Brian Cox on how black holes could unlock the mysteries of our universe

• Could black holes be the key to a quantum theory of gravity? A deeper theory of how reality, of how space and time works? Black holes are interesting because we have a place called the 'event horizon' where we think that we have full control of the physics. We understand what's happening, but there is a fundamental clash between our two basic theories of nature: both quantum theory and general relativity together. And the quest to unify those two great pillars of 20th- and 21st-century physics into what's often referred to as 'a quantum theory of gravity' is in some sense the holy grail to theoretical physicists.

So black holes are forcing us into a deeper understanding of what space and time are. And in that sense, I think it is fair to say that black holes are the keys to understanding the Universe. I'm Brian Cox; I am a professor of particle physics at the University of Manchester, and the Royal Society Professor for Public Engagement in Science, and amongst other things, I'm the author of the book, "Black Holes: The Key to Understanding the Universe."

The idea of black holes goes back a long way, actually, back into the 1780s and 1790s. There were two physicists, mathematicians, natural philosophers, whatever you want to call them, working at the time that had the same idea, apparently independently of each other. One was a clergyman called Mitchell, and the other was the great French mathematician, Laplace.

And they were both thinking in terms of an idea called 'escape velocity.' The escape velocity is the speed you have to travel to completely escape the gravitational pull of something, a planet or a star. So for the Earth, for example, the escape velocity from the surface of the Earth is around eight miles a second. If you go bigger, you make a bigger, more massive thing; let's go to a star, for example, like the Sun, it's somewhere in the region of 400 miles a second. The escape velocity increases because the gravitational pull at the surface increases.

What Mitchell and Laplace thought, and I think it's a very beautiful idea, is they imagined in their mind's eye, "Well, can you go bigger? Can you imagine more and more massive stars, giant stars such that the gravitational pull is so large at the surface that the escape velocity exceeds the speed of light?" And then you wouldn't be able to see them.

To understand why black holes cause so many conceptual problems, it might be worth just describing very, very briefly what a black hole looks like. So a black hole, what do you see from the outside? Well, there's an event horizon surrounding the black hole. In some sense, it defines the boundary between the external universe and the interior of the black hole. And if you go across the boundary into the interior of this sphere, then even if you can travel as fast as the speed of light, you can't escape the gravitational pull of the black hole.

But another description of the event horizon, which confused people all the way through the history of black hole research, actually, was the idea that the event horizon, when viewed from the outside, is a place in space where time stops. And that's a direct prediction of Einstein's theory of relativity.

From the external perspective, there is a central problem, though, which is still not solved, which is what lies at the center of a black hole? You might think of it as an infinitely dense point to which this massive star collapses. So what are we picturing? It's this thing called the 'singularity.' Actually, it's not right to talk about the center of a black hole really.

Just even in pure general relativity, when you look at a nice map of a black hole, a so-called 'Penrose diagram' named after Roger Penrose, what you see is that space and time is so distorted, that in some sense, their roles swap; the singularity is not really a place in space at all, it's a moment in time, and actually, it's the end of time. But the nature of that thing was not and is still not understood, and for many, many years actually until quite recently, then people thought, "Well, there we are, we have a proble...