Tag Archives: LIGO

LIGO founder Rainer Weiss talks gravitational waves at UW

There has been a great deal of talk about gravitational waves since scientists with the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced back in February that they had collected the first evidence of the phenomenon in September 2015. Dr. Rainer Weiss, professor emeritus of physics at MIT and one of the founders of LIGO, talked about the history, discovery, and future of LIGO Tuesday at the University of Washington. The event was part of the Frontiers of Physics lecture series of the University’s College of Arts and Sciences.

Was LIGO really the first?

Rainer Weiss at UW

Dr. Rainer Weiss, a co-founder of LIGO, gave a lecture this week at the University of Washington about the detection of gravitational waves. The logos represent the more than 80 organizations involved in the LIGO Scientific Collaboration. Photo: Greg Scheiderer.

Weiss said that it might not be totally accurate to say that LIGO was the first to spot gravitational waves. Joseph Weber at the University of Maryland claimed to have detected them way back in 1969, but no other scientists could duplicate his observation, and the claim was eventually discredited. Weiss said much credit should go to Russell Hulse and Joseph Taylor, Jr., of the University of Massachusetts. They used a radio telescope to study what is now called the Hulse–Taylor binary, and noticed that the orbits of these two neutron stars around each other have decayed since they were discovered in 1973. A graph of the decay matches up precisely with a plot of the loss of energy predicted due to gravitational waves.

“It’s a dead ringer,” Weiss said. “That was, as far as I’m concerned, the first real evidence that there were gravitational waves. It was a very important moment, because there had been endless discussions in the scientific community about whether the gravitational waves that Einstein had predicted were real or not.”

In a way the detection of gravitational waves is like the story of an “overnight sensation” who hits the big-time after decades toiling in obscurity. The first glimmerings of LIGO go back more than 40 years, and the basic design of the observatory was actually created well before Einstein dreamed up gravitational waves as part of the general theory of relativity.

The beginnings of LIGO

Back in 1967 MIT asked Weiss to teach a course about relativity. He didn’t tell them that he wasn’t really up on the math of relativity, and joked that it was all he could do to keep a day ahead of his students. Weber was doing his experiments at the time, and Weiss had his class do a thought experiment—what Einstein would call a Gendankenexperiment—about how to detect gravitational waves using light beams. Their solution was essentially a Michaelson Interferometer, a device developed in 1880s. (An animated view of a simple interferometer is below; also check our recent post about LIGO from an Astronomy on Tap Seattle event.) A few years later, after the Weber findings were dismissed, Weiss started to think about the detection of gravitational waves a little more seriously.

“I wanted to convert that Gedankenexperiment into a real apparatus,” he said.

An animation of how LIGO works. A laser beam is directed through a splitter into two
equal-length arms, and reflected back. If the length remains the same, the reflected beams
cancel each other out. But if a gravitational wave distorts the beams, they do not cancel and
light reaches a detector. Image credit: LIGO/T. Pyle.

This was easier said than done. As noted, many in the scientific community doubted that gravitational waves existed, and even Einstein had expressed doubt that they could ever be detected. This made getting funding for the work a challenge. The technical obstacles were greater still. The device had to detect preposterously small distortions in spacetime—along the order of a thousandth of the width of a proton—and it had to do so in an environment in which there is a tremendous amount of noise. The Earth itself is spinning and vibrating, ocean waves lap up on the shore, a train goes by. They had to figure out a way to get the interferometer mirrors to hold still. That problem was solved by suspending the mirrors from multiple pendula, which themselves hang from a noise-reducing feedback system. Even a little heat or a molecule of oxygen in the interferometer tube could distort the light beam.

“The way you get rid of it: you make a very good vacuum, and that costs a lot of money,” Weiss noted. They also added mirrors to the basic design that make the light path longer and keep more light in the system, both ways to amp up the sensitivity of the instrument.

It’s no wonder this “overnight” discovery was more than 40 years in the making, and didn’t happen until a century after Einstein first proposed gravitational waves. Weiss spent a lot of time recognizing the many scientists who contributed to LIGO over the years, and noted that today the LIGO Scientific Collaboration includes more than one thousand people from 83 different organizations.

