A Nobel for your black hole thoughts

Black holes are in the news, and I could hardly be more excited. As phenomena in our universe go, these are about the strangest. Now we have pulsars and supernovas and fast radio bursts out there, and each of them defies the imagination in different ways. But black holes first fascinated me for this reason: they were theoretically conjectured before we actually managed to detect them.

They are in the news because three scientists have shared this year’s Nobel Prize in Physics for their work on black holes. Roger Penrose gets half for showing that black holes must exist — as Albert Einstein’s general theory of relativity predicted. Andrea Ghez and Reinhard Genzel share the other half for their discovery of a vast black hole not particularly far from us, as celestial distances go: it’s at the centre of our Milky Way galaxy.

But what does it mean to predict that black holes exist?

Or let’s start by wondering, just what is a black hole? I explored that in this space some years ago (“Stop the world and let me off”, 16 February 2012), but with this Nobel news, let’s explore a bit more.

We can think of a black hole as just another celestial object, which is usually OK. But it is not an object in the sense of being tangible, or even visible. It has no “surface” or “boundary” like the Earth, and the Sun do. Consider it, instead, as a certain volume of empty space that has a gravity all its own. I realize that’s hard to imagine, but if you follow the logic of its birth, it makes complete sense.

The thing is, that particular region of space used to contain a more tangible object — a star. The star collapsed on itself, succumbing to its own gravitational attraction. This is a vicious cycle because the star’s contraction strengthened its gravitational power, and that pulled the material in even more. This “gravitational collapse” of the star eventually could not be stopped, and the material making up the star fell below what’s known to black hole theorists as an “absolute event horizon”. And it is this horizon that defines a black hole.

Again, this is not a horizon as you know it, nor a physical surface of any kind. This is a purely mathematical construct that gives us the radius of the black hole. Outside it, it is still possible for objects to evade this once-star’s gravitational tug. Inside that horizon, nothing at all can escape. That’s a black hole.

But let’s take a step back to ask, how and why would a star collapse like this? Take our Sun. It’s now about 4.5 billion years old, and will probably shine for another 3 billion years. We know enough about the evolution of stars to predict what it will do when those years have passed. In its death throes, it will first expand hugely, swallowing up at least its first three planets—yes, that means us on Earth—and perhaps Mars as well. But this “red giant” phase won’t last. Broadly: As its fuel starts running out, it will run out of the energy that counteracts its gravity and keeps it enormous. It contracts dramatically, pulled in by its own gravity. The theory is that it will shrink to become a “white dwarf”—about the size of the Earth. Think of our entire Sun, with all its gases and minerals, now packed into a globe one hundredth its original diameter. Put it another way: this is a body about 500,000 times heavier than the Earth, now packed into an Earth-sized sphere. What if you filled a matchbox with stuff dug from this white dwarf? The stuff is so dense that the little box will weigh several tonnes. You can imagine just how strong gravity would be on this white dwarf.

Why does the contraction stop at this white dwarf stage? Because of a law from quantum mechanics, Pauli’s Exclusion Principle. This tells us that you cannot push electrons closer together than a certain limit, even with the enormous gravity of a white dwarf. When that limit is reached, the star stays as a white dwarf and then slowly cools down and turns into another enormous but dead rock out there.

But as strange as a white dwarf is, it is not a black hole. The great physicist S. Chandrasekhar asked: Is it possible that a star, more massive than the Sun, might collapse on itself so much so that its gravity will overcome even the force that keeps electrons apart, implicit in Pauli’s Exclusion Principle? His answer: Yes. It only needs to be about 1.4 times heavier than the Sun — a number that’s known as the Chandrasekhar limit. A star more massive than this will overwhelm the Pauli Principle and compress till it is only a few kilometres in diameter. Many million times more dense than a white dwarf, this is known as a neutron star.

Still not a black hole, though! Particularly massive neutron stars — a few dozen times heavier than our Sun, for example—are subject to such extreme gravity that they contract still further, to the point where the material in the star falls below that absolute event horizon. Now, none of it can escape. At this point, we have a black hole.

Newton’s theory of gravitation is no longer able to explain this object’s behaviour. To understand, physicists have to use Albert Einstein’s general theory of relativity.

That’s beyond what I can put into this column, but nevertheless, some broad brush strokes. Einstein theorized the idea of spacetime, and how it is like a vast canvas on which sit massive objects like planets and stars. They bend the canvas to different degrees, depending on their mass. Light, he predicted, would follow this curvature of spacetime, and we have actually observed this happening (look up “gravitational lensing”). The more massive the object, the more it “bends” light. And what if we have a black hole? A passing ray of light bends towards it, strays past that event horizon—and then?

Think of it like this: to escape the Earth’s gravity and fly off into space, we would need to accelerate to the Earth’s “escape velocity”: about 11 km per second. Anything slower, and we’d be stuck to our planet (as you and I are). On larger Jupiter, with its far stronger gravity, the escape velocity is about 60 km per second. Well, what if we have an object whose gravitational force is so strong that its escape velocity is greater than the speed of light, 300,000 km per second?

On such an object, not even light can escape. Take a moment to grasp that. That, folks, is a black hole.

In a 1973 lecture, Roger Penrose himself told us about a “blue supergiant” star in the constellation Cygnus, HDE 226868, which is about 30 times more massive than our Sun. What’s special about this star, though, is not its size. Instead, it orbits around a “mysterious companion” that’s half its weight, but only about — get this — 50 km across. That companion, Penrose suggested, is a black hole.

Ghez and Genzel found something similar in the Milky Way. About that, another time. But let me leave you with this thought from Tom McLeish, a professor at the University of York: “Penrose, Genzel and Ghez together showed us that black holes are awe-inspiring, mathematically sublime, and actually exist.”

Spare some awe, today, for that sublimity.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun