16/01/2013 05:15 GMT | Updated 17/03/2013 05:12 GMT

Paul Emsley and Kate Middleton

Paul Emsley's much-mocked painting of Kate Middleton makes her look old, reflective, a tad chubby, and impish. There's a hint of mischief about her smile, helped by the askance gaze and the slightly enlarged left eye. The hair is long, but limp; it hangs like a heavy curtain. The dark, menacing background threatens to suck all life out of the person, except for the strong persistence of the flat, white visage. It's pretty disturbing.

This is, quite simply, the most brilliant piece of public art in recent years. It's performance art, and mass outrage is an integral part of the show. It doesn't look like Kate Middleton - but then nor do the fuzzy telephoto pics from France, or, for that matter, the airbrushed poses condoned by the palace and blazoned by the royalist press. In fact, you or I have no reason to think that we know what the Duchess really looks like. We're all skilled-up readers of media imagery; none of us takes representation for reality.

The unsettling questions the portrait raises have nothing - nothing at all - to do with realism. It's obvious to anyone with eyes that Elmsley is a highly skilled painter. He hasn't painted Kate this way because he can't see what's in front of him. Technically, it's a gorgeous picture; it's just that it doesn't look like a young, beautiful princess.

The fact that it doesn't depict reality shouldn't be a surprise. Art is not, and never has been, realistic, Noone ever got upset that the Simpsons are yellow and have three fingers on each hand. The best guess is that babies in Giotto's era weren't actually shrunken adults, fifth-century Athenians weren't red, and ancient Egyptians weren't matchstick people with huge, yearning eyes. No culture has really believed that the job of art is to represent reality exactly.

The real problem is that, despite all the real progress that's been made in attitudes towards aging, we just can't deal with an artwork that confronts us with decay and mortality. This is actually as realistic as art gets; it's just that the reality it depicts is the upsetting truth that youth and beauty are impermanent.

Most controversial is that the decaying subject the portrait celebrates is a woman. The reaction is a reminder that we are, as a culture, in no sense post-feminist. Harry may have had some schtick for cavorting naked, but Kate wins the nipple-google contest hands-down. The picture discomfits because Emsley, a man, has chosen to show us a woman who would not appear on this side-bar of the Daily Mail online. The problem is not that the portrait doesn't look like Middleton, it's that it doesn't look like the photoshopped images of female beauty that we've learned to expect from the media.

Even worse, it portrays a woman who seems comfortable with her withering. She enjoys the confident assuredness of age. She is intelligent, thoughtful, and controlled. Or, to put it in more conventionally misogynist terms, bossy, interfering and nagging. Emsley's Middleton is actually a confident prediction of the media's inevitable falling out of love with her. She will, equally inevitably, find her own voice during the years to come. She'll become a real human being with a voice and opinions. She'll be a nuisance, and the establishment will come to disillusion. I'm not claiming this as prophetic guess-work on my part; it's in the script. Emsley's Kate knows that it's her destiny to be judged against the Diana narrative. That story haunts every brush-stroke of this picture, down to its disturbingly old-fashioned idiom and the death-mask pallour of its subject.

I am just back from New York, where I saw the Picasso show at the Guggenheim. As we chugged up the famous spiral, my friend and I mused on how difficult it is for art now to generate the kind of public scandal that surrounded so much surrealist art, given that our world is already saturated with and sensitized to shocking imagery. How wrong we were. An understated portrait of an elegant, mature woman has horrified a nation, in all the right ways that great art should do. Paul Emsley should be very proud - and Kate Middleton too.