October 2017 was a good month for gravity.
The intellectual leaders of the Laser Interferometer Gravitational-Wave Observatory (LIGO), Rai Weiss of MIT and Kip Thorne and Barry Barish of Caltech, were announced as the winners of the 2017 Nobel Prize in Physics on October 3. This was a surprise to no one. LIGO detected the merger of two big black holes back in September 2015, a long-sought result that capped almost 50 years from the imagination to the execution of this National Science Foundation-sponsored project. That was too late in the year to be nominated for the 2016 Prize, but the LIGO team was a shoo-in for 2017.
Even though the project is a collaboration of 1,000 scientists, the Swedes have their rules, and no more than three can share the prize. As we like to say, it doesn’t really matter so much who gets to dress up in a frock coat and bow to the monarch of a democracy – the real fun is the science itself. And this has been a great month for science.
LIGO has been busy. Joined by the VIRGO device in Italy, the LIGO detectors in Washington State and in Louisiana can combine their measurements of the jiggles in spacetime to figure out where they are coming from. Instead of vaguely gesturing toward a quadrant of the sky, the combined LIGO/VIRGO system can point like an English setter. Look over there!
Analyzing the waves from previous events showed they were the last yelp of merging black holes. These are extremely interesting to astronomers who, frankly, did not expect that there were many pairs of black holes orbiting one another. But you cannot expect to see anything at the site of the collision. It doesn’t help to know where to look if there’s nothing to see. Black hole mergers are very good at making the powerful thunder of gravitational waves, but you don't expect to see the lightning of the merger from those events.
When the stars align, or in this case, collide
For years astronomers had predicted that the most common type of gravitational wave event would be the collision of two neutron stars. Neutron stars are the dense clinkers left over when a star’s core collapses to power a supernova. A typical neutron star has the mass of the Sun, but the size of a city, about 10 miles across. A teaspoonful of neutron star stuff has the mass of a mountain.
Neutron stars are not rare. The Crab Nebula, the shreds of a supernova seen in 1054 AD, has a spinning neutron star in its center. We can find neutron stars all over our galaxy from the flashes of radio emission they emit. What’s more, we know of binary pairs of neutron stars where one is zipping around the other. Even better, because the spinning neutron star is a super-stable clock, we can see that the orbit is shrinking. For decades, astronomers had attributed that to energy sapped out of the orbit by gravitational waves. The prediction was that a pair of neutron stars would whirl into a death spiral, emitting stronger and stronger gravitational waves as they grew closer, finally orbiting a hundred times a second at nearly the speed of light as they smashed into one another. That’s what LIGO was designed to see!
Astronomy progresses by observation, but also by thinking. We have a set of grantees at Caltech, UC Santa Barbara and UC Berkeley who have been thinking about neutron star collisions and other forms of cosmic catastrophes. Starting a decade ago, Dan Kasen and his student Brian Metzger (now on the faculty at Columbia) had the idea that the collision would eject a plume of radioactive neutron-star stuff that could create a burst of light and, as a bonus, manufacture the heavy elements beyond iron. They emphasized the elements of greed: platinum and gold. Personally, I was more interested in the dysprosium and the uranium. Gary Greenburg, science program officer at our foundation, developed the grant to Kasen and his team at Berkeley.
The Berkeley team predicted these events would not be as energetic as a supernova, but they were a thousand times as energetic as ordinary nova explosions. I suppose they considered calling them “pretty good nova,” but settled on the name “kilonova.” In recent months, our grantees have been perfecting their models, making more definite predictions for the color and brightness of the light when two neutron stars smash together.
While a gang of U.S.-based scientists sweated to improve the performance of LIGO and an entourage of European physicists got VIRGO working, the herd of cats of observational astronomers got organized to follow up the promised events. One among them was our grantee at UC Santa Cruz, Ryan Foley. He has access to a modest telescope at Lick Observatory and another small one at Las Campanas, in Chile. Both of these have modern detectors that can take an image of a pretty good chunk of sky, to look for the emission from a new object that might just be the light from a kilonova.
Ryan was at a scientific meeting in Copenhagen, bicycling over to take PTO at the Tivoli Gardens when his cellphone buzzed. It was an alert from LIGO/VIRGO. The setter was pointing! Pointing south, to a place where the one-meter Swope telescope could search for the source of the gravitational waves. Once evening came to Chile, they spotted it on their ninth exposure. They were the first. It’s not so important who was first, but what followed. Once you know where to look, big telescopes could analyze the light at every wavelength across the electromagnetic spectrum.
Long story short, the kilonova story, which could have been proved wrong by these observations, passed every challenge. The gravitational waves showed the August event was the death spiral of two neutron stars, the brightness and color showed they produced a kilonova, and the broad outlines of the story of element production looks right.
The currency in science is not gold, platinum, or bitcoin. It is a refereed paper in a leading journal. There are many papers flooding out from this wonderful event, but a short one that synthesized all the observations was in The Astrophysical Journal Letters. When you look inside all the collaborations, there were 3,500 authors. A journal editor needs to find somebody competent who is not on that list. I do not wish to betray any confidences, but when the archives are opened in 75 years, the constructive comments of a chief program officer at a major philanthropic organization will be revealed.
What’s ahead for LIGO
LIGO has been running for two years. They’ve found one neutron star merger. Good, but to put the O for observatory in LIGO they need to find many of these events. They are not all going to be the same!
Nature's variety is richer than human imagination. The siren-like scream of the merger can be used to measure the expansion rate of the Universe – where today we use supernova explosions. But to play this game at today’s precision, LIGO will need many events. What would it take to push the rate up from roughly one a year to one a month? One answer is “better coatings” on the mirrors inside LIGO’s long arms. The NSF has done a great job in supporting LIGO. It was hundreds of millions of dollars, expended over decades for a risky project that could have failed.
But times are tough for the NSF today – they can keep LIGO running, but they don’t have the resources to do all they would like to make LIGO better. Ernie Glover, science program officer at the foundation, has developed a grant to a Stanford-led team that aims to develop new coatings for LIGO’s mirrors. If they succeed in a timely way, these new mirrors will be incorporated into a future upgrade of LIGO. And if they work as promised, LIGO will have the sensitivity to hear merging neutron stars over eight times the volume they are searching today.
Then we can have once-in-a-lifetime events every month!
Robert Kirshner, Ph.D. is the chief program officer for science at the Gordon and Betty Moore Foundation.
Image: Artist's depiction of a neutron star collision after inspiral. Credit: NASA/ Swift/ Dana Berry