Kip Thorne, a Princeton Graduate School alumnus, is one of three recipients of the 2017 Nobel Prize in Physics. Thorne joins Rainer Weiss and Barry Barish in winning the prize “for decisive contributions to the LIGO detector and the observation of gravitational waves.”
In announcing the award, the Royal Swedish Academy called the direct measurement of the ripples in spacetime, predicted by Albert Einstein a century ago but never before directly observed, "a discovery that shook the world."
Thorne, the Feynman Professor of Theoretical Physics, Emeritus, at the California Institute of Technology, earned his Ph.D. in physics from Princeton in 1965. He stayed on as a postdoctoral fellow until 1966 before leaving to teach at Caltech. Thorne overlapped at Princeton with Weiss, a professor of physics, emeritus, at the Massachusetts Institute of Technology, who was a postdoctoral researcher from 1962 to 1964.
Under the mentorship of John Archibald Wheeler, Thorne became interested in black holes. Wheeler, the longtime Joseph Henry Professor of Physics, had worked with scientific luminaries including Einstein and Enrico Fermi and is credited with adding the terms “black hole” and “wormhole” to the scientific lexicon.
Thorne's 1973 graduate-level textbook "Gravitation," co-authored with Wheeler and Charles Misner, has just been re-issued by Princeton University Press.
Weiss worked in a separate research group, under Robert Dicke. “In the late 1950s, two Princeton University professors, John A. Wheeler and Robert Henry Dicke, decided gravity physics was too important to ignore,” said James Peebles, the Albert Einstein Professor of Science, Emeritus, at Princeton. “John took a theoretical point of view, Bob took an experimental point of view.” He described LIGO as “a branch growing out of those roots, set back in the late '50s by John and Bob.”
Peebles recalled Thorne and Weiss from their Princeton days. “Kip was very impressive as a graduate student — you could tell he was headed for great things,” he said. “Ray Weiss ... I remember him very well. Brilliant guy, even then.”
Weiss and Thorne will be returning to Princeton on Apr. 12 for the dedication of Palmer Physical Laboratory, now Frist Campus Center, as an APS Historic Site. Thorne will give the Hamilton Lecture that evening.
The discovery
On Sept. 14, 2015, the two enormous instruments that make up LIGO were undergoing final preparations when, suddenly, they recorded a peculiar signal, usually described as a “chirp” or “thump.” Five months later, the researchers announced that just as they had hoped, that chirp was the distant echo of the cataclysmic collision of two black holes merging into one, larger black hole.
The collision itself had happened 1.3 billion years ago, and, because of the ongoing expansion of the universe, somewhat more than 1.3 billion light-years away.
“The physics community was electrified,” wrote Princeton physics professors Steve Gubser and Frans Pretorius in their new book, "The Little Book of Black Holes," coming out Oct. 10 from Princeton University Press. “It was as if we had lived for all our lives blind to the color red, and at the moment the veil was lifted we saw a rose for the first time. And what a rose it was! Best estimates from LIGO indicated that the faint thump they recorded was the result of the coalescence more than a billion years ago of two black holes, each of them roughly 30 times the mass of the sun.”
The signal was detected by two nearly identical instruments, one in Washington state and the other in Louisiana, each shaped like a carpenter’s right angle but with arms that stretch almost 2.5 miles each.
As Einstein predicted a century ago, gravity waves propagate outward, pushing up-and-down and side-to-side with respect to their forward motion, rather like the ripples in a pond push the surface of the water vertically while the ripples spread horizontally. The twin instruments of the Laser Interferometry Gravitational-Wave Observatory (LIGO) have laser light running up and down their arms continually, always searching for a ripple in spacetime that will distort the light differently in the two directions.
As Gubser and Pretorius explained, “The gravitational wave is simultaneously ‘stretching’ space in the north-south direction and then ‘squeezing’ space in the east-west direction, and then vice versa.”
What makes measuring these differences extraordinarily difficult is that even a gravitational wave as enormous as one generated by two colliding black holes creates only an infinitesimal ripple in spacetime — just enough to squeeze and stretch the light waves “about one thousandth the size of a proton,” explained Barish, the Linde Professor of Physics, Emeritus, at Caltech, in an interview on the Nobel Prize website. Even if the arms of the LIGO detectors reached across the full diameter of the Earth — about 8,000 miles — the gravitational ripple would change their length by about one trillionth of a millimeter, or the size of an atomic nucleus.
Even with such a sensitive instrument as LIGO, the researchers had to know exactly what they were looking for, and that key came from Pretorius, noted Lyman Page, the James S. McDonnell Distinguished University Professor in Physics at Princeton.
“That was another tour de force,” said Page. “People have been trying to solve Einstein’s equations for 40 years, and it was Frans who found the solution — that’s what fits the data. It’s experiment and theory coming together — just amazing.”
When LIGO detected the signal that Pretorius had predicted, back in September 2015 — a feat it reproduced again on Dec. 25, 2015, Jan. 4, 2017 and Aug. 14, 2017 — it set the stage for a whole new field: gravitational wave astronomy.
The laureates “really are the pioneers of what is now a new field in astrophysics,” said Joseph Taylor, the James S. McDonnell Distinguished University Professor of Physics at Princeton, who won the Nobel Prize in Physics in 1993 with Russell Hulse, principal research physicist at the Princeton Plasma Physics Laboratory, for indirect measurements of gravitational waves.
Their discovery “confirmed that these waves predicted by Einstein’s theory do exist, do carry energy and therefore the experiment to detect them directly is not chasing a will-o’-the-wisp, but there’s actually something there,” Taylor said. “I think that the fact that our experiment was successful made it much easier for the National Science Foundation to decide to go out on a limb and fund this big project, the success of which, of course, is honored today.”
“This is a truly historic moment, opening up this entirely new window into the universe,” said Pretorius. “History has shown — starting with Galileo, who discovered the satellites of Jupiter and spots on the sun, things that no one would have expected — every time astronomers have used a new tool to look at the universe in ways that we can’t see with our eyes on Earth — X-rays, gamma rays, radio waves — something new and interesting has been discovered.”
Pretorius added: “There are certain things we expected to see, or hoped to see, like black holes colliding. But I think what we’re also all hoping for is, how will this new window on the universe surprise us? ... I hope that we see something really astonishing and surprising soon.”
