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Questions for Stefan Ballmer on Discovery of Collision of Neutron Stars and the Origins of Gold
Stefan Ballmer, associate professor of physics in the College of Arts and Sciences, is one of the Syracuse physicists on the LIGO team that has made groundbreaking discoveries on gravitational waves and, now, the collision of two massive neutron stars that confirms the origins of gold and other heavy metals.
Below, he talks about his work and what this latest discovery means for the future study of the universe.
01What was your very first reaction when the team discovered the collision of these stars?
I first learned about it from my graduate student Thomas Vo, who is currently at the LIGO Hanford Observatory as a LIGO Scientific Collaboration fellow. We were carpooling to the site in the morning of this fateful Aug. 17. At first I assumed that this was yet another black hole merger. It took the whole 20-minute drive to the observatory for the news to sink in.
02How does this discovery differ in significance from the historic 2015 discovery of gravitational waves?
The biggest difference is that this source—colliding neutron stars—involved ordinary matter and can also be observed using traditional telescopes: optical, gamma-ray and radio. We always expected to see them eventually. In fact, we promised finding them to our funding agency, the National Science Foundation. But we never dreamt of being able to locate the optical afterglow and corresponding gamma ray burst for our very first neutron star collision.
This combined observation is what makes our discovery so huge: For the first time, we now know exactly what type of explosion happened where and when. We know how much and what type of matter was involved in the collision. We know the exact time delay from the collision to when it was observed in gamma rays. And we have about a thousand follow-up observations of the collision afterglow, pictures and spectra, across multiple bands, spread out over several weeks. This data set will undoubtedly form a Rosetta stone for high energy astrophysics, keeping researchers busy for years to come.
There are other unique features: This is the longest, loudest and closest gravitational wave event ever observed. But who is counting?
03Please walk us through the process of how gold and heavy metals are created in the universe and end up as the jewelry we wear?
Most of the elements we are familiar with that form everyday life—such as carbon, nitrogen and oxygen—are forged in the central furnace of stars. Here lighter elements are fused together into heavier ones, in the process giving off the energy that allows stars to shine. But this only works up to the element iron.
Once a heavy star converted its fuel to iron, its core will collapse, and the star explodes in a supernova. A neutron star is the remnant of such a supernova: an atomic nucleus about the size of the city of Syracuse but with the mass of the sun.
What we saw on Aug. 17 is the collision of two such neutron stars at about one third the speed of light, ejecting a huge amount of nuclear matter. Within minutes of the collision the existing lighter nuclei capture neutrons from this extremely neutron-rich environment. The nuclei start climbing up the periodic table of elements, until they become the elements we use in jewelry: gold and—in the case of my wedding ring—platinum.
04You were instrumental in building the advanced LIGO detectors that made these discoveries possible. Can you give us a thumbnail sketch of how they work?
The gravitational wave causes the distance between two objects at rest—glass mirrors in our case—to fluctuate in the rhythm of the two neutron stars orbiting each other. The LIGO interferometers have two perpendicular four-kilometer-long arms with mirrors at the ends. We use a laser beam to compare the length of the two arms, looking for these telltale fluctuations.
The distance fluctuations are incredibly tiny—about one one-thousandth the size of an atomic nucleus. Of course, we need to be able to measure fluctuations of that size, which by itself is a challenge. But that is not good enough. The much harder problem is making sure that nothing else on our noisy planet moves the mirrors more than a gravitational wave. In fact, most of the construction cost of Advanced LIGO went into seismic isolation systems keeping our mirrors from moving.
05How do you translate your work with LIGO into teachable moments and research opportunities for your students?
We have several research projects at Syracuse University that directly aim at improving the Advanced LIGO detector sensitivity. The two dominant sources of disturbance are due the thermal motion of the mirror surfaces, and the quantum uncertainties of the laser light we use to measure the mirror separation. We are currently building a test facility that should allow us to test new mirror coating materials. And we are collaborating with MIT and Caltech on the installation of new hardware that will allow us to use tricks from the quantum mechanical toolbox to further increase the detector sensitivity.
We also have Syracuse University graduate students present at the observatories, helping with the installation of new hardware in the detectors, and with the operation of the detectors during observation runs. We also started planning the future, designing the detectors we want to build after Advanced LIGO.
06What does this discovery mean for future study of the universe? What do you think the next big discovery in the universe will be?
We are at the very beginning of a new chapter in astrophysics. As we speak, we are further improving the detectors. When we start observing again in about a year, we fully expect to see about one of these collisions a week. Many of them will start to look familiar. But there will be surprises. We might see a stellar collision at the edge of the observable universe—magnified by the gravitational lens of a relatively nearby galaxy cluster. Or something even more exotic.
We just opened our gravitational-wave eyes to a yet unknown part of the universe. There will be work for generations of scientists to come, improving the detectors and understanding the observations made along the way. Maybe the ultimate goal is the observation of gravitational waves from the big bang itself. We know that our current detectors are not sensitive enough for that, but we also know that gravitational waves could provide the clearest picture yet of the beginning of our cosmos.