The road to the 2017 Nobel Prize in physics, awarded for the discovery of gravitational waves produced by colliding pairs of black holes, was long and convoluted. The detection took place after decades of planning by the three recipients: Rainer Weiss, Kip Thorne, and Barry Barish. They had to struggle with much skepticism about whether the Laser Interferometer Gravitational-Wave Observatory (LIGO) experiment would work.
At one time, belief in the existence of gravitational waves and in the actuality of black holes were controversial positions. Albert Einstein, for example, swayed back and forth in his support of gravitational waves and deemed the idea of black holes (which were earlier called "gravitationally collapsed stars") unphysical. It was Thorne's research adviser, Princeton physicist John Wheeler, who encouraged the pursuit of far-reaching ideas.
Wheeler began his study of the general theory of relativity, Einstein's masterful vision of how the warping of space and time produce the effects we know as gravitation, in the early 1950s, when Einstein was in his final years. Wheeler discussed his views with Einstein and even invited the elderly physicist to speak to his relativity classes.
Einstein first predicted gravitational waves in 1916 but had spent little time exploring the implications of his proposal. By 1936, he'd changed his mind. He and his research assistant Nathan Rosen performed a calculation purporting to rule out a simple kind of gravitational wave.
Einstein and Rosen submitted their results to the prestigious journal Physical Review, which passed the draft paper to an anonymous referee who rejected it on the basis of mathematical errors. Historian Daniel Kennefick revealed many years later that the referee was accomplished Princeton astronomer Howard P. Robertson.
Einstein was incensed by the rejection, withdrew the draft, and resubmitted it to the Journal of the Franklin Institute. In the process, Robertson managed to pass along word about the errors. When they were corrected, Einstein changed his conclusion once more and supported the existence of gravitational waves.
Although Rosen's name was on the new paper, he still didn't quite believe in such waves. He began to talk them down at conferences. Einstein died in 1955, before anyone really thought such entities could be detected.
Enter Wheeler and his talented students Richard Feynman and Joseph Weber. Feynman, who had worked with Wheeler in the early 1940s and who still kept in close touch, developed a simple thought experiment in 1957 for how gravitational waves could jostle a bead on a stick, releasing energy in the process. His conjecture, dubbed the "stick bead" argument, proved convincing. Meanwhile, Weber began a multidecade search for finding such waves in nature, using room-size apparatus he built himself. Wheeler was supportive of Weber's endeavors, which unfortunately proved not sensitive enough for such detection. Weber would claim success, but the rest of the astronomical community was dubious.
In the late 1960s, Wheeler came to embrace the notion that the collapse of a massive enough star would create an object so compact and gravitationally powerful that nothing, not even light, could escape its grasp. An audience suggestion during a talk led him to call such objects black holes. Thorne studied under Wheeler during that critical time, and then became a prominent physics professor at Caltech. He similarly became a leading proponent of gravitational physics and black holes, which were finally discovered in the 1970s.
Thorne's time at Princeton under Wheeler overlapped with a postdoctoral research stay there by Weiss. Weiss was working under one of Wheeler's colleagues, Robert Dicke. Weiss took an experimental approach to the detection of gravitational waves. He started to advocate the thesis that the collision of a pair of black holes would be powerful enough to produce signals that could be detected on Earth.
Together with researcher Ronald Drever, Weiss and Thorne developed the LIGO project. Detectors far larger than Weber's design were constructed near Livingston, La., and Hanford, Wash. Wheeler was deeply interested in the project and visited the Hanford site in 2000.
After years of fruitless searching, the detectors were updated to what was called Advanced LIGO in 2015, under the direction of Barish. In September 2015, soon after they flipped the switch on the upgraded machines, they picked up the weak but unmistakable signals of a binary black hole collision.
When the LIGO team announced success in February 2016, the world took note, and a Nobel Prize seemed virtually inevitable. It was late in the game that year, however, for nominations. The group found more events, and were rewarded with a Nobel announcement on Oct. 3. On Monday, the team announced yet another major discovery: gravitational waves due to colliding neutron stars that could also be detected independently by means of other forms of radiation.
Wheeler died at 96 on April 13, 2008. Though he didn't live long enough to witness the discovery, there is no doubt he would have been extraordinarily proud if he had. His long life, by terrestrial standards, was but a moment's flash compared with the billion years the gravitational waves from the black holes took to help prove his bold conjectures. Though he is gone, his legacy in gravitational physics persists.