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We don’t know what gravity is.

Say that to the average person, and the answer you’ll probably get is some version of: “What are you talking about? Gravity is the force of attraction that makes things fall straight down.” But say it to a physicist, and the answer you’ll get is, “That’s right.”

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I know, because those are the two answers I’ve been getting for the past few years, ever since I figured out that nobody knows what gravity is, and that just about nobody knows that nobody knows what gravity is. The exception is physicists: They know that nobody knows what gravity is, because they know that they don’t know what gravity is.

The assumption that they do — that we all do — is understandable. Unless you think hard about gravity, your brain does what it evolved to do: It associates gravity with your relationship to the ground beneath your feet. You don’t have to think about gravity because you know it in your bones. But if you do think about it, you can begin to realize that what you “know” is, in fact, a series of illusions. These misunderstandings make the universe more navigable — physically and psychologically — but also leave it less mysterious.