He would one day develop the theory of relativity and the most famous equation ever written, E=mc2. He would help lay the foundations for modern quantum theory, win a Nobel Prize, and become synonymous with the word “genius.”

But Elsa Einstein once confided to a visitor that she fell in love with her handsome cousin Albert for quite a different reason: “because he played Mozart so beautifully on the violin.”

Or maybe it wasn’t such a different reason. Music was far more than a sideline to Einstein’s work; it was central to everything he thought and did.

“Music helps him when he is thinking about his theories,” said Elsa, who became his second wife in 1919. “He goes to his study, comes back, strikes a few chords on the piano, jots something down, returns to his study.” (Read "Einstein’s Evolving Universe: Beyond the Big Bang" in National Geographic magazine.)

The great physicist himself once said that if he hadn’t been a scientist, he would certainly have been a musician.

“Life without playing music is inconceivable for me,” he declared. “I live my daydreams in music. I see my life in terms of music … I get most joy in life out of music.”

It was a love affair that required time to truly spark. Einstein was six when his mother Pauline, herself an accomplished pianist, arranged for him to take violin lessons. But the instrument was a dutiful chore until he discovered the violin sonatas of Mozart at age 13. From that moment on, music became an enduring passion.

Mozart continued to be his favorite composer, along with Bach, for the rest of his life. That was probably no coincidence: As many of Einstein’s biographers have pointed out, the music of Bach and Mozart has much the same clarity, simplicity, and architectural perfection that Einstein always sought in his own theories.

That may also explain his distaste for the less organized, more emotive music of late 19th-century figures such as Wagner. (“For the most part, I can listen to him only with disgust,” Einstein once said of the German composer.)

In those pre-iTunes days, Einstein took pains to carry his music with him in physical form. He rarely went anywhere without his battered violin case. It wasn't always the same instrument inside—Einstein owned several throughout his life—but he reportedly gave each one in turn the same affectionate nickname: "Lina," short for violin. On his travels, he would frequently bring Lina along for an evening playing chamber music in someone’s home, and he had many musical friendships.

In the 1930s, he and Elsa settled in Princeton, New Jersey, rather than go home to Nazi Germany, and they hosted chamber music sessions at their own home every Wednesday night. Those sessions were sacrosanct: Einstein was forever rearranging his schedule to make sure he could be there.

On Halloween evenings, he was known to come outside and surprise trick-or-treaters with impromptu violin serenades. And at Christmastime, he would come out to play along with groups of carolers. (Find out five ways Einstein was a regular guy.)

Because there are no authenticated recordings of Einstein’s playing, a lively debate continues to rage about how good he was. One photograph shows him exhibiting terrible form, with his violin sagging downward, his bow crossing the strings at an angle instead of being perpendicular—all the faults that make violin teachers cringe.

Einstein was also notorious for not staying in sync. Legend has it that when he missed yet another entrance while playing in a quartet with Fritz Kreisler, the great violin virtuoso turned to him and asked, “What’s the matter, professor? Can’t you count?”

Still, the evidence suggests that Elsa wasn’t being sentimental about the quality of his playing. At 16, her cousin took a music examination at his local school, and the inspector wrote that “a student called Einstein shone in a deeply felt performance of an adagio from one of the Beethoven sonatas.”

Much later, a friend wrote that “there are many musicians with much better technique, but none, I believe, who ever played with more sincerity or deeper feeling.”

Einstein continued playing almost until the very end. Only when his aging left hand could no longer manage the fingering did he put Lina away for good. But he never lost that passion for music.

View Images Einstein had many musical friendships and was often joining in with chamber music recitals. Photograph by The New York Times, Redux

In a profile published a few months after Einstein’s death in April 1955, the writer Jerome Weidman recalled being at an opulent dinner party where he was trapped listening to chamber music. During a lull, he confessed to the man seated next to him that he was virtually tone deaf.

“You will come with me,” declared Einstein, who immediately dragged the chagrined Weidman out of the concert and led him upstairs to a study that contained an extensive collection of phonograph records.

There, Einstein played snippets of Bing Crosby, Enrico Caruso, and more — the 1950s pop equivalents of Bruno Mars and Lady Gaga. He insisted that Weidman sing each snippet back to him as a way of training his ear.

Once Einstein was satisfied, they returned downstairs—where, to Weidman’s astonishment, he was able to appreciate Bach’s aria “Sheep may safely graze” for the first time. (Also see "Making Music Boosts Brain's Language Skills.")

Afterward, the hostess asked where the two men had gone.

They had been engaged “in the greatest activity of which man is capable,” Einstein replied, “opening up yet another fragment of the frontier of beauty.”

Genius premieres globally on National Geographic on April 25.