The pop power couple, armed with a personal stash of glowsticks, got their (Weekapaug) groove on at Long Beach Arena and THR has the photographic proof.

Talk about a strange collision of worlds. Teen phenom turned international pop star Justin Bieber, along with girlfriend Selena Gomez, hung out with jam band giants Phish at the group's Long Beach Arena show on Wednesday, Aug. 15.

First spotted near the soundboard and surrounded by a group of friends, Bieber was brought to the concert by his guitarist and musical director Dan Kanter, who happens to be a massive and very devoted Phish fan. Kanter had apparently been asking Bieber to check out the band's lyric-light brand of psychedelia and rockabilly for almost as long as the two had known each other. Finally, with the band playing a venue some 45 minutes south of Los Angeles, the stars aligned. (It also helped that Bieber was finally 18, notes an insider).

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Bieber and crew stayed put all through the first set (Phish typically plays two, each of which is long enough to qualify as its own show) and were brought backstage to meet the band during the intermission.

Wearing a green tie-dye T-shirt and a Philadelphia Flyers cap, Bieber was first introduced to Phish bassist Mike Gordon, then guitarist, frontman and Kanter's personal idol Trey Anastasio. According to photographer Jeff Kravitz, they found common ground in talking music, of course, and also in singing the praises of Phish's revered lighting director Chris Kuroda, who will be designing Bieber's next tour. The couple also posed for pictures backstage with various members of the Phish extended family -- and their children, naturally (see photo above, taken by longtime Phish road manager Brad Sands).

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For the second set, Bieber and Gomez really got into the (Weekapaug) groove, hurling glowsticks from a virtual arsenal they had stashed at the soundboard riser. Traditionally, a glowstick war breaks out at every Phish show. This one took place during the first two songs, "Rock & Roll" and "Ghost," according to witnesses who noted, "they were having a blast."

Indeed, Bieber and friends ended up staying three-quarters of the way through the second set, when they were expected to leave after intermission.

Twitter: @shirleyhalperin