Take a trip to a New York ice-rink at the weekend and you're likely to see a middle-aged Japanese man jumping and spinning in perfect harmony with Newtonian physics. This is Michio Kaku's idea of relaxation; no relativity, little friction - just one man and the three laws of motion. He moves almost effortlessly, his blades carving perfect patterns in the ice; but then the three-dimensional world should present few problems to a man who is equally at home in 11.

Kaku is the Henry Semat professor of theoretical physics at the City University in New York, and the man who, in the late 1960s, co-founded the field theory of strings - the equation that united a complex series of equations which described the behaviour of sub-atomic particles into a coherent whole.

Mention string theory - the notion that an electron is not a dot, but a rubber band which, if vibrated enough times, can turn into every single sub-atomic particle in the universe - to most people and you get a glazed incomprehension: but to theoretical physicists it is just about the hottest topic around. "It either explains everything, or it explains nothing," Kaku shrugs, though his expression tells you exactly which side of the fence he is on.

Over in the US you can buy a T-shirt with Kaku's equation printed on the front, but it has taken a while for him to gain this recognition. Back in 1974, he was laughed off campus when it was discovered that strings could only vibrate coherently in 10-dimensional hyperspace, and Kaku groans as he remembers being taunted by the Nobel laureate Richard Feynman with "How many dimensions are you living in today?".

Kaku started work on another theory, only to realise he was looking at the same phenomenon at a higher vibration on the rubber band. And no one is laughing any more. For years, physicists and cosmologists have been searching for the grand theory that united quantum physics and gravity theory. Einstein spent the last 30 years of his life on just this problem, trying, in his words, "to read the mind of God".

"He got to the fourth dimension and dabbled with the fifth," says Kaku, "but there wasn't yet the understanding of the nuclear force and quark model to allow him to progress. String theory is now the only real contender for the grand unified theory; everything else has fallen by the wayside. If Einstein had never lived, we would have been able to determine all his ideas from string theory."

Kaku slips into overdrive as he explains all the implications. "String theory predicts the universe is like a soap bubble that is expanding and dying," he says. Billions of years from now stars will blink out; the night sky will be dark and the oceans will freeze over. But we may have an escape route. Our soap bubble co-exists with other soap bubbles; every time a black hole forms it may be creating a baby universe. The matter being sucked in may be blown out the other side, creating a white hole in a twin universe, which will expand very rapidly, like our own Big Bang.

"Perhaps, also, a Type III civilisation, which can harness the Planck energy, will open a hole in space and tunnel through a wormhole to a parallel, warmer universe. There is no other hope. Either leave the universe or die with it. If the wormhole is microscopic in size then we may send a nanobot that can reproduce itself indefinitely and create cloning factories to recreate the dead civilisation through it."

This sounds like something out of Star Trek, but Kaku is deadly serious as he goes on to talk up his latest variation of string theory - M (membrane) theory, which operates out of 11 dimensions - and to speculate on multi-worlds theory, a new version of quantum theory, which predicts there may be clones of ourselves with separate lives. "It sounds crazy," he admits, "but we can't apologise for it as it's a possibility." He notices my eyebrow rise. "It's a theory that's being pushed strongly by physicists over here in Oxford," he adds, as if to show it's not just some nutty Stateside notion.

Kaku's voice washes easily over you. Each sentence makes sense. It's only when you get to the end of each paragraph that you realise you're hopelessly lost. But you don't really care, because it is never less than entertaining. And - academic credentials aside - this is Kaku's strong point. In the US, he's a regular contributor to the Wall Street Journal and has his own weekly radio show that explains the ins and outs of hyperspace and parallel universes.

It's a skill that was born out of necessity. "I remember being at the final congressional hearing in 1994 to decide whether the US government would fund the building of an atom smasher outside Dallas that would be twice the size of that at Cern, Geneva," he says. "A physicist was asked, 'Will we find God?'. The reply came back, 'We will find the Higgs Boson [sub-atomic particle].' That answer cost US physics the £12bn project.

"Thinking back, I would have said, 'This machine will take us as close as humanly possible to his or her greatest creation - genesis itself. It will give us a window on the instant of creation.' Before 1990, physicists only had to say 'Russia' to get their hands on cash, but since the end of the cold war, we have had to learn the language of the taxpayer."

It's a language that Kaku now speaks fluently, and the financial rewards have been forthcoming. Tempting as it is to caricature theoretical physicists as a bunch of geeks happy to operate in a mathematically pure vacuum, the reality is rather different. For the theoretical is rapidly becoming practical, as billions of dollars are now being spent to test the validity of string theory.

When Cern opens for business in two years' time, Kaku expects physicists to find evidence of sparticles - higher vibrations of the superstring, echoes from the 11th dimension. But his biggest hopes are pinned on the Laser Inferometry Space Antenna (Lisa), which will be launched in 2011. "Lisa is three satellites connected by laser beams which stretch 3m miles across space. This may pick up shock waves from the instant of creation, and maybe even pick up the umbilical cord of our universe."

Kaku's bet is that string theory will then be verified, but there's plenty happening here on earth to keep him occupied. Both string and M theory predict that gravity can seep across parallel universes - which means their existence can be proven by looking for deviations from Newton's inverse square law of gravity. One such experiment has already been conducted in Denver. "The results came back negative," he smiles, "but this just shows there are no parallel universes in Denver. Physicists in Atlanta are already planning to repeat the experiment at the atomic level."

It's hard to keep track of parallel universes, especially when Kaku keeps insisting they could be here, right now, in your living room and that there could be a universe in which Elvis Presley never died and in which Hitler was never born. But maybe Kaku's own life encourages him to believe in his own parallel world. For in most normal worlds, the first-born son of Japanese immigrants who were interned outside San Francisco during the second world war don't go on to refine and redefine Einsteinian physics.

His awakening came on hearing the news of Einstein's death when he was eight years old. "I saw this picture of his unfinished work lying on his desk," he says, "and it became like a real-life murder mystery that I wanted to solve." By the time he was 16, Kaku had bought 400lb of steel and 22 miles of copper wire and had built his very own atom smasher in the family garage. It was powerful enough to pull fillings out of teeth, but the only thing it smashed was the house. "It broke every fuse and ruined every circuit breaker," he admits ruefully.

But it had the desired effect. Kaku came to the attention of the renowned US physicist Edward Teller, who took him under his wing and secured him a scholarship to Harvard. Even then there was a parallel twist. "I discovered later that all Teller's scholarship students were earmarked for the Star Wars programme at Los Alamos," Kaku says. "I was offered a chance to work there, but I've always thought science was about creation, not destruction. So I turned it down."

Kaku's legacy remains unclear. He could go down in history as a brilliant but misguided mathematician or he could be the first person to read the mind of God. But you sense that he isn't too bothered either way. "String theory is the only game in town," he says, "and you can't afford not to play it to the end." In any case, isn't there a parallel universe in which there's a Michio Kaku who reckons string theory is a load of nonsense?

"Quite possibly," he laughs, before teleporting himself out of the room.

The CV

Name: Michio Kaku

Age: 57

Job: Henry Semat professor in theoretical physics, City University of New York

Before that: visiting professor at Institute for Advanced Study, Princeton

Radio: hosts Explorations, an hour-long programme in the US

Publications: Nine books, including Parallel Worlds, Hyper space and Visions, and more than 70 articles

Likes: figure skating

Dislikes: hiding in foxholes for the US army

Married: with two children