Why Does E=mc2_ - Brian Cox [4]
To find the flaw in Aristotle’s answer, let us accept his picture for a moment and see where it leads. According to Aristotle, we should fill space with imaginary grid lines centered on the earth and work out where everything is, and who is doing the moving. If we accept this picture of space as a box filled with objects, with the earth fixed at its center, then it is obvious that you, the passenger on the plane, have changed your position in the box, while the person watching you fly by is standing still on the surface of the earth, hanging motionless in space. In other words, there is such a thing as absolute motion and therefore absolute space. An object is in absolute motion if it changes its position in space, as measured against the imaginary grid fixed to the center of the earth, as time ticks by.
A problem with this picture, of course, is that the earth is not standing motionless at the center of the universe; it is a spinning ball in orbit around the sun. In fact, the earth is moving at about 67,000 miles per hour relative to the sun. If you go to bed at night and sleep for eight hours, you’ll have traveled over half a million miles by the time you wake up. You could even claim that, in about 365 days, your bedroom would have returned to exactly the same point in space since the earth would have completed one full orbit around the sun. You might therefore decide to change your picture a little, while keeping the spirit of Aristotle’s view intact. Why not center the grid on the sun? It’s a simple enough thought, but it’s wrong too because the sun itself is in orbit around the center of the Milky Way galaxy. The Milky Way is our local island of over 200,000 million suns, and as you can probably imagine it’s very large and takes quite a while to get around. The sun, with the earth in tow, is traveling around the Milky Way at 486,000 miles per hour, at a distance of 156,000 trillion miles from the center. At this speed, it takes 226 million years to complete one orbit. And so, perhaps one more step might be sufficient to save Aristotle. Center the grid at the center of our Milky Way galaxy and you could be led to another evocative thought: As you lie in your bed, imagine what the world looked like the last time the earth was “here” at this very point in space. A dinosaur was grazing in the early morning shadows, eating prehistoric leaves at the place where your bedroom now stands. Wrong again. In fact, the galaxies themselves are racing away from each other, and the more distant the galaxy, the faster it recedes from us. Our motion among the myriad galaxies that make up the universe appears to be extremely difficult indeed to pin down.
So Aristotle has a problem, because it seems to be impossible to define exactly what is meant by the words “standing still.” In other words, it seems impossible to work out where to center the imaginary grid against which we can work out where things are, and thereby decide what is standing still and what is moving. Aristotle himself never had to face this problem because his picture of a stationary Earth surrounded by rotating spheres was not seriously challenged for almost 2,000 years. Perhaps it should have been, but as we have already said, these things are far from obvious even to the greatest of minds. Claudius Ptolemaeus, known today as Ptolemy, worked in the great Library of Alexandria in Egypt in the second century. He was a careful observer of the night sky, and he worried about the apparently strange motion through the heavens of the five then-known planets, or “wandering stars,” from which the word “planet” is derived. When viewed from Earth over many months, the planets do not follow a smooth path across the starry background,