Dan Harlow spends a lot of time thinking in a boomerang universe.
The MIT physicist is looking for answers to one of the biggest questions in modern physics: how can our universe respect two incompatible rules?
The first, the Standard Model of Physics, is the quantum mechanical theory of particles, fields and forces and the ways they interact to build the universe we live in. The second, Einstein’s theory of general relativity, describes the influence of gravity and how the fundamental force brings matter together to build planets, galaxies and other massive objects.
Both theories work very well in their respective lanes. However, Einstein’s theory breaks down when trying to describe how gravity works on a quantum scale, while quantum mechanics makes reality-bending predictions when applied to enormous cosmic dimensions. For more than a century, physicists have searched for ways to combine the two theories and get to the truth about what makes our universe work.
Harlow suspects that any common thread may be too delicate to grasp in our existing universe. Instead, he’s looking for answers in a boomerang version, an alternate reality that folds back on itself, much like the trajectory of a boomerang, rather than endlessly stretching and expanding like our real universe does. Quantum gravity in this boomerang universe turns out to be easier to understand, as it can be rephrased in terms of conventional (gravity-free) quantum theory using a powerful idea called holographic duality. This makes it much easier to contemplate, at least from a theoretical perspective.
In this boomerang environment, Harlow made some exciting and unexpected revelations. He has shown, for example, that the equations that describe the behavior of gravity in this toy universe are the same equations that control the quantum error-correcting codes that will hopefully soon be used to build real-world quantum computers. That the mathematics describing gravity should have anything to do with the protection of information in quantum computers was a surprise in itself. The fact that both phenomena shared the same physics, at least in this alternate universe, suggests a potential connection between Einstein’s theory and quantum mechanics in the real universe.
The discovery, which Harlow made as a postdoc at Princeton University in 2014, sparked new lines of inquiry in the study of quantum gravity and quantum information theory. Since joining MIT and the Center for Theoretical Physics in 2017, Harlow has continued his search for fundamental connections between general relativity and quantum mechanics and how they may intersect in the contexts of black holes and cosmology.
One of the things that was fun is that, even though physics and science more generally was all about different systems and experiments, a lot of the ideas are the same, says Harlow, an associate professor who received a professorship in 2022. So, I try to have an open mind and keep my ears open, and look for how things can be related.
A humanist philosophy
Born in Cincinnati, Harlow moved with his family to Boston as a child, where he spent several years before the family moved again, setting roots in Chicago. When he was 10, he took piano lessons, focusing first on classical music, then on rock. In middle school, he played keyboard in various bands before finding his groove in the looser, more improvisational style of jazz.
I love to sit and play with people and see where things go, Harlow says.
His love of jazz was in part what brought him to New York City after high school where he attended Columbia University which was located near some of the best jazz clubs in the city. The university’s core curriculum, which required students to read classic works of literature and philosophy, also appealed.
You can’t graduate from Columbia without reading The Iliad, Harlow says. This gives you a shared community of things you can talk about. I liked the humanist philosophy that drives the place. Even if I chose to be a physicist, I would still have this broader cultural experience.
Harlow worked for three years as a research assistant professor in an experimental cosmology laboratory on campus, where she learned how to work in a clean room and run simulations to improve the performance of filters designed to detect the subtle radiation signatures left behind by the Big Bang.
Harlow especially appreciated the general approach of the lab leader, Amber Miller, who was then a junior faculty member.
She had this great way of running her group, where she wasn’t so wrapped up in publishing or getting things done in a short amount of time, Harlow recalls. She just let us play.
That mental freedom to explore new ideas would stay with Harlow throughout her career. From Columbia, he went west to Stanford University in 2006. Within the physics department, he found he aligned very naturally with Professor Leonard Susskind, a theoretical physicist and leader in the study of string theory.
Her strong desire to identify things that aren’t important and push them aside so you can focus on the essence of the problem that was also how I try to think, says Harlow, which ended up choosing Susskind as her advisor. Lenny said, he works on what you want, and I’ll tell you about it.
With this invitation open, Harlow has been keeping tabs on conversations within the Susskinds group to get a sense of the big questions in the field. What he heard was an issue that would shape the rest of his research career: the question of how to relate quantum mechanics to general relativity, in the context of cosmology, and scientists’ understanding of the large-scale structure and of the evolution of the universe.
Searching for an answer, Harlow read everything he could find on both theories. His readings have poured primarily into quantum information science, a field that focuses on applying the principles of quantum mechanics and information theory to the study and development of quantum computers.
Whenever I feel like a tool is going to be important to a problem I’m trying to solve, I learn a lot more about it than I think I need to, Harlow says. More often than not, that investment pays off.
At the end of her time at Stanford, Harlow decided to take a difficult turn, moving from cosmology to black holes, which she considered a simpler system to study for any fundamental thread linking quantum mechanics and general relativity.
In 2012, he returned east to Princeton for a three-year postdoc, during which he began exploring the quantum behavior of gravitational black holes. To simplify the problem, he did it in a boomerang universe, what physicists know as anti-de Sitter space, named after the physicist who studied the curvature of the universe. As Harlow read more about quantum information, he noticed, and eventually confirmed, an unexpected overlap in the physics of gravity around black holes and quantum error-correcting codes designed to protect the information.
It was a very exploratory and transformative time, Harlow says. I’m still exploring many of the trails I started there.
After a second postdoc at Harvard University, Harlow joined MIT as a junior faculty member in 2017, where he continues to make amazing connections in the study of quantum gravity and quantum information science. At the Institute, and more generally in theoretical physics, he enjoyed a collegial and productive contempt for authority.
This is a community where I can go to the world’s most famous theoretical physicist, tell them they’re wrong, and if I have an argument, they’ll listen to me, Harlow says. People are open. There’s this basic shared agreement that, what matters is that we find the right answer. It matters less who finds it.
Among Harlow’s achievements since arriving at MIT are evidence that there are strong restrictions on the possible symmetries of quantum gravity, a deeper understanding of the nature of energy in gravitational systems, and a concrete mathematical framework for understanding the interiors of holes. blacks of quantum mechanics.
Beyond research, Harlow is working to bring more diverse voices and perspectives to the field of physics. In addition to mentoring and advocacy work outside of MIT, she is leading a program within the physics department that invites students from underrepresented and underprivileged backgrounds to do physics research at MIT each summer.
Unfortunately physics remains quite white and masculine, and making it more welcoming and accessible to a wider slice of humanity is one of my priorities for the future, she says.
Looking to the future, Harlow is considering taking a new turn in her research path, perhaps to focus less on black holes in a hologram universe, and more on cosmology, and the quantum structure and evolution of our current universe.
I’ve been living in the anti-de Sitter space for a long time, Harlow says. Okay, but I also want to understand the world we live in. And it should be fun.