The Royal Swedish Academy of Sciences awarded its 2017 Nobel Prize in Physics to Drs. Rainer Weiss, Barry Barish and Kip Thorne, researchers who made decisive contributions to the Laser Interferometer Gravitational-Wave Observatory (LIGO) and its observation of gravitational waves.
On September 14, 2015, LIGO observed gravitational waves for the very first time. The waves, which were predicted by Albert Einstein a hundred years ago, came from a collision between two black holes 1.3 billion years ago. The signal was extremely weak when it reached Earth, but is already promising a revolution in astrophysics. Gravitational waves are an entirely new way of observing the most violent events in space and testing the limits of our knowledge.
LIGO is a collaborative project with over one thousand researchers from more than twenty countries, but the Royal Swedish Academy of Sciences singled out the 2017 Nobel Laureates as being the most invaluable to the successful four-decade-long effort to observe gravitational waves. “I was hoping that the prize would go to the LIGO-Virgo collaboration, which made the discovery, or to the LIGO laboratory, the scientists of the LIGO laboratory, who designed and built and perfected the gravitational wave detectors and not to Barish, Weiss and me,” says Thorne. “We live in an era where some huge discoveries are really the result of giant collaborations, with major contributions coming from very large numbers of people.”
The project had its genesis in efforts worldwide to develop wave detectors, including experiments by Joe Weber in the 1960s. Weber announced that he had seen gravitational waves in his Weber bars in 1969, but no other group was able to replicate the experiment. The effort did, however, inspire Weiss’s curiosity. “I was teaching a course in general relativity…and I couldn’t explain the way a Weber bar worked. Mostly because I just didn’t know enough,” he says. “I thought that there must be an easier way to explain how a gravitational wave interacts with matter.”
By the mid-1970s, Weiss had already designed a laser-based interferometer. Early on, both Thorne and Weiss were convinced that gravitational waves could be detected and bring about a revolution in our knowledge of the universe. “Gravitational waves are the only other kind of wave, besides electromagnetic, that propagate across the universe,” says Thorne. “So initially we will see not just binary black holes. We will see neutron stars collide, tear each other apart, we will see black holes tearing neutron stars apart, we will see spinning neutron stars, pulsars, when the space-based LISA mission is operating, hopefully by about 2030; we’ll be exploring basically the birth of the universe, the earliest moments of the universe. And there will ever so much more I’m sure, including huge surprises, as the years wear on.”
Gravitational waves spread at the speed of light, filling the universe, as Albert Einstein described in his general theory of relativity. They are always created when a mass accelerates, like when an ice-skater pirouettes or a pair of black holes rotate around each other. Einstein was convinced it would never be possible to measure them. The LIGO project’s achievement was using a pair of gigantic laser interferometers to measure a change thousands of times smaller than an atomic nucleus, as the gravitational wave passed the Earth.
So far, all sorts of electromagnetic radiation and particles, such as cosmic rays or neutrinos, have been used to explore the universe. However, gravitational waves are direct testimony to disruptions in spacetime itself. This is something completely new and different, opening up unseen worlds. “I think this couldn’t have been done 50 years ago, or 20 years ago, or 30 years ago,” says Barish. “It’s taken the best modern lasers and control and engineering to be able to do it. The technical challenges were technical challenges that were not unbeatable—it was just that we had to learn how to do things and how to build a sensitive enough device. That took us 20 years after we built the first version of the LIGO detector. And of course the science is unbelievable, so I think it is not hard to be motivated for 20 years to do the kind of science we’re starting to be able to do.”