DiscoveryOfSubatomicParticles

The Discovery of Subatomic Particles (Steven Weinberg)

  • Great supplement to a Modern physics course.
  • This book is written for readers who may not be familiar with classical physics, but who are willing to pick it up as they go along in order to understand the rich tangle of ideas and experiments that make up the history of 20C physics
  • The discoveries of science are among the most precious elements of 20C culture, it seems a tragedy that so many otherwise well-educated people are cut off from this part of culture by lack of familiarity with the basics of science. Generally the student or reader who wants to become literate in physics is offered only one path: he or she must follow the same time-honored sequence of courses followed by generations of professional scientists. My starting point in this book was the assumption that, whether or not readers enjoy calculating the collisions of billiard balls, they do generally want to have a cultural background in the revolutionary scientific ideas and discoveries of our time. Instead of starting with classical physics, I invite the reader to plunge immediately into a sequence of key topics in 20C physics using each topic as an entree into just those concepts and methods of classical physics needed to understand that topic. The reader is asked to learn aspects of classical physics or chemistry only when it has been made clear that these specific concepts and methods are needed to understand the progress of 20C physics.
  • Most of what I know about physics and mathematics I have learned only when there was no alternative, when I simply had to learn something in order to get on with my own work. So the plan of this sort of book may be closer to the actual education of working scientists than many of the books and courses we design for students who specialize in science. My hope is that this book may contribute to a radical revision in the way science is brought to the non-scientist.
  • How better can we understand the dissolving of a piece of salt in a pot of water than by supposing that the atoms of which the salt is composed spread into the empty spaces between the atoms of water? How better can we understand the spread of a drop of oil on the surface of water, out to a definite area and no further, then by supposing that the oil spreads until it is a few atoms thick?
  • Oddly the general acceptance of the atomic nature of matter came about through the discoveries of the constituents of the atom, the electron and atomic nucleus – discoveries that undercut the old idea that atoms are indivisible.
  • In Chemical, optical and electrical phenomena, the nucleus of the atom is essentially inert.
  • Protons and neutrons in nuclei, like the electrons surrounding the nuclei, can be excited to states of higher energy or, if excited , can fall back to a state of lower energy, but the energies needed to excite the nuclear particles are typically a millions times those needed to excite electrons in the outer part of the atom.
  • The special property that makes electrons, protons and neutrons ubiquitous ingredients of ordinary matter is their relative stability. These particles, elementary or not, will concern us in this book.
  • There is one place with which the discovery of the constituents of the atom is especially associated : the Cavendish Laboratory of the University of Cambridge. -) JJ Thomson performed the experiments on cathode rays that discovered the electron -) Rutherford began his work on radioactivity, was later the leader of the laboratory and established it as the center for nuclear physics. -) Chadwick discovered the neutron
  • Although Maxwell's work lent great prestige to the Cavendish Professorship, the Laboratory did not develop into a leading center of experimental physics during his tenure.
  • As a consequence of its lightness, charge and stability, the electron has a unique importance to physics, chemistry, and biology.
  • JJ Thomson was not skillful in the execution of experiments. His success was largely due to the use of better vacuum pumps, which lowered the pressure inside the cathode-ray tubes to the point where interactions with the residual gases became negligible.
  • We still dont fully understand why when amber is rubbed with fur the electrons go from the fur to the amber or why when glass is rubbed with silk the electrons go from the glass to the silk. The so-called "Trobolectric" sequence is not well understood theoretically, and even a change in the weather can affect the relative placement of those substances.
  • How to remember anode vs cathode: cathode rays (electrons) come from the cathode and are attracted to the anode.
  • The gram was originally defined as the mass of a cubic centimeter of water.
  • Ampere reached the conclusion that all magnetism is electromagnetism and that it is tiny electric currents circulating in the particles of a lodestone that give it its magnetic properties. This is the sort of unification that brings joy to the hearts of physicists.
  • The discovery of electromagnetism had an immediate effect on technology. The powerful magnets used in steel mills and particle accelerators are electromagnets. The application that probably had the greatest historical importance was the telegraph. Ampere saw at once, that the deflection of a compass needed can tell whether a current has been switched on in a wire even if the switch is far away; so a massage (reduced to a series of "on" and "offs") can be sent as far as the wire will carry a sufficiently strong current.
  • A problem that has been encountered often in physics: Energy is defined so that it is conserved in some limited context and then in a larger context it is found not to be conserved. The response to this problem that has proved the most fruitful has been not to abandon the concept of energy, but to widen int to define new kinds of energy in such a way that the total value of all kines of energy is conserved.
  • Zeemans measurements had provided a rough estimate of the mass-to-charge ratio of whatever it is that carries electric currents in atoms, Thomsons work on cathode rays showed that these charge carriers are not just part of the architecture of the atom, but have a separate existence of their own outside the atom. Thus whatever what electrons were, they contain at least one common constituent that is emitted from metals as cathode rays. The universality was soon verified when the beta rays were seen to be the same particles.
  • Atoms in solids could be presumed to be packed closely together, so measurements of the densities of solid materials provided values for the densities of atoms.
  • The mole is a useful unit because a mole of any substance always contains the same number of molecules.
  • JJ Thomson discovered that ordinary nonradioactive light elements also have isotopes. Not surprisingly he used the technique of electric and magnetic deflection of rays produced in a cathode-ray tube. (but using the rays coming off the anode.
  • In electrolysis definite numbers of molecules of each type are produced when a given amount of electric charge flows through the wires and the battery.
  • "I am now convinced that we have recently become possessed of experimental evidence of the discrete or grained nature of matter, which the atomic hypothesis sought in vain for hundreds and thousands of years" The evidence quoted by Ostwald was Perrins experiments on Brownian motion and Millikan's measurement of the electron charge
  • The great task after the discovery of the electron was to identify the positive material within the atom and described how electrons are arranged within it.
  • Xrays are emitted when electrons in the outer part of atoms drop into the inner orbits to replace electrons knocked out of the atom by the cathode ray,
  • Rutherford's discovery of the nucleus raised new questions as formidable as the ones he had answered. What determines the sizes and energies of the orbits of electrons in an atom? Why dont the orbiting electrons radiate EM waves? What keeps the nucleus from flying apart?
  • Radium is 1e6 times more radioactive than uranium.
  • The reason that alpha particles are commonly emitted by radioactive elements is the same reason that helium is common in our universe: The helium nucleus is by far the most tightly bound of the lightest atomic nuclei.
  • There are some radioactive elements (such as radium) whose 1/2 lives are short enough to be measured form the rate at which their radioactivity decreases, and yet long enough to also be measured by assembling a sample of known mass and counting the radioactive disintegrations. These must agree, if the one has calculated the number of radioactive elements present in the sample correctly. One can turn this around and use the 1/2 from the decay of radioactivity together with the number of disntigrations per second to determine Avogadro's number.
  • The electric charge of the nucleus can be determined by measuring the scattering cross section of alpha.
  • IF the "plum pudding" model was correct, then classical calculations of the alpha scattering cross section would have given the wrong answer and it would have been impossible to interpret the results until the development of QM.
  • Bohrs atomic model could be used to measure the nuclear charge from the x-ray wavelengths.

