What is Gravity?
We're all familiar with gravity as "the reason objects fall", but there's much, much more to gravity than that. Gravity is a long range attractive force between all objects with mass: it's what keeps us from falling off the earth, it's what keeps the earth in orbit around the sun, and it's what caused the sun itself to form four and a half billion years ago! It's amazing to think that every massive object attracts every other in the universe – that means that your dog, the earth, and a black hole in the Andromeda galaxy 2.5 million light years away are all gravitationally attracted to you, and you to them. In the 17th century, Isaac Newton discovered that the strength of the gravitational force decreases by the square of the distance between two objects – so if you're twice as far away, gravity is only one fourth as strong! He also discovered that the strength of gravity is proportional to the mass of the objects in question: the more massive an object, the stronger the gravitational force. That's why we can all feel the Earth pulling on us, but don't really notice the pull of the moon – it's smaller, and farther away! (the moon's gravity is strong enough to cause the tides, though!) And when I said earlier that gravity is an attraction between objects with mass, I lied. I meant objects with Energy. Because in addition to massive objects, gravity also attracts light and other massless (but energetic) particles, so that a photon of light can be bent slightly passing the sun, or trapped completely by a black hole. Now do you understand the Gravity of the situation?
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