Editing A Brief History of Time (section)

=== Chapter 5: Elementary Particles and Forces of Nature ===
Hawking traces the history of investigation into the nature of [[matter]]: Aristotle's four elements, [[Democritus]]'s indivisible [[atoms]], [[John Dalton]]'s idea of atoms combining to form [[molecule]]s, [[J. J. Thomson]]'s discovery of the [[electron]], [[Ernest Rutherford]]'s discovery of the [[atomic nucleus]], [[James Chadwick]]'s discovery of the [[neutron]] and finally [[Murray Gell-Mann]]'s theorizing of  [[quark]]s which constitute protons and neutrons (collectively called [[hadrons]]). Hawking discusses the six different "flavors" ([[up quark|up]], [[down quark|down]], [[strange quark|strange]], [[charm quark|charm]], [[bottom quark|bottom]], and [[top quark|top]])  and three different "[[color charge|colors]]" of quarks (red, green, and blue). Later he discusses [[anti-quark]]s, which are outnumbered by quarks due to the expansion and cooling of the universe.

[[File:Arrow west.svg|thumb|A particle of [[quantum spin|spin 1]] needs to be turned around all the way to look the same again, like this arrow.]]
Hawking introduces the [[Spin (physics)|spin]] of particles. Particles can be divided into two groups. [[Fermions]], or matter particles, have a spin of 1/2. Fermions follow [[Wolfgang Pauli]]'s [[Pauli exclusion principle |exclusion principle]]: they cannot share the same [[quantum state]] (for example, two "spin up" protons cannot occupy the same location in space). Without this rule, atoms could not exist. [[Bosons]], or the force-carrying particles, have a spin of 0, 1, or 2 and do not follow the exclusion principle. 

[[File:Quark_structure_proton.svg|thumb|right|A [[proton]] consists of three [[quark]]s, which are different colours due to [[colour confinement]].]] Gravity is thought to be carried by [[gravitons]], massless particles with spin 2. The [[electromagnetic force]] is carried by [[photons]]. The [[weak nuclear force]] is responsible for [[radioactivity]] and is carried by [[W and Z bosons]]. The [[strong nuclear force]], which binds quarks into hadrons and binds hadrons together into atomic nuclei, is carried by the [[gluon]]. Hawking writes that [[color confinement]] prevents the discovery of quarks and gluons on their own (except at extremely high temperature) as they remain confined within hadrons.

Hawking writes that at extremely high temperature, the electromagnetic force and weak nuclear force behave as a single [[electroweak force]], giving rise to the speculation that at even higher temperatures, the electroweak force and strong nuclear force would also behave as a single force. Theories which attempt to describe the behaviour of this "combined" force are called [[Grand Unified Theories]], which may help us explain many of the [[List of unsolved problems in physics|mysteries of physics.]]