David Cassidy

Moving the Earth

Galileo’s discoveries about motion formed a major part of a much larger development across all of the sciences, a development now known as the Scientific Revolution. In the study of the physical world, the science of motion, or mechanics, joined with the science of astronomy to form the basic approach to modern physics. Paralleling the revolution in mechanics, the revolution in astronomy involved an extremely difficult transition for most people from the common-sense view of the Universe in which the Earth is stationary at the center of the Universe to our current, more abstract, view that the Earth is actually spinning on its axis as it orbits around a star, our Sun, as the third planet. Since the Earth was now seen as a moving object, the revolution in mechanics helped to encourage the revolution in astronomy, and vice versa. This chapter looks at the parallel developments in astronomy, before turning to the causes of motion in the next chapter.

Bomb Apologetics: Farm Hall, August 1945

A little over a month after the ten German physicists had settled into Farm Hall, the British manor where they would be held and observed for six months (see the preceding article on page 27), they were astounded by the news of the atomic bombing of Hiroshima. Believing themselves far ahead of the Allies in nuclear research, the German scientists were suddenly shaken by the realization that they were in fact far behind. How had the Allies done it? Why had the Germans made so little progress in comparison? How could they explain this to themselves, to their countrymen, to their former enemies?

A Historical Perspective on Copenhagen

Copenhagen, a thought‐provoking drama by Michael Frayn that has played to sold‐out audiences in London, has now opened in New York with much well‐deserved attention. Despite the sparse set, the three‐person cast, and the technical nature of the subject matter, this play appears to speak on some level to almost everyone. (See PHYSICS TODAY, May, page 51.)

Heisenberg's first paper

Werner Heisenberg had just celebrated his twentieth birthday when he presented his first paper for publication in 1921. This paper, a long and complex study entitled “On the Quantum Theory of Line Structure and of the Anomalous Zeeman Effects,” immediately placed its young author on the forefront of theoretical spectroscopy. “He understands everything,” Niels Bohr remarked. But, as often happens with brilliant first papers, its unique proposals were as controversial and perplexing as the phenomena they purported to explain. Figure 1 is a reproduction of part of the first page of this paper.Werner Heisenberg had just celebrated his twentieth birthday when he presented his first paper for publication in 1921. This paper, a long and complex study entitled “On the Quantum Theory of Line Structure and of the Anomalous Zeeman Effects,” immediately placed its young author on the forefront of theoretical spectroscopy. “He understands everything,” Niels Bohr remarked. But, as often happens with brilliant first papers, its unique proposals were as controversial and perplexing as the phenomena they purported to explain. Figure 1 is a reproduction of part of the first page of this paper.

Einstein and Relativity Theory

Following Newton’s triumph, work expanded not only in mechanics but also in the other branches of physics, in particular, in electricity and magnetism. This work culminated in the late nineteenth century in a new and successful theory of electricity and magnetism based upon the idea of electric and magnetic fields. The Scottish scientist James Clerk Maxwell, who formulated the new electromagnetic field theory, showed that what we observe as light can be understood as an electromagnetic wave. Newton’s physics and Maxwell’s theory account, to this day, for almost everything we observe in the everyday physical world around us. The motions of planets, cars, and projectiles, light and radio waves, colors, electric and magnetic effects, and currents all fit within the physics of Newton, Maxwell, and their contemporaries. In addition, their work made possible the many wonders of the new electric age that have spread throughout much of the world since the late nineteenth century. No wonder that by 1900 some distinguished physicists believed that physics was nearly complete, needing only a few minor adjustments. No wonder they were so astonished when, just 5 years later, an unknown Swiss patent clerk, who had graduated from the Swiss Polytechnic Institute in Zurich in 1900, presented five major research papers that touched off a major transformation in physics that is still in progress. Two of these papers provided the long-sought definitive evidence for the existence of atoms and molecules; another initiated the development of the quantum theory of light; and the fourth and fifth papers introduced the theory of relativity. The young man’s name was Albert Einstein, and this chapter introduces his theory of relativity and some of its many consequences.

Healing the mental scars

Few groups of scientists can have been studied more intensely than 20th-century German physicists, thanks to both their extraordinary successes and their involvement in the upheavals of the first half of the century. That attention has produced a number of first-rate accounts of these physicists and their community through the Third Reich (1933–1945), thus enabling us to gain a considerable grasp of their science and its broader cultural and political context. The same cannot be said, however, for the postwar period, especially the chaotic years following the defeat of Nazi Germany in 1945 through to the founding of East and West Germany – then under Allied occupation – in 1949.

Integrating Historical and Societal Contexts in the Computing Curricula

This paper provides instructors of computing a method to integrate computing history in the computing curriculum and to elevate the awareness of the social context of the subject. It provides suggestions by which instructors can enrich the curriculum by including history in the subjects they teach, even though they may not have had formal education in computing history or the history of science. Using history in computing very often stimulates discussion and dialogue among students and makes them aware of the social consequences of the computer systems they will use, help design, or create. Instructors can enrich the courses they now teach by integrating social and historical interludes within them.

Probing the Nucleus

We saw in Chapter 14 that studies of the atom indicated that the atom consists of a very small, positively charged nucleus surrounded by negatively charged electrons. Experiments on the scattering of a particles revealed that the nucleus has dimensions of the order of 10-14 m. Since the diameter of an atom is of the order of 10-10 m, the nucleus takes up only a minute fraction of the volume of an atom. The nucleus, however, contains nearly all of the mass of the atom, as was also shown by the scattering experiments.

The Nucleus and Its Applications

The discoveries of radioactivity and isotopes were extraordinary advances. And as usual, they also raised new questions about the structure of atoms, questions that involved the atomic nucleus. We saw in Chapter 17 that the transformation rules of radioactivity could be understood in terms of the Rutherford-Bohr model of the atom. But that model said nothing about the nucleus other than that it is small, has charge and mass, and may emit an a or a β particle. This implies that the nucleus has a structure that changes when a radioactive process occurs. The question arose: Can a theory or model of the atomic nucleus be developed that will explain the facts of radioactivity and the existence of isotopes?

The Quantum Model of the Atom

One of the first important clues to understanding atomic structure involved the study of the emission and absorption of light by the atoms of different elements. Physicists knew from Maxwell’s theory that light is emitted and absorbed only by accelerating charges. This suggested that the atom might contain moving charges. Patterns and regularities in the properties of the light emitted we expected to provide valuable clues about the precise nature of the motions of the moving charges. The results of this study were so important to the unraveling of atomic structure that we review their development here in some detail.