DOCSLIB.ORG

By Willis Lamb

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
By Willis Lamb FIVE ENCOUNTERS WITH FELIX BLOCH by Willis Lamb ABSTRACT The impact of Felix Bloch's work on the fields of parity non- conservation, the Mossbauer effect, nuclear induction, chemical shifts, and laser theory is described from a personal point of view. I have benefited enormously from contacts with a number of the great theoretical physicists of the twentieth century. One such relationship far exceeds all the others in respect to duration and meaning to me. I am going to describe five encounters with Felix Bloch that involved purely scientific matters. Personal matters come into the account only to set the times and places of the episodes related. The first two and the last of the five en- counters involved suggestions Felix made to me in connection with my re- search. If I had had the wit, energy, and luck to folIow up properly the earli- est two of these leads, I might have made some very good discoveries. The third encounter involved a request from him for help on his research. It turned out that he did not, after the fact, need this help. Still, the story has a certain interest for me and might provide a footnote for a history of modern physics. The fourth encounter was very slight. I had worked on a certain prob- lem in the early forties. Bloch was working in the same area in the earIy fifties. I had missed following up some interesting aspects of my work. %loch and I talked about his problem and its relationship to my earlier re- search. It later turned out that his work was capable of enormous applica- tion, in chemistry rather than in physics, so that he did not follow it up as far as he might have done. WiIlis Lamb is Professor of Physics at the University of Arizona. 134 RICE UNIVERSITY STUDIES The fifth encounter led me to the development of the theory of lasers and masers, which has been a very productive and useful field of research for over twenty years. I received a B.Sc. at Berkeley in chemistry in 1934, and began the study of theoretical physics with Robert Oppenheimer. In those days, the Physics Departments at Berkeley and Stanford occasionally had joint picnics fol- lowed by a colloquium. One that I remember vividly took place in the spring of 1934 at the Lick Observatory on Mount Hamilton. The speaker was someone from Stanford named Felix Bloch. Solid state physics was not taught at Berkeley then, and I had no notion of the enormous contributions made to that field by the speaker. He had just come from Rome and told us about Fermi's new work on the theory of beta decay. In the fall of 1934, Oppenheimer and Bloch began to conduct joint weekly theoretical physics seminars alternating between Berkeley and Stanford. During the next year I gradually got to know Bloch, who accepted me with an unusual degree of tolerance. In the summer of 1935, I went to the theoretical physics school at Ann Arbor. This was organized by George Uhlenbeck and Samuel Goudsmit and operated during the eleven years before the U.S. entry into WorId War 11. Bloch was one of the lecturers that summer, and I learned from him the elements of his theory of electrical conductivity. I also acquired a little understanding of the theory of beta decay from other lecturers at the summer school, among whom were Enrico Fermi and David Dennison, as well as Uhlenbeck and Goudsmit. PARITY NON-CONSERVATION One day in the autumn of 1935,I was driving with Bloch somewhere in Berkeley, and he told me that the shape of a beta decay electron energy spectrum would depend on the change of angular momentum of the nuclear states. He suggested that 1 calculate this effect in more detail. The angular momentum dependence contemplated by Bloch came from the variation across the nuclear radius of the plane wave electron and neutrino wave functions in the transition matrix element. This brought in additional momentum-dependent factors and thereby changed the shape of the theoretical electron energy spectrum. At that time, the theory of beta decay was very primitive and poorly formulated. Competing with Fermi's originally postulated interaction for the emission of an electron and a neutrino was the proposal of Konopinski and Uhlenbeck that explicitly involved derivatives of lepton wavefunctions, and also changed the momentum dependence of the matrix element for the emission process. Gamov and Teller had also suggested an interaction that differed from Fermi's by the presence of nucIeon and lepton spin operators. FIVE ENCOUNTERS WITH FELIX BLOCH 135 By that time, I had learned from Pauli's 1933 Handbuch der Physik article' on Quantum Mechanics how he had classified the covariant quanti- ties that