The Hidden Reality_ Parallel Universes and the Deep Laws of the Cosmos - Brian Greene [46]
Moreover—and this is the truly daunting part—beyond the specific examples of black holes and the big bang, you can calculate how massive and how small a physical system needs to be for both gravity and quantum mechanics to play a significant role. The result is about 1019 times the mass of a single proton, the so-called Planck mass, squeezed into a fantastically small volume of about 10–99 cubic centimeters (roughly a sphere with a radius of 10–33 centimeters, the so-called Planck length graphically illustrated in Figure 4.1).6 The dominion of quantum gravity is thus more than a million billion times beyond the scales we can probe even with the world’s most powerful accelerators. This vast expanse of uncharted territory could easily be rife with new fields and their associated particles—and who knows what else. To unify gravity and quantum mechanics requires trekking from here to there, grasping the known and the unknown across an enormous expanse that, for the most part, is experimentally inaccessible. That’s a hugely ambitious task, and many scientists concluded that it was beyond reach.
You can thus imagine the surprise and skepticism when, in the mid-1980s, rumors started racing through the physics community that there had been a major theoretical breakthrough toward unification with an approach called string theory.
Figure 4.1 The Planck length, where gravity and quantum mechanics confront each other, is some 100 billion billion times smaller than any domain that’s been explored experimentally. Reading across the chart, each of the equally spaced tick marks represents a decrease in size by a factor of 1,000; this allows the chart to fit on a page but visually downplays the huge range of scales. For a better feel, note that if an atom were magnified to be as large as the observable universe, the same magnification would make the Planck length the size of an average tree.
String Theory
Although string theory has an intimidating reputation, its basic idea is easy to grasp. We’ve seen that the standard view, prior to string theory, envisions nature’s fundamental ingredients as point particles—dots with no internal structure—governed by the equations of quantum field theory. With each distinct species of particle is associated a distinct species of field. String theory challenges this picture by suggesting that the particles are not dots. Instead, the theory proposes that they’re tiny, stringlike, vibrating filaments, as in Figure 4.2. Look closely enough at any particle previously deemed elementary and the theory claims you’ll find a minuscule vibrating string. Look deep inside an electron, and you’d find a string; look deep inside a quark, and you’d find a string.
With even more precise observation, the theory argues, you’d notice that the strings within different kinds of particles are identical, the leitmotif of string unification, but vibrate in different patterns. An electron is less massive than a quark, which according to string theory means that the electron’s string vibrates less energetically than the quark’s string (reflecting again the equivalence of energy and mass embodied in E = mc2). The electron also has an electric charge whose magnitude exceeds that of a quark, and this difference translates into other, finer differences between the string vibrational patterns associated with each. Much as different vibrational patterns of strings on a guitar produce different musical notes, different vibrational patterns of the filaments