Black holes and LIGO on Bainbridge Island

The Laser Interferometer Gravitational-wave Observatory—LIGO—is leading scientists to discoveries at an impressive clip. Just two years ago we wrote about UW Bothell physics professor Joey Shapiro Key’s talk to the Seattle Astronomical Society about the detection of gravitational waves from the merger of two stellar-mass black holes—a discovery that won the Nobel Prize. Last week at Bainbridge Island Open Mic Science Key talked about LIGO, its latest detections, and plans for even bigger science in the future.

Joey Shapiro Key

Interferometers are a simple idea. They have two perpendicular arms of equal length. Laser light is split into the two arms, hits mirrors at the far ends, and returns to the source. If something changes the length of an arm, the light waves interfere with each other. LIGO in Hanford and a twin observatory in Louisiana are huge observatories with arms four kilometers long, and they are making amazing measurements.

“When we detect the gravitational waves they are quite pristine, even from billions of light years away,” Key explained. “But it was a challenge because gravitational waves interact so weakly with matter—that’s why they’re so pristine when they reach us—they’re very hard to detect.”

How hard? Einstein, who thought up the notion of gravitational waves and did the math to explain how they would work, thought the effect was too small to ever detect. It took a century to develop the technology to do it. LIGO can detect unbelievably minute changes in the length of its arms when a wave passes through.

“This is the most sensitive measuring device in the world,” Key said of LIGO. “For those four-kilometer arms, the change in the length in the arms we measure is a thousand times smaller than the width of a proton in the center of an atom.”


Simulation by SXS

The big discovery by LIGO since Key’s previous talk came in August of 2017.

“We detected a gravitational-wave signal from two neutron stars colliding, followed immediately by a detection of a gamma-ray burst by NASA’s Fermi satellite, and this set off a worldwide search for the source of that gravitational wave signal,” Key said. More than half a dozen observatories were involved in the work, observing the event in many wavelengths across the electromagnetic spectrum and pinning down the galaxy in which the collision occurred.

“This is the first ever multi-messenger detection with gravitational waves where we’re doing observations using gravitational waves and light,” Key said. Being able to see light from the event taught us a lot.

“We really learned from this one in particular that most of the heavy elements in our universe, including what solar systems are made of, what planets are made of, and what we are made of, comes from neutron stars colliding and kilonova events,” Key noted.

Just as light has a wide range of wavelengths, so do gravitational waves. Key said LIGO can only detect a limited slice of those wavelengths. It would be not able to find gravitational waves from the collisions of supermassive black holes or from the early universe. That will take a different tool.

“The future of gravitational wave astronomy lies in experiments such as LISA, the Laser Interferometer Space Antenna, that will do laser interferometry in space,” Key said. LISA is a joint venture between NASA and the European Space Agency, but there will be a bit of a wait for it. LISA’s planned launch isn’t until 2034. In the meantime, LIGO has plenty to do, with planned upgrades that will make the detector even more sensitive.

“We really are in a brand new era of gravitational wave astronomy, and there’s a lot to be discovered,” Key said.

LIGO and the era of multimessenger astronomy

Multimessenger astronomy is a fairly new buzz word in the science. Dr. Joey Key, an assistant professor of physics at the University of Washington Bothell and a member of the LIGO Scientific Collaboration, talked about the concept at last month’s meeting of the Seattle Astronomical Society.

Dr. Joey Key of the UW Bothell gave a talk about LIGO and
the era of multimessenger astronomy at the Dec. 21 meeting
of the Seattle Astronomical Society. Key made the same
presentation to the Everett Astronomical Society Jan. 7.
Photo: Greg Scheiderer.

As you probably know LIGO—the Laser Interferometer Gravitational-wave Observatory—made the first detection of gravitational waves, as predicted by Einstein’s theory of general relativity, back in December 2015 and announced the findings last February. So now what?

“The next big goal for LIGO is to have a gravitational wave detection where we also get an electromagnetic signal from the same source,” Key explained. She noted that various wavelengths of light, from gamma ray to radio, require different types of tools to detect and reveal different things about objects observed. Key said gravitational-wave astronomers refer to such science as “electromagnetic astronomy.” The big hope, then, is to learn even more if there can be an electromagnetic observation as well as a gravitational wave observation of the same event.

“That’s what we would call a multimessgenger source,” Key said.

A difficult search

Einstein never thought gravitational waves could be detected because he figured they would be too small. It took a century of technological advances to prove him right—again. Finding a multimessenger source may be an even more elusive needle in the cosmic haystack.

Key explained that, right now, it’s hard for LIGO to detect with precision from whence a source signal originates. When they detect a source they send an alert to about 60 electromagnetic astronomy partners and give them a general direction in which to look. In addition to the challenge of pinpointing the source, they also don’t really know what to look for. Key said their models aren’t very good, not yet anyway. Light from a source may have already passed, but there could be x-rays, gamma rays, afterglow, or shock waves under certain conditions.

Fortunately, LIGO is getting better. The addition of more Earth-based observatories will help better locate sources and discover collisions of neutron stars or stellar-mass black holes. Project LISA, scheduled to launch in 2029, will look for supermassive black hole collisions and “extreme mass ratio in-spirals” which occur when a little star or black hole falls into a big black hole. Pulsar timing arrays could detect when supermassive black holes collide in galaxy mergers. There’s even study of the cosmic microwave background to try to detect gravitational waves from early universe.

“Just like electromagnetic astronomy, different sources are detected by these different kinds of experiments,” Key said. “We need all these different kinds of gravitational-wave experiments to be able to study the gravitational-wave sky.”

The LIGO Scientific Collaboration includes more than a thousand scientists from 15 countries and 90 institutions. Four of the institutions are in Washington: The University of Washington, UW Bothell, Whitman College, and Bellevue College.

Unknown discoveries ahead

Key said it is an interesting time to be involved in the field as LIGO is just into its second observing run.

“We’re really going to be able to map out and explore the population of black holes in our universe,” Key said.

“We don’t know what we’ll discover, and that is always the story of a new astronomy,” she added. ”We do not know very much about black holes in general, and so this is a new way to study the universe and study what is out there. It will be very exciting!”

LIGO could discover new kinds of sources like cosmic strings, study supernovae, and maybe even lead to the detection of dark matter and dark energy.

“We are lucky we live in the era of gravitational-wave astronomy, and we hope soon that it will be the era of multimessenger astronomy,” Key concluded.