could be made out of binary combinations of Dirac wave func- tions: scalar, polar vector, tensor, axial vector, and pseudovector. A par- ticular combination of these interactions was used by Fermi, and, stimu- lated by Bloch, I began to investigate the effect on the beta spectrum of taking wild mixtures of the various Pauli building blocks. I remember that Robert Serber (then at Berkeley) showed me that some of the mixtures I was considering did not conserve parity! This might have been responsible for a missed opportunity, but 1936 was simply too early for the discovery2 of non-parit~conserving interactions. In fact, it took over twenty years before physics was ready for the theoretical analysis of Lee and Yang. The experimental state of measurement of beta spectra in 1936 was very confused because of thick source effects. I never really accomplished anything very useful with beta decay theory, and the work was publi~hed'.~ only in two abstracts of talks given before meetings of the Pacific Coast Section of the American Physical Society. SUMMERS AT STANFORD There wasn't much theoretical physics going on in Berkeley during the summers. Oppenheimer invariably spent about six weeks in the late spring at Cal Tech. I usually folIowed him there, but never to his summer retreat in New Mexico. Bloch stayed working at Stanford during the summers, and kindly provided me with a desk and library facilities. I was there during the summers of 1436-1939 and 1941. Each year there was a visiting lecturer, and in these years I came to know George Gamov, Edward Teller, Victor Weisskopf, J. H. Van Vleck, George Placzek, Samuel Allison, I. I. Rabi, George Kistiakowski, and many others who passed through Pa10 Alto on their summer travels., I also met most of the Stanford physicists: David Webster, Paul Kirkpatrick, Norris Bradbury, William Hansen, Russell Varian, Arnold Siegert, and Arnold Nordsieck (I had already known Nord- sieck at Berkeley, and we later were colleagues at Columbia for many years). My meeting with Rabi led to an appointment as Instructor in Physics at Columbia in the fall of 1938 after I received a Ph.D. THE M~SSBAUEREFFECT At Ann Arbor, I had heard Fermi lecture on the effect of chemical binding of a hydrogen atom on its scattering of slow neutrons. This interest- ed me, and I began to work on related problems. It seemed that there might also be an effect of the binding of a hydrogen atom on the capture cross 136 RICE UNIVERSITY STUDIES section for slow neutrons. At first I thought the effect would be large, but finally had to settle for a very rough estimate of the cross section for a very unlikely process: the radiationless capture of neutrons by bound protons to form deuterons, with the excess energy and momentum going into vibra- tional motion of the deuteron instead of a gamma ray. The normal capture process was very little affected by the chemical binding. Even today, this radiationless capture has never been seen, but I am still hoping that some- day it may be. This work5 formed part of my doctoral thesis. The other part6dealt with electromagnetic properties of nuclear matter. That summer, I came across a paper by Hans Bethe and Placzek7 on the resonant capture of slow neutrons by nuclei in a gas. The resulting com- pound nucleus could decay in a number of ways that contributed to rhe natural width of the resonance curve. The Lorentzian resonance curve of the Breit-Wigner theory of capture of slow neutrons to form a compound nucleus was modified because of the Doppler effect. In the limit of small gas atom velocities, the resonance curve was Lorentzian, while for rapid motions it became a Doppler-broadened Gaussian. In the general case, Bethe and Placzek obtained the Voigt8profile, which was well known in the theory of Doppler broadening of naturally broadened spectral lines emitted or absorbed by atoms in a gas. Since most of the experimental studies on resonant neutron capture in- volved atoms in a solid, I began to generalize the Bethe-Placzek theory to deal with nuclei bound in a crystal lattice. For the case of a simple harmonic oscillator (Einstein) model of the solid, it was clear that the neutron reso- nance curve would be a sum of Lorentzians with centers displaced by the separation of the vibrational energy levels of the oscillator. Then using what was learned from Bloch in a previous summer, I worked out the theory for a Debye model solid. Instead of a single oscillator frequency, there was a fre- quency continuum extending from zero up to a maximum value simply related to the Debye temperature of the solid. I derived a general, but complicated, expression for the neutron capture cross section as a function of neutron energy.
