How can the ripples from a collision of two black holes help us understand the universe? For our XxXX interview series with ten leading female scientists, Nergis Mavalvala, Professor of Physics at the Massachusetts Institute of Technology, talked to us about imploding stars, Einstein's gravitational waves, and the building blocks of our galaxy.

What do you do?

I am a physicist working on detecting gravitational waves, which are waves in the fabric of spacetime itself. They were predicted by Albert Einstein in his general theory of relativity a hundred years ago, but it's only in the past year that we've been able to observe them.

What's so special about gravitational waves?

What makes gravitational waves amazing is that they are emitted by objects that are inherently dark, like black holes. We've never seen a black hole, because our telescopes are designed to observe light. We've never seen two black holes collide. But by measuring the waves they emit in a collision, we can observe black holes and understand their life cycle.

An illustration of two black holes colliding, with gravitational waves rippling outward
Image: LIGO/T. Pyle

When stars collapse, they turn into neutron stars or black holes. They run out of the nuclear fuel that makes them shine, and they implode due to their own gravitational pull. Our own sun will do that at some point. These neutron stars or black holes have a huge amount of gravity, they are very dense. Now imagine two black holes coming closer, orbiting each other and ending that dance in a big collision. They emit waves that we can measure with our detector, LIGO, which is short for Laser Interferometer Gravitational Wave Observatory.

When did you first observe a collision of two black holes?

In September 2015, our detector in Louisiana measured a strong signal that indicated a passing gravitational wave, and 7 milliseconds later our second detector in the state of Washington showed the same signal as the wave passed through it. I remember seeing the signal and getting goosebumps. I've been working on gravitational waves for 25 years, and when I first heard about their possible detection as a grad student, it sounded completely insane - and completely captivating.

The signal came from two black holes orbiting each other. Their respective mass was about 30 times the mass of our sun, but they measured only 150 kilometres across, which is small by astrophysical standards. For comparison, our sun has a radius of 700,000 kilometres. Now imagine these extremely massive black holes whipping around each other, almost at the speed of light. When they collided, they released more energy than all the shining stars in the universe emit. That energy was put out in the form of gravitational waves. We saw all of that in the data, and we also saw that a new black hole had formed as a result of the collision.

The black holes were 1.3 billion lightyears away, so we observed something that happened 1.3 billion years ago.

Are black holes dangerous?

Only if you get too close! The ones we observed are a billion lightyears away. They don't pose any danger to our solar system - there are lots of other things we should worry about more, most of all human behaviour and what we are doing to our planet. The probability of an asteroid hitting the earth, for example, is really small compared to the damage we can do by ourselves.

So why do we need to understand black holes?

Black holes are important building blocks of our universe. We are learning that at the centre of every galaxy, there is a supermassive black hole with a million or even a billion times the mass of our sun. We don't really know much about black holes, but we live in a galaxy and we must try to understand it.

In fact, most of the elements of the periodic table were formed in the explosions of stars, the same event that gives birth to a black hole. All of these things are related, and understanding them is part of the big questions: What are we made of? Where do we come from?

What has been the biggest challenge in your research?

We've faced three major challenges. One was building detectors that could make the very precise measurements we need. Gravitational waves cause spacetime itself to stretch and shrink. Even as these waves pass through you and me, which they do all the time, they cause us to stretch and shrink, but only by a tiny amount.

Our LIGO detector is L-shaped, and each side of the L is 4 kilometres long. That's big enough for our technology to - just barely - allow us to make the measurements. On that scale, the change caused by the waves is one thousand times smaller than the nucleus of an atom. So there's the beauty of seeing two black holes collide for the first time, but there's also the beauty of seeing the precision of the measurements.

Another challenge was solving the underlying theory, and understanding what the signal should even look like. The third challenge was extracting the signals from very noisy data. Some of the signals were clear and strong, but others were weak, and we wanted to see them all.

What are your next goals?

Building ever better detectors, that's the direction we're going in - using better technology to get more sensitive measurements. There is this whole universe out there, waiting to be observed.

This fall, we are going to restart our detectors with improved sensitivity and run for six months. By our estimates, we can expect to observe five more black hole collisions during that time.

As a female astrophysicist, you entered a field dominated by men. You also grew up as part of a religious minority, the Parsi community in Pakistan. From your experience, how do you think we could attract more women and minorities to the STEM fields?

One thing that shaped me was that I was an outsider in every milieu I moved in. So I was always comfortable being different, and that informed my ability to thrive on the fringes of the mainstream. When I walked into a physics lecture hall as one of three women among a hundred men, I thought nothing of it, because I was used to being the Other. I then worked in this experimental field that for a long time was considered pretty maverick.

In terms of attracting more women, the biggest difference would be to increase the number of women doing science, but that's easily said. How do we make this happen? Perhaps every scientist should mentor young people and show them that this is fun and exciting, and that they would enjoy doing it.

Interview by Sophie Hardach.

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