Q&A with Andrea Ghez, who discovered the black hole Sagittarius A*

This week, for the first time ever, the world got a glimpse of Sagittarius A*, the supermassive black hole at the center of our galaxy. The image of a fuzzy golden ring of superheated gas and diffracted light was captured by the Event Horizon Telescope, a network of eight radio observatories scattered around the globe.

Feryal Özel, a University of Arizona astronomer and founding member of the EHT Consortium, said seeing the image of the black hole is like finally meeting a person in real life who you’ve only interacted with online.

For Andrea Ghez, an astrophysicist at UCLA, perhaps the encounter was more like a biographer meeting her subject after decades of pursuit.

In 2020, Ghez was awarded the Nobel Prize in Physics for her role in discovering a supermassive object at the core of the Milky Way. This object is now known as Sagittarius A*, or Sgr A* for short.

Ghez studies the center of our galaxy and the orbits of thousands of stars orbiting the dense object at its heart. Though she wasn’t involved with the EHT project, she said its “impressive” achievements — including the 2019 revelation of the black hole anchoring a distant galaxy called Messier 87 — offer intriguing new possibilities for exploring the cosmos.

The Los Angeles Times spoke to her about black holes, cosmic surprises and what Einstein has to do with the GPS app on the phone. The interview has been edited for length and clarity.

How does it feel to finally see what you’ve studied throughout your career?

It’s super exciting. We live in a really interesting time, with technology advancing so rapidly in so many areas, giving us new insights into these incredibly exotic objects.

Does it look different than expected?

Not really. It’s remarkably similar. You should see a ring that has a radius about two and a half times the Schwarzschild radius [the radius of the event horizon, the boundary around a black hole beyond which no light or matter can escape]. That’s predicting where gravity should bend, and that’s where you see it. This is impressive.

A blurred image of a glowing ring.

This is the first image of Sagittarius A*, the supermassive black hole at the center of our galaxy.

(EHT collaboration)

How much have the technological possibilities for researchers changed since you started studying black holes?

Huge, huge progress. I often say that we ride a wave of technological development. Everything we do can truly be described as technology-enabled discovery.

One of the things I love about working in these fields, where technology is advancing very quickly, is that you have the opportunity to see the universe in a way that you couldn’t see before. And so often this reveals unexpected discoveries.

We’re really fortunate to live in an age where technology is advancing so rapidly that you can really rewrite the textbooks. The Event Horizon Telescope is a similar story.

What unanswered questions about the universe excite you the most?

I have a few favorites at the moment. What I’m really looking forward to is our ability to test how gravity works near the supermassive black hole, using stellar orbits, and also as a probe for dark matter at the center of the galaxy. Both should be reflected in the orbits.

Andrea Ghez, professor of physics and astronomy at UCLA.

UCLA astronomer Andrea Ghez has won the 2020 Nobel Prize in Physics for her work on black holes.

(Aron Ranen/Associated Press)

A simple way I think of it is: first time, these orbits tell you the shape. And after that, you can ask more detailed questions, because somehow you know where the star is in space.

For example, S0-2 (my favorite star in the galaxy and probably the universe) rotates around every 16 years. Now we’re at the second passage, and that gives us an opportunity to test Einstein’s theories in a different way than what the Event Horizon Telescope is studying, as well as to narrow down the amount of dark matter you might expect to find at the center of the galaxy. There are things we don’t understand about the early results, and for me that’s always the most exciting part of a measurement – when things don’t make sense.

How do you deal with these moments?

You must have complete integrity with your process. Things can’t make sense because you make a mistake, which is the uninteresting result, or they don’t make sense because there’s something new to discover. That moment when you are not sure is super interesting and exciting.

We’ve just discovered these objects at the center of the galaxy that appear to expand as they get closer to the black hole and then become more compact. They are called tidal interactions. If you think of the movie Interstellar with that big huge tidal wave, it would be like a big tidal wave just taking off the planet. If we see stars with such interactions, that means the star must be, I don’t know, a hundred times bigger than anything we’ve predicted in that region. That makes you scratch your head.

Yes. Absolutely. Black holes represent the breakdown in our understanding of how gravity works. We don’t know how to make gravity and quantum mechanics work together. And you have to work those two things together to explain what a black hole is, because a black hole is strong gravity plus an infinitesimally small object.

wait what I thought black holes were huge.

no The image shows the phenomena happening around the black hole. The black hole doesn’t have a finite size, but it does exist abstract Size of the event horizon, which is the last point for light to escape. And then the gravitational interaction with local light is concentrated in this ring, which is two and a half times larger than the event horizon.

However, we know that black holes represent the collapse of our knowledge. So everyone there keeps testing Einstein’s ideas about gravity, because at some point you expect to see what you might call the expanded version of gravity, just as Einstein was the expanded version of Newton’s version.

It’s fair to say that Newton’s laws explain quite well how gravity works here on our small planet, but we need Einstein if we’re going to venture out into the universe?

Yes, except for what we take for granted today: our cell phones. The fact that we’re so good at finding each other on Google or Waze or your favorite traffic app is because GPS systems position your phone in relation to satellites orbiting the earth. These systems must use Einstein’s version of gravity. So yes. We could use Newton until such things matter to us. Q&A with Andrea Ghez, who discovered the black hole Sagittarius A*

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