Editing A Brief History of Time (section)

=== Chapter 4: The Uncertainty Principle ===
Hawking begins by discussing nineteenth-century French mathematician [[Laplace]]'s belief in [[scientific determinism]], where [[scientific law]]s would be able to perfectly predict the future of the universe. A crack in classical physics appeared with the [[ultraviolet catastrophe]]: according to the calculations of British scientists [[Lord Rayleigh]] and [[James Jeans]], a hot body should radiate an infinite amount of energy. In 1900, the ultraviolet catastrophe was averted by [[Max Planck]], who proposed that [[energy]] must be absorbed or emitted in discrete packets called [[Quantum|quanta]].

Hawking discusses [[Werner Heisenberg]]'s [[uncertainty principle]], according to which the speed and the position of a [[particle]] cannot be precisely known due to Planck's quantum hypothesis: increasing the accuracy in measuring its speed will decrease the certainty of its position and vice versa. This overturned Laplace's idea of a completely deterministic theory of the universe. Hawking describes the development by Heisenberg, [[Erwin Schrödinger]] and [[Paul Dirac]] of [[quantum mechanics]], a theory which introduced an irreducible element of unpredictability into science, and despite Einstein's strong objections, has proven to be very successful in describing the universe at small scales.
[[File:Simple wave.png|thumb|A representation of a [[light wave]]]]
Hawking discusses how Heisenberg's uncertainty principle implies the [[wave–particle duality]] of light (and particles in general). [[File:Colours reflected from a thin water film depending on thickness and angle of incidence.png|right|thumb|[[Light interference]] causes many colours to appear.]]
He describes the phenomenon of [[Wave interference|interference]], where multiple light waves interfere with each other to give rise to a single light wave with properties different from those of the component waves, as well as the interference within particles, exemplified by the [[two-slit experiment]]. Hawking writes that American scientist [[Richard Feynman]]'s [[sum over histories]] is a useful way of visualize quantum behavior. Hawking explains that Einstein's general theory of relativity is a classical, non-quantum theory as it ignores the uncertainty principle and that it has to be reconciled with quantum theory in situations where gravity is very strong, as in a singularity.