|Albert Einstein around the time he was |
working on general relativity
But all this centennial hoopla would have been a surprise to Einstein. It’s little remembered today, but right before Einstein’s death in 1955, general relativity was actually in the doldrums. He even remarked to his collaborator Leopold Infeld that fellow physicists now “regard me as an old fool.”
That’s because few universities were teaching general relativity, believing it had no practical applications. The theory was more admired as an exquisite mathematical sculpture. After the flurry of excitement in 1919, when a famous solar eclipse measurement triumphantly provided the proof for Einstein’s new take on gravity (turning Einstein’s name into a synonym for genius), general relativity came to be largely ignored. Isaac Newton’s law of gravity worked just fine in our everyday world, so why be bothered? “Einstein’s predictions refer to such minute departures from the Newtonian theory,” noted one critic, “that I do not see what all the fuss is about.” As a consequence, the best and the brightest moved into other realms of physics.
But what his colleagues didn’t realize is that Einstein had devised a theory years ahead of its time. Experimental measurements had to catch up to a model of gravity fashioned from pure intuitive thought. Not until the final decades of the twentieth century when new astronomical tools revealed unexpected, highly energetic phenomena in the universe, did scientists take a second look at Einstein’s view of gravity. Newton’s laws fail when gravity is extremely strong, such as in quasars, neutron stars, and the celestial objects that did more to bring general relativity back to the forefront of physics than anything else—black holes.
The German astronomer Karl Schwarzschild started it all in 1915. Just a few weeks after Einstein introduced his completed theory to the Prussian Academy of Sciences, Schwarzschild sent Einstein the first full solution, a way to map the gravitational field around a star. But, in doing so, Schwarzschild came upon an unexpected outcome: when the stellar mass was assumed to be a point in the center, a spherical region of space suddenly arose around that “singularity” out of which nothing—no signal, not a glimmer of light nor bit of matter—could escape. If our Sun were compressed into a dot, this sphere (now known as the “event horizon”) would be about four miles wide. Schwarzschild’s sphere wasn’t yet the bottomless pit of space-time known today but more an enigmatic boundary where matter vanished and time simply stopped. But no one worried. Everyone, including Einstein, believed it wasn’t physically meaningful. No star would ever collapse to a point, they grandly proclaimed, with other forces surely stepping in to save the day if such a fate were looming.
However, by 1930 Subrahmanyan Chandrasekhar at Cambridge University did prove that a star could collapse drastically, if weighty enough. The young graduate student from India didn’t speculate on what happened to such a star, but nine years later at the University of California, Berkeley, J. Robert Oppenheimer and his student Hartland Snyder picked up the thread. They saw that an aged stellar core, depleted of fuel and heavier than a certain mass, would enter into a state of permanent free fall, collapsing to a point and closing itself off from the rest of the universe. They called it “continued gravitational contraction,” the first modern description of a black hole.
Their finding swayed hardly anyone. Astronomers still faced serious psychological hurdles in accepting such outrageous stellar behavior, as preposterous to them as continents moving around the Earth. Moreover, the Oppenheimer-Snyder paper was published the very day that Hitler marched into Poland, starting World War II. Collapsing stars seemed of little import at this tumultuous time; physicists had more urgent topics on their mind.
It was not until the late 1950s that general relativity gradually revived after its decades-long lull. A brief and misguided hope to discover “antigravity” led to private and military funding into general relativity, while the emergence of powerful computers allowed physicists to better simulate the death of stars. The epicenters for this relativistic Renaissance were in Moscow, under the guidance of the noted Soviet theorist Yakov Zel’dovich, and at Princeton University, where John Archibald Wheeler led forays into general relativity with a small army of students and post-docs.
|Illustration of black hole Cygnus X-1 stealing|
gas from its companion star
Where once the field of general relativity was a cozy backwater, it now flourishes, thanks to the black hole. No longer oddities, black holes are a vital component of the universe. One is formed somewhere in the universe with each tick of the clock. More than that, every fully developed galaxy appears to have a supermassive black hole at its center; it may be that the very existence of a galaxy—and, in turn, us—depends on it.
John Wheeler once remarked that he never read science fiction. “All the science fiction I need,” he aptly noted, “is right out there in front of us.”