Stephen Hawking at 70: Exclusive interview

When he was diagnosed with motor neurone disease aged just 21, Stephen Hawking was only expected to live a few years. He will be 70 this month, and in an exclusive interview with New Scientist he looks back on his life and work

STEPHEN HAWKING is one of the world's greatest physicists, famous for his work on black holes. His condition means that he can now only communicate by twitching his cheek (see "The man who saves Stephen Hawking's voice"). His responses to the questions are followed by our own elaboration of the concepts he describes.

What has been the most exciting development in physics during the course of your career?
COBE's discovery of tiny variations in the temperature of the cosmic microwave background and the subsequent confirmation by WMAP that these are in excellent agreement with the predictions of inflation. The Planck satellite may detect the imprint of the gravitational waves predicted by inflation. This would be quantum gravity written across the sky.

New Scientist writes: The COBE and WMAP satellites measured the cosmic microwave background (CMB), the afterglow of the big bang that pervades all of space. Its temperature is almost completely uniform – a big boost to the theory of inflation, which predicts that the universe underwent a period of breakneck expansion shortly after the big bang that would have evened out its wrinkles.

If inflation did happen, it should have sent ripples through space-time –gravitational waves – that would cause variations in the CMB too subtle to have been spotted so far. The Planck satellite, the European Space Agency's mission to study the CMB even more precisely, could well see them.

Einstein referred to the cosmological constant as his "biggest blunder". What was yours?
I used to think that information was destroyed in black holes. But the AdS/CFT correspondence led me to change my mind. This was my biggest blunder, or at least my biggest blunder in science.

NS: Black holes consume everything, including information, that strays too close. But in 1975, together with the Israeli physicist Jakob Bekenstein, Hawking showed that black holes slowly emit radiation, causing them to evaporate and eventually disappear. So what happens to the information they swallow? Hawking argued for decades that it was destroyed – a major challenge to ideas of continuity, and cause and effect. In 1997, however, theorist Juan Maldacena developed a mathematical shortcut, the "Anti-de-Sitter/conformal field theory correspondence", or AdS/CFT. This links events within a contorted space-time geometry, such as in a black hole, with simpler physics at that space's boundary.

In 2004, Hawking used this to show how a black hole's information leaks back into our universe through quantum-mechanical perturbations at its boundary, or event horizon. The recantation cost Hawking a bet made with fellow theorist John Preskill a decade earlier.

What discovery would do most to revolutionise our understanding of the universe?
The discovery of supersymmetric partners for the known fundamental particles, perhaps at the Large Hadron Collider. This would be strong evidence in favour of M-theory.

NS: The search for supersymmetric particles is a major goal of the LHC at CERN. The standard model of particle physics would be completed by finding the Higgs boson, but has a number of problems that would be solved if all known elementary particles had a heavier "superpartner". Evidence of supersymmetry would support M-theory, the 11-dimensional version of string theory that is the best stab so far at a "theory of everything", uniting gravity with the other forces of nature.

If you were a young physicist just starting out today, what would you study?
I would have a new idea that would open up a new field.

What do you think most about during the day?
Women. They are a complete mystery.