$$\require{cancel}$$

# 3.13 The Plurality of Worlds

Until the Copernican Revolution and the invention of the telescope, the Earth seemed the unique center of everything. The Copernican Revolution displaced the Earth forever from its central role. The excitement of this revolution at the time is hard to overemphasize. It seemed for the first time that humans could understand the "machinery" that runs the universe. Johannes Kepler recalled the moment around 1604 when he studied the observations of Mars’ position from earlier years. He was the first person in history to realize that planets moved not in circles around Earth but in ellipses around the Sun. "I awoke as if from sleep, a new light broke on me," he said.

Illustration of Johannes Kepler. Click here for original source URL

The Copernican Revolution switched a light on for a lot of people. Messengers ran through the streets of European cities to homes of famous scientists shouting in front of their houses about the latest discoveries and what they might mean. New knowledge can cause turmoil too. The conflict between scientists and the Catholic Church occurred because the heliocentric model broke the cozy pact between man and God that placed the Earth at the center of creation. The revelations of the telescope did not sit comfortably with some people. Early telescopes were not perfect, but the images they produced became more and more reliable. Yet we hear of people who were unwilling to believe the evidence of their own eyes because it did not accord with their preconceived notions.

?Early depiction of a ?Dutch telescope? from the ?Emblemata of zinne-werck? (Middelburg, 1624) of the poet and statesman Johan de Brune (1588-1658). Click here for original source URL

When Kepler read Galileo's descriptions of stars and planets as viewed through a telescope, he "awoke" even further. He realized that Giordano Bruno might be right after all in speculating that stars and planets were not supernatural bodies, but worlds. Galileo's work showed that planets were spherical worlds like Earth, and the Moon could even be seen to have mountains and valleys. Kepler also cited Bruno and went further to conclude that stars are distant luminous objects like the Sun, whereas planets are moons or Earths. There might be many worlds. Perhaps some were even habitable! The hypothesis that Earth is not the only world in the universe is called plurality of worlds.

Scientists also speculated freely about the size of the universe. In the Copernican model the stars had to be very distant because no change in their brightness or relative positions was observed as the Earth orbited the Sun. More and more stars became visible as the light grasp of the telescope improved. Bruno speculated that there was "not one, but countless suns, not a single earth, but a thousand, I say, an infinity of worlds." Such extravagant and heretical thinking cost him his life. Newton used logic to conclude that the universe was infinite. Gravity is a universal and attractive force, so if there were an edge to the distribution of stars then stars near the edge would be pulled by gravity towards the center of the distribution. Newton reasoned that the universe could not be stable unless it was infinite.

These ideas spread rapidly among intellectuals after the Renaissance. Just to take one example, we can paraphrase the questions raised by an English scholar named Robert Burton:

"...who dwells in these vast bodies, these Earths and worlds?
Rational creatures? Have they souls to be saved?
Do they inhabit a better part of the Universe than we do?
Are we or they the Lords of the Universe?"

Those were pretty heavy-duty questions for the 17th century, and they are unanswered today. The Copernican revolution was the first major step in our awareness of our true place in the universe.