Gravitational wave discovery ushers in new era in astronomy

“This is beginning a new era in astronomy,” said Ethan Siegel about the publication in February of a paper announcing that scientists had detected gravitational waves. Siegel has taught physics and astronomy at Lewis & Clark College and the University of Portland in Portland, Oregon. He is creator of the science blog Starts With a Bang, and is the author of Beyond the Galaxy: How Humanity Looked Beyond Our Milky Way and Discovered the Entire Universe (World Scientific, 2015). Siegel gave a talk at this month’s meeting of the Rose City Astronomers in Portland about what he calls the discovery of a lifetime.

Ethan Siegel

“This was something, when it was first proposed, that was really taken to be a preposterous consequence of a theory and something that we never really thought we were going to be able to test,” Siegel said. “We have gone in 101 years from pure theory to concrete, direct detection of gravitational waves.”

Einstein’s theory of relativity states that mass and energy bend spacetime, and that’s why objects orbit each other. Relativity explained anomalies in the orbits of planets in our solar system, but Siegel said there is an “extra weird” effect because the orbits decay.

“Another consequence of Einstien’s relativity is that as things spiral in, and it takes a long time to do, but as they do they emit a special type of radiation; they emit radiation that goes through the fabric of space itself,” Siegel said. “This is gravitational radiation.”

It takes way too long for that to happen here in the solar system. For Earth’s orbit to decay completely and merge with the Sun would take 10¹⁵⁰ years, according to Siegel. He said we’ll have to look elsewhere to see the effects happen on human-length time scales.

“You need to find heavy masses; heavier mass in relativity means a stronger effect,” Siegel said. “You need them to have small distances, where small distance is a few kilometers, not a few million miles. And you need them to orbit at fast speeds, where fast is kind of close to the speed of light.”

Luckily these conditions exist. Black holes, neutron stars, and pulsars can do the trick; the gravitational waves detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) last fall were generated by merging black holes. One of those black holes started out at 36 solar masses and the other at 29. After the merger they weighed in at 62 solar masses. That’s simple arithmetic: 36+29=65; what happened to the other three solar masses? Siegel said, oddly enough, this was a prediction by Einstein as well. It’s the flip side of e=mc².

“When these two black holes merged, three solar masses, about five percent of the total mass, was converted into pure energy,” he said. “That energy is the gravitational radiation and is why we here on Earth were able to detect this huge event of two black holes merging from over a billion light years away.”

Siegel is amazed that we were able to figure the mass, spin rate, merging speed, mass loss and other characteristics of these distant objects.

“We learned all of this information from one 20-millisecond signal that moved two laser arms by less than 10^-18 meters,” he marveled. “What I’d say we have now is a whole new way to discover our universe.”


That way is improving rapidly. The LIGO detectors at Hanford, Washington, and Livingston, Louisiana, are being tweaked to even greater sensitivity. New detectors are planned for Italy, Japan, and India. Siegel said the ultimate would be to build three huge LIGO detectors in space, forming an equilateral triangle in Earth’s orbit and having detector arms hundreds of millions of kilometers long.

“If you do that, you can not only watch things merge with supermassive black holes, you can find mergers of ultramassive black holes,” Siegel said. We might even be able to spot gravitational waves from cosmic inflation within the light of the cosmic microwave background. Siegel said if that happens, it would prove that gravity is a quantum force.

“There’s no way to make these fluctuations unless gravity is inherently a quantum force,” he explained. “The process that makes these fluctuations is a quantum process.”

Siegel said it’s a thrilling time to be involved in astronomy.

“This is the first time we’ve seen something astronomical without using a telescope or light of any type,” he said. “This is the dawn of astronomy beyond light-gathering telescopes.”