Load more

Recommended publications
  • Charles Hard Townes (1915–2015)
    ARTICLE-IN-A-BOX Charles Hard Townes (1915–2015) C H Townes shared the Nobel Prize in 1964 for the concept of the laser and the earlier realization of the concept at microwave frequencies, called the maser. He passed away in January of this year, six months short of his hundredth birthday. A cursory look at the archives shows a paper as late as 2011 – ‘The Dust Distribution Immediately Surrounding V Hydrae’, a contribution to infrared astronomy. To get a feel for the range in time and field, his 1936 masters thesis was based on repairing a non-functional van de Graaf accelerator at Duke University in 1936! For his PhD at the California Institute of Technology, he measured the spin of the nucleus of carbon-13 using isotope separation and high resolution spectroscopy. Smythe, his thesis supervisor was writing a comprehensive text on electromagnetism, and Townes solved every problem in it – it must have stood him in good stead in what followed. In 1939, even a star student like him did not get an academic job. The industrial job he took set him on his lifetime course. This was at the legendary Bell Telephone Laboratories, the research wing of AT&T, the company which set up and ran the first – and then the best – telephone system in the world. He was initially given a lot of freedom to work with different research groups. During the Second World War, he worked in a group developing a radar based system for guiding bombs. But his goal was always physics research. After the War, Bell Labs, somewhat reluctantly, let him pursue microwave spectroscopy, on the basis of a technical report he wrote suggesting that molecules might serve as circuit elements at high frequencies which were important for communication.
    [Show full text]
  • Wolfgang Pauli Niels Bohr Paul Dirac Max Planck Richard Feynman
    Wolfgang Pauli Niels Bohr Paul Dirac Max Planck Richard Feynman Louis de Broglie Norman Ramsey Willis Lamb Otto Stern Werner Heisenberg Walther Gerlach Ernest Rutherford Satyendranath Bose Max Born Erwin Schrödinger Eugene Wigner Arnold Sommerfeld Julian Schwinger David Bohm Enrico Fermi Albert Einstein Where discovery meets practice Center for Integrated Quantum Science and Technology IQ ST in Baden-Württemberg . Introduction “But I do not wish to be forced into abandoning strict These two quotes by Albert Einstein not only express his well­ more securely, develop new types of computer or construct highly causality without having defended it quite differently known aversion to quantum theory, they also come from two quite accurate measuring equipment. than I have so far. The idea that an electron exposed to a different periods of his life. The first is from a letter dated 19 April Thus quantum theory extends beyond the field of physics into other 1924 to Max Born regarding the latter’s statistical interpretation of areas, e.g. mathematics, engineering, chemistry, and even biology. beam freely chooses the moment and direction in which quantum mechanics. The second is from Einstein’s last lecture as Let us look at a few examples which illustrate this. The field of crypt­ it wants to move is unbearable to me. If that is the case, part of a series of classes by the American physicist John Archibald ography uses number theory, which constitutes a subdiscipline of then I would rather be a cobbler or a casino employee Wheeler in 1954 at Princeton. pure mathematics. Producing a quantum computer with new types than a physicist.” The realization that, in the quantum world, objects only exist when of gates on the basis of the superposition principle from quantum they are measured – and this is what is behind the moon/mouse mechanics requires the involvement of engineering.
    [Show full text]
  • Turning Point in the Development of Quantum Mechanics and the Early Years of the Mossbauer Effect*
    Fermi National Accelerator Laboratory FERMILAB-Conf-76/87-THY October 1976 A TURNING POINT IN THE DEVELOPMENT OF QUANTUM MECHANICS AND THE EARLY YEARS OF THE MOSSBAUER EFFECT* Harry J. Lipkin' Weizmann Institute of Science, Rehovot, Israel Argonne National Laboratory, Argonne, Illinois 60^39 Fermi National Accelerator Laboratory"; Batavia, Illinois 60S10 It is interesting to hear about the exciting early days recalled by Professors Wigner and Wick. I learned quantum theory at a later period, which might be called a turning point in its development, when the general attitude toward quantum mechanics and the study of physics was very different from what it is today. As an undergraduate student in electrical engineering in 19^0 in the United States I found a certain disagreement between the faculty and the students about the "relevance'- of the curriculum. Students thought a k-year course in electrical engineering should include more electronics than a one-semester 3-hour course. But the establishment emphasized the study of power machinery and power transmission because 95'/° of their graduates would eventually get jobs in power. Electronics, they said, was fun for students who were radio hams but useless on the job market. Students at that time did not have today's attitudes and did not stage massive demonstrations and protests against the curriculum. Instead a few of us who wished to learn more interesting things satisfied all the requirements of the engineering school and spent as much extra time as possible listening to fascinating courses in the physics building. There we had the opportunity to listen to two recently-arrived Europeans, Bruno Rossi and Hans Bethe.
