get back to your seat, and when you returned your book was where you left it. Now think a little more deeply about the concept of “the same place.” This might seem a little pedantic, because it’s intuitively obvious what we mean when we describe a place. We can call a friend and arrange to meet up for a drink in a bar, and the bar won’t have moved by the time we both arrive. It will be in the same place that we left it, quite possibly the night before. Many things in this opening chapter will appear at first sight to be pedantic, but stick with it. Thinking carefully about these apparently obvious concepts will lead us in the footsteps of Aristotle, Galileo Galilei, Isaac Newton, and Einstein. How, then, could we go about defining precisely what we mean by “the same place”? We already know how to do this on the surface of the earth. A globe has a set of grid lines, lines of latitude and longitude, drawn onto its surface. Any place on the earth’s surface can be described by two numbers, representing the position on this grid. For example, the city of Manchester in the UK is located at 53 degrees 30 minutes north, and 2 degrees 15 minutes west. These two numbers tell us exactly where to find Manchester, given that we all agree on the locations of the equator and the Greenwich Meridian. Therefore, by simple analogy, one way to pin down the location of any point, whether on the earth’s surface or not, would be to picture an imaginary three-dimensional grid, extending upward from the earth’s surface and into the air. Indeed, the grid could also carry on downward through the center of the earth and out the other side. We could then describe where everything in the world sits relative to the grid, whether in the air, on the surface, or below ground. In fact, we needn’t stop with just the world. The grid could extend outward beyond the moon, past Jupiter, Neptune, and Pluto, beyond even the edge of the Milky Way galaxy to the farthest reaches of the universe. Given our giant, possibly infinitely large, grid we can work out where everything is, which to paraphrase Woody Allen, is very useful if you’re the kind of person who can never remember where you put things. Our grid therefore defines an arena within which everything exists, a kind of giant box containing all objects in the universe. We may even be tempted to call this giant arena “space.”
Let’s get back to the question of what is meant by “the same place” and return to the aircraft example. You might suppose that at 12:00 and 12:15 you were at the same point in space. Now imagine what the sequence of events looks like to a person sitting on the ground watching the plane. If she sees the plane fly overhead at 600 miles per hour, she would say that between 12:00 and 12:15 you had moved 150 miles. In other words, you were at different points in space at 12:00 and 12:15. Who is correct? Who has moved, and who has stood still?
If you can’t see the answer to this apparently simple question, then you are in good company. Aristotle, one of the greatest minds of ancient Greece, got it dead wrong. Aristotle would have answered unequivocally that it is you, the passenger on the aircraft, who is moving. Aristotle believed that the earth stands still at the center of the universe. The sun, moon, planets, and stars rotate around the earth attached to fifty-five concentric crystalline spheres, stacked inside each other like Russian dolls. He therefore shared our intuitively satisfying concept of space: the box or arena in which the earth and the spheres are placed. To modern ears, this picture of the universe consisting only of the earth and a set of spinning spheres sounds rather quaint. But think for a moment about what conclusion you would draw if nobody had told you that the earth rotates around the sun and that the stars are distant suns, some many thousands of times brighter than our nearby star but billions and billions of miles away. It certainly doesn’t feel like the earth is adrift in an unimaginably large universe. Our modern worldview was hard-won and is often counterintuitive.