Composer Philip Glass turns 75 today. His birthday is being celebrated with festivals and performances around the globe and the premiere of his 9th symphony at Carnegie hall tonight. Glass is easily the most famous composer of his generation. How many other composers have received commissions from the Metropolitan Opera and inspired a knock-knock joke? Philip Glass began playing works to tiny, often hostile audiences back in the 1960’s. Since then, Glass has achieved an almost dizzying output of operas, symphonies, and works for his own ensemble. In this 2009 interview, Glass talks about his many collaborations.