Stephen Hawking's early career

Stephen Hawking’s earliest work, in the 1960s, coincided with an exciting period in astronomy and cosmology: this was the time when evidence began to emerge for black holes and the Big Bang. Chris Smith spoke with Professor Lord Martin Rees, the Astronomer Royal and a contemporary of Stephen Hawking...

Martin - I knew him from that time. I started two years after him and he was very lucky, as I was, to have a supervisor called Professor Dennis Sciama, and Dennis Sciama had a good feel for what was important. He gave Stephen some good advice which was to go to London to hear lectures by Professor Roger Penrose, who was developing new ideas of understanding black holes. And Stephen took this idea and ran with it, as it were, and his early work was on applying Penrose’s ideas to show that inside a black hole a so-called "singularity" developed where everything would go infinite and was a signal for new physics.

And he also, at that time, got some new ideas about the nature of black holes because he, and others, showed that any black hole that existed in the universe would be described by a very standard equation and this was a very big idea and especially important because this was a time when people were starting to observe evidence for black holes, called quasars, objects which outshone an entire galaxy even though they were not bigger than a star, discovered in 1963, and it was realised later that they probably involved big black holes.

Chris - So people did have an insight into the existence of black holes but they had not really any way of grappling with how they behaved or what their evolution was likely to be? And it took Stephen Hawking to apply the equations of Roger Penrose to then work out how we could grapple cognitively with what these entities might be?

Martin - That’s right. The evidence that they actually existed really came up rather gradually. After 1970 most people believed black holes existed but Stephen was one of those who really told us what black holes were like, and that they were standardised objects. And George Ellis and Stephen Hawking wrote the classic textbook on this subject in the early 1970s.

Chris - You  said that at the centre of a black hole there’s this concept of a singularity, what’s that and why was that such a breakthrough for Stephen Hawking to begin to get grips with?

Martin - To explain why this is important, if you imagine something which is completely spherical and collapses, then no-one's surprised that it goes to a point. But the important result of Penrose and Hawking’s work that was even if something collapses in an irregular way, then once it gets passed a point of no return it will actually form a singularity where things go infinite.

Now, of course that’s just what the theory says and when we have a singularity in physics that just means we have the signal that the physics we have is incomplete and something else comes in. So that was the first indication that places existed in the universe where we would have to modify Einstein’s theory and perhaps bring in quantum theory as well.

Chris - One of the other guests here this week is Andrew Pontzen, who I think famously said on this programme that you have to be very careful with theoretical physics because you can prove anything is right. Is that one of the issues with Stephen’s work in the sense that you could prove on paper that something might be happening, but actually having evidence for it happening and observation is a very different thing, and that was what we had to wait for?

Martin - Well, it’s been harder because we’ve had pretty good evidence that many objects like quasars are powered by gas swirling down into something which is like a black hole. So something with a deep gravity potential. But whether that was exactly the kind of black hole which Einstein's theory predicted, according to the work of Hawking and others, that took a long time and, even now, it’s not completely clear.

There is some evidence that those models do work quite well, but the most important proof that black holes did behave in Einstein's way was only just a couple of years ago when gravitational waves were found. This was a phenomenon where two black holes were spiralling together into one and they shake around and, eventually, settle down into a single black hole, and in that process they emit ripples in space as it were - gravitational waves. These were detected for the first time just two years go and this was a really strong confirmation of Einstein’s theory in a context where it’s very important.

In most of astronomy, Einstein’s theory was just a small correction to Newton’s theory, which is good enough for most purposes. But here we have phenomena that Newton couldn’t explain at all and Einstein’s theory seemed to be borne out and black holes seem to behave in a way that was consistent with what people have discovered partly due to Stephen Hawking’s work.

Chris - What did Stephen Hawking make of the LIGO experiments that detected gravitational waves? Did you talk to him about that; what was his reaction?

Martin - Yes. He was delighted because this was an observation which could, in principle, have refuted one of his key ideas. He has shown that a black hole had as surface area that could never decrease, and if two black holes merged then the black hole that resulted would have to have an area which is bigger than the sum of the first two.

Now that could have been refuted by this experiment if it had found that the merged black hole was radiating at a high frequency and applying a low mass. And it wasn’t, and he was happy that that was the nearest that astronomers had got to actually testing one of his key ideas.

Chris: Thanks very much, Astronomer Royal, Professor Lord Martin Rees.