Editing A Brief History of Time (section)

=== Chapter 11: The Unification of Physics ===
[[Image:Open and closed strings.svg|right|thumb|alt=A wavy open segment and closed loop of string.|The fundamental objects of string theory are open and closed [[string (physics)|strings]].]]

Quantum mechanics and general relativity describe the universe with astounding accuracy within their own domains of applicability (atomic and cosmic scales, respectively). However, these two theories run into problems when combined. For example, the uncertainty principle is incompatible with Einstein's theory. This contradiction has led physicists to search for a theory of [[quantum gravity]].

Hawking is cautiously optimistic that such a unified theory of the universe may be found soon, in spite of significant challenges. At the time the book was written, [[superstring theory]] had emerged as the most popular theory of quantum gravity, but this theory and related string theories were still incomplete and had not yielded testable predictions (this remains the case as of 2021). String theory proposes that particles behave like one-dimensional "strings", rather than as dimensionless particles. These strings "vibrate" in many dimensions. Superstring theory requires a total of 10 dimensions. The nature of the six "hyperspace" dimensions required by superstring theory are difficult if not impossible to study.

Hawking thus proposes three possibilities: 1) there exists a complete unified theory that we will eventually find; 2) the overlapping characteristics of different landscapes will allow us to gradually explain physics more accurately with time and 3) there is no ultimate theory. The third possibility has been sidestepped by acknowledging the limits set by the uncertainty principle. The second possibility describes what has been happening in physical sciences so far, with increasingly accurate partial theories.

Hawking believes that such refinement has a limit and that by studying the very early stages of the universe in a laboratory setting, a complete theory of Quantum Gravity will be found in the 21st century allowing physicists to solve many of the currently unsolved problems in physics.