What was Bohr's response to the problem of wave-particle duality? Before considering this question in detail it will be useful to survey briefly Bohr's work in physics between 1911 and 1923. Bohr and the ‘old’ quantum theory In 1911 Bohr completed his doctoral dissertation on the classical electron theory of metals. Developed by Sir J.J. Thomson and Lorentz, the theory explained electrical, magnetic and thermal properties of metals in terms of the motions of free electrons. Although Bohr made no use of the quantum hypothesis in his dissertation, he noted that classical electrodynamics was incapable of explaining Planck's radiation law and entailed the ultraviolet catastrophe. After completing his dissertation Bohr pursued his post-graduate studies under Thomson at Cambridge University. Finding that Thomson was little interested in his work, Bohr went on to Manchester University in March 1912, to work under Ernest Rutherford. Bohr's move to Manchester was very opportune: the quantum hypothesis was just beginning to receive a wider recognition, thanks largely to the first Solvay Conference in 1911, which Rutherford attended; Rutherford's laboratory, moreover, was one of the most active centres in the investigation of radio-activity. In order to explain the results of experiments on the passage of alpha and beta particles through matter Rutherford had revived Hantaro Nagaoka's (1904) planetary model of the atom. On this model an atom consists of a positively charged nucleus which is orbited by negatively charged electrons, the number of which corresponds to the number of charges on the nucleus.
What was Bohr's response to the problem of wave-particle duality? Before considering this question in detail it will be useful to survey briefly Bohr's work in physics between 1911 and 1923. Bohr and the ‘old’ quantum theory In 1911 Bohr completed his doctoral dissertation on the classical electron theory of metals. Developed by Sir J.J. Thomson and Lorentz, the theory explained electrical, magnetic and thermal properties of metals in terms of the motions of free electrons. Although Bohr made no use of the quantum hypothesis in his dissertation, he noted that classical electrodynamics was incapable of explaining Planck's radiation law and entailed the ultraviolet catastrophe. After completing his dissertation Bohr pursued his post-graduate studies under Thomson at Cambridge University. Finding that Thomson was little interested in his work, Bohr went on to Manchester University in March 1912, to work under Ernest Rutherford. Bohr's move to Manchester was very opportune: the quantum hypothesis was just beginning to receive a wider recognition, thanks largely to the first Solvay Conference in 1911, which Rutherford attended; Rutherford's laboratory, moreover, was one of the most active centres in the investigation of radio-activity. In order to explain the results of experiments on the passage of alpha and beta particles through matter Rutherford had revived Hantaro Nagaoka's (1904) planetary model of the atom. On this model an atom consists of a positively charged nucleus which is orbited by negatively charged electrons, the number of which corresponds to the number of charges on the nucleus.
“How wonderful that we have met with a paradox. Now we have some hope of making progress,” said Niels Bohr with characteristic irony. The quantum view of the atom began with Bohr. Coming from Denmark, he worked with Rutherford, who although famously skeptical about theoreticians, supported him. Max Planck had previously introduced the idea of the quantum, and Einstein had extended it from matter to light, but Bohr’s proposal that electrons can adjust their orbits only by quantum leaps was revolutionary. The next revolution was Schrödinger’s description of electrons as waves instead of particles. This led to recognition of the wave–particle duality. The rest of the 20th century was marked by the realization that the apparently fundamental particles of the nucleus, the proton and the neutron, each actually consisted of a series of sub-particles. The wave–particle duality created a baffling series of paradoxes. Following revolutions tended to be driven by theoreticians, and massive team efforts were required to identify the sub-particles experimentally.
The aim of Niels Bohr’s “great” paper was to try to explain how Ernest Rutherford’s planetary model of the atom could exist. Bohr never used the words “quantum jump” in the paper, but it became very much in vogue shortly after this. Max Planck and even A. Einstein had not yet made the full evolution beyond classical physics, and the purely classical atomic views of Rutherford and H. Nagaoka did not provide the explanation of the absorption and emission spectra of hydrogen. In truth, Bohr’s “theory” consisted of a very few equations, with very great assumptions. As de Broglie’s work became known, he was hailed for his advances and he would join the list of Nobel Prizes for advances in quantum theory. Wolfgang Pauli did not particularly like the wave and particle approach that de Broglie introduced with his pilot waves in 1924, and would forcefully express these objections a few years later at the Solvay conference.
