A Brief History Of Time

The Big Bang theory states that the universe started as an infinitely dense point, or singularity, which contained all the matter and energy of our universe and then rapidly expanded outward. The Big Bang expansion went through several stages, as matter and energy ballooned outward; these stages are better understood because of Hawking’s contributions.

The term “big bang” came from British astronomer Fred Hoyle, a believer in the Steady State theory of the universe who disagreed with the singularity theory: In a BBC interview in 1949, he dismissively referred to the proposed singularity expansion as a “big bang,” and the term stuck.

The Big Bang is widely accepted by the modern scientific community, and its implications for the nature of time and all matter in the universe are essential to Hawking’s exploration of alternate and ancillary theories.

A black hole is a region in space where densities are so great because of gravity that matter condenses down to a point called a singularity. The black hole’s gravitational field is so intense that, within a certain range called the event horizon, nothing—not even photons of light—can escape. Black holes form when very large stars use up their nuclear fuel and stop generating the heat that keeps the star expanded against its own powerful gravitational field. The star collapses and explodes in a supernova, and some of the material at the core becomes so pressurized that it overcomes the resistance of matter against sharing space, and the matter shrinks down to a point.

Black holes were author Hawking’s area of specialization, and his discoveries changed science’s understanding of black holes. He also developed related theories that affected how cosmologists think about the origin of the universe. Because black holes offer unique conditions with enormous effects on the nature of time and space, they are rich areas of exploration for scientists to learn about gravity and the mechanics of the universe.

The universe contains four forces: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force. Gravity and electromagnetism are distributed by massless particles, the graviton and the photon, respectively, that travel at the speed of light, often for great distances. The strong force binds together the quarks that make up the protons and neutrons at the centers of atoms; this force is delivered by gluons, and its power is limited to a tiny sub-atomic range. The weak force is delivered by three particles, W-, W+, and Z°. These particles have mass, and their effects occur only within atoms. They’re responsible for nuclear fission—the breakup of certain large atoms into smaller ones—and nuclear fusion, the formation of larger atoms out of smaller ones, as when stars fuse hydrogen atoms into helium.

The elusive Grand Unified Theory seeks to combine all four forces in one scientific principle, of which each force is an aspect. So far, gravity has not been reconciled with the other three forces under one theory. Possible solutions to this quandary comprise much of Hawking’s exploration and discussion in the book.

A photon is a massless particle that delivers the electromagnetic force, which brings us light, electric power, and chemistry. Photons keep particles separate so that they bump against each other and don’t simply blend together. Because photons are sub-atomic particles, the uncertainty principle prevents both their position and velocity to be fully known at the same time. This means that sometimes photons are observed as particles and sometimes as waves. Photons of low energy appear as radio waves and microwaves; at medium energy, they show up as visible light; and at high energies, they’re known as ultraviolet rays, x-rays, and gamma rays.

The behavior of photons undergirds many of the scientific experiments and theories explored by Hawking in the book.

A singularity is a point, as inside a black hole, where matter condenses to an infinitely dense point. At a singularity, gravity is so strong that density and energy become infinite, and the laws of physics no longer work. Thus, singularities are places where the universe contains something that doesn’t quite exist in space and time. The inside of a black hole cannot be observed, so the concept remains theoretical.

The modern Big Bang theory of the beginning of the universe contains a singularity where matter and energy were infinite, and the laws of physics do not apply. If science can learn more about the singularities inside black holes, it might better understand the beginnings of our universe.

In the classical physics of Newton and Galileo, space and time are separate entities. In Einstein's theory of relativity, space and time are blended together into a single entity called space-time. In very strong gravitational fields, space becomes warped, and the passage of time slows down; during high-speed travel, the time sense of passengers similarly slows, while the mass of their spacecraft increases.

Space-time represents a major change in how scientists conceive of the universe. Hawking used Einstein’s equations to discover much about the nature of black holes and also to figure out aspects of the creation of the universe during the Big Bang.