This Is Why Understanding Space Is So Hard

The overwhelming success of Einstein’s theory was the final blow for Newton’s absolute space—but without absolute space, we continue to struggle to make sense of the forces evinced by Newton’s spinning bucket of water.. Photograph by Comstock / Getty Images
If all the matter in the universe suddenly disappeared, would space still exist? Isaac Newton thought so. Space, he imagined, was something like Star Trek’s holodeck, a 3-dimensional virtual-reality grid onto which simulated people and places and things are projected. As Newton put it in the early pages of his Principia: “Absolute space, of its own nature, without reference to anything external, always remains homogeneous and immovable.”1

By Dan Falk | NAUTILUS

This seems persuasive in everyday life. I’m walking east, you’re walking west, and the post office stays put: The frame of reference remains static. But Newton’s contemporary, the German mathematician and philosopher Gottfried Leibniz, balked at this idea of absolute space. Take away the various objects that make up the universe, he argued, and “space” no longer holds any meaning. Indeed, Leibniz’s case starts to look a lot stronger once you head out into space, where you can only note your distance from the sun and the various planets, objects that are all moving relative to one another. The only reasonable conclusion, Leibniz argued, is that space is “relational”: space simply is the set of ever-changing distances between you and those various objects (and their distances from one another), not an “absolute reality.” 2

Au contraire, responded Newton. The effects of absolute space are quite observable. And Sir Isaac had just the experiment to prove it: a spinning bucket of water. Simple as the experiment may sound, it set off a debate about the nature of space, time, motion, acceleration, and force that continues to this day.

Ever since the formation of the solar system, billions of years ago, Earth’s been spinning around with its equator “bulging out,” just like the water in the spinning bucket.

In the Principia, Newton asks us to imagine a bucket of water, suspended by its handle from a rope. Turning the bucket clockwise, the rope winds up. What happens when you let go? The bucket begins to spin counter-clockwise, slowly at first, then faster. But something else happens, too: As Newton writes, the surface of the water “will gradually recede from the middle and rise up the side of the vessel, assuming a concave shape.” For a while, the bucket and the water spin together. Eventually, the bucket slows and its spin reverses; the water slows too, gradually flattening again.

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1. Cohen, I.B. & Whitman, A. Isaac Newton / The Principia: Mathematical Principles of Natural Philosophy: A New Translation University of California Press, Berkeley CA (1999). All quotations from The Principia are from this edition.

2. Alexander, H.G. The Leibniz-Clarke Correspondence, Together With Extracts from Newton’s Principia and Opticks Manchester University Press, Manchester (1956).