MIT physicist Daniel Harlow is searching for answers to one of the biggest questions in modern physics: How can our universe abide by two incompatible rulebooks? The first – the Standard Model of Physics – is the quantum mechanical theory of particles, fields, and forces, and the ways in which 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 pulls together matter to build the planets, galaxies, and other massive objects. Both theories do remarkably well in their respective lanes. However, Einstein’s theory breaks down when trying to describe how gravity works at quantum scales, while quantum mechanics makes reality-bending predictions when applied at massive, cosmic dimensions. For over a century, physicists have searched for ways to unite the two theories and get to the truth of what makes our universe tick.
Harlow suspects that any connecting 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 a boomerang’s trajectory, rather than stretching and expanding without end as our actual universe does. Quantum gravity in this boomerang universe turns out to be easier to understand, as it can be reformulated in terms of conventional quantum theory (without gravity) using a powerful idea called holographic duality. This makes it far simpler to contemplate, at least from a theory perspective.
In this boomerang environment, Harlow has made some exciting, unexpected revelations. He has shown, for instance, that the equations that describe how gravity behaves in this “toy” universe are the very 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 protecting 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 fresh lines of inquiry in the study of both 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.
Harlow is working to bring more diverse voices and perspectives into the field of physics. In addition to mentoring and advocacy work outside of MIT, he is running a program within the physics department that invites students from underrepresented and underprivileged backgrounds to carry out physics research at MIT each summer.
Looking ahead, Harlow is considering taking a new turn in his 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 actual universe.
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