Something is rotten in the state of Denmark. Shakespeare , Hamlet, Act 1 Scene 4 The rise and fall of the old quantum theory The principles of the old quantum theory were first articulated by Niels Bohr in 1913 in a three-part paper titled “On the Constitution of Atoms and Molecules.” Bohr had adopted Ernest Rutherford's model of the atom, according to which the mass of the atom is concentrated in a small central nucleus, while the electrons orbit the nucleus in planetary trajectories. The key challenge facing Rutherford's model was that it was unstable: according to classical electrodynamics, the electron, which is a moving charged body, should radiate energy and so eventually collapse into the nucleus. Bohr's solution was to incorporate Max Planck's theory of radiation, according to which “the energy radiation from an atomic system does not take place in the continuous way assumed in ordinary electrodynamics, but that it, on the contrary, takes place in distinctly separated emissions” (Bohr 1913, p. 4; BCW 2, p. 164). In this early paper, and most subsequent accounts, Bohr summarized his quantum theory by means of two assumptions or postulates. According to the first postulate, electrons cannot travel in any path they like around the nucleus, rather atomic systems can only exist in a series of discrete “stationary states,” in which the electron is in a particular allowed stable periodic orbit and is not emitting radiation.
Niels Bohr's theory of complementarity was an attempt to solve the enormous problems of interpretation – especially the problem of wave particle duality – that beset the quantum theory in the mid-twenties. By 1920 electromagnetic radiation could be conceived of either in terms of the wave model or in terms of the particle model, though neither conception alone was wholly adequate to the empirical data. By the mid-twenties the duality problem was quite general, applying to matter as well as to radiation. Quantum mechanics, the new quantum theory, could be interpreted partially in terms of one or other of the two models but not comprehensively in terms of either. Bohr's theory can be fully understood only against the background of the interpretative problems which it was intended to solve; and the root of these problems goes back to the very origin of the quantum theory, viz. the quantum hypothesis to the effect that in certain physical processes energy is transferred discontinuously in discrete amounts. Wave-particle duality was a development of the continuity-discontinuity duality which the quantum hypothesis introduced into physics. How did wave-particle duality arise?
The results of Bohr model can be used to explain the motion of macroscopic objects or big objects (like orbiting planets and sun, falling stone etc.). These macroscopic objects have essentially a particle nature and Newtonian laws of motion can be applied on them. But it fails when applied to explain the motion of microscopic objects or small objects (like electrons, atoms, molecules etc.). This is because: According to Bohr model, an orbit is a well-defined path and this path can completely be defined only if both the position and velocity of the electron are known exactly at the same time. But this is not possible according to the Heisenberg’s uncertainty principle. Thus, Bohr model contradicts the Heisenberg’s uncertainty principle . According to Bohr model, an electron is regarded as a charged particle moving in well-defined circular orbits concentric about the nucleus. The wave character of the electron is not considered. Thus, Bohr model ignores the de-Broglie dual nature of matter .
The results of Bohr model can be used to explain the motion of macroscopic objects or big objects (like orbiting planets and sun, falling stone etc.). These macroscopic objects have essentially a particle nature and Newtonian laws of motion can be applied on them. But it fails when applied to explain the motion of microscopic objects or small objects (like electrons, atoms, molecules etc.). This is because: According to Bohr model, an orbit is a well-defined path and this path can completely be defined only if both the position and velocity of the electron are known exactly at the same time. But this is not possible according to the Heisenberg’s uncertainty principle. Thus, Bohr model contradicts the Heisenberg’s uncertainty principle . According to Bohr model, an electron is regarded as a charged particle moving in well-defined circular orbits concentric about the nucleus. The wave character of the electron is not considered. Thus, Bohr model ignores the de-Broglie dual nature of matter .