When Michio Kaku was eight years old he heard that a great scientist had died, and when he looked in the newspaper there was an image he found impossible to forget.

It was a picture of Einstein’s desk, and on it there was an open notebook. Its pages revealed that the man some judge to be the smartest human being who ever lived had been working on a problem that even he had been unable to solve.

The young Kaku was overwhelmed. “What could possibly be so hard that even the great Einstein could not solve it?”

Later, training as a physicist himself, Kaku learnt what it was. The universe seems full of separate forces and events. There’s the blowing power of winds, the crackling of electricity, the glaring heat of the sun. But were they truly separate or was there some hidden unity behind them that we could, one day, finally see?

The intense, religious Isaac Newton had made a start. Apples fell to the ground in Lincolnshire the same way as the moon “fell” around the entire planet in its orbit. From his 17th century equations, much of the 19th and 20th centuries’ bridges, skyscrapers and rockets could be built.

Around the time of the US civil war, the whimsical Scottish thinker James Clerk Maxwell showed that the seemingly disparate forces of electricity and magnetism were just manifestations of a single, deeper force — the somewhat unimaginatively named “electromagnetism”. Craftsmen had already made simple versions of electric motors and lights. Maxwell accelerated that, and today’s offshore wind turbines and electric cars are the result.

Einstein kept pictures of Maxwell and Newton above his desk. He wanted to take the next steps in unifying the seemingly disparate forces in our universe.

A lot of his work was successful. Even mass and energy, which seemed as far apart as any two things could be, turned out to be just different aspects of a deeper entity. (That link is specified in his famous E=mc², with the atomic bomb just the most famous result.)

Much beyond that, however, Einstein could not go. He was too locked in to the past; too lacking in knowledge about how subatomic particles worked.

The latest effort to do better is string theory — an attempt to link the forces that govern the smallest particles within a nucleus with the gravitational force that operates on the largest scale of all: stretching the length of galaxies and beyond.

It’s a majestic story, and Kaku tells it well in The God Equation. (It helps that — having done his eight-year-old self proud — he is a noted authority in string theory himself.)

Unfortunately, it’s a difficult theory to get across to the general reader, for it inherently operates in more dimensions than we can possibly visualise. But perhaps a metaphor can give an impression of what its achievement might produce.

Our universe is expanding, and at an ever-increasing rate. In the far future either it will tear apart in the equivalent of a great “rip” or — just as uninviting — its gradual dissipation of energy will make the continuation of any form of life impossible.

But what if some future beings could create a lifeboat to another universe? In string theory that possibility is not closed off, not least because the theory is consistent with our universe being just one of a great multitude of bubbling universes in existence.

Even with the limited tools of Einstein’s era it was possible to imagine wormholes that could stretch from one point in our universe to another. They wouldn’t stay open for long; nor could they link with other universes. With string theory, however, all that could, conceivably, be possible.

From the 21st century that sounds implausible. But note that our cell phones are little more than carefully sculpted clumps of metal and sand (silicon). Would anyone in Newton’s era — aside, perhaps, from the great man himself — have believed that whispering into such a metal/sand object in London could produce sounds heard near-instantly in South America?

The God Equation: The Quest for a Theory of Everything, by Michio Kaku, Allen Lane, RRP£16.99, 240 pages

David Bodanis is the author of ‘The Art of Fairness’

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