"What is Inside a Black Hole?"

A theoretical physicist explores how asking a simple question can lead to interesting places.


Jens Boos giving a 3MT talk in Convocation Hall. 

“Wow, that’s a lot of people!” It is a rainy evening in March 2019 when I find myself in blinding lights on the stage at Convocation Hall. I have three minutes to explain my work on black holes to an audience of fellow students, parents, friends, alumni, professors, and families. I have practiced my little speech over and over again, just like the other fourteen three-minute thesis (3MT) finalists. How did I get here?

“Why?” had always been my favorite question, and after high school I was determined to become a physicist. Have you ever asked yourself what the universe is made of, at the most fundamental level? That was the question that piqued my ultimate interest. The strangest thing is that we can try to answer it with methods and calculations in theoretical physics.

One answer is: spacetime. Everything that happens, has happened, or will happen, anywhere in our universe, is a part of an object that we call “spacetime.” Think of it like a calendar: when and where, those numbers, are one point in spacetime. We take that for granted and it determines our lives, like when we have to get up early to catch an 8 am class, or when we have plans for the weekend.

But what if spacetime could end?!

Apparently that happens inside black holes. When I first read about this in my teens in a popular science book, I was shocked! How could that be? How could space and time themselves end? I mean, it’s not like an explosion, or something like that, that takes place somewhere and eventually you can build something stronger and more resilient. Inside black holes, so Albert Einstein tells us, space and time come to an end. It’s called a “singularity.”

I was hooked and wanted to investigate! Understanding black holes shaped much of my career (all the way from my undergraduate studies on General Relativity, to aspects of quantum gravity during my first master degree, to my second master degree at Perimeter Institute), and when it was time for my PhD I was sure that I wanted to learn even more on black holes, which is why I applied to the University of Alberta.

Why the U of A?

The Edmonton gravity group has a long history. It was founded by renowned physicist Werner Israel in the 1960s, and has touched and shaped many influential gravitational physicists: Valeri Frolov and Don Page (professors in the Edmonton gravity group), Sharon Morsink (astrophysics at the U of A), Eric Poisson (Guelph), David Kubiznak (Perimeter), Frans Pretorius (Princeton). So I was excited when in the fall of 2016 I was accepted as a PhD student. Together with my advisor Valeri Frolov and research associate Andrei Zelnikov I began my PhD studies.

Into the depths of graduate research… These are some formulas I worked with during my PhD.

We wanted to find a model of gravity where black holes exist, but inside they no longer have these singularities where space and time end. Now how would that work?

You can think of spacetime as a kind of flexible fabric. Heavy and dense objects, like our planet or the sun, curve this fabric of spacetime. We’ve all seen the pretty pictures and listened to Neil deGrasse Tyson explain it. But inside a black hole, this curving of spacetime becomes too much and a singularity is formed. This singularity rips spacetime apart, and you can think of it as a sharp “singularity needle.” That’s why in our work we wanted to find a way to smooth that needle with “mathematical sandpaper” so that spacetime is safe and no longer ends inside of black holes. This sandpaper is a concept that is called nonlocality, and the following picture shows our hope:


Left: A black hole of Einstein’s theory of gravity in red. It has a singularity and that singularity, just like a needle, rips a hole into spacetime. Right: A black hole in a new theory of gravity, in blue, where the singularity needle is softened and spacetime is safe.

Did we succeed?

During my PhD thesis we applied this principle of nonlocality (“mathematical sandpaper”) to a wide variety of physical problems, and found that it works rather well. This was very exciting because the idea was taking shape, and we were past the initial viability check. The full theory of black holes is very complicated because gravity interacts with everything, including itself, so we don’t have a definitive answer yet.

Defending my PhD thesis during a pandemic

Then COVID-19 happened. All of a sudden in-person discussions, which are so important for progress in theoretical physics, were a thing of the past. Sure, you can talk online or on the phone, but the immediacy was different. Luckily, at that time I had to start typing my PhD thesis, so that kept me busy. On the few occasions when I wanted to give a talk on my work it was all done remotely—something I had to get used to. Later, my PhD defense was also an online event. After a few hours my defense was over, the committee told me that I had passed, we all signed off, and I just sat there with my PhD, not really knowing what to do or how to celebrate it. But I think many of us students felt that way, and that helped.

Giving an invited online talk during the summer of 2020, during the COVID-19 pandemic, at my future postdoctoral institution William & Mary in the US, from my desk in Edmonton.

Looking forward

As a theoretical physicist, I always try to learn something new. So what have I learned from my time at the University of Alberta? And what advice would I give future graduate students?

  • A good supervisor goes a long way. I was very fortunate that my advisor, Dr. Valeri Frolov, was not just available whenever I needed him, but he also gave me critically important feedback when he saw that I was getting off track. Even though it took some practice, we learned to communicate effectively with one another. I would recommend to always always always strive for a supervisor with whom you can talk, and who you feel listens to you as well.

  • Pick a topic you are passionate about. In my case, it was black holes (it’s probably something else for you). Sure, not everything is super exciting all of the time—just look at that screen of equations up in the story, do you think I enjoyed that?—but overall your topic of research should be something you care about.

  • Make some friends. Edmonton has a wonderful graduate community, so get involved. Go dancing, go to the gym, join a hiking club— there are dozens of things you can do (even online). It’s important because it will ground you in life whenever things don’t go smoothly.

Now that I am done with my PhD I am still not done with black holes, and I probably never will be. I am grateful for my time in Edmonton, and hopefully, when it will again be possible to travel more freely, I will come back for a visit.


About Jens

Jens is a former PhD student and Vanier scholar at the University of Alberta and graduated in September 2020. His PhD thesis was awarded the 2021 PR Wallace Thesis Prize by the Canadian Association of Physicists, Division of Theoretical Physics, and the Winnipeg Institute for Theoretical Physics, as well as a University of Alberta Faculty of Science Dissertation Award. Currently, Jens is a postdoctoral researcher at William & Mary in Virginia, United States.