Gravity is what holds the universe together. It keeps the galaxies from flying apart, the planets in orbit around the Sun, and our feet firmly on the ground. But we didn’t always know what gravity is. To some extent, it’s still a mystery.

The world according to Newton

In 1687, Sir Isaac Newton proved that gravity is the mutual force of attraction between all masses in the universe and that it extends infinitely across space but gets weaker with distance.

For us, the most massive object around is the planet we live on. The Earth’s mass is about 6 trillion trillion kilograms-that’s ""6"" followed by 24 zeros. That much mass puts out a lot of gravitational force, and all that gravity pulls down towards the centre of the Earth. That’s why no matter where you live on the Earth, you can’t fall off the "bottom"-gravity always pulls you towards the Earth’s centre.

Gravity also pulls the Moon toward the Earth. But because of its orbital motion, the Moon doesn’t hit the Earth, it basically falls around it. Likewise, Newton concluded that the Sun’s gravity pulls the Earth toward the center of the solar system and keeps all the planets in orbit.

Einstein’s general theory of relativity

About two centuries later, in 1915, Albert Einstein predicted how gravity would act near massive objects like stars, which have very strong gravity. He suggested that we think of space as a flat rubber sheet that is stretched tight. A large ball placed on this sheet will make a dent. When a smaller object is placed nearby, it rolls into the dent and "gravitates" toward the larger object. The more massive the object is, the deeper the dent, and the greater the gravitational attraction.

A black hole is an extremely massive object embedded in the sheet. It makes such a deep dent that even light can’t escape.
Canadian Heritage Information Network
Australian Museums & Galleries Online, Australia; Centre of the Universe; Gemini Observatory, Hawaii; Glenbow Museum; The Manitoba Museum; National Research Council Canada; Planétarium de Montréal

© Canadian Heritage Information Network, 2003

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