Extreme astrophysics: new Nobel laureate Andrea Ghez's work on supermassive black holes

Originally published on November 7, 2020.

In October 2020, Andrea Ghez received the early morning phone call that any physicist dreams of.

It was the Royal Swedish Academy of Sciences, telling her that she was one of the winners of the 2020 Nobel Prize for Physics, for her discovery that a supermassive black hole resides at the centre of the Milky Way. 

Since that discovery 25 years ago, Ghez has been hard at work trying to understand all she can about the conditions at the heart of our galaxy. She's used the world's largest telescopes to study that extremely massive, monstrous, but invisible black hole — and the stars that swirl around it — to potentially unlock some of the fundamental secrets of the universe itself.

Prof. Andrea Ghez spoke with Quirks & Quarks host Bob McDonald about her prize-winning research, and how she continues to make discoveries about the centre of our galaxy. Here is part of their conversation.

You won the Nobel Prize for discovering a black hole at the centre of our Milky Way. What made you decide to focus your research on that spot? 

I've always been interested in black holes, and I started to be interested in this question of whether or not these really massive black holes, the supermassive black holes, exist. And it turns out that if you want to ask that question, the centre of our galaxy is just the best place to ask it, because it's the closest centre of a galaxy that we'll ever have to study.

What made you believe that there would be one of these supermassive black holes at the centre of the galaxy? 

There are a class of galaxies that show a tremendous amount of activity in their centres called active galactic nuclei. They're exactly what their name suggests, the nuclei or their centres are very active. And these are 10 per cent of all the galaxies. And people had started to suggest that that activity was driven by these supermassive black holes that are dining on a tremendous amount of the material that's around them. 

And that led people to suggest that maybe all galaxies harbour supermassive black holes, and that in other galaxies they're just quiet, or you could say stealth black holes, or black holes on a diet. So that's what led people to think about these lurking black holes. 

How did you go about proving that the black hole is actually there at the centre of our galaxy? 

The key to our work has been developing techniques that allow us to use very large telescopes to get the sharpest images ever of the centre of the galaxy, in which we've been able to discover a set of stars that are close enough to the heart of the galaxy that their motion can reveal the answer of whether or not there's a supermassive black hole. 

So over the last 25 years, we've been able to discover a set of stars and track their motion and to observe the orbits that they trace out. And  the orbits themselves tell you how much mass is inside any star's orbit. And the size of the orbit tells you the region that you can find that mass, too. 

I have my favourite star in the entire galaxy. Its name is S02 and it goes around every six years, and its closest approach is about the size of the solar system. And that tells us very clearly that there's four million times the mass of the sun worth of mass, inside a region that corresponds to the scale of our solar system, and that has increased the evidence for a supermassive black hole by a factor of 10 million, and that has moved the idea of supermassive black holes from a possibility to really a certainty. 

Do you have a sense of what it would be like if you could go there and see it? 

It'd be a very busy place. I like to think of the centre of the galaxy like the crowded urban centre. We're out in the calm suburbs where things move quite slowly. It takes the sun 200 million years to go around the centre of the galaxy. And we're seeing things that are whipping around roughly a decade or two — a human lifetime. 

So at the centre of the galaxy, it's an extreme place. Everything is more extreme at the centre of the galaxy, the density of stars. So, again, kind of like the population and an urban centre, things are moving faster. The magnetic fields are much higher. So it's just an exciting, energetic place. 

How is that helping you understand gravity, and especially Einstein's special theory of relativity? 

Black holes represent the breakdown of our understanding of the laws of physics. They represent the fact that we don't know how to make the laws of gravity, general relativity, which is what Einstein is so famous for, work together with the laws of quantum mechanics, which describe the things that are very small. So black holes, which have lots of mass and are infinitesimally small, force you to reconcile our inability to to make these two areas of physics work together. So in a sense, black holes point you in the direction of an important area or an important frontier to make progress in our fundamental understanding of how gravity works.

Q&A has been edited for length and clarity. Produced and written by Amanda Buckiewicz.