You won’t need much training in physics to parse Simon Stephens’s sweet and slight Heisenberg at Manhattan Theater Club. It takes the uncertainty principle – the idea that you can precisely measure a subatomic particle’s velocity or its position, but not both at once – and applies it to a two-character play. Sort of. As in his earlier works, Stephens’s concern is less with quantum mechanics generally than with the human heart’s particulars.  In Heisenberg, the hearts reside Alex Priest (Denis Arndt), an Irish-born butcher on the cusp of retirement, and Georgie Burns (Mary-Louise Parker, of Weeds fame), an American-bred woman who might be an assassin and might be a waitress but is probably a low-level administrator at a ritzy school. She describes herself as “Possibly winsome, maybe psychotic.” She tends to pile on two or three conflicting facts, often in the same breath. Speaking of a former husband, who likely never existed, she says: “We never had children. Which is one thing. I don’t regret it. I do really.”  They meet when Georgie spies Alex in a train station and promptly smooches the back of his neck. This may be a spontaneous demonstration of affection or the inauguration of a nefarious scheme. (Or just a playwright’s kooky effort to shove two unlikely people into the same scene?) Winsome or psychotic: you decide. But Alex is sufficiently charmed, and Georgie sufficiently persistent, that they embark on a kind of romance.  Bits of physics do crop up from time to time. Georgie describes the spaces between people in cities and Alex lectures on gaps between notes in music in ways that sound a bit like the movement of atomic particles. And Georgie comes right out and all but types the address for Heisenberg’s Wikipedia page when she discusses the sudden disappearance of her child. “If you watch something closely enough you realise you have no possible way of telling where it’s going or how fast it’s getting there,” she tells Alex. “Did you know that?” Yes, says anyone and everyone who has Googled the play’s title.  But all this chatter about particles and matter doesn’t really matter much. Stephens’s great interest, as in previous plays like Harper Regan, Bluebird and The Curious Incident of the Dog in the Night-Time, is how our perception of people and relationships alters depending on what we know, what we see and whose side we see it from. Seemingly senseless actions that seem inexplicable reveal reasonable motives; solid facts dissipate and disappear. Stephens likes surprises and Heisenberg has a fair number of them.  But that doesn’t mean that the play is wholly credible, particularly the character of Georgie with her tangled motivations and wild mood swings, and this July to December romance more generally. But under Mark Brokaw’s uncomplicated direction, Parker is an appealing and adroit actress and she just about pulls it off. Her wittering plays nicely against Arndt’s slightly dour placidity.  Heisenberg is only 80 minutes, performed with no set to speak of and hardly any props. It’s a flimsy piece, but it rewards watching, both where it is and where it’s going.
Can an affair between a middle-aged woman and an older man be explored through science? Simon Stephens springs some surprises, though his play is not entirely credible