 Suppose I took two positive charges and put them next to each other, like this. What would I see? Well, they're positive charges, so they'll just repel each other and fly apart. Now, it turns out that it was this simple thought experiment that Einstein used to show that a Newtonian understanding of physics was incomplete. But to see how this works, we first need to talk about inertial reference frames. To represent the positions of objects in three-dimensional space, we use coordinates x, y, and z. Now, because it's easier to draw, we'll stick to two-dimensions for now, but everything we're about to go through is just as valid in the three-dimensional case. One day, the dread pirate Alice sailed to an uninhabited island. Disembarking onto the shore, she had a look around and saw a gigantic forest to the south and a tremendous mountain range to the east. Alice trekked deep into the heart of the island and buried her treasure here, X marks the spot. Returning to her ship, she drew a map so that she could find her treasure the next time she returned. Labelling her axes in kilometers, the coordinates of the treasure were 6.5 minus 4. Unknown to Alice, however, the island was inhabited, and Bob, whose people lived in the forests, watched her bury the treasure. Bob hurried back to his village and drew a map so that his people could see where it was buried. What made sense to Bob, however, was to place the origin where his village was, so that on Bob's map, the coordinates of the treasure were at 6.60 kilometers.