1.The table below is an elaboration of Table 1.1. It records the masses and force charges of the particles of all three families. Each type of quark can carry three possible strong-force charges that are, somewhat fancifully, labeled as colors — they stand for numerical strong-force charges values. The weak charges recorded are, more precisely, the "third-component" of weak isospin. (We have not listed the "right-handed" components of the particles — they differ by having no weak charge.)
2. Strings can also have two freely moving ends (so-called open strings) in addition to the loops (closed strings) illustrated in Figure 1.1. To ease our presentation, for the most part we will focus on closed strings, although essentially all of what we say applies to both. 3. Albert Einstein, in a 1942 letter to a friend, as quoted in Tony Hey and Patrick Walters, Einstein's Mirror (Cambridge, Eng.: Cambridge University Press, 1997). 4. Steven Weinberg, Dreams of a Final Theory (New York: Pantheon, 1992), p.52. 5. Interview with Edward Witten, May 11, 1998.
3. Albert Einstein, in a 1942 letter to a friend, as quoted in Tony Hey and Patrick Walters, Einstein's Mirror (Cambridge, Eng.: Cambridge University Press, 1997). 4. Steven Weinberg, Dreams of a Final Theory (New York: Pantheon, 1992), p.52. 5. Interview with Edward Witten, May 11, 1998.
4. Steven Weinberg, Dreams of a Final Theory (New York: Pantheon, 1992), p.52. 5. Interview with Edward Witten, May 11, 1998.
5. Interview with Edward Witten, May 11, 1998.
