Interactions in Space: An Interview with Philip Hopkins

Philip Hopkins, Caltech's newest assistant professor of theoretical astrophysics, describes his work as studying the formation of really big things—like stars, galaxies, black holes, and planets. Although these "big things" may seem wildly different from one another, Hopkins creates models of these events that focus on the interconnectedness of the universe—such as how the formation of a single star can have an impact on the galaxy as a whole.

Originally from Cleveland, Hopkins received his bachelor's in astronomy from Princeton in 2004 and his doctorate from Harvard in 2008. After completing several postdoctoral fellowships at UC Berkeley, Hopkins joined the Caltech faculty in September 2013. Recently, he sat down with us to talk about his work.

Why are you excited to be at Caltech?

It's a fantastic astronomy department. And although there really aren't any other theorists here who do the same kind of work that I do, a good portion of the department is working on the observational side of the things I'm working on. That's super exciting to me, because I feel like now I'm in the heart of where all the observations are coming from. It also helps that my wife got a job next door at IPAC [the Infrared Processing and Analysis Center]. She's an astronomer, too—a planet hunter.

Can you tell us a little bit about your research?

I work on a broad range of topics, but basically I like studying how big things form. I study how galaxies form, how stars form, and how supermassive black holes form. Recently, I started studying how planets form. When you study the formation of entire galaxies and the formation of single planets, it's really a wide range of scales, but a lot of those problems involve the same basic physics—gravity and fluid dynamics—just on larger scales and smaller scales. Right now, I'm mostly focused on how the formation of stars, galaxies, planets, and black holes feed back on one another. The big realization in the past few years in almost all of those fields has been that you can't cleanly separate these big events. You can't say, "My research is focused on galaxies, so I don't have to care about individual stars."

We're trying to study the interplay in detail. We want to see if you can put these interactions in a model, where you start in the very early universe and try to evolve everything through to today.

For example, a star exploding as a supernova has a big impact back on the entire galaxy, and then that, in turn, changes how the next generation of stars, black holes, and planets form. There is some kind of constant feedback loop between all of these processes. We study a lot of those interactions, and in our study, there is a lot of crossing between different fields of astronomy. I think it's a good time to be doing this interdisciplinary work because those fields have been separated for a long time.

What is your relationship with the observers on campus?

It's a lot of back and forth—so it's a little feedback loop of its own. They want to know what they can do with their data, and they want to be able to test models, so sometimes I go to them and I say, "I have this model. Here are the predictions it makes." And sometimes they come to me, and they say, "We saw this weird thing. Do you have an explanation, or can you think of one?" Those are the most exciting: when something is unanticipated, and you get a whole new project out of trying to figure out what's going on.

These are always messy problems because there is always a huge range of possible models out there. I think the observers on campus are looking forward to having a theorist there to help them decide how they can really discriminate between the different models and what properties we need to measure.

Is there anyone in particular that you're looking forward to working with?

In the past, I've worked with Richard Ellis and Chuck Steidel—both do observations of galaxies in the very early universe—and many observers at IPAC and JPL. I'm also thinking about other possible collaborations, but it's still early; I've only been here a few weeks.

How did you get started in this field?

My parents are an art history major and a sociology major who never took a math or science class after junior year of high school, so they don't know quite what happened with me. I always liked science, but I also really liked any subject that was removed from reality. I feel like biology was too practical to me. When I went to college, I started taking courses to be a physics major. I had read a lot of books on string theory, and I thought that was cool. But then I had the "good fortune" to have a terrible adviser for my first physics project who basically convinced me that I didn't want to do physics anymore. I was about to switch to becoming a classics major when my roommate convinced me to take an astronomy class. I didn't even have the requirements for the class, but the professor said it was fine, so I took it, and I loved it. And then I took the second one. When I look back on my first experiences in physics and my first experiences in astronomy, it is like night and day.

What's most exciting in your research right now?

There's so much happening that's new. Observations are just pouring in—new planets are being discovered, and new galaxies are being discovered at farther and farther distances and earlier times—and the theory is way behind the observations. So, I'm constantly asking if we are even in the right ballpark. Are we qualitatively near some explanation that actually works for all of this? It's exciting. Unlike in a lot of fields, there's so much new data that a single person can write an interesting paper or make an interesting measurement in just a few months. That's definitely something that's not true in a lot of the sciences. 

Is there a certain research question that keeps you up at night?

The boring answer is: "Where is the newest bug in my simulations?"
For all the romance of looking into the skies, the truth is that I spend most of my day sitting at a computer, debugging code. These big simulations have a couple hundred thousand lines of code that you have to worry about, so it's quite a process.

Does the type of work that you are doing carry over to other fields?

I think it does and not always in the ways I would expect it to. Some of the things I've been working on recently are really more about fluid dynamics. For example, if you think about the gas in galaxies and the gas that forms stars, turbulence is really important. Turbulence is a problem in a whole range of fields—and it turns out there are some interesting problems in turbulence that the astrophysicists have really highlighted.

Surprisingly, I've also found myself talking to people who create models of smog formation. My research involves the dust grains inside of the disks in which planets form and how the dust grains get concentrated in certain regions after swirling around in little turbulent vortices. Although this is a very new topic in astrophysics, there is a whole field studying the phenomenon in smokestacks. The two fields are addressing different problems, but we're sort of converging on the same place from our different sides.

Do you have any interests outside of astronomy?

I really like skiing, and I'm also a big movie nerd. As for a genre, my highbrow answer is that I enjoy film noir; my lowbrow answer is that I'm a big fan of stupid action movies. I will get into long discussions about why Die Hard is the greatest movie ever made.