Black holes sit at the crossroads of the two most powerful ideas in physics: relativity and quantum theory. Physicist Brian Cox explains why the mysterious giants force us to confront the deepest questions about space, time, and the structure of reality itself.
Cox traces the unlikely history of black hole thought, from the 18th-century notion of “dark stars” to Stephen Hawking’s breakthroughs.
BRIAN COX: Could black holes be the key to a quantum theory of gravity, a deeper theory of how reality, of how space and time works? Well, I think so.
- [Host] What are black holes hiding?
- It's interesting. These objects which have been known, I would say, for 40 or 50 years, but theoretically for the best part of a century, have always been fascinating. They're odd things. The simplest way to describe a black hole would be, a region of space from which even light can't escape. Predictions that such objects existed go all the way back to the beginnings of relativity back at the turn of the 20th century. But actually, really, I would say, into the 1960s, perhaps even into the 1980s, many physicists felt that because of the intellectual challenges that these predicted objects pose, many physicists felt that maybe nature would not create them. I even saw the great physicist Steven Weinberg say that he in some sense hopes that these things would not exist, because they're so confusing. But we now know that they do exist, and so, we have to face the challenges that they pose. Black holes are interesting because going back to the work of Stephen Hawking in the 1970s, it turns out that they demand that we think about 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 a holy grail for theoretical physicists. But the problem has always been, well, is there anywhere in nature that we can look to observe something that requires us to merge those two theories together, and black holes really are the unique place, as far as we can tell, in nature where we can see a thing just sitting there in the sky that demands that we consider those two theories working together to hopefully reveal a deeper theory. 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, an English 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. So the escape velocity is the speed you have to travel to completely escape the gravitational pull of something, a planet or a star. For the Earth, for example, the escape velocity from the surface of the Earth is around eight miles a second, 11 kilometers a second. If you go bigger, you make a bigger, more massive thing, let's go up to a star, for example, like the Sun, then the escape velocity increases because the gravitational pull at the surface increases. And actually for the Sun, it's somewhere in the region of 400 miles a second. It's really fast. 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. There's a wonderful quote actually in Laplace's paper where he says that the largest objects in the universe may go unseen by reason of their magnitude, and this is back in the 1780s or 1790s. So he's imagining stars where the gravitational pull is so vast that even light can't escapee and you couldn't see it, dark stars, I think he referred to them as. Now we know that such objects do not exist in the universe in that sense, in the sense that Mitchell and Laplace meant.
