Models of the Atomic Nucleus

Models of the Atomic Nucleus Unification Through a Lattice of Nucleons von Norman D. Cook 2nd. ed. Models of the Atomic Nucleus – Cook schnell und portofrei erhältlich bei beck-shop.de DIE FACHBUCHHANDLUNG Springer-Verlag Berlin Heidelberg 2010 Verlag C.H. Beck im Internet: www.beck.de ISBN 978 3 642 14736 4 Inhaltsverzeichnis: Models of the Atomic Nucleus – Cook Chapter 2 Atomic and Nuclear Physics Two important areas where the concepts of quantum theory have been success- fully applied are atomic physics (concerned primarily with the interactions between nuclei and electrons) and low-energy nuclear physics (concerned primarily with the interactions among protons and neutrons). Despite some strong theoretical connections, the practical issues in atomic and nuclear physics can be dealt with separately because the amount of energy required to bring about nuclear effects is generally many thousands of times greater than that required for electronic effects. For this reason, atomic physicists can work in an energy range where nuclear reactions do not occur, and nuclear physicists can work in an energy range where electron reactions occur, but are usually negligible in comparison with the much stronger nuclear phenomena. This separation of electronic and nuclear effects means that atomic and nuclear physicists are normally housed in different university departments, but there are, nevertheless, many conceptual links and similarities in the theoretical techniques employed in both realms. Most importantly, in both nuclear and atomic physics, quantum mechanics is the theoretical framework for most quantitative work. Quantum mechanics is, above all else, a theory of the discreteness of physical quantities. Whereas classical mechanics assumed that all quantities of mass and energy are continuous, experimental findings obtained near the end of the nineteenth century suggested that, at the atomic level, there is quantization of some physical quantities into integral units, and quantities in-between do not occur. The theoretical efforts that followed the first experimental indications of discrete units of mass and energy led eventually to atomic theory with quantum mechanics at its heart – a comprehensive, self-consistent theory of the microphysical world, where indeed quantal jumps and a certain discreteness of physical quantities are the general rule. The development of that theory was an unanticipated conceptual revolution at the turn of the century, but is today fully established. Although the discreteness of the physical world at the atomic level is not evident at the macroscopic level of classical physics, both classical macroscopic physics and quantal microscopic physics are correct, when applied to their respec- tive realms. Today, it is no longer a controversial issue that both kinds of physics have their own applications, formulated in somewhat different terms. Both are correct in the very real sense that both can be used to understand natural phenomena and to predict physical events in the material world. N.D. Cook, Models of the Atomic Nucleus, 2nd ed., 11 DOI 10.1007/978-3-642-14737-1_2, C Springer-Verlag Berlin Heidelberg 2011 12 2 Atomic and Nuclear Physics The validity of classical mechanics is demonstrated again and again every time a rocket is launched, a skateboarder wiggles down a sidewalk or a 50 story building refuses to come tumbling down. Similarly, classical electromagnetic theory may not be “easy,” but macroscopic electromagnetic phenomena are no longer a mys- tery; it is a known world and, as a consequence, heavily exploited in countless practical ways. When we arrive at the microscopic realm of quantum mechanics, however, we run into a world where much is known, but little is understood. At least, quantum theory is not “understood” in the visceral way that we understand that a brick dropped on a toe will hurt more than a wad of cotton. Within the framework of classical mechanics, an analysis of force, mass and acceleration of the brick and the cotton wad would lead to technical conclusions that fully support our common sense. But in quantum mechanics, technical conclusions stand alone, and loose analogies and imperfect models based on common sense notions are never more than suggestions. The visceral understanding that we normally have with everyday objects is simply missing in the quantum world. It is this lack of a visceral understanding of quantum mechanics that makes it an interesting and difficult intellectual puzzle and why it is the source of endless specu- lation along the lines of “What does quantum mechanics mean for our understanding of the universe, consciousness or human existence?” In contrast, there is really noth- ing of interest to discuss about the workings of a classical clockwork mechanism. It works, and the cause-and-effect throughout the entire system can be traced to what- ever level of precision we wish to pursue. But quantum mechanics is different. The concepts of causality that we know (and