Conductor-less, Phila. Orchestra plays on

With no conductor for Thursday night's Philadelphia Orchestra concert, the ensemble was left to confront Mozart on its own. Little surprise that the

Eine kleine Nachtmusik

was small and bland, the

Symphony No. 25

erratic. And yet, the composer's

C Minor Piano Concerto

glowed with a point of view of such blinding beauty that we probably won't hear its likes again anytime soon.

It was all planned, actually - even rehearsed that way. Conductors are a relatively modern invention, especially the breed of whom celebrity is expected, and for this one program the orchestra constructed a concert that asked musicians to find leadership elsewhere.

In Imogen Cooper they found it in spades. The pianist exercised mysterious powers from the keyboard. She left the job of opening Mozart's Piano Concerto No. 24 in C Minor, K. 491, to concertmaster David Kim raising his instrument for the upbeat, and after that, her visible leadership amounted only to a facial expression or spare gesture (much of which must have been blocked to players by the piano's open lid).

But the fact that the interpretation was so thoroughly worked out spoke to a major myth of what the conductor does, and when. Much of the job really happens in rehearsal, refining dynamics, balance, phrasing, and bowings.

I'm not privy to Cooper's rehearsal methods, but clearly a lot happened before curtain time. The orchestra echoed her pacing, slowing down in increasing steps toward the end of certain phrases. The strings imitated her color. This is one of only two Mozart piano concertos in a minor key, and Cooper infused the first movement with all of the uncertainty and fragility it suggests. The sense of risk and inquest missing in the Eine kleine Nachtmusik (led by Kim) was here.

The first-movement cadenza (in part written by Alfred Brendel, with whom she studied) was a stunning act of time travel, looking back to Bach or Scarlatti, and to darker Romantic concertos to come.

Were the woodwinds in the third movement so precise and buoyant because they were inspired by Cooper's commanding presence? It seemed so.

Ideally, opening up a blank spot on stage where you normally find a podium would mean sparks flying among musicians, so it was enervating that chamber music failed to break out in Mozart's Symphony No. 25 in G Minor, K. 183, shaped minimally by Kim. After Cooper, it emphasized how crucial it is to have a singular personality on stage - if not on the podium, then somewhere.

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