QED []

At the Vivian Beaumont Theater, Lincoln Center, 150 W. 65th St.; (212) 787-6868.

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WHO knew quantum electrodynamics could be so much fun?

Those who loved “Copenhagen” – and even those who didn’t – should rush to Lincoln Center to see Peter Parnell’s new play, “QED,” which opened last night.

Alan Alda plays Nobel laureate Richard Feynman, considered by many to be America’s greatest physicist of the 20th century – and one of our great eccentrics.

It is June 1986, and Feynman is in his office at the California Institute of Technology in Pasadena. He is dying of cancer. He is not going to die for another 19 months, but he doesn’t know this and neither do we.

But he does manage to tell us almost everything else.

At present, apart from his teaching and research, he is considering more cancer surgery, a possibility he examines with scholarly detachment.

Meanwhile, his life continues – he’s playing a drum in an amateur production of “South Pacific” and trying to arrange an expedition to the remote Soviet republic of Tuva. (He inspired his friend Ralph Leighton to write a book “Tuva or Bust!” that partly informs this play.)

He is also concerned with filing a significant report on the explosion of the space shuttle Challenger during lift-off.

So that is where Feynman is. But where are Parnell and his play? And where is the audience?

We are listening to Feynman, and Parnell has rightly gambled that that will be entertainment enough.

Feynman was a fabulous lecturer, and for most of the play lectures the audience as if we were his favorite students.

He is talking chiefly about quantum electrodynamics (that’s the QED of the title) for which he won his Nobel. It’s dazzling, and the show makes the basics crystal clear.

No, it’s not all lecture. Parnell also manages to weave in Feynman’s life – including his time at Los Alamos working on the atomic bomb and watching it explode, the death of his first wife and, of course, his cancer.

The monologue is interrupted by phone calls and the brief appearance of a nubile girl student, with whom Feynman charmingly flirts, and . . . well, I don’t want to spoil the rest.

This fine production by Mark Taper has been unerringly staged by its artistic director, Gordon Davidson, and is faultlessly cast.

Kellie Overbey blends cuteness with vulnerability and sagacity as the tempting student, while, bearing the whole weight of the evening, Alan Alda’s magisterial portrait of the genius physicist is so utterly convincing that you shiveringly feel that Feynman is there in front of you.

If Feynman in life was not the spitting image of Alan Alda, in presence, voice and gesture, he should have been.

One snag: For some obscure reason the play is only being performed on Sunday and Monday nights, the days when the musical “Contact” is disconnected. So better stampede for available tickets rather than merely rush. “QED” is worth such trouble.