A professor returns to Columbia University, where she first explored dark matter when she was an undergraduate
Kirsten Perez (CC’05) first came to Columbia from Philadelphia to study physics as an undergraduate. After receiving her Ph.D. at Caltech, Perez received a National Science Foundation fellowship in astronomy and astrophysics at Columbia. Now, after stints at Haverford College and MIT, Perez returned to Columbia this summer as the Lavigne Family Associate Professor of Natural Sciences in the Department of Physics. Perez is an astroparticle physicist. They make instruments that look into space and try to measure what the universe is made of on the smallest subatomic scales. She focuses specifically on determining the particle nature of dark matter.
Columbia News Perez met to discuss dark matter, her upcoming research project in Antarctica, and how science needs to change in the wake of the pandemic.
How would you explain your field to non-physicists?
If you zoom out and think of the universe at its largest—galaxies and whole galaxy clusters—these are the things that we know are bound together by gravity, just as gravity holds me in a chair now, gravity holds things together. Dark matter is the matter in the universe, which creates gravity. It makes the universe assemble into the structures that we observe. We don’t know what it is, but we know it’s there.
How do we know it exists?
Basically, if you look at a galaxy, and you count all the visible matter, and you see how fast it’s spinning, the observed matter isn’t enough matter to hold it together, so there must be something else, and that’s another thing that is what we call dark matter.
Imagine you have a bucket of bricks on a string and you are spinning it around and around. And if you see this bucket going, you can infer something about how strong this thread is. Gravity is the thread that binds them together. But if you add up all the visible matter that makes gravity, it won’t produce enough gravity to hold it together for the speed you’re moving. Galaxies should orbit in space, and diverge from each other, but they are not.
We have this whole set of particles on Earth that everything around us is made of: we know that in an atom, electrons orbit a nucleus made up of protons and neutrons. These protons and neutrons are made of smaller things called quarks. The electron has some of its heavier cousins, the muon and the tau; Those are bound to some very light particles called neutrinos.
But in the case of astrophysics, only those particles can’t be there: we know how to detect them. We know how to figure out what structures those particles will make. And it’s not just about holding things together, so we know we need something new.
You are currently working on the GAPS experiment, an Antarctic balloon mission due to launch next year. What is that experience looking for?
This experiment is looking for low-energy antinuclear agents. It measures antiprotons and looks for antihelium and deuterons, which have never been seen coming from outer space.
Why are we looking for it? I always say, you’ve probably never thought of an antideuteron in your life. And there’s a very good reason for that: astrophysics can’t really make antideuterons. Outer space creates all kinds of particles. Stars are born and die, emit protons, and produce carbon, hydrogen, and helium. But we’ve never seen a complex antinuclear such as an antideuteron or an antihelium come from outer space.
But there are plenty of well-motivated theories for what dark matter is that predict that you’ll see some antideuterons coming from space. That’s the basic idea of that mission: We’re building an instrument to look for antideuterons, and if we see a few of them, it will confirm that dark matter or some other new physics we haven’t discovered yet is causing them to appear.
What happens if the experiment detects no antideuterons?
It is quite possible.
We’ve built a lot of dark matter experiments, and we haven’t conclusively found dark matter. So what I usually tell my students is, look, we all want to build the experiment that finds dark matter, or evidence of dark matter interactions, go to Stockholm, meet the king, win a Nobel Prize, everything.
Perhaps this will not happen. And basically, we have to be happy with our jobs of telling theorists they’re wrong, and saying go back to the drawing board, and come up with more ideas about what dark matter is, because it isn’t. This is where we are now. We all plan to find it. We all hope to find it. But basically, a lot of our measurements are checking things off the list, like, “No, no, no, no, no, not that.”
In addition to the GAPS experiment, I’m using data from NuSTAR, an X-ray telescope in outer space, to find evidence of dark matter. I’m also working at the International Axion Observatory, a new experiment looking at particles emitted from the Sun for the same purpose, and observing dark matter interactions.
Early in the epidemic, I wrote an editorial for Within higher education, where you said that a sense of “cultural disharmony” rather than a lack of ability is why some students from minority backgrounds are leaving STEM programs and that professors need to change the way they teach in the classroom. What do you think should be changed in particular?
I think the biggest thing to change is the attitude to us as professors being custodians of science, and that our job is to force the obstacles we call standards between students and what they want to study. We need to change to a mindset: We have these students who are right there in our institution; They’re obviously smart, you can’t get here by accident. They obviously know how to handle school work. And so the question is why some of the students who come on the first day of physics don’t study it at the end of four years. I think what a lot of research shows us is that it makes a difference whether your attitude is “Hey, you’re here, prove yourself” versus “You’re here, you care about physics, and I’m here to help you reach the high standards I set”. These are two completely different messages.
If you’re a student who hears, “Prove yourself, I’m here to decide if you’re good enough,” then there will be a subset of students who are like, “Wow, I love a challenge.”
But there are a lot of students out there who will say, “Well, maybe I’m not good enough.” And what the research will show is that if you are a woman, a student of color or a woman of color, you are more likely to react in this way.
This is a loss for physics. Physics is not a done deal. We have a lot of open questions that we don’t understand. So making people turn away is a loss.
Have you seen any change in the last two years since you wrote it?
Well, the whole world has changed. I remember writing that and I wrote it from a somewhat hopeful point, because I’ve already seen how the pandemic has made us able to talk about this in STEM departments in general. Suddenly, we can no longer be like, “Physics is objective, you either do it or you can’t.” Every professor I knew in the early days of the pandemic spent time on the phone with an individual student who had nowhere to go when campus was closed, for example. This lie really shredded that problems are problems and that everyone has an equal chance of doing well with them. In fact, everyone brings a lot of their lives into the classroom with them.
One other thing I’ve seen is that we have to be aware of the fact that the students who are coming to us from the pandemic have very different backgrounds than their education has had over the past few years. You could have students who were in a private school and were in person in class from fall 2020 onwards, no problem. Then you had students whose entire high school day was three hours online, and physics was 20 minutes of that. Should we take these two groups of students and say, OK, those who’ve been in class all day, those are the ones who get to be physicists? This is a whole conversation we just started.
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