Putting Tried-and-True Theories to the Test

Albert Einstein's general theory of relativity is a triumph of science, celebrated by scientists around the world for, among other truths, explaining how gravity works. Whereas Isaac Newton proposed in the late 17th century that gravity is a force tugging on objects, Einstein taught us, in the early 20th century, that gravity is in fact a warping of space and time. The more massive an object, the more it curves space and produces stronger gravity.

Now, many decades later, scientists are using a prediction of Einstein's general theory of relativity—gravitational waves—to study the universe and to even poke holes in the theory itself. Einstein predicted these ripples in spacetime 100 years ago but they were not directly detected until 2015, when the National Science Foundation-funded LIGO (Laser Interferometer Gravitational-wave Observatory) detected the waves from a collision between two black holes.

Assistant Professor of Physics Katerina Chatziioannou, who joined the Caltech faculty in 2020 and is part of the LIGO team, is using gravitational waves to explore these extreme events. She is using the waves to study the space-bending objects themselves, such as black holes and neutron stars, as well as to look for places where our current knowledge of gravity might break down. Any deviations from the tried-and-true theory could lead to new discoveries in physics.

"We know general relativity breaks down at some point because it is not compatible with what we know about the universe at quantum scales," she says. "We just don't know at what point it breaks down or even how this breakdown might appear."

Chatziioannou was born in Greece, where she obtained an undergraduate degree in physics from the University of Athens and then earned a PhD in physics from Montana State University. Before joining Caltech, Chatziioannou held academic positions at the Canadian Institute for Theoretical Astrophysics (CITA) at the University of Toronto and at the Flatiron Institute in New York.

We met with her over Zoom to learn more about her scientific pursuits and pandemic pastimes.

What made you decide to come to Caltech?

I'm from Greece, so I love warm weather. In the past two years or so, I've lived in Montana, Toronto, and New York, and those were all cold places. I don't like the cold; if I have to wear a jacket, I'm not happy! So, I really like the weather here.

But more importantly, there are the science reasons for coming here. Caltech has basically been involved in every aspect of general relativity and gravitational-wave research since the beginning. When it comes to theory, observation, and experiment, general relativity has always been present at Caltech. In fact, my advisor at the University of Athens was a graduate student of Kip Thorne's. [Thorne (BS '62) is the Richard R. Feynman Professor of Theoretical Physics, Emeritus, at Caltech and one of three scientists who won the 2017 Nobel Prize in Physics for their contributions to LIGO].

Were you surprised when LIGO made its first historic detection of gravitational waves?

I was a fifth-year graduate student when the first detection occurred. At that point, I felt sure that it was going to happen. Our advisors kept telling us a detection would happen soon, and they were right! We didn't have enough time to doubt that this would come true. We were the lucky ones, in that we arrived on the scene right when the data came rolling in. I was probably more surprised when we made our first detection of a neutron star collision, which was observed not only in gravitational waves but also in light waves by telescopes around the world. I was leading a discussion at a workshop on neutron stars that same morning we learned of the detection. I knew that half the audience would know about the result and the other half were not supposed to know. We all had to regroup and compose ourselves, because it was very exciting.

Why are neutrons stars a good place to study Einstein's general theory of relativity?

Neutron stars are the most extreme material objects we know of. In fact, they are one of only two types of objects whose structure is strongly affected by general relativity. The other type is black holes. But what makes them even more intriguing is that neutron stars live at the interface of our most advanced theories of gravity and matter. A conversation about neutron stars will routinely involve concepts such as the warping of spacetime around them and the properties of quarks, or fundamental subatomic particles, at their centers. The neutron stars are so dense that they distort the very fabric of space and time in extreme ways that we can measure. When two neutron stars collide, they send gravitational waves traveling all the way to Earth, across millions of light-years, where LIGO can detect them.

It is within these extreme regimes where we do not fully understand the properties of matter. Nor do we understand if and how general relativity might break down. And, while we have come a long way in understanding how matter affects the observed gravitational-wave signals, we also want to understand the fundamental properties of general relativity so that we can pinpoint what observational outcomes might signal that the theory is breaking down.

If the theory broke down, what would that look like?

It is actually not straightforward to predict what happens when general relativity breaks down because we don't have any good examples of this happening, but this is what I find most fascinating about this endeavor. Trying to think about how general relativity could break helps us understand the theory itself and its predictions better.

One possible avenue for finding a break in the theory is to look at its fundamental predictions, such as the polarization of the gravitational-wave signals. Polarization is something that applies to waves: it is a fundamental property related to the geometrical structure of the wave in which the waves are limited to one plane. General relativity makes very precise predictions about the polarization of gravitational-wave signals, such that, when we get the actual measurement, we can look for deviations from the theory. This would help guide us to the correct underlying theory.

What are you expecting to see if LIGO's next observing run? [The run is scheduled to begin at the end of 2022.]

LIGO's sensitivity and the area it can probe will be improved when it turns back on, so we expect to see a lot more collisions between black holes and neutron stars. So far, we've seen about 85 or so collisions between black holes; two collisions between neutron stars; and two collisions between a neutron star and black hole. Only one of the neutron star collisions was seen in both gravitational and light waves, what we call a multi-messenger event.

This time, we will see even more events. At the time we discontinued observing operations in March 2020 due to the pandemic, we were observing an average of about 1.5 collisions between two black holes every week. When the detectors recommence operations, we expect one detection every day or so.

The more events we have, the more likely we will see rare events, such as neutron star mergers that are detected in gravitational waves and light like the one we saw before. We hope to see that again, but it's impossible to predict.

Did you pick up any new hobbies during the pandemic?

I did not bake! I did, however, learn to make normal food. And I really like hiking and biking and being outdoors. Once the parks opened, we spent a lot of time there, too. So far, we are really enjoying the quality of life here in Pasadena.