“A theoretical investigation which Einstein published in 1917 provides a third conspicuous landmark. It connected up he two great landmarks already mentioned by showing that the disintegration of radioactive substances is governed by the same laws as the jumps of the kangaroo electrons in the theory of Bohr. In fact radioactive atoms were now seen merely to contain a special breed of kangaroos, much more energetic and ferocious than any that had hitherto been encountered.”

— James Jeans

Physics and Philosophy (1942)

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James Jeans 54

British mathematician and astronomer 1877–1946

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“A second conspicuous landmark… is the enunciation of the fundamental law of radioactive disintegration by Rutherford and Soddy in 1903. This law was in no sense a consequence or development of Planck's theories; indeed fourteen years were to elapse before any connection was noticed between the two. The new law asserted that the atoms of radioactive substances broke up spontaneously, and not because of any particular conditions or special happenings. This seemed to involve an even more startling break with classical theory than the new laws of Planck; radioactive break-up appeared to be an effect without a cause, and suggested that the ultimate laws of nature were not even causal.”

James Jeans (1877–1946) British mathematician and astronomer

Physics and Philosophy (1942)

“The laws which governed the spontaneous jumps of the kangaroos were shown to be of the simplest; out of any number of kangaroos a certain proportion always jumped within a specified time, and nothing seemed to be able to change this number. Also, before the jumps took place, there was nothing in the world of phenomena to distinguish those kangaroos that were about to jump from those that were not… to help fill the quota demanded by the statistical law. As discontinuity marched into the world of phenomena through one door, causality walked out through another.”

James Jeans (1877–1946) British mathematician and astronomer

Physics and Philosophy (1942)

“In 1925 Heisenberg made a new attempt, on entirely novel lines, to obtain an explanation of atomic spectra. Working in collaboration with Bohr, he had come to the conclusion that the imperfections of Bohr's theory had been a consequence of assuming too simple a model for the atom. For Bohr had not only assumed that the atom consisted of particles moving through space and time, but also that the particles inside atoms were of the same kind as the electrons outside atoms.”

James Jeans (1877–1946) British mathematician and astronomer

Physics and Philosophy (1942)

“In the interior of the atom, Bohr had tried the plan of retaining the particle-electron and modifying the classical mechanics. Heisenberg took the opposite course, his procedure amounting in effect to retaining the classical mechanics, at least in form, and modifying the electron. Actually, the electron dropped out all together, because it exists only as a matter of inference and not of direct observation. For the same reason, the new theory contains no mention of atoms, nuclei, protons, or of electricity in any shape or form. The existences of all these are matters of inference, and Heisenberg's purely mathematical theory could no more make contact with them than with the efficiency of a turbine or with the price of wheat.”

James Jeans (1877–1946) British mathematician and astronomer

Physics and Philosophy (1942)

“Bohr's investigation had typified what had become a standard procedure in problems of theoretical physics. The first step was to discover the mathematical laws governing certain groups of phenomena; the second was to devise hypothetical models or pictures to interpret these laws in terms of motion or mechanism; the third was to examine in what way these models would behave in other respects, and this would lead to prediction of other phenomena-predictions which might or might not be confirmed when put to the test of experiment. For instance, Newton had explained the phenomena of gravitation in terms of a force of gravitation; a later age had seen the luminiferous ether introduced to explain the propagation of light and, subsequently, the general phenomena of electricity and magnetism; finally Bohr had introduced electronic jumps in an attempt to explain atomic spectra. In each case the models had fulfilled their primary purpose, but had failed to predict further phenomena with accuracy.”

James Jeans (1877–1946) British mathematician and astronomer

Physics and Philosophy (1942)

“The scientist tries to understand the origin of our solar system, the structure of the universe and the laws governing the atom. He has discovered X rays, the radioactive substances, and he has built cyclotrons. He has foreseen the existence of electromagnetic and electronic waves. Out of his thought has grown the technique of our century. But not until today has he begun to notice that the earth on which he moves is covered with sweat and with blood and that in the world in which he lives "the son of man has nowhere to lay his head."”

Leopold Infeld (1898–1968) Polish physicist

Quest: An Autobiography [1941] (second edition, 1980), Book III, "Search and Research", p. 338

“I have satisfied myself that the [cosmic] rays are not generated by the formation of new matter in space, a process which would be like water running up a hill. Nor do they come to any appreciable amount from the stars. According to my investigations the sun emits a radiation of such penetrative power that it is virtually impossible to absorb it in lead or other substances…. This ray, which I call the primary solar ray, gives rise to a secondary radiation by impact against the cosmic dust scattered through space. It is the secondary radiation which now is commonly called the cosmic ray, and comes, of course, equally from all directions in space. [The article continues: The phenomena of radioactivity are not the result of forces within the radioactive substances but are caused by this ray emitted by the sun. If radium could be screened effectively against this ray it would cease to be radioactive, he said. ]”

Nikola Tesla (1856–1943) Serbian American inventor

Quoted in 'Tesla, 75, Predicts New Power Source', New York Times (5 Jul 1931), Section 2, 1.

“Whereas, our tenet ever was, and, indeed, it is almost the only landmark which now divides the federalists from the republicans, that Congress had not unlimited powers to provide for the general welfare, but were restrained to those specifically enumerated;…”

Thomas Jefferson (1743–1826) 3rd President of the United States of America

1810s, Letter to Albert Gallatin (16 June 1817)

“An extension of Planck's ideas, due to Prof. Niels Bohr of Copenhagen, went on to suggest that… the ultimate particles of matter would be seen to move not like railway trains running smoothly on tracks, but like kangaroos hopping about in a field.”

James Jeans (1877–1946) British mathematician and astronomer

Physics and Philosophy (1942)

“Science Of Energetics. Although the mechanical hypothesis just mentioned may be useful and interesting as a means of anticipating laws, and connecting the science of thermodynamics with that of ordinary mechanics, still it is to be remembered that the science of thermodynamics is by no means dependent for its certainty on that or any other hypothesis, having been now reduced, to a system of principles, or general facts, expressing strictly the results of experiment as to the relations between heat and motive power. In this point of view the laws of thermodynamics may be regarded as particular cases of more general laws, applicable to all such states of matter as constitute Energy, or the capacity to perform work, which more general laws form the basis of the science of energetics, — a science comprehending, as special branches, the theories of motion, heat, light, electricity, and all other physical phenomena.”

William John Macquorn Rankine (1820–1872) civil engineer

Source: A Manual of the Steam Engine and Other Prime Movers (1859), p. 31

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