A Brief History of Time | Study Guide

Stephen Hawking

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A Brief History of Time | Chapter 5 : Elementary Particles and the Forces of Nature | Summary



Just as the history of our perception of the universe has expanded from the confined spheres of the Aristotle/Ptolemy model, our understanding of the building blocks of all matter has taken us past Aristotle's four elements of Earth, air, fire, and water that either sink or rise. While some of his fellow Greek observers agreed with Aristotle that matter could be infinitely divided into smaller and smaller pieces of itself, others believed all matter was made up of different combinations of atoms (meaning "indivisible" units).

The ideas of what constitutes matter was not proven until 1803, when chemist John Dalton demonstrated that the behavior of chemical compounds could be explained as the bonding of atoms into molecules. Subsequent discoveries throughout the 20th century, supported by advances in technology, proved that not only do atoms exist but they are made up of increasingly smaller building-block particles, the smallest of which is called a quark (the most elementary particle in the present standard model). Six "flavors" of quarks come in three "colors," and quarks may be "up" or "down." But in order to go any further down this line of investigation, Stephen Hawking tells us that physicists must incorporate the wave-particle duality concept.

In so doing, physicists describe what all particles look like from different directions as having the property of "spin." However, it was not until physicist Paul Dirac developed a theory in 1928, the first to be consistent with both quantum mechanics and the special theory of relativity, that spin could be understood. Understanding this property, all known matter in the universe as particles have a spin of ½, while particle spins of 0, 1, and 2 create forces between particles of matter. In addition, it was found that matter obeys Pauli's exclusion principle, which states that "two similar particles cannot ... have both the same position and velocity" as limited by the uncertainty principle.

Stephen Hawking goes on in this chapter to describe the four fundamental forces (the force-carrying particles) that are classified by their strength and the interactions they have with particles that have mass. The first is the gravitational force, which is the weakest at the quantum level, even though it is always attractive over great distances at the relativity level (or has the property of drawing a smaller object toward a larger one, as is consistent with Newton's laws of physics). The second and stronger force is the electromagnetic force, which interacts with charged particles and may be attractive or repulsive. The third force is the weak nuclear force, which is responsible for radioactivity. Insight into this force was pioneered by Abdus Salam and Steven Weinberg in 1967. The fourth is the strong nuclear force, which holds subatomic particles together, believed to be carried by gluons.

Hawking next explains the attempts to unify the four forces in a unified theory. So far, only three of the four (gravity being the exception) have been modeled into one theory. A model that accommodates both gravity and quantum mechanics continues to be elusive.

Hawking concludes the discussion of this chapter with three symmetries, called C, P, and T, which were (until 1956) believed to have been separate. While P (parity transformation) states that "the laws are the same for any situation and its mirror image," C (charge conjugation) "means that the laws are the same for particles and antiparticles." T (time reversal) means that "the laws are the same in the forward and backward directions of time. This is important because it has been determined "that any theory combining quantum mechanics and relativity must always obey the combined symmetry of CPT." But physicists have determined that the laws of physics do not obey the symmetry of T. Understanding these symmetries provides at least provisional answers for a number of questions about the universe, such as why the universe has very little antimatter.


Although Stephen Hawking discusses the four fundamental forces in this chapter, the classifications of the four forces are artificial. That is, they were invented as a means of working with partial theories in the search for the unified theory that explains all four. If such a theory can be found, it may show that all four forces are actually the same force working in different circumstances. While three are unified into grand unified theories (GUTs; potential stepping stones to a grand unification theory that brings together quantum and relativity), the fourth force (gravity) remains elusive.

It is reported that the American physicist Murray Gell-Mann came up with the term "quark" from a line in James Joyce's Finnegans Wake (1939). "Three quarks for Muster Mark! / Sure he has not much of a bark / And sure any he has it's all beside the mark." Joyce himself might have been referring to the common pub call of "Three quarts for Mister Mark," but the word also resembles a German word for something like cottage cheese and another amounting to "trivial nonsense." Reportedly, Gell-Mann said "the line struck him as appropriate, since the hypothetical [quark] particles came in threes."

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