On July 22 the Einstein Papers Project, located at the California Institute of Technology in Pasadena, will release the 12th volume of letters written or received by Albert Einstein—791 of them—plus transcripts of several notable lectures and interviews the physicist gave, covering the year 1921. It was a momentous 12 months. You might think there are no new revelations to be made about him, but for Einstein groupies the current volume addresses at least one key question: what did Einstein know about an 1887 experiment that discovered that the speed of light is invariant, regardless of the observer's speed or direction of motion—an idea that forms the core of special relativity and that Einstein did not mention when he laid out the theory of special relativity in a 1905 paper?

Called the Michelson-Morley experiment, it disproved the existence of the ether, a substance once thought to carry light waves and form an absolute reference frame for space. In their namesake experiment, Albert Michelson (a physicist who won the Nobel Prize in 1907) and Edward Morley (a chemist) showed that the speed of light is always the same (now known to be 186,282 miles per second) relative to stationary observers as well as moving ones. Nothing but light has this property: if you are approaching a car that's moving 30 miles per hour, and you're moving 30mph as well, the approaching car appears to be coming at you at 60 mph. Not so with light. If you are traveling at the speed of light, designated c, toward a light beam moving directly toward you, it appears to be approaching at c, not 2c.

Where did Einstein get the idea that the speed of light is invariant, the key claim of special relativity (which also asserts that motion causes objects to contract and time to slow down, both of which depend on the speed of light remaining the same even for observers in motion)? In his autobiography, he recalled how, at age 16, he imagined riding on a light beam to chase another light beam, and realizing that his quarry would move at the speed of light, unchanged by his own motion relative to it. Einstein was born in 1879, and so would have been 16 in 1895, eight years after the Michelson-Morley experiment. It's highly unlikely that a teenager with no connection to the world of science would have heard about the experiment by then; the real question is whether he knew of it before 1905, when he was 26 and putting the finishing touches on special relativity.

Over the years, as Walter Isaacson recounted in his wonderful 2007 biography, Einstein issued "a trail of contradictory statements" about whether the Michelson-Morley experiment played any role in his discovery of special relativity, which requires an invariant speed of light. In 1922, Einstein credited Michelson-Morley as "the first path that led me to what we call the principle of special relativity." But later he said he read the Michelson-Morley experiment only after his 1905 paper, and that he "just took it for granted that it was true"—the "it" being the invariant speed of light.

The new volume of Einstein papers includes a previously unknown transcript of an address Einstein delivered at the Parker School in Chicago on May 4, 1921. There, he made what Caltech science historian Diana Buchwald, director of the Einstein Papers Project, calls "a most intriguing remark." "On this occasion," she writes in her introduction to the new volume, "and perhaps to please the local audience [the Michelson-Morley experiment was done in nearby Cleveland], Einstein stated that, already as a student, he had come across the Michelson-Morley experiment: 'But when I was a student I saw that experiments of this kind had already been done, in particular by your compatriot, Michelson.'" She notes that the role played by the experiment "in the development of Einstein's thinking on relativity has long been the subject of scholarly debate. Some researchers have even entertained the possibility that in 1905 Einstein had been unaware of the experiment, to which there is no reference in his celebrated paper." The new evidence from his 1921 lecture shows once and for all that he was indeed aware of the Michelson-Morley work.

There is no insinuaton of plagiarism, or even of failure to credit an influential earlier discovery. Instead, the question of what Einstein knew about the speed of light, and when he knew it, has long intrigued historians. For as Isaacson so astutely pointed out, physicists before Einstein questioned the idea of absolute motion, absolute space, and absolute time. Yet it was Einstein who took the ultimate leap, forging that insight into special relativity. In light of the newly discovered transcript, it seems safe to add the invariant speed of light to the list that many physicists knew about, but which only Einstein was able to forge into special relativity.