## What is gravity?

A common answer to a child’s question of “Why do things fall?” is, “Because of gravity.” But knowing the name of something does not mean that we understand it. Originally, “gravity” was simply the built-in tendency of most objects to seek the center of Earth.

In 1687, Sir Isaac Newton dramatically increased our understanding when he published his law of Universal Gravitation. Newton deduced that anything made of matter is attracted to everything else made of matter. The strength of the attraction depends on the amount of matter in each object (its mass) and how far apart they are. This attraction is easily noticed when one of the objects is very massive, like a planet. The attraction between small objects (like people) is so weak that it can only be seen easily in cleverly designed experiments like those done by Henry Cavendish in 1797-98. I repeated a modified version Cavendish’s experiment long ago in a college physics lab. Our goal was to determine the gravitational constant by observing and measuring the gravitational attraction between small lead balls. Turns out, the lead balls did attract!

Newton’s mathematical description of this attractive force makes possible accurate predictions of the motions of the Moon, planets, comets, asteroids and spacecraft. While we give the name “gravity” to this force, no one understands how gravity works or in other words, why objects with mass attract each other. Isaac Newton wrote, “The reason for these properties of gravity, however, I have not yet been able to deduce from the phenomena . . .  it is enough that gravity really exists, and acts according to the laws set forth by us, and is sufficient [to explain] all the motions of the heavenly bodies and of our sea.”

The gravity of this galaxy cluster acts as a "lens" in space, bending and magnifying the light of galaxies located far behind it, distorting their shapes and creating multiple images of individual galaxies.

Another change in the description of gravity was provided by Albert Einstein in 1915. Previously, Einstein’s Special Theory of Relativity had united three dimensional space with time into the four dimensional playing field of the Universe that we call “spacetime.” It also declared that mass and energy were different forms of the same thing which I shall call “mass-energy.” Einstein proposed that gravity results from the curvature of spacetime. The presence of mass-energy curves spacetime. Curved spacetime controls the movement of mass-energy. One prediction of Einstein’s description of gravity different from Newton’s was the gravitational bending of light. Observations made of bending starlight during the 1919 total eclipse of the Sun confirmed Einstein’s prediction and made him a world celebrity.

Bending starlight now appears on a gigantic scale, with a cluster of galaxies acting like a giant lens, smearing out light into numerous cosmic streaks seen in this image taken by the Hubble Space Telescope.

While Einstein’s description of gravity is more complete than Newton’s, we still do not know how gravity works or in other words, why mass-energy bends or curves spacetime.

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