Copenhagen; Benefactors, Sheffield Theatre, review Historians have long argued about the purpose of this trip, which ended badly. Was Heisenberg tipping off his old friend that Germany was developing atomic weapons? Was he hoping to receive his former tutor's blessing for his nuclear research? Or was he in some way delivering a message of reassurance that was misinterpreted? What's so brilliant is that the play actually seems to exemplify Heisenberg's uncertainty principle, which suggested that the act of observing a particle in some way affects its behaviour. In a limbo outside time, with all the protagonists now dead, we watch three re-enactments, and three different versions, of the encounter between Heisenberg, Bohr and Bohr's wife Margarete. What becomes thrillingly and movingly clear is that if the movements of an electron around an atom are unpredictable and mysterious, how much more extraordinary are the workings of the human mind and heart. This is a play that illustrates and celebrates the complexity and uncertainty of life. And in its consideration of mortality - the Bohrs lost a beloved son in a drowning accident - it has emotional as well as intellectual profundity. The final scene also achieves a beautiful poetic resonance about "the final core of uncertainty at the heart of things" that seems like a secular version of TS Eliot's Four Quartets. David Grindley directs a lucid yet atmospheric production, with the haunting quality of a recurring dream. And the three characters in the play's limbo-like world are played with a superb mixture of intellect and depth of feeling. Henry Goodman occasionally stumbled over his long, complex speeches as Bohr, but still powerfully brings to life this fiercely intellectual yet engagingly good-hearted character. Geoffrey Streatfeild is mesmerising as the conflicted Heisenberg, desperately trying to understand his own motivation, while Barbara Flynn as Bohr's wife acts as an often witty moderator of the debate, and constantly raises the questions to which the audience want to know the answers. You leave the theatre in no doubt that you have seen a masterly production of a modern classic. Benefactors seems small beer in comparison, as Frayn contemplates the decay of friendship among two middle-class couples and the conflict between high-rise architectural ambition and the kind of homes people actually want to live in. It's an intermittently entertaining but curiously cold and bloodless piece, not helped by Rebecca Lacey's over-the-top performance as an anxious, twittering housewife which seems to have been bizarrely modelled on Sybil Fawlty. Charlotte Gwinner's workaday production just about holds the attention, but the play never takes flight.