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To talk to Stephen Hawking you need to start several days in advance, sending him your questions and then allowing him the time and effort to key the answers into his famous voice machine. It is the aural equivalent of waiting for light from a very remote source, even though he is just up the M11 in Cambridge. A few years ago, the last finger that he was able to use finally passed from his control. This left him – leaves him – with nothing that responds to his physical prompting except a tiny muscle in his cheek. By clenching it, he causes a minute infra-red detector attached to his spectacle frame to register the light coming off the upper slope of his right cheek. When he tenses the muscle to a certain degree, he sends a signal into his computer and can construct sentences by responding to the range of options displayed on the screen.

Because we never see this labour but only hear its flat and otherworldly results, it is hard to imagine the rigour of it all. Think of texting without fingers, think of speaking without a voice, think of almost any conversational assault course you can, and you would struggle to trump the reality. And the awful fact is that there is so much to ask him. He is now 65, having been told when he was 22 that he would be dead within 18 months. There are no other known sufferers from MND (motor neurone disease) to have lived this long with the condition. He is divorced from his second wife, Elaine, who had been his nurse and also the subject of (unsubstantiated) allegations of abuse. There is said to have been a rapprochement between him and his first wife, Jane, who has over the years written devastatingly candid accounts of the difficulties of marriage to such a man (“It was unnatural even to feel desire for someone with the body of a Holocaust victim and the undeniable needs of an infant”).

Now Hawking and his 36-year-old daughter Lucy, one of his three children with Jane, have collaborated on a scientific adventure story for children, George’s Secret Key to the Universe. Is it autobiographical? Well, there is a girl called Annie who lives with her scientist father, Eric, who has the world’s most intelligent computer, Cosmos, which can whizz people off to anywhere in the universe. The book is a kind of mirror image of Hawking’s bestseller A Brief History of Time, published 19 years ago. If that retained a childlike amazement in the service of a seriously adult argument, this one takes time out from the whizz-bang narrative in order to instruct young readers, old ones too probably, on the facts of cosmic life. For example: “What is a Black Hole? A Black Hole is a region where gravity is so strong that any light that tries to escape gets dragged back… Falling into one is like falling over Niagara Falls; there’s no way of getting back the same way you came.”

Hawking did the science and Lucy, already with two novels to her name, did the fiction. Were there times when he overruled her? “Oh, yes. Occasionally he said, ‘You can’t do that, Lucy. The laws of physics don’t allow it.’ I remember saying, ‘Oh, can’t they just be bent a bit?’ and he said, ‘No.’ When he says no, there’s no arguing.” The truth of this last statement is confirmed many times in the course of the day.

The book was her idea. When they visited Hong Kong together a year ago, with the first few chapters already written, they mentioned their plans at a press conference and soon had contracts in 20 languages. This is to be the first of a trilogy. So, unusual though his circumstances may be, Hawking is doing what superstars in every field have done at one time or another – talking to the press when they have product to shift.

He can be found in his office, which occupies a set of rooms in the Centre for Mathematical Sciences, a silent, modern building near the ring road on the west side of Cambridge. For the past 28 years, he has been the university’s Lucasian Professor of Mathematics. The post was once held by Sir Isaac Newton, whose modern counterpart Hawking is said to be. This is a place of muted areas; even canteen conversations are conducted in whispers. On the noticeboard are fliers for a meeting of the Polymers and Complex Fluids Group, and a talk on gauge fields and strings at the Isaac Newton Institute for Mathematical Sciences.

On the wall outside Professor Hawking’s door are some lavish equations squiggled on a whiteboard. No one is entirely sure what they refer to. There is also a photo of him, incongruously absent from his wheelchair, lying horizontally several feet above the ground with no visible means of support. It was taken in April this year, high above the Kennedy Space Center in Florida, in an aeroplane simulating the weightlessness of space travel by performing gigantic rollercoaster arcs. Hawking has a blissed-out, slightly triumphant look on his face, as if for this instant, this 25 seconds of deferred downpull, there is nothing to choose between him and the able-bodied others. He whirled “like a gold-medal gymnast”, said one crew member.

Over the decades, as he has surrendered one element after another of his physical independence, the wheelchair has become an ever heavier and more complex affair. It looks massive now beneath the slight, frozen zed-shape of his body. An ever-present carer, one of a rotating team, is in the corner. On the desk in front of him is a bowl with an arrangement of seashells in it, and thick white vapour rising through them – a humidifier to aid his breathing.

It has taken him the best part of a day to answer the questions that I had e-mailed to his office. There were 20 of them, including how he and Lucy collaborated on their book, whether he missed married life, how on earth (or on Mars?) mankind is to rescue itself from itself, and what happens to us when we die. Having generated the answers, he can send them out in sound form. There is a series of beeps and then the electric voice: “I will answer questions 1 to 11. The rest are personal.”

His personal assistant Judith Croasdell puts my questions in front of me so that I can read them and Hawking can then activate his answers one by one. The idea is to make contact, to sit face to face, so that we can at least simulate the standard experience of conversation. “Was it strange,” I read, “to have this other area of your imagination called into play [for the writing of the book]?”

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