Over a thousand people packed into Jadwin Hall on Thursday, April 12, filling five auditoriums, to attend the 43rd Donald R. Hamilton Lecture delivered by Kip Thorne, Professor Emeritus at the California Institute of Technology.
Thorne, who won the 2017 Nobel Prize in Physics along with Barry Barish of Caltech and Rainer Weiss of MIT, spoke of his momentous discovery of gravitational waves, detected by the Laser Interferometry Gravitational wave observatory from a black hole merger 1.3 billion light years away.
Thorne opened by narrating the events which led to this historic finding in 2015.
“When multi-cell life was just forming on Earth 1.3 billion years ago, but in a galaxy far, far away, two black holes crashed together, creating a giant burst of gravitational waves, that traveled out … into the great reaches of intergalactic space,” he said.
These gravitational waves reached the outer edges of the Milky Way 50,000 years ago, during the age of the Neanderthals.
“On 14 September 2015, they reached the Earth. Touching down first on the Antarctic Peninsula, they traveled up through the Earth, unscathed by all the matter of the Earth, and emerged in Livingston, La., at one of two LIGO detectors,” Thorne continued.
Gravitational waves such as the ones detected in 2015 are actually incredibly difficult to pick up, mostly because of their minute effect on spacetime. When cosmic monstrosities like black hole collisions and neutron star collisions occur, the gravitational interactions with the environment around them are so violent that they bend spacetime.
These ripples in spacetime travel enormous distances to be detected by LIGO, so much so that the ripples in space that we observe are minuscule compared to the ripples surrounding the collision.
LIGO uses an intricate system called an interferometer, or a laser beam splitter reflected by 40-kilogram mirrors to find these tiny undulations in reality.
Thorne elaborated on the size of those undulations.
“Begin with the thickness of a human hair, divide by 100 and you get the wavelength of the light that is used to measure the [gravitational waves]. Divide by 10,000 and you get the diameter of an atom,” said Thorne. “Divide by 100,000 and you get the diameter of a nucleus of the atom. Divide by another factor of 1,000 and you get the factor of the mirror motion.”
Earlier that day, Thorne and Weiss paid homage to the late Robert Dicke, a former physics professor whose work on gravity was an integral precursor to Thorne’s and Weiss’s work on gravitational waves. Both attended the dedication of a plaque outside Frist Campus Center, called the Palmer Physical Laboratory during Dicke’s tenure, where in the 1960s and ’70s Dicke and fellow physics professor John Archibald Wheeler proposed the existence of gravitational singularities, coining the term “black holes.”
Dicke passed away in 1997.
“Thorne’s passion was infectious, and despite his scientific stature being quite towering at the present, he still presented himself as very approachable,” said Andrew Wu ’20, an astrophysics concentrator.
Wu asked Thorne a question about the way gravitational waves affect time.
“Despite having had some exposure to the concepts of relativity before, his answer still astounded me: gravitational waves appear to change the flow of time only by affecting space, and therefore, how light travels through it, which is how we observe their effect on time,” Wu said.
“He specifically said he found the first few years of undergrad very challenging,” said Elliot Davies ’20, also an astrophysics concentrator. “Meeting him gave me hope that I could follow in his footsteps; it inspired me to work hard even when I’m struggling at Princeton.”
The possibilities for further study of gravitational waves, according to Thorne, are endless.
“By the mid-part of this century I think the biggest effort is to explore the first second of the universe with gravitational waves,” Thorne said.
Thorne explained that when the universe was a trillionth of a second old, the forces described Maxwell’s equations began applying to the universe. Force separation occurred inside bubbles that produced bursts of gravitational waves.
These waves should be detected by the Laser Interferometer Space Antenna, a system of three satellites in space designed to detect gravitational waves from the primordial universe.
“It was two years ago that LIGO discovered gravitational waves with colliding black holes,” said Thorne. “I invite you to speculate on the next 400 years with combined electromagnetic and gravitational waves.”