  • It was not expected that nuclear charge simply goes up by one unit as we go from one element to the next in a list by atomic weights. Thus, with few exceptions the number that gives the place of an element in the list when they are ordered by weight is the same as the electrical charge of the nucleus in units of the charge of the electron, now called the "atomic number"

    IT was now possible to determine the nuclear charge of any element, and by inference the number of electrons in its atom, but just looking at the list of elements ordered by weight.

    Even more important it was now understood that the particular set of elements found on earth is not some random sampling of an infinitely diverse menu of elements, but comprises essentially all the elements that could exist.

    "Moseley called the roll of the elements, so that for the first time we could say definitely the number of possible elements between the beginning and the end, and the number that still remained to be found." (Mosley was killed in WW1 during the Gallipoli campaign)

  • The atomic weight of an element is not just the number of nuclear particles it has; it also receives a contribution from the internal energy of the nucleus. This it is not expected to be precisely a whole number.

    By looking at the difference between the measured atomic mass and the nearest whole number, we conclude that the internal energy (per particle) is lowest for the nuclei of medium atomic weight and larger for both lighter and heavier nuclei. Hence common isotopes of medium-weight nuclei are not radioactive, because they have no surplus of internal energy to get rid of; common isotopes of lighter nuclei are not radioactive, because the lighter nuclei into which they might decay have an ever larger surplus of internal energy. However, nuclei of large atomic weight have more surplus energy than the lighter nuclei into which they might decay, so they can and do release this energy in radioactivity.

  • For 20 years after the discovery of the atomic nucleus, physicists generally thought that the nuclei of all elements consisted of protons and electrons.

  • Stable nuclear valley: (nuclear energies plotted against the number of protons and neutrons in the nucleus)

    The stable valley grows steadily deeper as we move toward heavier elements, this is because the attraction caused by the nuclear forces increase with increasing numbers of protons and neutrons. Because of this, energy will be released if the nuclei of light elements fuse to form heavier nuclei; this is the source of the energy of stars like our sun.

    For light nuclei the repulsive electrical forces among the protons are much weaker than the very strong nuclear forces among the neutrons and protons, but the electrical forces build up for larger nuclei much more rapidly than the nuclear forces do, and they become important when there are more than about 20 protons. This has two effects: The floor of the stable valley start to rise again; and the valley veers toward the neutron-rich side of the map, since it is the charge of the protons that matters.

    The pattern of radioactive decays is thus a sequence of alpha decays interspersed with beta decays, the alpha decays taking the nucleus down the valley toward lead but also somewhat out of the valley toward the neutron-rich side and the beta decays returning the nucleus to the neighborhood of the valley floor.

  • The energy of naturally radioactive elements was put into them by the neutrons released in the explosions of stars, and can ultimately be traced to the force of gravitational attraction which provided the energy for the stellar explosions.
  • The electro-weak theory proposed that the dissimilarities between charged leptons and neutrinos are more apparent than real: the equations governing their fields are actually symmetric between the two, the apparent difference arise from the interactions of these fields with the cosmic environment (Higgs)
  • The large mass of the top quark is not surprising. The W and Z can be regarded as defining a natural mass scale and the top quark is only about twice as heavy. Also the masses of the neutrinos is not surprising, the EW theory in its simplest form require them to be massless, and so their masses can only come from tiny corrections to that theory, due to processes at extremely high energy. What is surprising is the fact that all the other quarks besides the top and all the charged leptons have masses much less than the W or Z particles. From this point of view the most mysterious of all the elementary particles is the electron, the first discovered among them all.
  • The deepest aim of science: to understand the complexity of nature in simple terms

Backlinks