IT'S A TREMENDOUS technical feat to turn a play that deals with electrons, neutrons and photons, quantum mechanics, fission and wave equations, into a totally absorbing detective thriller about what precisely two people said to one another on a 10-minute walk in 1941. This is what Michael Frayn achieves in his superb Copenhagen.

On the evening in question, the German physicist Werner Heisenberg visits the home of his old mentor, Niels Bohr, in German-occupied Copenhagen. He has dinner there, with Bohr and his wife, Margrethe, and then, because the Nazis have hidden microphones in the house, the two physicists go for a walk. It ends abruptly. No one has since been able to agree on what was said.

At that point, four years before the atomic bomb, any breakthrough in nuclear physics might have changed the course of the war. These two Nobel prize-winners, colleagues in the 1920s, whose relationship had been as intense as that of any father and son, find themselves on opposing sides. Was Heisenberg trying to find out about the Allied bomb programme? Was he offering to slow down his researches if, in Los Alamos, Oppenheimer could be persuaded to do the same? Did Heisenberg fail to build the bomb because he didn't want to, or because he failed to do his sums?

Bohr, Heisenberg and Margrethe died in 1962, 1976 and 1984 . Frayn brings them together in a kind of present, as they discuss and revisit that night in Copenhagen. In purely structural terms, it is a masterly exercise in exposition. Frayn's characters agree on so little that they are able to dispute ideas and anecdotes that would otherwise be too familiar to them for debate. Margrethe's role is essential here: as the non-scientist, scientific matters have to be explained to her (and therefore to us).

The script of Copenhagen hasn't a single stage direction, yet it is intensely theatrical. Michael Blakemore's powerful production is stark and focused. Members of the audience sit on the back wall, lined up like jurors at a war tribunal. Only three plastic chairs stand on a white circular floor on which the cast circle round and round. There are a few sound effects: a train, a seagull, a doorbell, the rumble of an atomic bomb. Otherwise switches in lighting denote shifts in time, place and point of view. Thankfully, there's no naturalistic guff with the cast shuffling scientific papers, pouring out drinks for each other or clearing away the plates at dinner. Changes in atmosphere come down to the expert nuances of the actors.

The three give wonderfully unstarry, transparent performances as they chase these ideas with an unrelenting rigour. David Burke's Bohr has a clubbable charm and a dose of irony, while Matthew Marsh plays Heisenberg with a furrowed earnestness and a rapid deadpan delivery: one might be a Ludovic Kennedy, the other a young Martin Bell. As Margrethe, Sara Kestelman has a caustic intelligence that sees events in refreshingly personal terms.

Don't be put off Copenhagen by the science. It's the emotional currents that give this play its own questing energy. The programme includes bits of scientific information, but it could just as easily have included examples of other father/son relationships, or of biographers who have never been able to puzzle out events in their subjects' lives, or of chains of events that lead from an innocent action to an evil consequence, or of any number of people making compromises during wartime. It is unquestionably a play for non-scientists too: it deals with politics on the largest scale imaginable, and personal relationships at their most private and unspoken.

It was Heisenberg who discovered the uncertainty principle. This states that it's impossible to measure precisely the position and momentum of a particle, as the means by which the observer measures the particle will inevitably affect it. Frayn takes this principle and makes it a central theme of his play. Copenhagen is about - among other things - the impossibility of understanding people's actions. Knowing thyself is an impossible task; the one person unable to observe an action is the person taking it. Uncertainty is a condition of knowledge.

After the war, Heisenberg was reviled by Western scientists because he worked for Hitler. Yet he never built a bomb or killed a person. Bohr was revered by the Los Alamos team as a father figure and helped design the trigger for the Nagasaki bomb ("my small but helpful part in the deaths of 100,000 people"). Copenhagen is full of ironies, contradictions and uncertainties. They are presented with exhilarating clarity. Of a few things we can be reasonably certain: this is a fascinating and profound piece of theatre - the best new play this year.

The audience work hard at the Globe. The afternoon I saw As You Like It, they cheered when Orlando beat the wrestler, they hissed when Oliver plots his brother's downfall, they wolf-whistled when Rosalind and Celia fling up their skirts and - a very modern touch, this - they sighed when the stag was slain. Performances at the Globe have to be extrovert - if only to match the audience.

The sun was in our eyes, helicopters flew past and it wasn't only the actors who kept moving about. Lucy Bailey's engaging new production meets such challenges with a boisterous, energetic approach that centres on Anastasia Hille's awesomely skittish Rosalind. Hille physicalises her emotions with an extreme form of nervy femininity. She flicks back her hair, rolls up her sleeves and widens her eyes. When she gets into her breeches, she keeps hitching them up too. It's a restless, attractive performance that would benefit from some moments of stillness. In this well-cast production John McEnery is perfect as a weary defiant Jaques ("Let's meet as little as we can"), David Fielder is unstoppable as randy Touchstone and - doing the most costume changes - Jonathan Cecil switches from fop to rustic to vicar with benign aplomb.

The musical team Kander and Ebb are best known for Chicago and Cabaret. The Orange Tree Theatre scored a big hit (for a tiny theatre) when it revived - first once, then twice - the little-known Kander and Ebb musical Flora, the Red Menace. Now they have revived The Rink, which first starred Chita Rivera and Liza Minnelli. How well you get along with this Seventies musical, about a mother who is selling the family skating rink and the daughter who comes home to save it, will largely depend on how you feel about the person who wrote the book, Terrence McNally. I rate him highly when it comes to fake emotion. (He wins many awards on Broadway.) Here, John Gardyne directs a cast of eight with an almost comic resourcefulness. The ensemble work, in particular, between the Wreckers, the guys who've come in to take the Rink apart, has a likeable easy-going charm. Their rollerskating number is the high point. I only wish these wreckers in their boiler suits would take the script apart instead.

'Copenhagen': RNT, Cottesloe, SE1 (0171 452 3000); 'As You Like It': Globe, SE1 (0171 401 9919), in rep to 20 Sept; 'The Rink': Richmond Orange Tree (0181 940 3633), to Jul 4.