**Co-authored with Diana K. Buchwald and Jürgen Renn**

The confirmation of gravitational waves, announced this week, arrives almost exactly one century after they were first predicted in Albert Einstein's theory of general relativity. The detection of gravitational waves has been one of the great challenges of modern astrophysical research. Its success will open a new window on the universe and allow us to trace its evolution almost to its beginning.

Collaborative work on the historiography of 20th century physics by the Einstein Papers Project at Caltech, the Hebrew University of Jerusalem, and the Max Planck Institute for the History of Science, carried out over many years, has recently shown that the early story of gravitational waves was more dramatic than has hitherto been realized.

The first debates about the existence of gravitational waves even preceded the completion of general relativity by Einstein in November, 1915. The physicist Max Abraham, one of Einstein's competitors in creating a field theory of gravitation, discussed gravitational waves as early as 1912, noticing that they must be different from electromagnetic waves. When Einstein presented his theory of general relativity on Nov. 25, 1915 in Berlin, the question of whether such waves would constitute a consequence of his theory remained untouched. Einstein mentioned gravitational waves for the first time in a letter of 19 February 1916 to Karl Schwarzschild, a pioneer of astrophysics. After some obscure technical remarks, he laconically stated:

"There are hence no gravitational waves that would be analogous to light waves."

But within a short time, he revised his opinion. On June 22, 1916 he published a follow-up paper to his recently formulated theory of the gravitational field in which he predicted the existence of gravitational waves traveling at the speed of light, in analogy with electromagnetic radiation (i.e. light, radio waves, etc.). In it, he derived a formula for the emission of gravitational waves that was marred by an error which he corrected in a subsequent paper in 1918. His calculations showed, however, that these waves were too weak to be observed with the technology then available. In addition, he also argued that a future theory of quantum gravity would be required to guarantee the stability of atoms.

Nevertheless, for decades afterwards, Einstein and many others remained uncertain about the actual, physical existence of these waves. The subject receded into the background of physics research until it was revived in the mid-1950s.

It has now taken center stage after a joint effort of the international physics community. On Feb. 11, 2016, physicists at the Laser Interferometric Gravitational Observatory confirmed the existence of ripples in the fabric of spacetime produced by colliding black holes, gigantic astronomical objects also predicted by Schwarzschild a century ago.

The beginnings of this dramatic story have remained obscure. But now a reexamination of primary documents contained in the Einstein Archives and elsewhere, following up on the historical investigations by Daniel Kennefick, has shed new light on the earliest explorations of this subject.

The first to squarely address the issue of gravitational waves in general relativity was Schwarzschild. He had also been the first to find an exact solution to Einstein's brand new theory while stationed at the Eastern Front during World War I, corresponding with Einstein and other physicists.

On Feb. 6, 1916 Schwarzschild wrote to Einstein about properties of a special solution to the gravitational field equations that later turned out to be crucial for the understanding of black holes. Einstein, however, showed little interest, excusing himself for his belated answer because "the special cases treated [by you] have raised my interest to a lesser degree." But he did react with excitement to something else that Schwarzschild had mentioned. It is now clear that a second communication from Schwarzschild, now lost, had fired Einstein's curiosity anew: "But your new communication I do find interesting. I confirmed your calculation." This remark is followed by Einstein's first statement about gravitational waves quoted above. So, what did Schwarzschild write that Einstein found so much more interesting than an exact solution to his field equations?

Even at that time, in the midst of a devastating war, physicists maintained a tight, albeit small, network of scientific communication, a circumstance that now allows historians to reconstruct the full story. At about the same time as he was corresponding with Einstein, Schwarzschild also communicated with another colleague, Arnold Sommerfeld, a pioneer of quantum physics: "I am rummaging around further in Einstein's field equations. Today I am totally flabbergasted."

He had tried in vain to deduce gravitational waves from an approximation to these equations and was puzzled by a conundrum he had encountered. This must have been the same problem with which he also confronted Einstein in the letter now lost. Einstein repeated the calculation and he too was unable to find gravitational waves. In his response to Schwarzschild, Einstein blamed this failure on the approximation method which he himself had come to abandon. He also referred to an argument first published by his one-time competitor Abraham against the very existence of such waves.

But the conundrum must have remained on Einstein's mind even after Schwarzschild's premature death in May 1916.

Soon afterwards the astronomer Willem de Sitter, Einstein's main interlocutor in the debate on the structure of the universe, wrote to Einstein, spurring him to return to the challenge of gravitational waves. Einstein wrote to De Sitter: "Highly esteemed Colleague, your letter pleased me very much and inspired me tremendously." Einstein went on to explain that he had now found how the gravitational equations can be solved by circumventing the problem that had prevented him earlier from overcoming Schwarzschild's conundrum. De Sitter's letter has not survived either, but we know its effect on Einstein. On the same day he submitted his milestone paper along the long road to understanding gravitational waves as part of our physical reality.

As we now realize, these first steps were also a joint effort, even if not to the same extent as the most recent achievement.

**Diana Buchwald** is director of the Einstein Paper Project at California Institute of Technology.

**Hanoch Gutfreund **is chair of the Physics and Advanced Studies institutes at The Hebrew University of Jerusalem and director of its Einstein Archives.

**Jurgen Renn** is director of the Max Planck Institute for the History of Science in Berlin.