feel in our bones!) from classical mechanics don’t seem to work at the quantal level. We are left in an abstract cerebral world with its own logic and rules, but with only weak connections to familiar dynamics, gut feelings, and common-sense cause-and-effect. The puzzles of the atomic level motivated a huge intellectual effort in the philos- ophy of science over the entire twentieth century. Since quantal systems – atoms or nuclei – cannot be individually measured, nearly all knowledge of these small systems is obtained from experiments dealing with huge numbers of similar systems. This makes all of quantum mechanics inherently probabilistic. Our knowledge and intuitions from classical physics might still sometimes be relevant to aspects of the world of quantum physics, but we do not know for certain how quantal causality will relate to classical causality. This dilemma has not yet been resolved by philo- sophical discussion over the course of a century – with illustrious figures such as Bohr and Einstein defending diametrically opposing views. Popular science writers sometimes declare that Bohr (or Einstein or Bohm or another player in the quantum debate) was correct – and go on to elaborate on one particular view, but the historical reality is that the scientists who initiated the quantum revolution (Planck, Einstein, Bohr, Schrodinger, Heisenberg and Pauli), as well as the next generation of theorists who struggled with quantum mechanics in the 1920s and 1930s, did not arrive at a consensus view. For all we know, “the correct” view might be outlined in unpublished memoirs from that era, but the unification of conflicting views was not in fact achieved in “real time” – suggesting that pieces of the puzzle were, and possibly still are, missing. As much could perhaps be said 2.1 The Atom 13 of the current disarray in neuroscience concerning consciousness, but a failure to resolve differences is not an inevitable part of research – as witnessed by the virtual unanimity of opinion in molecular biology concerning cellular life. Resolution of apparent contradictions will undoubtedly eventually be reached in microphysics, but it serves no useful purpose to hastily rewrite history by falsely declaring a “winner” when the reality of the 1920s was “stalemate.” So, without committing ourselves to either the Einsteinian or the Heisenbergian interpretation of quantum mechanics, let us begin with a brief review of electron physics and the issues of atomic structure in order to understand the conceptual problems. From there, we can proceed to the nuclear realm, where related, but even more controversial topics remain unresolved. 2.1 The Atom Some modern textbooks treat quantum theory as a recent revolution in scientific thinking, but that is truly no longer the case. For more than five generations of scientific inquiry, it has been known that the basic building block of all matter on earth is the atom; and for almost as long, the quantal nature of atomic phenomena has been studied. The atom contains a centrally-located nucleus and a covering of electron “clouds”. Although more than 99% of the atomic volume is due to the electron clouds, more than 99% of the atomic mass is contained in the protons and neutrons (nucleons) locked inside the nucleus. The density of matter in the nucleus and in the electron clouds is thus very different, but the nucleus and the electron periphery also differ with regard to their electric charges. Each electron contains one unit of negative charge, whereas the nucleus contains a number of positive charges (from 1 to more than 100) equal to the total number of protons. The housing of so much positive charge in the small volume of the nucleus already tells us one important fact about the nuclear realm: the forces that hold those charges together, despite their mutual electrostatic repulsion, must be quite strong. This is the so-called “strong force” or the “nuclear force.” The idea that all matter might be built from elementary subunits has a long history, but progress in understanding the reality of atomic substructure began in the nineteenth century with the gradual systematization of chemical knowledge. As more and more pure substances were isolated, laws and regularities of chemical reactions became apparent, thus setting the stage for turning the art of chemistry into the science of atomic physics. The first major theoretical breakthrough came with the proposal of the Periodic Table of elements by Mendeleev in 1869. While several blanks in the chart remained unfilled and all questions concerning why such periodicity would exist went unanswered, the sequence of elements of increasing mass and the curious periodicity of some important properties were indication that there exists a finite number of physical laws that underlie the huge diversity of chemical phenomena. The importance of the Periodic Table can hardly be over- stated, for it brought considerable order to the world of chemistry.

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