Benjamin T. Rossen’s opera “The Unknowable: An Operatic Ballet in Two Acts” makes its world premiere at Lowell House Opera on Feb. 10 and 11. “The Unknowable” aims to reinvent the opéra-ballet first popularized during the Baroque era. In addition to Rossen’s score, the opera features song cycles “Les nuits d’été” by Berlioz and “Lieder eines fahrenden Gesellen” by Mahler {…}