This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Down here; up there: Cleaving the universe in two is as old as civilization. “In the beginning,” begin innumerable creation myths before enumerating what was in the beginning. Those primordial contents are almost always the same across cultures and continents, seas and millennia—what 20th-century mythology scholar H. J. Rose called “the primeval pair”: earth and sky.
The reason we conceive of the universe as two parts is gravity. We unthinkingly identify with down here because that’s where gravity anchors us, and we unthinkingly don’t identify with up there because that’s where gravity won’t let us go. The two realms are, to our intuition, in opposition to each other. If we flew like a bird or swam like a fish or dissolved time and space like a photon, our physical relationship to the part of the universe that’s not us would be different.
But this fundamental division isn’t just physical; it’s psychological. Gravity renders up there not only distinct but distant, and that distance can blind us to a fundamental unity. With a little effort—and a little historical context—however, we can bridge that abyss.
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Creation myths were just the start, so to speak. Up there remained mysterious and hence the province of gods, while down here remained mundane—from the Latin mundānus, for “of the earth.” Then came those two eternal extrapolations of positive and negative values, heaven and hell. When Aristotle applied philosophy to physics in On the Heavens, he brought along the same unthinking assumptions. In the Aristotelian universe, down here consisted offour imperfect substances moving straight up, straight down or any combination thereof, while up there consisted of one perfect substance, ether, moving in a single perfect, circular way.
Even Isaac Newton couldn’t uncleave the universe. By the time he was overthrowing Aristotelian physics, Nicolaus Copernicus had devised the math allowing Earth to be a planet, and Galileo had made the decisive observations validating that prediction. Newton’s achievement, however, wasn’t only to derive equations that capture the motions of matter down here and up there but to discover that the same equations apply to both realms—thereby redefining them as one realm. A universal law of gravitation that is truly universal renders the up there/down here distinction meaningless, at least physically.
Psychologically, though, the divide remained as wide as ever—and with good reason. Counterintuitive concepts, by definition, defeat our brain. We know that Earth orbits the sun, not vice versa, but at the end of the day, we still say the sun goes down. And when we think about gravity, what comes to mind isn’t black holes or the big bang but airplanes and apples and us.
Even physicists can’t help but enshrine that divide. When thinking about gravity, they refer to “big G” and “little g”—G being a universal constant (the correlation, in Newtonian terms, of mass with the inverse square law of distance or, in Einsteinian terms, the geometry of spacetime with the energy-momentum tensor) and g being the local rate of acceleration.
That distinction serves a valid scientific function: G represents the cumulative effect that the late theorist John Archibald Wheeler once described as spacetime telling matter how to move and matter telling spacetime how to curve, while g represents, loosely speaking, how fast things fall. Still, it’s merely a convenience for physicists, a reflection of a fundamental psychological divide that might never go away, because they—we—are only human.
Which is not to say the abyss must remain unbridgeable. The next time you find yourself standing outside far from city lights on a cloudless night, by all means, be sure to marvel at the spectacle—the vastness, the majesty and, yes, the distance. But also try keep in mind that you’re looking in a mirror and that if you squint just right, you might even see yourself.
