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Professor Xavier Calmet

Black holes emit radiation that gives clues to the dying star

Physicist Professor Xavier Calmet describes how the ill advice of his father sent him on a journey to black holes  - and a scientific breakthrough.

I had a telescope in the back yard when I was a child. It was enough to dream, to look at the stars and wonder what could be out there. I had a broad interest in science, but I always thought it was so beautiful to watch the stars and to realise that we were so little and there’s so much out there that we don’t understand. 

My father told me physics was dead. He trained as a physicist but became quite depressed by the research limitations at the time and moved into computer science. He said there might be a huge desert between the Standard Model [the theory of particle physics that was formulated in the 1970s to explain electromagnetism, strong and weak forces], and a Grand Unified Theory [which would explain these energies as a single force]. He believed unification physics will never be able to be probed because it involves such massive energies. He said there were more exciting things in other fields and that I might become bored!

When you’re 18 and your dad tells you not to do something, you have a teenage rebellion.  I went to the University of Karlsruhe in Germany, thinking I would become an astronomer. But I became more interested in physics and mathematical modelling, and how to describe the world with equations. 

When I came to Sussex in 2009, I started working more on quantum gravity and ‘black holes’ – a name given as a joke in the 1950s by an American physicist, John Archibald Wheeler to what we believe are stars that have died and collapsed inwards. The idea of them as objects so dense that not even light could escape from them had been around for two centuries before they were rediscovered in Einstein’s theory of General Relativity (GR) in 1915. The fact that not even light can escape from them is why we can’t see them.

In the 1960s black holes were considered to have smooth or ‘bald’ horizons, which was also Wheeler’s joke. But in the 1970s, Stephen Hawking applied the rules of quantum physics. He suggested that black holes evaporate with time and, as they shrink, they emit radiation. According to Hawking, this radiation is featureless and contains no information about the original star. It became known as Hawking’s Paradox as it seemed to pose a problem for quantum physics, which posits that any process in physics can be reversed.  We should be able to reverse the movie and see how Hawking radiation comes back together, recreates the black hole and ultimately the star that gave birth to the black hole. But how is that possible if information is lost and not contained in the radiation? After Hawking’s discovery, it was believed that either GR or quantum physics would need to be modified.  

This year myself and colleagues published two breakthrough papers that showed black holes are not bald and how information is encoded in Hawking radiation after all. They have what we have called ‘quantum hair’, which preserves the memory of the celestial body that collapsed to form the black hole. The mathematical methods needed for these calculations are something I have been working on for about ten years. We have developed methods that enable us to do calculations that merge Einstein’s theory and quantum mechanics. 

We have also proved that any object with gravity will have quantum hair, not just black holes. There is always information that leaks out. We were able to show that we can do these calculations in a controlled manner, which is not something people thought was possible.  We don’t have a full understanding of quantum gravity, but there are mathematical techniques we can use to do these calculations in a model, independent way that does not depend on the details of the correct theory of quantum gravity.

When I got the result, my first thought was I’m crazy. I thought I must have missed something. Smart people have been working on this a long time, and I expected to be crushed. But our work was reviewed, and we gave talks, and there was no serious challenge. 

I feel this is an exciting time in physics, so I am so glad that I didn’t listen to my father. In three years, we have cracked problems that were there for 50 years and really changed the field. Compared to when I was growing up, we now have new and exciting ways of studying the Universe, not just with light and telescopes, but with the discovery of gravitational waves and neutrinos. We have new messengers to look at the Universe.  

My hope is that we are getting closer to understanding gravity at the quantum level, and that, by pushing the theories we currently have, we can understand where they fit and where they don’t. I am hopeful that we will find a complete theory of how all the fundamental forces of nature unify. 

This  profile  is part of our This Sussex Life series.

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