    [Show full text]
  • Laser Spectroscopy to Resolve Hyperfine Structure of Rubidium
    Laser spectroscopy to resolve hyperfine structure of rubidium Hannah Saddler, Adam Egbert, and Will Weigand (Dated: 12 November 2015) This experiment had two main goals: to create an absorption spectrum for rubidium using the technique of absorption spectroscopy and to resolve the hyperfine structures for the two rubidium isotopes using saturation absorption spectroscopy. The absorption spectrum was used to determine the frequency difference between the ground state and first excited state for both isotopes. The calculated frequency difference was 6950 MHz ± 90 MHz for rubidium 87 and 3060 MHz ± 60 MHz for rubidium 85. Both values agree with the literature values. The hyperfine structure for rubidium 87 was able to be resolved using this experimental setup. The energy differences were determined to be 260 MHz ± 10 MHz and 150 MHz ± 10 Mhz MHz. The hyperfine structure for rubidium 85 was unable to be resolved using this experimental setup. Additionally the theory of doppler broadening was used to make measurements of the full width half maximum. These values were used to calculate a temperature of 310K ± 40 K which makes sense because the experiments were performed at room temperature. I. INTRODUCTION in the theory section and how they were manipulated and used to derive the results from the recorded data. Addi- tionally there is an explanation of experimental error and The era of modern spectroscopy began with the in- uncertainty associated the results. Section V is a conclu- vention of the laser. The word laser was originally an sion that ties the results of the experiment we performed acronym that stood for light amplification by stimulated to the usefulness of the technique of laser spectroscopy.
    [Show full text]
  • Laser Spectroscopy Experiments
    Hyperfine Spectrum of Rubidium: laser spectroscopy experiments Physics 480W (Dated: Sp19 Paper #4) I. OBJECTIVES FOR THESE EXPERIMENTS We wish to use the technique of absorption spec- troscopy to probe and detect the energy level structure of atomic Rubidium, Rb I, whose ground state is split by a tiny amount on account of nuclear magnetism. In effect, the spectroscopy we do today tells us about nuclear prop- erties and so combines atomic and nuclear physics. The main result of this experiment, the 4th of the semester, is to 1. measure the hyperfine splitting for each isotope, and compare with accepted values, with the fol- lowing details in mind: (a) what is the hyperfine splitting of the ground 2 state, S1=2 term? Do we need saturation- absorption techniques for this? (b) what are the hyperfine splittings of the ex- 2 cited state, P3=2 term, that can be reached with a nominal wavelength of 780nm from the ground state? Here we need saturation- absorption techniques to perform sub-Doppler FIG. 1. Note the four 'blobs'. Why are there four? Which spectroscopy, certainly. Help the reader un- 85 are associated with Rb37, and so on. If all goes swimm- derstand what is entailed in the technique, ingly, we'll get an absorption spectrum that looks much line both experimentally and theoretically. You the figure below the setup. The etalon data will be needed to will need to explain what `saturation' means. make the abscissa something proportional to frequency. The The saturation intensity is an important fig- accepted value of the gap between the 2 outermost dips is ure of merit.
    [Show full text]
  • The Concept of the Photon—Revisited
    The concept of the photon—revisited Ashok Muthukrishnan,1 Marlan O. Scully,1,2 and M. Suhail Zubairy1,3 1Institute for Quantum Studies and Department of Physics, Texas A&M University, College Station, TX 77843 2Departments of Chemistry and Aerospace and Mechanical Engineering, Princeton University, Princeton, NJ 08544 3Department of Electronics, Quaid-i-Azam University, Islamabad, Pakistan The photon concept is one of the most debated issues in the history of physical science. Some thirty years ago, we published an article in Physics Today entitled “The Concept of the Photon,”1 in which we described the “photon” as a classical electromagnetic field plus the fluctuations associated with the vacuum. However, subsequent developments required us to envision the photon as an intrinsically quantum mechanical entity, whose basic physics is much deeper than can be explained by the simple ‘classical wave plus vacuum fluctuations’ picture. These ideas and the extensions of our conceptual understanding are discussed in detail in our recent quantum optics book.2 In this article we revisit the photon concept based on examples from these sources and more. © 2003 Optical Society of America OCIS codes: 270.0270, 260.0260. he “photon” is a quintessentially twentieth-century con- on are vacuum fluctuations (as in our earlier article1), and as- Tcept, intimately tied to the birth of quantum mechanics pects of many-particle correlations (as in our recent book2). and quantum electrodynamics. However, the root of the idea Examples of the first are spontaneous emission, Lamb shift, may be said to be much older, as old as the historical debate and the scattering of atoms off the vacuum field at the en- on the nature of light itself – whether it is a wave or a particle trance to a micromaser.
