…for the planets and look at the planets to see if they attract each other? And this experiment, The direct test was made by Cavendish on equipment which you see indicated on the next slide. I got my slides right. Well, I made a mistake. I was talking…

…is very, very tiny indeed. And there it was. And it was possible then to measure the force between these two balls. Cavendish called his experiment weighing the Earth. We're pedantic and careful today. We wouldn't let our students say that. We would have…

…the ball on which we stand. It's kind of an amazing achievement to find that out, and I think that's why Cavendish named his experiment that way instead of determining the constant in the gravitational equation. Weighing the Earth. He incidentally was…

…thrown out by inertia, one will be pulled toward the sun and the other away. And so hanging on another one of those Cavendish quartz fibers, the thing will twist toward the sun. It doesn't twist to this accuracy, so we know that the sun's attraction for…

…that Newton, in his mind worrying about the solar system, was able to predict what would happen in an experiment of Cavendish, where Cavendish's little model of the solar system, the two balls attracting, has to be expanded 10 million million times to…