Quantum theory holds that a cat can be alive and dead. Alison Goddard talks to Roger Penrose, who plans to confront physics with common sense - no matter whom it upsets

Roger Penrose is a worrier. He does not worry about the kinds of things most worriers churn over - being late, making mistakesI Rather, he worries passionately about the physical world, about why things are the way they are - and why, maybe, they are not.

Inconsistencies in the laws of physics, for instance, niggle away at him. "My wife worries about people when they are inconsistent, I worry about 'things' when they are," he says, laughing. It is a habit that has pitted him against other scientists and brought him aggressive criticism as well as international respect.

When Penrose recently entered a debate about the nature of human consciousness - the question of how our awareness of everything, including colour, smell and pain, is formed - he hugely upset computer scientists.

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In his 1989 book, The Emperor's New Mind, Penrose argues that human understanding is generated by quantum events - bizarre effects that are seen only at extremely small scales - in the brain's nerve cells. As such, consciousness is not something computers could ever emulate.

Artificial intelligence researchers were enraged - accusing Penrose of making a bizarre case for human "specialness". They insist that human consciousness will one day be replicated in a computer - because it is generated by the collective behaviour of nerve cells in the brain.

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Penrose, the Rouse Ball professor of mathematics at Oxford University, admits to having been surprised by the vehemence of the attacks. "There were a lot of rude responses from people in the artificial intelligence community, mainly due to misunderstandings," he says ruefully.

But he is not backing down. "I think that the claim that if you train artificial neural networks long enough they will begin to develop qualities that look like human understanding is simply wrong. It is not that I do not want computers to take over the world. It's just that it is not the right theory, it is incorrect."

Nor has the controversy put him off pursuing other original lines of inquiry. He is tackling a new problem, one at the heart of the fashionable quantum physics. The question, put simply, is: how can a cat be both alive and dead?

It cannot, would be most people's response. And it is Penrose's too. Common sense, he says, tells us it cannot - but common sense does not play a very large part in quantum physics.

Penrose sees this failing as the problem at the heart of modern physics - mathematics, he says, dominates the subject at the expense of common sense. To solve it, he is writing a new book. "I am addressing various modern theories that people have about physics, so I shall probably get myself into trouble with various different sets of people," - again.

"There are all sorts of ideas going round, and the people who have them are confident that they are right. I am sceptical. There must be some major change in how we look at physics."

There is a gap between the classical physics developed by Isaac Newton in the 17th century and the quantum physics developed in the early 20th century. Newtonian physics describes the behaviour of large objects - from a ball rolling down a hill to the movement of galaxies in the universe. Quantum mechanics, on the other hand, deals with much smaller scales, such as interactions between the fundamental building blocks of nature.

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"The bridge between classical and quantum mechanics is a major missing part in our picture," Penrose says. "Understanding how quantum mechanics merges with classical mechanics is the major revolution that we may expect to see in the next century. We are not even close to it yet."

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Penrose believes that the problem lies with quantum mechanics itself. "I am slightly unconventional in the belief that quantum mechanics is going to need a major change," he says. "The way it is described nowadays - people think the formula is completely accurate. Yet there is this puzzle that it seems to apply to very small objects but not to apply to large objects. People have different ways of arguing around that, but in my view there is an essential ingredient missing from the theory and it is not known yet."

In particular, Penrose dislikes the Copenhagen interpretation of quantum mechanics, which says that a system can exist simultaneously in two states until observed, when it must become one or the other. An extreme example of this is a thought experiment proposed by the physicist Erwin Schrodinger. A cat is kept in a box with a phial of poison gas that could be opened by some truly random process such as radioactive decay - killing the cat. While the cat is unseen inside the box, it exists in a superposition of being alive and dead - only when the box is opened is the cat's fate decided.

"If you read accounts of standard quantum mechanics, they have no end of ways of getting round this," Penrose says. "One point of view is that the cat does indeed exist in both states, the 'many worlds' view. You look at it and there is one copy of you seeing the live cat and another copy of you seeing the dead cat. If you are purist about quantum mechanics, then that is the view you would be driven to."

"But in my opinion, superposition does not last forever. It's common sense. If you imagine putting a cat into a superposition of being alive and dead, you do not really believe that it is going to be alive and dead at the same time. I think that is the most improbable scheme of things. So I shall make some rude comments about that in my book, and that shall get me into trouble."

In fact Penrose plans to test whether a system can remain in superposition indefinitely or whether it collapses into one state or another within a fraction of a second. The experiment would take place in space within the next ten years (see box below).

Particle physics and cosmology also come in for criticism from Penrose in his new book. "The standard model of particle physics is all right as far as it goes, but it is not really a fundamental theory," he says. "There are a certain number of parameters that are just put in there, and it is a bit of a mess."

"I have more of a quarrel with inflationary cosmology, which says that the universe expanded rapidly immediately after the Big Bang, then the rate of expansion slowed down. When I first heard of it, I thought: 'Oh, that is a nice idea but you don't really believe that do you?' But it became one of the big ideas of the early universe. To me it doesn't really resolve the problems it set out to resolve. People bring it in because they think it resolves the issue of why the universe is so uniform. But I don't think it does: it is just a mistake. It doesn't really resolve the problem as it depends upon the universe's already being uniform."

The title of Penrose's new book has yet to be agreed: Penrose has called it The Road to Reality. It is to be published by Vintage in September.

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