As scientific theories go, Albert Einstein’s general relativity has had an amazing run. James Clerk Maxwell’s theory of electricity, magnetism, and light gave way to a quantum theory of light within 50 years. Quantum mechanics was superseded by quantum field theory in less than 20 years. But after 100 years, Einstein’s theory of gravity, space, and time is still state of the art.
The longevity of general relativity owes as much to its complexity as it does to the wide variety of strange new phenomena it predicted. In fact, my late colleague and Nobel laureate Subrahmanyan Chandrasekhar often said that Einstein’s understanding of general relativity was quite limited! Chandra was not trying to cut Einstein down to size; he was trying to indicate how deep and profound the theory is.
While the mathematical solution describing the simplest black hole was discovered in 1916, the solution describing the more interesting spinning black hole was not found until 1963. And it wasn’t until the late 1960s, when John Wheeler coined the term “black hole,” that the mathematics describing these mysterious entities were fully understood and their reality was no longer doubted.
Likewise, debate raged into the 1970s about whether general relativity really predicted gravity waves and, if so, whether the waves actually carried energy. Singularities, places where space and time literally end, remained a mystery until Stephen Hawking and Roger Penrose clarified the subtle mathematics involved. Their singularity theorems made black holes and the Big Bang more understandable and more beautiful.
By the end of the 20th century, advances in technology allowed most of general relativity’s predictions to be tested. It began in 1919 with Sir Arthur Eddington’s apparent confirmation that gravity bends starlight as it passes near the sun. Ten years later, astronomer Edwin Hubble verified the basic tenet of Einstein’s theory—that space is flexible—when he discovered that galaxies are racing away from each other, like raisins in a rising loaf of raisin bread. In 1960 a Harvard laboratory measured the warping of time in a gravitational field, and some 20 years later researchers found indirect evidence for the existence of gravity waves.
