

This article is an excerpt from the Shortform book guide to "A Brief History of Time" by Stephen Hawking. Shortform has the world's best summaries and analyses of books you should be reading.
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What is the theory of relativity? What phenomena does the theory attempt to explain? How has it revolutionized our understanding of the workings of our universe?
Within Einstein’s theory of relativity, there are actually two theories: special theory and general theory. The former applies to all physical phenomena discounting gravity, and the latter explains the gravitational force in relation to the other forces of the universe.
Learn about Albert Einstein’s theory of relativity, explained in simple terms.
Albert Einstein’s Theory of Relativity
Einstein’s theory of relativity greatly enhanced understanding of the interplay between the speed of light and the passage of time. But it was still missing a crucial element—gravity. Gravitational effects ought to be immediate, meaning that gravity travels at infinite velocity (it shouldn’t take time for gravity to exert its effects). But how could this square with the idea that nothing could travel faster than the speed of light?
Proposed in 1916, Einstein’s general theory of relativity explained that gravity was a special force that existed because of the curvature of space-time itself. Space-time, according to this theory, is not flat. Thus, orbiting bodies travel in geodesics—the shortest distance between two points. This is the nearest thing to a straight line. Thus, the Earth actually follows a linear path through four-dimensional space-time, but it appears to us to be an elliptical orbit.
The mass and energy of bodies like the sun actually curve the fabric of space-time itself, creating slight variations in the “elliptical” orbits, variations that couldn’t be fully explained by Newton’s theory. It is analogous to placing an object on a stretched-out piece of fabric. The weight of this object will cause the fabric to sink—this is the same mechanism by which gravity warps the curvature of space-time.
The test of a good theory is if it describes observed reality and makes reliable and accurate predictions about the future. By these standards, general relativity is a good theory—it successfully predicts these deviations in orbits, bolstering our faith in the theory.
Even light itself is impacted by the curvature of space-time. Light from distant stars appears to us in a different position than it actually originated from, because the gravitational mass of the sun reflects the angle of the light.

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- The search for a theory that explains the history and evolution of our universe
- Stephen Hawking's discussions about time, space, dimensions, and quantum theory
- How time travel would theoretically work