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mod1 - Physics 54 Atoms and Quantum Mechanics Physics isn't...

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Physics 54 Atoms and Quantum Mechanics Physics isn't a religion. If it were, we'd have a much easier time raising money. — Leon Lederman Overview By the 1880’s many physicists had become rather complacent about their description of the natural universe, thinking that most of the basic principles had been found, and that what remained was mostly a matter of filling in the details. There were indeed some interesting details to be considered, especially in areas that were under study by chemists. The atomic theory of chemical elements seemed more and more to fit the facts, but no one had much of an idea what kind of physical system an atom might be. Then there was the matter of characteristic line spectra, the light emitted by each element consisting of a discrete set of wavelengths peculiar to that element. Why only these wavelengths? Why are they different for different elements? Maxwell’s theory of electromagnetism, still new but gaining acceptance, seemed to require the existence of a mysterious substance (the aether) as the medium for the transmission of electromagnetic waves. It would be important to know in which reference frame the aether is at rest, and methods were devised to determine the velocity of the earth through the aether. The experiments gave a puzzling result: the earth seems to be always at rest relative to the aether. Finally, there was the matter of specific heats of gases. The prevailing model, which included the equipartition of energy theorem, gave wrong answers (at room temperatures) for most of those specific heats, except for inert gases such as He. Nevertheless, to many in the 1880’s these puzzles seemed to be details which would probably be understood in good time. Things changed dramatically during the next decade, and not until about fifty years later would anyone again suggest that we were in possession of most of the basic principles. New discoveries came at a rapid pace: 1895: X-rays discovered by Roentgen 1896: Radioactivity discovered by Becquerel 1897: Electrons discovered by Thompson PHY 54 1 Modern Physics 1
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1900: Planck proposed “quanta” to explain spectrum of radiation from heated objects 1905: Einstein explained away the aether by inventing special relativity 1905: Einstein used Planck’s quanta (photons) to explain photoelectric effect 1911: Nuclei of atoms discovered by Rutherford 1913: First workable model of atoms by Bohr 1923: Compton effect showed photons behaving like both particles and waves 1924: DeBroglie suggested that “particles” such as electrons also behave like waves 1925-26: Heisenberg, Born and Schrödinger developed quantum mechanics 1928: Dirac developed relativistic quantum theory of electron, predicting its antiparticle (positron, found in 1932) 1932: Neutron discovered by Chadwick In the nearly seventy years since then many new things have been found, but they have not altered fundamentally our understanding of the principles governing the structure of atoms and molecules.
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