He's Santa, God and Forrest Gump

Albert Einstein is Hollywood's favourite scientist. Great accent, strange clothes and lots of fluffy white hair. Only one problem really, says Jon Ronson - the real Einstein was too clever by half

Albert Einstein wanted to be cremated, he said, "so that people won't come to worship at my bones". Now, Walter Matthau is playing him in a movie, a light, romantic celebration of dimwittedness in which the Nutty Professor attempts to matchmake his beautiful niece (Meg Ryan) with an obtuse but lovable garage mechanic (Tim Robbins). "The heart," blurbs the blurb for IQ, "is more important than the theory of relativity" (a historically inaccurate claim, by the way: Einstein's marital relationships floundered pitiably against his passion for equations). And the film's poster announces with equally erroneous Capra-esque fervour: "With Einstein as cupid, what could possibly go wrong?"

In the 40 years since his death, Einstein's face has sold powerful computers, Carlsberg lager and women's stockings. Indeed, a Californian advertising agency has trademarked his name. He regularly appears on T-shirts, mugs and posters, in pop songs ("Einstein A Go-Go") and operas (Philip Glass's Einstein on the Beach). In Young Einstein, as a teenager with huge hair, he invents the electric guitar, and in Terry Johnson's erudite play Insignificance (soon to be revived at the Donmar in London) he flashes his legs to Marilyn Monroe.

Heinz Woolf, Magnus Pike and Patrick Moore have all done very well in showbusiness by being a little bit like him (especially on the hair front). And Michael Jackson apparently wants to buy his eyes, a lofty accolade indeed. It seems that Einstein, who famously sacrificed self for science, has got his self back.

The ways in which Einstein has been portrayed over the decades make up an intriguing barometer of the shifts in public trends and anxieties. In the diffident Seventies, for instance, his caustic cynicism was employed by Randy Newman for his Einstein-narrated rewriting of the national anthem:

"America, America, God shed His grace on thee,

You have whipped the Filipinos and you rule the seven seas".

Thereafter, in the post-Insignificance nuclear paranoia of the mid-Eighties, it was practically impossible to visit some tawdry Notting Hill fringe production that didn't incorporate a scene in which Einstein was dispatched tearfully to some post-apocalyptic landscape to mumble an apology to a two-headed baby, or something.

And now that peace and credulousness has broken out, how does he fare as a leading character in a romantic comedy? Perhaps unsurprisingly, IQ's director, Fred Schepisi, accentuated the kooky and oddball aspects of the scientist's legend. Once in a while, he spouts one of his Greatest Hits of elegant aphorism ("God doesn't play dice with the universe" etc), but mostly he rides fast on a Harley Davidson while yelling "wahoo", gobbles up a flamboyant triple-scoop ice-cream ("Mmm, it's great to be American"), and has his car transformed into a flashy convertible (wahoo again). It is, I am sorry to have to report, a very dumb film.

Curiously then, Albert Einstein appears to have become mythologised for everything he claimed to abhor, "the merely personal", as he called it. The hair, snow-white and delightfully unkempt, is comfortingly reminiscent of Father Christmas and God. The genius who couldn't tie his shoelaces, stuck his tongue out to the camera, and possessed scores of clothes exactly the same so that he wouldn't need to expend his mental capacities deciding what to wear, constitutes an alluring blend of innocence, eccentricity and magnificence.

In a secular age, Einstein fulfils many of our most basic needs: he has become Santa, God and Forrest Gump, all wrapped into one. What is the theory of relativity actually about? God knows. If Einstein's allure was based on the specifics of his work (as, say Marilyn Monroe's is: she was paid to stand on an air vent while her skirt sky rocketed over her head, whereas Einstein was never paid to stick his tongue out) he probably would only be remembered for inventing an equation that nobody understood, but everybody was wildly thrilled by.

"I spent three weeks sitting in Watford library trying to decipher the theory of relativity," says the playwright Terry Johnson, whose Insignificance was first commissioned by the Royal Court in 1981. "Three weeks of research and then I realised that I only knew half of it. I abandoned the whole play then and there."

Three days later, Johnson came up with a perfect solution: he got Marilyn Monroe to explain the theory to him in broad strokes using toy trains and balloons and, in return, Einstein flashes his legs. That is how he now appears - passive, amused, silent.

Indeed, nowhere in the play (nor in Nicholas Roeg's movie adaptation) does Einstein speak for more than ten lines at a time: he just listens, silently, while the other characters gabble themselves into a hole.

"It was obviously very hard to generate things for Einstein to say," says Johnson. "He was not a man who would chatter. So I didn't. Dramatically, then, it wasn't too tricky."

There is a scene in the play in which Marilyn Monroe turns up unexpectedly at Einstein's hotel room.

"Why are you here?" he asks.

"You're famous," she replies.

"So are you," he says.

"I know," she replies, "we have an awful lot in common."

So Insignificance played neatly on the perception that Einstein had become a celebrity - and that we like our celebrities to be at least as limited as us. Things have moved on a bit since then.

As the joy of idiocy sweeps across America (Forrest Gump is up for 14 Oscars, Jim Carrey's huge US hit Dumb and Dumber will be released here soon), Einstein is being re-defined yet again: this time as the dumbo's genius. His unfathomable sagacity is made palatable by his quasi dimwittedness (wahoo). IQ is telling us, in fact, that dumbness is such a fabulous state of being that even Einstein endorses it by trying to marry his niece off to a feeble-minded schmuck rather than her egghead fianc (Stephen Fry).

That's quite a feat: using Einstein to validate our stupidity. For all his insight, perhaps even he would be surprised at the many ways in which he'd continue to be of use to us, decades after his death.

n `IQ' is released on 17 March