In 1965, when James Edward Gunn was in graduate school at the California Institute of Technology, he figured out a way to tell when the universe first lit up with stars. Other astronomers had spotted strange sources of radio waves known as quasars at enormous distances — two-thirds of the way back to the universe’s beginning. Gunn and a fellow graduate student, Bruce Peterson, noticed that the light from these quasars was missing a signature of neutral hydrogen, meaning the hydrogen gas surrounding the objects must have been ionized by starlight. So, Gunn and Peterson argued, stars had formed by then.
The signature of neutral hydrogen from the time before stars, known as the Gunn-Peterson effect, was finally seen in 2001, in a faraway quasar that flared up when the universe was just shy of a billion years old. The quasar’s discovery established a critical transition point on the cryptic timeline of the early universe. The quasar turned up in the Sloan Digital Sky Survey, an unprecedentedly thorough and detailed catalog of the cosmos done with astronomical instruments that Gunn himself had principally designed and built.
Astronomers usually choose one of three paths: theory, observation or instrument building. Gunn, 80, the Eugene Higgins Professor Emeritus of Astrophysical Sciences at Princeton University, has done all three — gathering data on galaxies and clusters of galaxies (among other things), theorizing about how those structures formed and evolved (among other things) and, at the same time, designing and building instruments to extend telescopic capabilities. For his contributions to astronomy, he has won nearly every honor in the field, including the Crafoord Prize, astronomy’s version of a Nobel, plus a MacArthur Fellowship and the National Medal of Science.
Around 1980, though, Gunn gave up observing and theorizing to focus on instruments. For an astronomer of his talent and reputation, the choice was odd: Instrument builders don’t get much glory. They comprise less than 10 percent of astronomers and aren’t on the usual academic track. Despite this, progress in the field has always crucially depended on advances in instrumentation. And in recent decades, big questions about the structure and composition of the universe have called for specialized cameras and spectrographs capable of surveying large patches of the sky. Gunn wanted answers, and instruments were the sine qua non.
They are also his original love. Born in Texas during the Depression, Gunn has been cutting metal since his single-digit years. Observing and theorizing about the evolution of the cosmos are, he says, merely how he earned his “spurs” before getting to do what he wanted to do all along: build things. “I’ve been interested in building things from the year dot,” he told me.
He has built hardware for quasar and cluster surveys, the Hubble Space Telescope and the Sloan Digital Sky Survey, which produced one of the most highly used data sets in astronomy ever. He is now busy with what he considers his best instrument yet: a spectrograph to be installed on Japan’s 8.2-meter Subaru Telescope, for a survey that will help map the distribution of galaxies, dark matter and dark energy throughout the cosmos.
Because Gunn is an astronomer whom astronomy writers are advised to know, I’ve been interviewing him for about 30 years. This time, we talked in Princeton in his basement office, which has room for two chairs among the desks and benches and cases with a zillion tiny drawers full of tinier electronics. Our conversation about his life and love of astronomical instruments has been edited and condensed for clarity.
Your Princeton friend and colleague Jeremiah Ostriker says you are “UNIQUE [his caps] in having exceptional talents in all three directions” — meaning as a theorist, observer and instrument builder. Is he right?
I think it’s unusual, right. My impression is that most people who do instrumentation start out that way. And it’s common for people who start out as theorists to become either frustrated with their own abilities, or frustrated with the amount of data they need, and they become observers. And by and large, once they become observers, they don’t do much theory anymore.
But people don’t often drift over into being instrumentalists?
The problem is preparation. In physics, people are often trained to be experimental physicists, to do instruments. In astronomy, that just wasn’t the case. Astronomers were people who used instruments, but very seldom people who built them. The professors didn’t get their hands dirty.
Along with your work with Peterson, another theory paper that earned you your “spurs” was known as Gott, Gunn, Schramm and Tinsley, after its authors. Ostriker said that 1974 paper established the parameters for a model of an evolving universe and that it, like your other theory papers, is still seen as essentially correct.
What we were trying to do is gather all the evidence on this issue of the density of the universe. We were trying to assay how much matter there was — I mean, you can add up all the stars and it’s nothing — and trying to balance the fraction of stuff in clusters and the fraction of stuff outside them. And the number we came up with was, quite fortuitously, almost exactly the number that we believe today. Not quite “fortuitously”; I think we did a reasonable job.
Soon after that, you swerved into observation, starting what became a series of surveys of the sky. And, in fact, most of your work as an observer has not involved observations of individual objects, but surveys. Why is that?
The universe is basically a statistical beast. And so to understand any class of objects in the universe you have to do a survey. If you study the hell out of one cluster, you’re going to learn about it. But you’re not going to learn how it formed, because how it formed was part of some much larger statistical scenario that was forming lots of things. The same is true with galaxies, right? Every galaxy is a weird thing, and understanding it in detail does not tell you very much about galaxies. And so you had to understand the statistical universe to understand how these things form. I mean, that’s the story of astronomy.