Theoretical concepts in physics Book by Malcolm Longair

memo

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Richard Westphal's monumental biography Never at Rest was the product of a lifetime's study of Isaac Newton's life and work. In the preface, he writes: The more I have studied him, the more Newton has receded from me. It has been my privilege at various tiInes to know a number of brilliant men, men whom I acknowledge without hesitation to be my intellectual superiors. I have never, however, met one against whom I was unwilling to measure myself so that it seelned reasonable to say that I was half as able as the person in question, or a third or a fourth, but in every case a finite fraction. The end result of my study of Newton has served to convince me that with him there is no measure. He has become for me wholly other, one of the tiny handful of supreme geniuses who have shaped the categories of human intellect, a man not finally reducible to the criteria by which we comprehend our fellow beings. I In the next paragraph, he writes: Had I known, when in youthful self-confidence I committed myself to the task, that I would end up in similar self-doubt, surely I would never have set out. I Newton's impact upon science is so all pervasive that it is worthwhile filling in some of the background to his character and extraordinary achievements. The chronology which follows is that adopted in the Introduction of the volutne Let Newton Be.

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This work was not published, because there were a number of steps in the calculation which needed further elaboration. (i) Kepler had shown that the orbits of the planets are ellipses and not circles - how did that affect the calculation? (ii) Newton was uncertain about the influence of the other bodies in the Solar System upon each others' orbits. (iii) He was unable to explain the details of the Moon's motion about the Earth, which, as we now know, is influenced by the fact that the Earth is not spherical. (iv) Probably most important of all, there is a key assumption in the calculation, that all the mass of the Earth can be located at its centre in working out the acceleration due to gravity at its surface and its influence upon the Moon. The same assumption was made for all the bodies in the solar system. In his calculations of 1 665-6, Newton regarded this step as an approximation. He was uncertain about its validity for obj ects close to the surface of the Earth. Newton laid this work aside until 1679.

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If the formula derived is correct, then V must be a straight line function of the frequency of the incident light, when plotted in Cartesian coordinates, whose slope is independent of the nature of the substance investigated. Thus, the quantity h / e, the ratio of Planck's constant to the electronic charge, can be found directly from the slope of this relation. These were relnarkable predictions because nothing was known at that time about the dependence of the photoelectric effect upon the frequency of the incident radiation. After ten years of difficult experimentation, all aspects of Einstein's equation were fully confirn1ed experimentally. In 1916, Millikan was able to summarise the results of his extensive experiments: Einstein's photoelectric equation has been subj ected to very searching tests and it appears in every case to predict exactly the observed results.

Photoionisation of gases. The third piece of experimental evidence discussed in Einstein's paper was the fact that the energy of each photon has to be greater than the ionisation potential of the gas if photoionisation is to take place. He showed that the smallest energy quanta for the ionisation of air were approximately equal to the ionisation potential determined independently by Stark. Once again, the quantum hypothesis is in agreement with experiment. At this point, the paper ends. It is one of the great papers in physics and is the work described in Einstein's Nobel Prize citation.