Photographs by Caitlin Abrams
Nelson Christensen, professor of physics at Carleton College
Astrophysics doesn’t make headlines often. But in February, newspapers took notice when the Laser Interferometer Gravitational-Wave Observatory (LIGO) published a paper saying that long, long ago in a galaxy far, far away, when two black holes collided they’d set into motion detectable gravitational waves that LIGO had just recently detected. It was the first time gravitational waves had ever been observed. This is a big deal because it proves Albert Einstein’s century-old prediction that, in addition to being the force that keeps you from floating away, gravity was a wave, a fact that further bolsters his general theory of relativity. The reason that’s an especially big deal for Minnesotans is that a significant player in reaching the astrophysical milestone is Carleton College, the sleepy undergraduate-only institution in Northfield, a town known more for foiling Jesse James than being at the forefront of decoding the mysteries of the universe. Nelson Christensen is the George H. and Marjorie F. Dixon professor of physics at Carleton, and one of the LIGO scientists. We caught up with him between classes.
How did Carleton come to be a hotbed of cutting-edge astrophysics?
Joel Weisberg [another Carleton professor] is one of the two people who proved that gravitational waves exist in 1982. He was studying two neutron stars spiraling around each other, and calculated that what they were observing perfectly aligned with what you’d see with gravitational waves. In the 1980s I [researched] how detectors like LIGO could search for and detect these gravitational waves from the Big Bang. I [came to Carleton] in 1999 mainly because of what a neat school it is, and because Joel has always been someone I can talk to about my science. . . . About two-thirds of [our] students go on to get an advanced degree. There were two current students and five alums [credited] on that paper. Carleton is one of about 83 institutions on the LIGO project.
Carleton College's majestic 22-foot-long, 16-ton Brashear refractor
I have mass. Do I have gravity?
Yes, anything with mass or energy has gravity. And, yes, you warp space-time. The little region of space-time around you is distorted because of your mass. When you jump off a diving board, your mass is trying to follow the straightest path it can, it’s falling into the curved space-time of earth.
Who has more gravity, Prince or Babe the Blue Ox?
The thing that’s more massive will have more gravity, at rest. But if Babe the Blue Ox is not accelerating, he is not emitting gravitational waves. If Babe the Blue Ox is just sitting there, and Prince is dancing, Prince is emitting gravitational waves and Babe the Blue Ox is not.
What’s next, now that you helped find gravitational waves?
Looking for the Big Bang. I’m the co-chair of the group searching for evidence of what happened in the early universe. If two black holes colliding was one raindrop in a pond, creating echoes that we just recorded, the Big Bang was a huge rainstorm on a pond, creating ripples everywhere, from the little quantum fluctuations at the very start of the universe to all space getting shook up as the universe expanded. Those ripples are spreading out still, they’re getting smaller and smaller, and it’s going to be very difficult for LIGO to detect them. Even if we look and don’t find anything, that will be helpful. There are many theories of what happened in those earliest moments, some will produce more gravitational waves than others, and what we find or don’t find will eliminate some.
The echoes of the Big Bang are passing through me right now?
Indeed. Detecting them is the trick.