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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.
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.
High school students learn about “centrifugal force”—it’s nice to have a name for it—but what’s actually causing the water to rise up against the rim of the bucket? It can’t be the motion of the water relative to the bucket, Newton observed, because the water’s surface is most distorted when the water is spinning most rapidly, “in synch” with the bucket. Of course, the bucket and the water are spinning relative to Earth, but that can’t be the explanation, either, because a similar experiment performed in space, Newton believed, would produce the same results.
