Newton and Einstein

Two explanations

In 1687, Newton explained that objects pull on each other with an invisible force. In 1915, Einstein saw it differently. Heavy things bend spacetime, and other objects move along the bent paths. In everyday life the two give almost the same answers, but where gravity is very strong, Einstein’s explanation is the one that works.

Why?The two arrows on the screen are the same length. The apple pulls on Earth exactly as hard as Earth pulls on the apple, but Earth has so much mass that it barely moves, so only the apple seems to fall. Newton’s formula is still accurate enough to plan rocket and satellite flights.

Learn moreSmall differences, like the way Mercury’s orbit slowly turns or how much light bends near the Sun, could only be explained by Einstein’s theory. This theory is called general relativity. Ten years earlier, in 1905, Einstein had published special relativity, about speed and time. We meet them in Chapters 2 and 3.

Clock we are watching

A clock on Earth’s surface

How much slower

About 0.02 s slower per year

Clock rate

0.9999999993

seconds this clock ticks while a clock far away in empty space ticks one second · calculated with the formulas of relativity