Scientists know four forces - things that attract or repel one object from another. The strong force and the weak force operate only inside the centres of atoms. The electromagnetic force rules objects with excess charge, and gravity directs objects with mass.
People have long speculated about gravity. While ancient Greek and Indian philosophers observed gravity, it was the insight from Isaac Newton that made it possible to measure and predict the phenomenon.
In 1687, Isaac Newton's treatise Philosophiæ Naturalis Principia Mathematica showed that every object in the universe, small and great, pulled on every other object, from a grain of sand to the planets.
Newton could compute the attraction: Doubling the mass of one object makes its pull twice as strong, and bringing two objects twice as close quadruples their mutual pull.
While Isaac Newton's description of gravity was good enough to detect Neptune's existence in the mid-1800s, Newton's law was not perfect. Astronomers discovered a mismatch between his law and the laws of nature.
This mismatch was resolved in 1915 when Albert Einstein published his theory of general relativity. Before this theory, physicists knew how to calculate a planet's gravitational pull, but they didn't understand why gravity behaved this way.
Albert Einstein held that space and time together made up the universe's fabric that could stretch and compress. Einstein suggested that the shape of spacetime is what gives rise to the force of gravity. A concentration of mass, like the earth or sun, bends space around it, like water that bends around the flow of a river. When other objects move closeby, they follow the curvature.
Einstein's field equations of general relativity, which show how matter and energy warp spacetime, gained acceptance when they successfully predicted the changes in Mercury's orbit.
The current description of gravity is so accurate that it has become a guide for cosmic discoveries.
American astronomers noticed in the 1960s that galaxies appear to rotate fast enough to spin-off stars. However, they did not whirl apart, and something seems to help them stick together. Most physicists now suspect that "dark matter" warps spacetime enough to keep galaxies and galaxy clusters intact.
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