Comparing electric and gravitational forces on page 90.
In any case the answer is nothing short of flabbergasting. Plugging in the relative strengths of gravity and the electric force, it turns out that the mass you have to put on top of the second electron to counteract the electric repulsion is -get this!- 5 billion tons. This is not only more massive than either the Empire State Building or the twin towers of the World Trade Center, or any other Manhattan skyscraper, it is more massive than all of them put together!
Even though I have been for some time familiar with the relative strengths of gravity and the electric force, I was surprised by this particular result after obtaining it - so much that I had to check my calculations three times and then ask a graduate student who happened to be walking by my office to check them to make sure I hadn't done something foolish. This time, I hadn't.
Why, you may naturally ask, don't we just use small electric charges to levitate buildings or large flying saucers? The answer is that these objects, if they are on the Earth's surface, are not merely attracted downward by the gravitational force of the single electron that one might hope to levitate them with, they are attracted by the whole Earth.
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