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By Root 885 0

but appear to perform loop-the-loops in the sky. This strange motion is known as retrograde motion and had in fact been known for many thousands of years before Ptolemy. The ancient Egyptians described Mars as the planet “who travels backward.” Ptolemy agreed with Aristotle that the planets were rotating around a stationary Earth, but to explain the retrograde motion he was forced to attach them to smaller off-center rotating wheels, which in turn were attached to the spinning spheres. This rather complicated model was able to explain the motion of the planets across the night sky, although it is far from elegant. The true explanation of the retrograde motion of the planets had to wait for the mid-sixteenth century and Nicholas Copernicus, who proposed the more elegant (and more correct) explanation that the earth is not stationary at the center of the universe, but in fact orbits around the sun along with the rest of the planets. Copernicus’s work was not without its detractors and was removed from the Catholic Church’s banned list only in 1835. Precision measurements by Tycho Brahe, and the work of Johannes Kepler, Galileo, and Newton, finally established not only that Copernicus was correct, but led to a theory of planetary motion in the form of Newton’s laws of motion and gravitation. Those laws stood unchallenged as our best picture of the motion of the wandering stars and indeed the motion of all objects under gravity, from spinning galaxies to artillery shells, until Einstein’s general theory of relativity came along in 1915.

This constantly shifting view of the position of the earth, the planets, and their motion through the heavens should serve as a lesson to anyone who is absolutely convinced that they know something. There are many things about the world that appear at first sight to be self-evidently true, and one of them is that we are standing still. Future observations can always surprise us, and they often do. Perhaps we should not be too surprised that nature sometimes appears counterintuitive to a tribe of observant, carbon-based ape descendants roaming around on the surface of a rocky world orbiting an unremarkable middle-aged star at the outer edge of the Milky Way galaxy. The theories of space and time we discuss in this book may well—in fact, probably will—turn out to be approximations to an as yet undiscovered deeper theory. Science is a discipline that celebrates uncertainty, and recognizing this is the key to its success.

Galileo Galilei was born twenty years after Copernicus proposed his sun-centered model of the universe, and he thought very deeply about the meaning of motion. His intuition would probably have been the same as ours: The earth feels to us as though it is standing still, although the evidence from the motion of the planets across the sky points very strongly to the fact that it is not. Galileo’s great insight was to draw a profound conclusion from this seeming paradox. It feels like we are standing still, even though we know we are moving in orbit around the sun, because there is no way, not even in principle, of deciding what is standing still and what is moving. In other words, it only ever makes sense to speak of motion relative to something else. This is an incredibly important idea. It might seem obvious in some sense, but to fully appreciate its content requires some thought. It might seem obvious because, clearly, when you sit on the plane with your book, the book is not in motion relative to you. If you put it down on the table in front of you, it stays a fixed distance from you. And of course, from the point of view of someone on the ground, the book moves through the air along with the aircraft. The real content of Galileo’s insight is that these statements are the only ones that can be made. And if all you can do is speak of how the book moves relative to you as you sit in your aircraft seat, or relative to the ground, or relative to the sun, or relative to the Milky Way, but always relative to something, then absolute motion is a redundant concept.

This rather