How an Idea Abandoned by Newtonians, Hated by Einstein, and Gambled on by Hawking Became Loved.
By Professor Marcia Bartusiak
It was recently party time in the physics community. Scientists around the globe last fall dusted off their tuxes and hemmed up their ball gowns to celebrate the hundredth anniversary of Albert Einstein’s most momentous achievement. His general theory of relativity finally revealed to us how gravity works, something that Sir Isaac Newton was never able to do. Collections of mass, such as a star or planet, indent the very fabric of space-time, and we’re just sliding along the natural curved pathways feeling a gravitational attraction.
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 at that time 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, as my book Black Hole chronicles, 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. In carrying out this endeavor, Schwarzschild did what all good mathematicians do—devise a scheme that makes the mathematics of the problem simpler. He used spherical coordinates to more easily map the gravitational field and at the center squeezed all the stellar mass into a point.
But, in doing so, he came upon an unexpected outcome: when the stellar mass was assumed to be a point, 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 that Schwarzschild’s solution wasn’t physically meaningful. No star would ever collapse to a point, they grandly proclaimed. Other forces would surely step in to save the day if such a fate were looming, so why be concerned?
However, by 1930 Subrahmanyan Chandrasekhar at Cambridge University in Great Britain did prove that a star could collapse drastically, if weighty enough. Continuing on the problem, the young graduate student from India demonstrated in 1935 that a white dwarf star grew smaller and smaller with increased mass, until its radius was so small that it “would cease to have any practical importance in astrophysics,” as he reported in his scientific paper.
Chandrasekhar didn’t speculate on what happened to such a star, but four years later at the University of California, Berkeley, physicist 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. Oppenheimer dropped the subject altogether, eventually joining the Manhattan Project and becoming the father of the atomic bomb.
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.
Wheeler actually started out to prove that Oppenheimer’s stellar Armageddon could not possibly happen. Getting rid of the singularity was his goal. “I was looking for a way out,” he said in his memoirs. “I was convinced that nature abhorred a singularity.” But in the end Wheeler only convinced himself and others that the black hole was inevitable. Observations eventually backed him up. Starting in the 1960s, spaceborne sensors soon spotted powerful X rays and gamma rays streaming from points around the celestial sky, identifying where black holes release huge energies as matter plunges toward them, before disappearing behind the dark curtains of their event horizons. It took around half a century for physicists to at last cry uncle and admit that Schwarzschild’s singularity was real.
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.
And at the beginning of this year (exactly one hundred years after Schwarzschild first introduced the possibility), physicists announced they had at last garnered direct proof that black holes exist. Two massive black holes, which had been orbiting one another some 1.3 billion light-years away, finally merged in a fateful embrace. This momentous clash shook the fabric of space-time hard, generating a gravitational wave (a ripple in that fabric) that was detected last September by two gravitational-wave detectors in Louisiana and Washington state. This was a landmark moment. No longer was the belief in black holes based on theoretical models or indirect astronomical evidence.
The gravitational wave captured last autumn was a direct and collective shout from the two black holes themselves. Here we are, they were saying.
Here we are.