In 1956, a new pop culture phenomenon emerged. Rock ’n’ roll was shaking things up in the United States and Elvis Presley, 21, was its rising star.

Presley was unknown nationally, but he caused a stir at home in Memphis, Tenn., and was quickly ascending to stardom. Photographer Alfred Wertheimer was there to capture that whirlwind year and a half that not only changed Presley’s life, but also American culture.

As described so perfectly in the National Portrait Gallery’s new exhibit, “Elvis at 21,” under Presley’s new reign as the king of rock ’n’ roll, “time stopped and then exploded.”

The exhibit features 56 photographs taken by Wertheimer between March and July 1956. Elvis was wildly famous at home, but Wertheimer hadn’t even heard of the young singer when he was hired by RCA to take promotional photos of Presley rehearsing and performing on TV in New York City.

When his assignment for RCA was finished, Wertheimer asked if he could tag along on the tour. Presley didn’t mind, and Wertheimer followed, camera in hand.

“I didn’t know it would be so historical,” he said. “It’s like you’re in the middle of the forest and don’t know how big it is.”

From behind his camera lens, Wertheimer captured every conceivable aspect of Presley’s life in that period. From the stage and studio to inside a train’s bathroom, the exhibit shows Presley going about his life as if Wertheimer wasn’t there.

It’s easy to feel the energy Presley put out in his unique synthesis of rhythm and blues, gospel and country music in Wertheimer’s photos of performances. Singing in front of thousands of screaming teenage fans seemed perfectly natural for Presley. But Wertheimer’s images show something different behind the larger-than-life stage persona with dangerously gyrating hips.

“Elvis was shy,” Wertheimer said. “He was reclusive except for when he was on stage.”

When he wasn’t performing, Presley often sought out quiet spots to take a break from the madness going on around him, Wertheimer said.

In a few photos, Presley retreats to his hotel room to read fan mail and rest between rehearsals. In another, he sits alone at a piano in a large rehearsal space, singing gospel songs to relax.

In other off-stage photos, he takes advantage of his newfound fame and flirts with some of his many female fans. One series catches Presley backstage after sneaking away from the dressing room with his date for the day. The pair playfully kisses as if Wertheimer wasn’t watching.

Meanwhile, the world beyond was being rocked to its core as traditional values were challenged.

“The stewards of culture got nervous,” said Amy Henderson, a National Portrait Gallery historian and exhibit curator.

Presley, like his idols Marlon Brando and James Dean, started a revolution glorifying the so-called juvenile delinquency that terrified the older generations. While adults couldn’t handle him, the kids couldn’t get enough.

“The message he’s conveying is change,” Henderson said. “He is a symbol of enormous cultural transformation.”

The exhibit will be on display at the National Portrait Gallery until Jan. 23.