The silly automatic hyperlink in this NYT article on general relativity and LIGO seems to be gone, but I was irked by the paragraph it appears in:
Imagine a rubber sheet pulled taut horizontally and then tossing a bowling ball and a tennis ball onto it. The heavier bowling ball sinks deeper, and the tennis ball will move toward the bowling ball not because of a direct attraction between the two, but because the tennis ball rolls into the depression around the bowling ball.This use of the rubber-sheet metaphor to describe gravity in general relativity shows up a lot in popularizations and especially in news articles. It frequently leads people to conclude that general relativity is a stupid theory, because it's completely circular. Why did the tennis ball roll into the depression? Because gravity's pulling it downward. But isn't gravity what we're trying to explain? Does this mean that general relativity explains gravity as the consequence of some super-gravity pulling in a higher dimension?
Actually there is no super-gravity involved; it's just a bad metaphor. The rubber sheet only shows you what happens to space, but in general relativity, it's space-time that actually gets distorted, and it's the time part that is responsible for gravity in the everyday limit.
What this rubber-sheet picture is good for is to explain certain deviations from Newtonian gravity in general relativity: why, for instance, the deflection of starlight past the Sun is twice what you would calculate from a naive Newtonian treatment in which the photons are just particles traveling at the speed of light and responding to gravity. The other half of the effect comes from distorted spatial geometry. But you don't need to invoke super-gravity pulling downward on the photons to see this.