    [Show full text]
  • Advanced Information on the Nobel Prize in Physics, 5 October 2004
    Advanced information on the Nobel Prize in Physics, 5 October 2004 Information Department, P.O. Box 50005, SE-104 05 Stockholm, Sweden Phone: +46 8 673 95 00, Fax: +46 8 15 56 70, E-mail: [email protected], Website: www.kva.se Asymptotic Freedom and Quantum ChromoDynamics: the Key to the Understanding of the Strong Nuclear Forces The Basic Forces in Nature We know of two fundamental forces on the macroscopic scale that we experience in daily life: the gravitational force that binds our solar system together and keeps us on earth, and the electromagnetic force between electrically charged objects. Both are mediated over a distance and the force is proportional to the inverse square of the distance between the objects. Isaac Newton described the gravitational force in his Principia in 1687, and in 1915 Albert Einstein (Nobel Prize, 1921 for the photoelectric effect) presented his General Theory of Relativity for the gravitational force, which generalized Newton’s theory. Einstein’s theory is perhaps the greatest achievement in the history of science and the most celebrated one. The laws for the electromagnetic force were formulated by James Clark Maxwell in 1873, also a great leap forward in human endeavour. With the advent of quantum mechanics in the first decades of the 20th century it was realized that the electromagnetic field, including light, is quantized and can be seen as a stream of particles, photons. In this picture, the electromagnetic force can be thought of as a bombardment of photons, as when one object is thrown to another to transmit a force.
    [Show full text]
  • The Light That Shin
    THE LIGHT THAT SHIN by CHARLES H. TOWNES A Nobel laureate recounts ON JULY 21, 1969, astronauts Neil Armstrong and Edwin “Buzz” Aldrin set up an array of small reflectors on the invention of the laser the moon, facing them toward Earth. At the same time, two teams of astrophysicists, one at the University of California’s Lick Observatory and the other at the University of Texas’s and the birth of quantum McDonald Observatory, were preparing small instruments on two big telescopes. Ten days later, the Lick team pointed electronics. its telescope at the precise location of the reflectors on the moon and sent a small pulse of power into the hardware they had added to it. A few days after that, the McDonald team went through the same steps. In the heart of each telescope, a narrow beam of extraordinarily pure red light emerged from a synthetic ruby crystal, pierced the sky, and entered the near vacuum of space. The two rays were still only a thousand yards wide after traveling 240,000 miles to illumi- nate the moon-based reflectors. Slightly more than a second after each light beam hit its target, the crews in California and Texas detected its faint reflection. The brief time inter- val between launch and detection of these light pulses per- mitted calculation of the distance to the moon to within an inch—a measurement of unprecedented precision. The ruby crystal for each light source was the heart of a laser (an acronym for light amplification by stimulated emis- sion of radiation), which is a device first demonstrated in 1960, just nine years earlier.
    [Show full text]
  • Chronological Table for the Development of Atomic and Molecular Physics
    Chronological Table for the Development of Atomic and Molecular Physics ≈ 440 Empedocles assumes, that the whole world is constant proportions”: Each chemical element BC composed of 4 basic elements: Fire, Water, Air consists of equal atoms which form with sim- and Soil. ple number ratios molecules as building blocks ≈ 400 Leucippos and his disciple Democritus claim, of chemical compounds. BC that the world consists of small indivisible 1811 Amedeo Avogadro derives from the laws particles, called atoms, which are stable and of Gay-Lussac (Δp/p = ΔT/T ) and Boyle- nondestructable. Marriot (p · V = constant for T = constant) ≈ 360 Plato attributes four regular regular geometric the conclusion that all gases contain under BC structures composed of triangles and squares equal conditions (pressure and temperature) (Platonic bodies) to the four elements and the same number of particles per volume. postulates that these structures and their inter- 1820 John Herapath publishes his conception of a change represent the real building blocks of the Kinetic theory of gases. world. 1857 Rudolf J.E. Clausius develops further the ki- ≈ 340 Aristotle contradicts the atomic theory and as- netic gas theory, founded by J. Herapath and BC sumes that the mater is continuous and does D. Bernoulli. not consist of particles. 1860 Gustav Robert Kirchhoff and Robert Bunsen 300 Epicurus revives the atomic model and as- create the foundations of spectral analysis of BC sumes that the atoms have weight and spatial the chemical elements. extension. 1865 Joseph Lohschmidt calculates the absolute 200 BC no real progress in atomic physics. number of molecules contained in 1 cm3 of –1600 AC a gas under normal conditions (p = 1atm, 1661 Robert Boyle fights in his book: “The Sceptical T = 300 K).
