Before string theory was a theory, early astronomy provided some support for the existence of a plurality of worlds, a view that was so controversial that it contributed to at least one man’s death. These plurality of worlds, and the eventual parallel worlds, were rooted in the ideas of an infinite universe, as are the ideas of parallel universes.
The Italian philosopher Giordano Bruno (1548–1600) was executed for a variety of heresies against the Catholic Church. Though Bruno was a supporter of the Copernican system, his abnormal beliefs went far beyond that: He believed in an eternal and infinite universe that contained a plurality of worlds.
Bruno reasoned that because God was infinite, his creation would similarly be infinite. Each star was another sun, like our own, about which other worlds revolved. He didn’t feel that such viewpoints were in opposition to the scriptures.
In fairness to the Catholic Church, Bruno wasn’t executed merely for believing in other worlds. His list of heresies was long and varied, including denial of Mary’s virginity, the divinity of Christ, the Trinity, the Incarnation, and the Catholic doctrine of transubstantiation. He also believed in reincarnation and was accused of practicing magic.
This is not to say that any (or all) of these viewpoints warranted death, but given the time period, it would be hard to get out of such accusations alive.
In 1686, the French writer Bernard le Bovier de Fontenelle wrote Conversations on the Plurality of Worlds, which was one of the first books to address the popular audience on scientific topics, being written in French rather than scholarly Latin. In Conversations, he explained the Copernican heliocentric model of the universe and contemplated extraterrestrial life.
Though other enlightenment thinkers — possibly even John Adams and Benjamin Franklin, by some accounts — were agreeable to such viewpoints, it would be many years before the plurality of worlds extended to the plurality of universes.
In 1871, the French political malcontent Louis Auguste Blanqui wrote — while in prison — a brochure titled Eternity by the Stars: Astronomical Hypotheses, in which he said that an infinite universe would have to replicate the original set of combinations an infinite number of times to fill up the infinite space.
This is the first inkling of the transition from “plurality of worlds” to “parallel worlds” — copies of you sitting reading this article on another planet.
