But Ian McKellen isn’t really there. Neither are the other three actors accompanying him, at least not in flesh and blood form. He is speaking the opening line of “An Ark,” one of the first plays to use mixed reality, a technology that blends the real physical world with digital content. Written by Simon Stephens and opening at the Shed on Jan. 21 (previews have already begun), “An Ark” offers the illusion that the four actors — McKellen, Golda Rosheuvel, Arinzé Kene and Rosie Sheehy — are talking directly to each audience member, taking them on a journey through a life, evoking memories and emotional states that suggest our shared human experiences.