Third Thoughts: The Universe We Still Don't Know by Steven Weinberg

Author:Steven Weinberg [Weinberg, Steven]
Language: eng
Format: epub
ISBN: 9780674241473
Barnesnoble:
Publisher: Harvard Univ Pr

1. Marcus du Sautoy, Symmetry: A Journey into the Patterns of Nature (New York: Harper, 2008); Ian Stewart, Why Beauty Is Truth: A History of Symmetry (New York: Basic Books, 2007).

2. For reasons that are difficult to explain without mathematics, these symmetries imply important conservation laws: the conservation of energy, momentum, and angular momentum (or spin). Some other symmetries imply the conservation of other quantities, such as electric charge.

3. Lorentz had tried to explain the constancy of the observed speed of light by studying the effect of motion on particles of matter. Einstein was instead explaining the same observation by a change in one of nature’s fundamental symmetries.

4. The term “broken symmetry” is somewhat misleading. In these cases the symmetry of the underlying equations may be exact; it is solutions of these equations that do not respect the symmetry.

5. Chiral symmetry is like the proton-neutron symmetry mentioned above, except that the symmetry transformations can be different for particles spinning clockwise and counterclockwise around their direction of motion. The pi meson is in a sense the analog of the slow precession of an elliptical planetary orbit; just as small perturbations can make large changes in an orbit’s orientation, pi mesons can be created in collisions of neutrons and protons with relatively low energy.

6. Honesty compels me to admit that here I am gliding over some technical complications for mirror symmetry. But this remark about accidental symmetry does apply to matter-antimatter symmetry, without complications.

7. These particles are not observed experimentally, not because they are too heavy to be produced (gluons are massless, and some quarks are quite light), but because the strong nuclear forces bind them together in composite states like protons and neutrons.

8. Again, I admit to passing over some technical complications.

9. Lepton number is defined as the number of electrons and similar heavier charged particles plus the number of neutrinos, minus the number of their antiparticles. (This conservation law requires the neutrino to be massless because neutrinos and antineutrinos respectively spin only counterclockwise and clockwise around their directions of motion. If neutrinos have any mass then they travel at less than the speed of light, so it is possible to reverse their apparent direction of motion by traveling faster past them, hence converting the spin from counterclockwise to clockwise, and neutrinos to antineutrinos, which changes the lepton number.) Baryon number is proportional to the number of quarks minus the number of antiquarks. Protons are the lightest particles with nonzero baryon number, so if baryon number were always conserved there would not be anything into which protons could decay while conserving energy.

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