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 Key

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.

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