Chapter 4 - The Uncertainty Principle
Key concepts: scientific determinism (Marquis de Laplace), uncertainty principle (Heisenberg), quanta (Planck), Occam's razor, probability, randomness, duality/interference, "sum over histories" (Feynman)??
"The success of scientific theories, particularly Newton's theory of gravity, led the French scientist Marquis de Laplace at the beginning of the nineteenth century to argue that the universe was completely deterministic...that there should be a set of scientific laws that would allow us to predict everything that would happen in the universe, if only we knew the complete state of the universe at one time. For example, if we knew the positions and speeds of the sun and the planets at one time, then we could use Newton's laws to calculate the state of the Solar System at any other time" (55). This is all well and good, Hawking suggests, when the solar system is the object of study. However, when the object of study is human behavior, for example, or even takes place on anything other than a macro, Newtonian level, determinism becomes completely untenable.
In 1900, Planck put forth the idea of what he called quanta: the constituent "packets" in which waves must be emitted (56). In 1926, Heisenberg noticed (as I discussed vis-a-vis Chown) that we could only know the position or velocity of any particle with any degree of certainty. The more certainly we could ascertain the position of a particle, the less we would know about its velocity, and vice versa. This discovery essentially shoots scientific determinism to shit, because we can never "know" the position and motion of any one particle at any given time: "The uncertainty principle signaled an end to Laplace's dream of a theory of science, a model of the universe that would be completely deterministic: one certainly cannot predict future events exactly if one cannot even measure the present state of the universe precisely!" (57). Hawking further notes that "Heisenberg's uncertainty principle is a fundamental, inescapable property of the world" (57). So how does this affect Hawking's search for a "complete unified theory"? Is there no element of scientific determinism inherent in such a quest? None at all?
A fundamental result of uncertainty is that "quantum mechanics does not predict a single definite result for an observation. Instead, it predicts a number of different possible outcomes and tells us how likely each of these is" (58). The discourse of physics, then, becomes one not of certainty, but of probability--not of "complete," unified descriptions, but partial ones. "Quantum mechanics therefore introduces an unavoidable element of unpredictability or randomness into science" (58). Again, how does this jive with Hawking's goal? Is it merely to say that, in theory, a theory which describes the incompleteness of itself can, in this sense, be considered "complete"? If we can predict the level of unpredictability with fairly acute accuracy, have we satisfied our litmus test of predictability?
"Einstein's general theory of relativity seems to govern the large-scale structure of the universe. It is what is called a classical theory; that is, it does not take account of the uncertainty principle of quantum mechanics" (63). Is relativity then more akin to Newtonian physics, which it completely undermined, than it is to quantum mechanics ("...classical general relativity, by predicting points of infinite density, predicts its own downfall, just as classical...mechanics predicted its downfall by suggesting that atoms should collapse to infinite density," p. 63)? If so, how does one justify the switch from Einstein to Heisenberg at that moment when the backward tape of the universe's expansion crosses the apparently arbitrary boundary between macro and micro, classical and quantum?
