# Even Einstein Didn’t Think Gravitational Waves Existed

Gravitational-wave theorists (left to right) Robert Oppenheimer, Roger Penrose, Albert Einstein, Karl Schwarzschild, Arthur Eddington, Kip Thorne and Richard Feynman, whose work helped pave the way for LIGO’s big announcement last week. Photo illustration by Olena Shmahalo/Quanta Magazine; Kip Thorne via A.T. Service, Roger Penrose via Festival della Scienza

“There are no gravitational waves … ” … “Plane gravitational waves, traveling along the positive X-axis, can therefore be found … ” … “ … gravitational waves do not exist … ” … “Do gravitational waves exist?” … “It turns out that rigorous solutions exist … ”

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Original story reprinted with permission from Quanta Magazine, an editorially independent division of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences

These are the words of Albert Einstein. For 20 years he equivocated about gravitational waves, unsure whether these undulations in the fabric of space and time were predicted or ruled out by his revolutionary 1915 theory of general relativity. For all the theory’s conceptual elegance—it revealed gravity to be the effect of curves in “space-time”—its mathematics was enormously complex.

The question was settled once and for all last week, when scientists at the Advanced Laser Interferometer Gravitational-Wave Observatory (Advanced LIGO) reported that they had detected gravitational waves emanating from the violent merger of two black holes more than one billion light-years away. Picking up the signal—a tiny flurry of contractions and expansions in space-time called a “chirp”—required extraordinary technical finesse. But it also took 100 years for scientists to determine what, exactly, Einstein’s theory predicts: not only that gravitational waves exist, but how they look after crossing the cosmos from a coalescing pair of black holes — inescapably steep sinkholes in space-time whose existence Einstein found even harder to swallow.

Daniel Kennefick, a theoretical physicist at the University of Arkansas, began his career as a graduate student working with LIGO co-founder Kip Thorne to unravel the predictions of general relativity. Fascinated by the contentious history of gravitational-wave research, Kennefick began a sideline as a historian; he is the author of the 2007 book Traveling at the Speed of Thought: Einstein and the Quest for Gravitational Waves, and last year he co-authored An Einstein Encyclopedia. In discussions before and after Thursday’s big announcement, Kennefick recounted the journey leading up to it and explained where theorists must go from here. An edited and condensed version of the conversation follows.