The enormity of the accomplishment didn’t hit Patrick Brady until a couple days after it happened. The physics professor was walking to work and wondering why he was smiling. “And I thought, ‘Whoa: We measured gravitational waves from a pair of black holes that collided with each other a billion years ago,’” says Brady, director of UWM’s Leonard E. Parker Center for Gravitation, Cosmology and Astrophysics. “How cool is that?”
It was 2015 when Brady, several UWM colleagues and a worldwide consortium of scientists made the stunning breakthrough, one that would win a Nobel Prize. They did it using the Laser Interferometer Gravitational-Wave Observatory (LIGO). In 2019, Brady was elected spokesperson for the LIGO Scientific Collaboration; the spokesperson leads the collaboration.
Gravitational waves were predicted by Einstein 100 years ago, but they’re so small that he didn’t think they’d ever be found. The movement detected was the equivalent of the width of a human hair spotted a galaxy away. Five years later, detections are almost routine, with dozens of other instances documented. It’s a whole new way to explore the universe.
How have gravitational waves helped scientists better understand the universe?
We use them to study invisible parts of the universe, the things we can’t observe in other ways. They’re a great tool, for example, for examining aspects of black holes. The first detection back in 2015 is a fantastic example of this. While we expected to see pairs of black holes crashing into each other, we had never actually seen pairs of black holes crashing into each other, and so this was right away a change – we suddenly had this confirmation that these things happened in the universe.
What has been discovered so far?
The first detection and the data that we took around that time showed us that merging black holes in the universe are more common than we originally thought.
Then, on Aug. 17, 2017, we observed a pair of neutron stars crashing into each other. We saw within just 1.7 seconds that they emitted a flash of gamma rays, which are the most energetic photons in the universe. We were able to tell that this wasn’t just a fluke but that gamma rays and gravitational waves were associated. This had long been speculated, but literally that day, within minutes, we suddenly had solved this long-standing mystery.
In the past year, we observed two black holes crash into each other, the largest pair that we’ve seen do this. And those black holes pose a mystery. For black holes that size – around a hundred times the mass of the sun – how do they form? Do they come from stars, or are they the result of a pair of black holes crashing into each other before? We don’t know the answer, and this is a new mystery that we now have to study.
What else do you hope to learn in the next couple years?
We hope to better pinpoint the locations for some of these collisions of neutron stars, and with the help of other astronomers, to find the galaxy, the flash of light that goes with it, and further improve our understanding of what happens when these objects collide. Also, seeing the collision of a neutron star and a black hole would be very intriguing. If the black hole is small, it can kind of tear the neutron star apart as the two get closer together. It will tell us more about the internal structure of neutron stars – what they’re made of, what they tend to be like.
What would be your dream discovery?
There’s a concept known as a cosmic string. Pretty much any modern theory of particle physics or high-energy physics allows for the existence of cosmic strings. They’re very big, long filaments, very thin, very dense, and they distort spacetime in an unusual way. We’ve never seen direct evidence for them. Such things could generate gravitational waves, and we search for those waves. So far, we have not come up with any evidence, but it would be pretty cool if we did.