Stephen Hawking -- Joins Bachelor Party ... Black Holes, Anyone?!

Stephen Hawking took a break from being smarter than everyone else this weekend ... to hang out at a BACHELOR PARTY ... and dude even posed for a picture!

The Hawk was out in Cambridge on Saturday night ... when he was approached by a bunch of guys wearing superhero costumes (apparently, they're all dressed like something called "Bananaman).

Hawking, always the good sport, posed for a pic with the gang.

The most shocking part ... Hawking, who reportedly LOOOVES strip clubs (seriously), went back home after the pic ... and didn't accompany the guys on the rest of their debaucherous journey.