Martin Rees: Our changing view of black holes

Chris Smith asked Martin Rees to explain how the complexity of black holes was tamed by scientists...

Chris - Can you give us a flavour for how the space science domain has evolved over the course of the time when you finished your PhD to where we are today. What have been the really big changes and what have been the waves that you've most enjoyed surfing?

Martin - Well, of course, space science only really got started in the 1960s, looking at x-rays from cosmic objects. X-rays come from especially energetic objects and so they were able to highlight in our view of the sky the objects which are especially exciting. It started off with sounding rockets being sent, so they got a few minutes' observation before they came crashing down again, but then the first satellites were launched in about 1970 to study the sky in x-rays, and they found the first evidence for black holes orbiting stars. I had a lot of contact with Ken Pounds, who was then a young lecturer at Leicester, who managed through his energy and initiative to make Leicester a major international centre for space science. His group were important in x-ray astronomy. That was something that I was very interested in.

Chris - At that time, were black holes something that people had thought about but they hadn't got any demonstrable evidence for? They were a theory, but you actually have now seen over your career them manifest for real.

Martin - That's right. The theory was worked out - some bits were done very early on. Some bits incidentally were done by Oppenheimer in 1939 before he was otherwise distracted. A lot was done in the 1960s under the inspiration of Roger Penrose bringing new mathematics into it. But on the observational side, I think when these objects appeared in surveys of the sky, which couldn't be ordinary stars, they were too bright, varying too rapidly, etc., I think, from the mid to late sixties, everyone speculated that they could be black holes, but the evidence was rather indirect. Some of my early work was in trying to work out how gas swirling down into a black hole could produce x-rays and things varying rapidly and this could explain some of these things. But it wasn't until the late 70s, 10-15 years later, that there was a general agreement that what was causing the big energy output from the centre of some galaxies was flowing around a black hole. It gradually emerged.

While I've got one of the foremost experts on this sitting next to me, can you explain to me the differences between the massive black holes we see at the centres of galaxies and the little ones we see doing collisions that we can pick up with our gravitational wave detectors and so on. What's the difference between them apart from size? And what's their origin?

One feature of black holes is that they are scalable. In fact, a black hole of a few times the mass of the sun and a black hole a few million times the mass of the sun, like the ones in some galaxies, are exactly the same in their properties - it's just a simple scaling factor. Of course some stars, in their lives collapsing and leaving black holes behind, those are the ones which we see in binary star systems in our galaxy. And incidentally, those are the ones that give rise to the gravitational waves that have been detected to black holes weighing between 10 and 50 times as much of the sun crashing together. But there is a separate category of supermassive black holes which form somehow by an agglomeration of mass in the centre of a galaxy. It's still debated whether the mass accumulates by just gas falling in or do lots of stars collide and fall together. All those things play some part. All we know is that there are, in the sense of galaxies, black holes where the biggest ones are several billion times the mass of the sun. In our galaxy's centre, there's one which is about 4 million times the mass of the sun. That's small by the standards of these massive black holes, but of course much bigger than any star. And in fact, Don Lyndon Bell, who was my senior colleague in Cambridge, and I wrote a paper back in 1971 first arguing that there could be a massive black hole in our galactic centre, and that of course has been firmed up gradually over the years.