To launch her new theatre company, director Marianne Elliott has chosen to reunite the team that mounted The Curious Incident of the Dog in the Night-Time to overwhelming acclaim.

Yet this play by Simon Stephens, first seen in New York in 2015 and getting its UK premiere here, is a very different beast: a gentle, elegiac two-hander that relies for its effects on the delicacy and conviction of its playing.

Although it has a German physicist in its title, it wears its science lightly. In fact all you need to know about Heisenberg and his uncertainty principle (formulated in 1927) is that it dictated that if you precisely measure the position of something, then you cannot precisely measure its movement. Or, as gabby Georgie, the American heroine of the play puts it: "There's no possible way of telling where a thing is going, and how fast it is going." Furthermore, in attempting to ascertain speed or position, you stop watching the object itself – and thus lose sight of it.

In this way, physics becomes a metaphor for the vagaries of existence, for the blurriness, uncertainty and lack of predictability in nature and in all human relationships. Stephens also weaves Bach and the nature of music into the mix, in order to focus on his essential interest – human behaviour.

My problem with the play is that it begins with an act so improbable – that a 33 year-old woman would kiss a 75 year-old male stranger on the nape of the neck when she sees him on a bench at St Pancras Station – that the suspension of disbelief required for their developing relationship is enormous.

But once I had climbed over that hurdle, there is plenty to enjoy. Stephens is an elegant and witty writer and wrings plenty of wry humour from butcher Alex's sheer amazement at the energy and madness of Georgie's pursuit of him. Bunny Christie's set, of sliding white panels and furniture that rises from the smooth floor, is framed by Paule Constable's brilliantly coloured lighting in ways that make the air seem solid.

When the couple are falling drunkenly into bed, the set sways and swings as if giddy with exhilaration, while Steven Hoggett's stylised choreography takes them through the rituals of seduction. In the cold light of dawn, the light reflects the realities of the day.

As Georgie and Alex, Anne-Marie Duff and Kenneth Cranham disguise the slender nature of the material with performances of physical and emotional grace. He doesn't quite convince as a tango master, but he moves with considered weight, borne down by the dead sister who haunts him and the romantic opportunities that have passed him by. The slow, gentle dawning of possibility on his face as he watches Georgie laugh is a joy to watch; his benign wariness makes the character loveable rather than creepy.

As Georgie, Duff is luminous, giving off a vibrant life force as she weaves tall tales that hide her sadness at her permanent estrangement from her grown up son. As the play progresses, she seems to change and alter in front of our eyes – in exactly the unpredictable ways that have given Stephens his theme. It all adds up to a touching if low-key start to a brave new venture.

Heisenberg runs at the Wyndham's Theatre until 6 January.