's formula p=h/ embodied wave-particle duality. Energy and momentum are properties commonly associated with particles, whereas frequency and wavelength are both characteristics of waves. Each equation contained one particle-like and one wave-like variable. The meaning of this combination of particle and wave characteristics in the same equation was something that niggled Bohr. After all, a particle and a wave are two wholly distinct physical entities.
As he corrected Heisenberg's analysis of the microscope thought experiment, Bohr spotted that the same was true for the uncertainty relations. It was a finding that led him to interpret the uncertainty principle as revealing the extent to which two complementary but mutually exclusive classical concepts, either particles and waves or momentum and position, could be applied simultaneously without contradiction in the quantum world.63
The uncertainty relations also implied that a choice has to be made between what Bohr called a 'causal' description based on the conservation laws of energy and momentum (E and p in the uncertainty relations), and a 'space-time' description in which events are followed in space and time (q and t). The two descriptions were mutually exclusive but complementary so as to account for the results of all possible experiments. To Heisenberg's dismay, Bohr had reduced the uncertainty principle to a special rule exposing the limits inherent in nature on any simultaneous measurements of complementary pairs of observables such as position and momentum or on the simultaneous use of two complementary descriptions.
There was another difference of opinion. Whereas the uncertainty principle led Heisenberg to question the extent to which classical concepts such as 'particle', 'wave', 'position', 'momentum' and 'trajectory' were applicable in the atomic realm, Bohr argued that the 'interpretation of the experimental material rests essentially upon the classical concepts'.64 While Heisenberg insisted upon an operational definition of these concepts, a sort of meaning through measurement, Bohr argued that their meanings were already fixed by how they were used in classical physics. 'Every description of natural processes,' he had written in 1923, 'must be based on ideas which have been introduced and defined by the classical theory.'65 Regardless of any limitations imposed by the uncertainty principle, they could not be replaced for the simple reason that all experimental data, its discussion and interpretation, by which theories are put to the test in the laboratory, is of necessity