A play by Sir Tom Stoppard, connecting espionage with quantum physics.

Elizabeth Hapgood heads a branch of the British secret service. She runs the physicist Joseph Kerner as a double agent, with him passing false results from his space research work to his Soviet controller. But a drop goes wrong, even though British and American agents are following everyone, until Kerner works out it's equivalent to the Bridges of Königsberg problem, and the only solution admissible is that there were twins. A spy who is a twin can be both here and not here at the same time, like a quantum particle going through two slits. The Americans suspect Kerner of being a triple; but known to very few, Hapgood's son Joe is Kerner's. She defends him by setting a trap for her suspect. Twins multiply.

A lot of people found the physics too abstruse, but Stoppard is a writer, not a physicist, and he knew no more than what he swotted up, and he explains it very well, I think. The agents want certainty about whose side Kerner is on, and he tells them physics doesn't give you certainty. All sorts of things are woven in as illustration. Kerner uses imaginary bullets to explain the slit experiment, and of course the spies use real bullets. Hapgood plays chess in her head and works out where her son left a key at school by focusing on the uncertainty in his quantization of his time. The printed edition is prefaced with a quote from Richard Feynman.

Hapgood was first performed at the Aldwych Theatre in London on 8 March 1988, starring Felicity Kendal and Nigel Hawthorne.