More discovery to come

The future of gravitational wave astronomy is fascinating, according to Weiss. With the VIRGO interferometer in Italy and LIGO-India (INDIGO) joining the LIGO facilities at Hanford, Washington and Livingston, Louisiana, scientists will be able to triangulate to get a better idea about where detected gravitational waves originate. The eLISA mission of the European Space Agency would be a huge interferometer in space that could possibly spot gravitational waves with longer lengths, created by such events as the mergers of supermassive black holes. The LISA Pathfinder mission successfully tested some of the technology earlier this year, and the ESA just this week put out a call for concepts for the next phase of the project. Most interesting is the possibility to detect gravitational waves from almost the instant of the Big Bang, which could be spotted as density variations in the cosmic microwave background.

“I fully expect that if there are gravitational waves that come from inflation, in the next ten years they’ll be found,” Weiss predicted.

A full house at the UW enjoyed the engaging lecture by Weiss.

Learning about LIGO at Astronomy on Tap

The most recent gathering of Astronomy on Tap Seattle brought to town two scientists working in one of the most groundbreaking areas of astronomy: detection of gravitational waves.

Nature was kind to us

Jeff Kissel, a control systems engineer at the LIGO Hanford Observatory, talked about how exciting it was when they switched on advanced LIGO back in September 2015.

“Boom! Right out of the gate we saw this whopper of an event,” Kissel said, detecting gravitational waves from the merger of a pair of stellar-mass black holes. “Nature was very kind to us.”

What they spotted at Hanford and at LIGO in Livingston, Louisiana was a match.

“Inside our data, which is almost always noise, we saw this very characteristic wave form that was predicted by general relativity,” Kissel recalled. They found gravitational waves from a couple of other black-hole mergers in the following months.

“This is the beginning of gravitational wave astronomy,” Kissel said.

Gravitational waves oscillate through spacetime in a way
demonstrated
by this animation. Credit: ESA–C.Carreau

Kissel pointed out that LIGO only detects a small part of the gravitational wave spectrum. As with light, gravitational waves can come in a wide range of wavelengths with periods ranging from milliseconds to billions of years. Longer-length waves might come from the mergers of galactic nuclei, or even from quantum fluctuations from the early universe.

“There’s a whole zoo of things to find out there,” Kissel said. He anticipates more ground-based observatories as well as some space LIGOs that could have detector arms millions of kilometers long.

How LIGO works

LIGO sounds awfully complicated, but, broken down, the idea is pretty simple. Jenne Driggers
is a Caltech postdoctoral scholar stationed at the LIGO Hanford Observatory, where her gig is improving the sensitivity of the interferometers. Driggers explained that, essentially, they shoot a laser beam into a splitter that sends beams down two equal arms four kilometers long. The beams reflect from mirrors and return to be put back together.

A simplified look at how LIGO works. A laser beam is split and sent down two equal
arms four kilometers long, then reflected back by mirrors. When they return to be
recombined, they will usually cancel each other out and no light will get to the detector.
But if a gravitational wave distorts the system, the light will be spotted by the detector.
Credit: T. Pyle, Caltech/MIT/LIGO Lab

“When they recombine they can be exactly out of phase, and then there’s no laser light (at the detector),” Driggers said. “They cancel each other out totally. Or the lengths will change and these two electromagnetic waves can add up, and so we do get some light.”

When that happens it means that a gravitational wave has distorted the LIGO arms ever so slightly. They measure the light received at the detector to learn more about the wave.

In practice it’s a lot more complicated. It all happens in a total vacuum to avoid any distortion from air. The mirrors are suspended from a system of four pendulums, which helps to eliminate vibration. The mirrors are highly reflective pieces that each weigh around 100 pounds and cost half a million dollars. The laser is about the best there is.

“The laser wavelength itself is our ruler that we’re using to measure the distance between those two mirrors,” Driggers said, “and we need to be able to measure that distance to 10-19 meters.”

