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# 4.3 Early Greek Physics

In the age before science, people had no mental constructs for interpreting nature so they generally accepted the world as they found it. A rock was a rock, a flower was a flower, and a star was a star. Each had its own immutable nature. Humans were clearly special, the preeminent inhabitants of the world. The dawn of science meant that simple acceptance could give way to inquiry. Science accepts the challenge of looking below the surface for deeper meanings. Its goal is to answer the question of why things are the way they are. Starting in the 6th century B.C., a series of philosophers made bold speculations about the natural world. Thales supposed that the source of the universe was water, the substance from which all materials emerged. His student Anaximander extended this idea, but in his version the primal element was an infinite substance called apeiron. Since everything formed from one material and would return to it, constant recycling allowed for the possibility that other worlds might have existed at other times.

?Illustration from "Illustrerad verldshistoria utgifven av E. Wallis. volume I":?Thales. Click here for original source URL

Meanwhile, Pythagoras and his followers were experimenting with numbers and inventing the foundations of geometry. Pythagoras saw mathematics as a powerful tool to understand music — harmony resulted from the ratio of lengths of a plucked string or open flute. He extended this idea of mathematical perfection to the heavens. The Sun, Moon, planets and stars were carried overhead on crystalline spheres, and an enlightened person might even be able to hear their "harmony." Pythagoras knew that the Moon shone by reflected light, and its phases could only be explained if it was a sphere. The arcing motions of the stars overhead, and the fact that new stars appeared as one traveled south, meant that the Earth too was a sphere. We can understand why Plato inscribed "Let Only Geometers Enter Here" above the entrance when he founded the world's first university in an olive grove outside Athens.

Another Greek idea with profound implications was atomism. Initially proposed by Leucippus, the idea was developed more fully by his student Democritus. Suppose you cut a stone in half with a sharp knife, then in half and half again. Eventually, it will be reduced to a grain of sand and become too small to see or too small to cut. Democritus found it implausible that this process could continue infinitely, so he proposed tiny, indivisible, units of matter called atoms. It is a moniker that survives today: everything is made of atoms and the atoms are in constant motion. All the familiar aspects of matter — color, smell, taste, texture — are secondary properties of collections of atoms; the atoms themselves have none of these attributes. Atomism gave new impetus to speculations about life beyond Earth. In the theory, everything on Earth and in the heavens was made of indivisible atoms and there were an enormous number of them. The Greek idea of elements was rudimentary; there were only four: earth, air, fire, and water. Anaxagoras thought celestial bodies were made of the same elements as the Earth, and suggested that the Sun was a flaming rock as large as Greece. This was brave indeed, to suggest that the world was not unique.

Democritus went even further, speculating that the Moon had mountains and valleys, and that the Milky Way was an aggregation of stars. He postulated space as infinite and occupied by atoms with pure void in between. This is strikingly close to modern cosmological views. He had no trouble imagining the variety of worlds that an infinite number of atoms might provide: "On some worlds there is no Sun and moon, others are larger than our world, in some places they are more numerous. There are some worlds devoid of living creatures or plants or even moisture." Democritus was known as the laughing philosopher, content to think about puzzles of matter and space. He said " I would rather discover a single cause than become king of the Persians."

Pythagoras, the man in the center with the book, teaching music, in?The School of Athens?by?Raphael. Click here for original source URL.