“The field of gravitational wave astronomy is now real”

February 15, 2016
Two LIGO physicists explain gravitational waves and what it’s like make scientific history.

LIGO project scientists have observed gravitational waves, proving Einstein right 100 years after his theoretical prediction. Countless researchers collaborating from around the world contributed to the project. We asked two of them about the science behind the observation, the roles they played, and what lies ahead for LIGO. Angus Bell works at the Institute for Gravitational Research (IGR), where he is the project manager for upgrades to the LIGO project. He helped develop construction techniques for the final stages of the isolation suspensions used in the instruments used to measure the waves. Amber Stuver is a scientist at the LIGO Livingston Observatory, analyzing the data collected there to separate real gravitational wave signals from noise generated by the environment. She is also an educator at Louisiana State University and the LIGO Science Education Center.

ResearchGate: Could you please explain the recent findings and why they confirm that Einstein was right?

Angus Bell: The objects that caused the ripples we saw are two black holes. These black holes are the remains of massive stars. These two black holes have been circling each other, slowly coming together as they lose energy by giving off gravitational waves. As they get closer and closer, they circle faster and faster and lose energy increasingly quickly. And then, in the final fraction of a second, they are moving at half the speed of light and come together, giving out a final enormous burst of energy in the form of gravitational waves. The two black holes were each 30 times the mass of our sun. Together, they formed a single black hole that is only 57 times the mass of the sun. That missing mass was all radiated away as energy, in the form of gravitational waves, in much less than a second. For that brief period, our event released as much energy as every other star in the known universe combined.

Using Einstein’s equations from his theory of general relativity, we can predict exactly what signal we would expect to see when two black holes collide. When we compared these predictions with the signals from our detector, they matched perfectly. We saw exactly what Einstein’s equations predicted we would see. And even better, we can use the equations to tell us how massive the black holes are, which is how we know that they were thirty times the mass of the sun. And then, from the size of the signal in our detectors, we can tell how far away they are. We can say that these two black holes collided 1.3 billion light years away. This is so far away it means the event actually happened at a time when only simple single cell organisms lived on our planet.
“We saw exactly what Einstein’s equations predicted we would see.”
RG: How did LIGO researchers come to these results?

Amber Stuver: We were first alerted to this signal by our on-line (near real-time) data analyses just 3 minutes after it was collected. These won't find gravitational waves that are just strong enough to be detectable, but they will tell us when there is a signal that is exceptional. Two different kinds of analyses picked up on this signal:

1. A method that uses predicted signals from different kinds of binary sources and compares them to the data. If there is a high correlation, not only do we know there is a potential detection, but we also know an estimate of the system that made the signal based on the predicted signal that matched the data.

2. A method that makes no assumptions about what a signal looks like so that we don't limit ourselves to detecting sources that are well understood. It is expected that this kind of analysis will at least pick up on the end of a gravitational wave signal from a binary system like the method above seeks.

The fact that two very different data analyses found this signal bolstered our confidence that this is a real gravitational wave. Of course, that isn't enough to declare a detection (which took months of validation), but this made all of us sit back and think that this may really be "it."

“The first direct detection of gravitational waves can only encourage theory and thought experiments.”

RG: When you first started researching this topic, did you imagine the discovery would play out as it did?

Bell: When I first started working on the project in 2009, I knew it would be a long time before we saw any signals. But from the beginning of 2015, it was clear that the upgraded detectors were going to be as good as we had predicted they would be. That meant that if the sources were out there, then we would see them. I personally expected that we would see something within a year of turning the detectors on, but there is no way I thought we would see something within a week of operation.

RG: What do you think the relationship between Einstein’s theory and LIGO’s discovery says about the role theoretical approaches and thought experiments have to play in the research world?

Stuver: The first direct detection of gravitational waves can only encourage theory and thought experiments.  After all, relativity was born of a gifted, yet bored, patent clerk thinking about what would it look like if he could ride light. Einstein went on to develop special and general relativity and predict the existence of gravitational waves, starting from this creative thought. The discovery itself challenges our current understanding of stellar evolution, and creative thought will be needed to refine theory to account for what has been seen. Each black hole in this binary system is what remains of an extremely massive star that lived its life and collapsed under its own gravity. But this process is violent—so much so that the first star that collapsed into a black hole should have destroyed its partner star; we should not have seen gravitational waves from black hole pairs with these masses. But we did see them! How can that be? Creative thought experiments will be needed to reconcile theory to match our current observations.

Bell: In some ways, this is the archetypal way that science moves forward. Somebody has a theory to explain an observation and then uses that theory to predict something new. The rest of the scientific community then goes away to look for the new prediction. If they find it, it validates the theory, if they don’t it negates it. In this case, as well as the prediction of gravitational waves, there are also many other things we are testing. The existence of black holes in the mass range observed. Theories of how stars form and evolve actually find it relatively difficult to make black holes of this mass, so it is important for those that study stellar evolution that such black holes exist. The existence of two black holes circling each other is something we thought we might see, but it was not guaranteed that we would. But to go back to Einstein, I don’t think there has ever been a scientist who used thought experiments quite so effectively as he did to progress our knowledge of the world. To predict an effect theoretically that takes 100 years to be realized just shows what an amazing mind he had.

“My entire scientific career has been dedicated to this moment.”

RG: How do you feel personally about this historic discovery?

Bell: I’m super stoked! The gravitational wave community is full of dedicated, talented people, and I have been very lucky that I was able to join it at a time when I could play a small part in building the instruments that detected the first gravitational wave signal. I have so much respect for the work put in by those who have been in this field so much longer than me. The effort required both scientifically and politically to get to where we are now is amazing, and it could not have been done without those who have dedicated their lives to this quest. I salute them!

Stuver: I am awed to be a part of it. My entire scientific career has been dedicated to this moment—although my 17 years is paltry in comparison to many others in our collaboration. I am a part of something that is historic and much bigger than myself. How many scientists ever get to be there for a moment like this? Few! Even though I have known about this detection for some time, it is still hard to believe that the field of gravitational wave astronomy is now real. My children will never know a time when gravitational waves weren't something that we could detect. I will remember this for all of my life!

RG: What’s next for LIGO?

Stuver: LIGO now truly becomes the observatory that the "O" in LIGO stands for. We will continue to observe gravitational waves and expect to see them on a more routine basis especially as Advanced LIGO continues to get closer to its design sensitivity (it was only at 30% of its design sensitivity for the observations we have made so far—but that is also 3x better than our best days with Initial LIGO). I expect to continue to see systems that challenge our current understanding of how the universe works as well as systems we never dreamed of. Pioneering the field of gravitational wave astronomy is next for LIGO!

Featured image courtesy of LIGO Laboratory.

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