*November 01, 2023*

# Rachel Rosen Weighs Importance of Gravity on Quantum Physics

Associate Professor Rachel Rosen is a force when it comes to studying gravity and quantum physics.

Rosen, who joined Carnegie Mellon University's Department of Physics in January 2023, focuses on improving the understanding of gravity through the lens of particle physics. Einstein's theory of general relativity revolutionized thinking around gravity with the idea that gravity is not a force but the curvature of spacetime responding to the presence of mass.

Using quantum field theory, the theoretical framework of particle physics, to study gravity, a researcher could arrive at the theory of general relativity for reasons very different than Einstein's, Rosen said.

"What is interesting is that you can show that general relativity is the unique theory of a massless spin-2 particles," Rosen said. Spin-2 particles, also known as gravitons, are postulated to transmit the gravitational force in the same way that photons transmit the electromagnetic force. "From a particle physicist's point of view it's a natural question to ask, 'Could you have a theory of a massive spin -2 particle? What would it be like? Could it possibly have anything to do with gravity — or our theory of gravity — at all?'"

Researchers often pursue these questions by making the simplifying assumption that spacetime is flat, but Rosen is also interested in understanding how quantum field theory works in a curved spacetimes.

"Quantum field theory is this very powerful framework, but one of the interesting things about it is that it very much relies on the assumption that we live in a flat spacetime," Rosen said. "It starts to get interesting if you ask, 'What are the corrections due to the fact that if we do actually live in a curved spacetime at the end of the day?' It's surprisingly difficult."

As a theoretical physicist, Rosen joins a growing group in Carnegie Mellon's Department of Physics that seeks to address some of the most challenging problems in particle physics, condensed matter physics, biological physics, gravitation and cosmology.

"I've been familiar with the group for a long time," Rosen said. "Ira Rothstein has been a prominent figure in the field for a long time, and Riccardo Penco and Grisha Tarnopolsky were exciting recent additions to the group."

Rosen, along with Rothstein, Buhl Professor of Theoretical Physics, and Assistant Professors Riccardo Penco and Grigory Tarnopolskiy are part of a recent 3-year grant from the Department of Energy to fund theoretical high-energy physics at Carnegie Mellon.

During the fall of 2023, Rosen taught thermal physics I for undergraduates. She also has taught quantum mechanics II for graduate students. Teaching is part of what drew Rosen to Carnegie Mellon.

"I like the philosophy of the university and how science-minded it is, and the caliber of the undergraduate students and the graduate students," she said.

Rosen earned her doctorate in physics from New York University and her bachelor's degree in mathematics and physics from Brown University. In 2013 she received a Blavatnik Award, the largest, unrestricted award for early career scientists. Prior to joining Carnegie Mellon, she was an associate professor at Columbia University. She previously worked as a postdoctoral researcher both at Columbia and Stockholm University.