How Does Gravity Work?

You probably don’t think about it regularly, but in the back of your mind you know that gravity affects your every move. You see it at work each time you watch the rain fall, throw a ball into the air, or drop a pencil. Without this omnipresent, invisible “force” you would fly off into space, along with everything else on Earth. The universe itself would become a chaotic landscape of planetary bodies aimlessly hurtling through space and often colliding. What is gravity and how does it work? The answer is simple: We’re not quite sure. To this day, gravity’s mystery hasn’t been solved.

In 1687, Isaac Newton described gravity as a force, claiming that any two objects in the universe exert a force of attraction upon each other. The Sun exerts gravity on all the planets, keeping them in orbit. Similarly, the planets exert gravity on the Sun and on all the other planets as well. The strength of these relationships is determined by the mass of the objects and the distance between them. The greater the mass of the two objects and the closer the objects are to each other, the stronger the pull of gravity.

For more than 200 years, Newton’s theory of gravity went unchallenged. Then enter Albert Einstein. In early 1915, Einstein, in his groundbreaking general theory of relativity, explained that gravity is a curvature in the space-time continuum, or the “shape” of space-time. The mass of an object, Einstein claimed, causes the space around it to bend, or curve. To understand the phenomenon, imagine a heavy ball sitting on a rubber sheet. The area occupied by the ball sags, or becomes distorted, due to the mass of the ball. Other smaller balls on the sheet roll in toward the heavier object because the heavy ball warps the sheet. According to Einstein, celestial bodies are not feeling the force of gravity, but rather following the natural curvature of time-space.

A recent alternative hypothesis to Einstein’s theory of gravity states that particles called gravitons, emitted by Earth, cause a gravitational force between objects. But gravitons have never been observed and, to date, remain hypothetical. Yet another idea holds that gravity is the result of gravitational waves, generated by an interaction between two or more masses, such as the merge of two galaxies or the orbit of two black holes. Like gravitons, however, gravitational waves have never been detected. And so the great mystery of gravity remains unsolved. And without discovering its secret, humankind may never truly comprehend how the universe works.

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