which the rest of this report can be understood. Later chapters focus more directly on the opportunities now available to begin to answer the 11 questions on the nature, origin and makeup of our universe.

PHYSICS OF MATTER: THE STANDARD MODEL AND BEYOND

The Standard Model

The Standard Model is a modest name for a grand intellectual achievement. For it is no less than, and in many ways more than, the theory of the fundamental structure of known matter. At the beginning of the 20th century, physics was very different from today. The classical laws of that time allow one to predict, given the configuration of matter and force fields at one time, the configurations at all later times. For example, Newton’s laws of motion and gravitational attraction can predict the positions of planets and comets in the future once their current positions (and velocities) are known. However, nothing in Newton’s laws can predict the existence of, or determine the overall size or shape of, the solar system. The modern (20th century) laws of physics go well beyond simple extrapolation of known conditions to the future. They describe not only how things move, but also what sorts of things there can and cannot be.

The first theory of the new type was the mathematical atomic model proposed by Niels Bohr in 1913. At first glance this model appears to differ little in spirit from Newton’s solar system or Rutherford’s nuclear atom: electrons orbit an atomic nucleus just as planets orbit the sun; the relevant force is electric rather than gravitational but obeys a similar law that relates force and distance between objects. But Bohr postulated that only certain orbits of definite size and shape could actually occur—the orbits are quantized. With this idea it became possible to explain why all systems with one electron orbiting one proton have exactly the same properties, and to calculate those properties. Thus, the universal properties of the substance called hydrogen could be explained. The existence of such a substance, with all its properties, is a consequence of the allowed quantum solutions for the interactions between a proton and an electron.

Bohr’s original rules, though successful in describing many features of atomic spectra, were not entirely correct, nor even internally consistent. Later physicists, including Werner Heisenberg, Erwin Schrodinger, and Paul Dirac, produced a framework that corrected these problems for the dynamics of quantized systems. The new quantum mechanics of simple electrical forces between elementary electrons and nuclei could explain the main



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