Hawking, Stephen (1942-)

Physics: The Background

Hawking's ideas of time were influenced by the work of scientists and philosophers over two millennia. Aristotle, for example, made many statements about the arrangement of the universe. The whole world was spherical and finite, he postulated in the 4th century BCE. The earth was at the center of the Aristotelian universe, surrounded by one concentric sphere each for the sun, the moon, Mercury, Mars, Venus, Jupiter, and Saturn, and one sphere for the stars. The outermost sphere, containing the stars, was considered superior to the sphere in which the earth resided, and was supposed to be composed of an element called the ether. This concept of interstellar ether would persist in various forms through the early 20th century. The space occupied by the earth, however, was made of the four classical elements of earth, air, fire, and water. Furthermore, on the surface of the earth, the heavier of two bodies that had the same shape would fall faster. Many of Aristotle's postulates were eventually proved false, but his ideas nevertheless became firmly entrenched in science.

Nicolaus Copernicus, over a millennium later in the early 1500s, made the next great development in astronomy. His theory, based on observation, stated that the earth makes not only one complete rotation on its axis per day (as well as executing a slight wobble around this axis) but also one complete revolution around the sun each year, producing the seasons. The sun was placed uncompromisingly at the center of the universe. The earth took its place among the other planets in orbit around the star, with an orbital period of one year. Copernicus retained the celestial spheres of the Ptolemaic and Aristotelian theories, but he associated a greater orbital radius with a longer planetary year, which necessitated a rearrangement of the five known planets into their proper order.

Figure: British theoretical physicist Stephen Hawking, Cambridge University Professor and Fellow, circa 1985

Source: Getty Images.

Early in the tumultuous 17th century, an Italian scientist named Galileo Galilei was at the heart of a battle over the Copernican model of the universe. with his innovative new telescope, Galileo discovered Venus's phases, Jupiter's four largest moons, sunspots, and the moon's topographical features. Venus was of particular interest, since its phases proved that the planets revolved around the sun. In addition, Galileo's observations showed that not everything must orbit the sun directly. His discoveries were not limited to the heavens; Galileo conducted experiments that yielded the laws of both falling and projected bodies. His legendary experiment of dropping two objects from the Leaning Tower of Pisa is just one example of Galileo's many attempts to explain the universe. Galileo's work, together with that of Johannes Kepler, led directly to the epitome of classical physics that was Sir Isaac Newton's lifework.

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