It's All Relative

1915: Albert Einstein

For Albert Einstein, daydreaming was not an escape from the drudgery of everyday life but a journey into the secrets of the universe.

Eddington (right) with Einstein

In 1907, for example, during and after long days as a clerk in the Swiss patent office, the young physicist pondered the physics of falling. He wondered what a housepainter would experience if he fell off a roof and plunged toward the ground.

Just two years earlier, Einstein had published his Special Theory of Relativity, which demonstrated that time and distance are not a constant framework, as physicists believed, but vary with an observer's motion. As you move faster, your own clock would appear to tick more slowly than those of people who were moving more slowly. At the same time, distances would compress — a yardstick would grow shorter relative to those at rest.

Special Relativity explained the effects of acceleration, but not the effects of gravity, which was a far more difficult problem.

The 1919 solar eclipse and a New York Times headline

As he considered the painter falling off a roof, though, Einstein experienced what he described as one of the greatest insights of his life: The painter would experience weightlessness as gravity accelerated him toward the ground. This meant that the effects of acceleration and gravity were the same. Put another way, you couldn't tell the difference between standing still on Earth, where you are pulled downward by gravity, and accelerating smoothly in an elevator out in space — you would feel exactly the same effect.

In Einstein's new vision of the universe, then, gravity was no longer a "force" acting instantly across the universe, as Isaac Newton had envisioned it. Instead, any object with mass — from a pill bug to a galaxy — "warped" the spacetime around it.

A pill bug's effect is negligible, but the effect of more massive objects could be profound. A star, for example, should curve the space around it in a way that it would deflect the light of more-distant stars passing by it. This effect was confirmed during a solar eclipse on May 29, 1919, when an expedition led by British astronomer Sir Arthur Eddington measured stars that appeared near the Sun and found that their positions were shifted slightly, just as Einstein's equations predicted.

The discovery made Einstein an international celebrity, and allowed other scientists to carry his equations to their extremes. If mass warps spacetime, they reasoned, then enough mass packed into a small enough space should create an infinite warp. In other words, it would warp space so much that nothing could escape from it, including light, creating what would come to be known as a black hole.

Many experiments have confirmed Einstein's Theory of General Relativity, as his new theory of gravity was called upon its presentation in 1916. Yet Einstein himself never believed that black holes could exist. For once, imagination failed the scientist who daydreamed about the secrets of the universe.

Einstein's paper introducing General Relativity