“This is one of the highest-power, frequency stable, power-stable lasers on the planet,” she added.

Driggers invited people to tour LIGO Hanford. Public tours are held twice each month, and groups of 15 or more can arrange for a private tour.

Up next: LSST

Astronomy on Tap Seattle is presented and organized by astronomy graduates students at the University of Washington. Their next event is planned for Friday, October 28 at Peddler Brewing Company in Ballard and will feature UW scientists Dr. John Parejko and Dr. David Reiss, who are working on the Large Synoptic Survey Telescope project. The events are free. Enjoy beer and astronomy!

Predicting some big astronomical kabooms

X-ray binaries are out in the universe making gravitational waves, and Breanna Binder says we may well be on the verge of being able to detect such waves generated in distant star systems. Binder, a recent University of Washington astronomy Ph.D. who did her dissertation about the evolution of X-ray binary systems, gave a talk on the subject at the August meeting of the Seattle Astronomical Society.

Dr. Brianna Binder gave a talk about X-ray binary systems at the August meeting of the Seattle Astronomical Society. Photo: Greg Scheiderer.

Dr. Breanna Binder gave a talk about X-ray binary systems at the August meeting of the Seattle Astronomical Society. Photo: Greg Scheiderer.

Binder noted that it’s a bit of a longshot for an X-ray binary system to form. They start out as a pair of stars ten times or more massive than our own Sun.

“Almost all massive stars are born in binary systems,” she said. “Not only that, massive stars are more likely to be born with massive companions.”

However, these massive stars live relatively short lives and ultimately explode in supernovae. The more massive the star, the more rapidly it evolves, and so the larger of two massive stars in a binary system will be the first to expand into a blue giant. The more it expands, the weaker its gravitational pull on its outer atmosphere will be, enabling the smaller companion to steal some of its mass.

Eventually the larger of the pair goes supernova and leaves behind a compact object: either a neutron star or a black hole. This is often the end of the binary system, as only about one in 10,000 pairs remain gravitationally bound after the supernova. If they do stick together, that’s when the fireworks really get going. The sibling star, having siphoned off some of its companion’s mass, also begins to grow into a blue giant.

“As this happens, material flows from the giant star onto the compact object,” Binder explained, “and when this happens the system starts to heat up. All that material funneling onto the compact object gets incredibly hot and begins to glow in X-rays.”

These are easy for us to spot from Earth.

“These objects will emit X-rays at levels that are tens of thousands to millions of times above what a normal star like our Sun does,” Binder noted.

This high-mass X-ray binary phase doesn’t last long in astronomical terms, perhaps just 10,000 years or so. Eventually the second star goes supernova.

“If the system survives the second supernova explosion, which is a big if, you end up with two compact objects in orbit around each other,” Binder explained. While two neutron stars is the most likely formation, it can also be two black holes or one of each, she said.

With two neutron stars in a system they spiral rapidly around each other, creating powerful gravitational waves. Eventually the two objects merge, creating a big explosion that we can see as a gamma-ray burst. This is the aftermath of the merger of two neutron stars, and it’s also where the new science comes in.

“In the very near future, we’re hoping to be able to detect neutron stars in the process of spiraling into each other before the gamma-ray burst occurs,” Binder said. We will do that by actually detecting gravitational waves using LIGO—the Laser Interferometer Gravitational-Wave Observatory.

The challenge with LIGO is that there’s a lot of noise out there. Anything that moves through space generates gravitational waves. In its first runs LIGO in Richland was able to detect motion from ocean waves breaking on the Washington coast. So scientists have been busy modeling and tweaking, and expect to make the first science runs of a new version of LIGO some time this fall.

“If we’re going to detect gravitational waves, it’s going to happen as soon as we bring advanced LIGO on,” Binder said. “It could easily be within the next year that we are able for the first time to directly detect gravitational waves from the source.” That will give us some early warning about where to look to spot future gamma-ray bursts.

Ultimately the study of these systems will help us better understand stellar formation and evolution.