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Particles colored in white collide in Hadron Collider, and show an explosion of red and yellow lines.

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Co-hire uses particle physics to improve predictions for gravitational waves

Posted on August 14, 2025

UNIVERSITY PARK, Pa. — Penn State Institute for Computational and Data Sciences (ICDS) co-hire Radu Roiban uses particle physics techniques to improve state of the art theoretical predictions for the gravitational waves produced by astrophysical systems and observed by future experiments. 

Roiban, professor of physics in the Eberly College of Science, became an ICDS co-hire this past fall, and aims to use the high-performance computing resources and ICDS expertise for his computations as he expands his research efforts in the future. 

Particle physics, according to Roiban, studies interactions between particles such as protons or neutrons. One of the methods, which was employed to great effect at the Large Hadron Collider and led to the discovery of the Higgs boson, involves taking two particles, aiming them at one another and accelerating them causing them to collide and scatter. 

When two particles like protons or neutrons collide, thousands of particles come out of that interaction. 

“There are literal fireworks,” Roiban said. 

Researchers use detectors to observe the outcome of such collisions. 

By studying these outcomes, researchers extract information about the building blocks of the interactions of particles, including when particles are bound together. 

“By observing the products of the collision, we learn exactly how these particles interacted at the microscopic level,” Roiban said. “Researchers have noticed that the methods used to describe the scattering of quantum particles can be used to describe the scattering of black holes and other astrophysical bodies.” 

Roiban is using these techniques to make theoretical predictions for processes involving astrophysical bodies, and for the gravitational waves that these processes give out. 

Applying these methods to collisions of astrophysical bodies, particles are much bigger, like black holes. If they don’t get too close to each other, then they can still be modeled as point particles, much like protons and neutrons, Roiban said. In such collisions, the particles that come out of a collision are the same as the ones that go in. 

Possible predictions that could come out of Roiban’s work would be the angle between the black holes’ trajectories before and after their interaction — this is called the scattering angle. By comparing theoretical predictions with observations, researchers can infer properties of the bodies, such as their mass and their energy. 

Astrophysical bodies, according to Roiban, can also spin around themselves, like a spinning top. Possible predictions will show how the axis of rotation changes or whether they spin faster or slower because of the interaction of two such bodies. 

The work aims to develop a deeper understanding of how gravitational interactions work and what the underlying structures are behind these interactions. 

“How can we use them to our advantage? What can we predict about the motions of various types of bodies that interact gravitationally?” Roiban asked. 

Future gravitational wave experiments will likely probe these interactions to an incredible precision, he added. 

“Our goal is to improve theoretical precision to match that of the needs of future gravitational experiments,” Roiban said. 

Last year, Roiban and his collaborators had published their work on a new method that allows them to do new calculations to look for gravitational theories in which the quantum interactions of particles with very high energy have special properties. 

“Einstein told us that gravity couples to pretty much anything that has energy,” Roiban said. “If an object is held in the air and dropped, it falls because gravity pulls its mass. Now, if the object moves really fast, then the Earth will pull it stronger because it has more energy than just its rest mass.” 

Researchers have observed that theoretical predictions for quantities that we might observe, such as a scattering angle or the rate of emission of gravitational waves, can become infinite. 

“If we collide two particles, we can theoretically compute the probability for the same two particles to come out. Quantum mechanics’ Heisenberg uncertainty principle says that particles with any energy, no matter how high, can appear in such a process, as long as they do so for sufficiently short times. These short-lived infinite-energy particles feel an infinite force of gravity, effectively making probability for scattering infinite,” Roiban said. 

These infinities remove researchers’ predictive power, according to Roiban. 

“We want to study whether it is possible to construct gravitational theories that are free of such infinites. If such a theory exists, then it must have some hidden, currently unknown structures that are responsible for the disappearance of these infinities,” he said. “These structures, whatever they are, are what we are after.” 

The goal of this research is to shed light on basic structures and symmetries present in particular gravitational theories, revealing properties that would otherwise remain hidden. Ultimately, the goal is to understand if there exist consistent and predictive quantum gravitational theories that can be studied using particle physics methods, that is the same methods used to describe the interactions of protons and neutrons. 

A quantum theory of gravity would unify two cornerstones of modern physics, quantum theory and general relativity, and is essential for a complete understanding of our universe from the smallest to the largest scales,” Roiban said. 

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