    [Show full text]
  • Contributions of Civilizations to International Prizes
    CONTRIBUTIONS OF CIVILIZATIONS TO INTERNATIONAL PRIZES Split of Nobel prizes and Fields medals by civilization : PHYSICS .......................................................................................................................................................................... 1 CHEMISTRY .................................................................................................................................................................... 2 PHYSIOLOGY / MEDECINE .............................................................................................................................................. 3 LITERATURE ................................................................................................................................................................... 4 ECONOMY ...................................................................................................................................................................... 5 MATHEMATICS (Fields) .................................................................................................................................................. 5 PHYSICS Occidental / Judeo-christian (198) Alekseï Abrikossov / Zhores Alferov / Hannes Alfvén / Eric Allin Cornell / Luis Walter Alvarez / Carl David Anderson / Philip Warren Anderson / EdWard Victor Appleton / ArthUr Ashkin / John Bardeen / Barry C. Barish / Nikolay Basov / Henri BecqUerel / Johannes Georg Bednorz / Hans Bethe / Gerd Binnig / Patrick Blackett / Felix Bloch / Nicolaas Bloembergen
    [Show full text]
  • All in the Mind? Responsive Conditions” Involve Activation of Tioned and Unconditioned Stimuli)
    book reviews to my students who are considering graduate response in both depression and Parkinson’s but later in the book, as he gets rolling, he studies and possible careers in research, as disease. And some clinicians are beginning seems to assume that he has demonstrated well as to friends and colleagues who are to consider the placebo response as an its validity. looking for a starting point to explore and integral part of healing, and are seeking His case is not helped by inaccuracies understand the dynamics of natural and ways to harness and enhance it. ranging from trivial errors of fact (he tells technological networks. I Given the subtitle of Dylan Evans’ book, us that no study of depressed patients has J. J. Collins is in the Center for BioDynamics and one might expect it to offer a comprehensive included both a placebo and a no-treatment the Department of Biomedical Engineering, Boston assessment of the role of belief in the placebo group, but there is at least one such study, University, Boston, Massachusetts 02215, USA. effect. But that’s not what we get. Instead and it is cited in a paper he reviews in depth), there is a smorgasbord of facts and spec- to misinformation (he says that placebos ulation presented in support of the author’s don’t help asthma) and major misconcep- main — and in my view thoroughly tions (that classical conditioning entails implausible — hypothesis that “all placebo- belief in the connection between the condi- All in the mind? responsive conditions” involve activation of tioned and unconditioned stimuli).
    [Show full text]
  • 1 Historical Introduction to the Elementary Particles
    13 1 Historical Introduction to the Elementary Particles This chapter is a kind of ‘folk history’ of elementary particle physics. Its purpose is to provide a sense of how the various particles were first discovered, and how they fit into the overall scheme of things. Along the way some of the fundamental ideas that dominate elementary particle theory are explained. This material should be read quickly, as background to the rest of the book. (As history, the picture presented here is certainly misleading, for it sticks closely to the main track, ignoring the false starts and blind alleys that accompany the development of any science. That’s why I call it ‘folk’ history – it’s the way particle physicists like to remember the subject – a succession of brilliant insights and heroic triumphs unmarred by foolish mistakes, confusion, and frustration. It wasn’t really quite so easy.) 1.1 The Classical Era (1897–1932) It is a little artificial to pinpoint such things, but I’d say that elementary particle physics was born in 1897, with J. J. Thomson’s discovery of the electron [1]. (It is fashionable to carry the story all the way back to Democritus and the Greek atomists, but apart from a few suggestive words their metaphysical speculations have nothing in common with modern science, and although they may be of modest antiquarian interest, their genuine relevance is negligible.) Thomson knew that cathode rays emitted by a hot filament could be deflected by a magnet. This suggested that they carried electric charge; in fact, the direction of the curvature required that the charge be negative.
    [Show full text]