Chapter 1: Introduction to Gravitational Waves

Gravitational waves, first observed by LIGO in 2016, provide a groundbreaking perspective on the cosmos. They hold the potential to shed light on events ranging from the aftermath of the Big Bang to significant occurrences in galactic centers. Recent studies indicate that these waves may leave behind discernible "memories," allowing scientists to investigate them long after they have passed through.

As the billion-dollar Laser Interferometer Gravitational-Wave Observatory (LIGO) continuously monitors for these cosmic ripples, new findings suggest that gravitational waves can induce lasting modifications in the detectors even after their passage. According to Alexander Grant, a doctoral student and primary author of the study, “The fact that gravitational waves can create permanent alterations in a detector after they have passed is one of the intriguing predictions of general relativity.”

Chapter 2: The Legacy of Gravitational Waves

Physicists have long acknowledged that gravitational waves leave an imprint on the particles they encounter. They have previously identified five such signatures, but recent research has uncovered three additional “persistent gravitational wave observables” that could enhance our ability to recognize these waves traversing the universe. Grant notes that each newly identified observable offers unique avenues for validating general relativity while providing deeper insights into the fundamental characteristics of gravitational waves. These properties may even assist in retrieving information from the Cosmic Microwave Background (CMB), the residual radiation from the Big Bang.

Éanna Flanagan, the Edward L. Nichols Professor and chair of physics and astronomy, remarked, “We didn’t foresee the extent and variety of observations possible.”

Grant emphasizes, “What surprised me was how diverse concepts were often linked in unexpected ways. We explored a broad range of observables and discovered that understanding one often requires insight into another.”

Chapter 3: Identifying New Observables

The research team identified three distinct observables that demonstrate the effects of gravitational waves in a flat spacetime region that briefly experiences a gravitational wave burst before returning to its flat state.

The first observable, known as curve deviation, measures how much two accelerating observers diverge from each other compared to observers with similar accelerations in an undisturbed space. The second, termed holonomy, involves tracking the linear and angular momentum of a particle along two different paths through the gravitational waves and comparing the outcomes. The third observable examines how gravitational waves influence the relative positioning of two particles, especially when one has an intrinsic spin.

Each of these observables has been defined in a manner that allows for measurement with existing detectors. The methods for detecting curve deviation and the effects on spinning particles are described as "relatively straightforward," as they involve measuring separation while the observers monitor their respective accelerations.

Detecting the holonomy observable, however, poses more significant challenges—requiring observers to measure the local curvature of spacetime, potentially necessitating portable gravitational wave detectors. Given the size of the laser arms required for LIGO to detect even a single gravitational wave, this may be impractical at present. Nonetheless, the research team remains optimistic, with Flanagan stating, “We’ve already witnessed many exciting developments with gravitational waves, and we anticipate many more. Plans are even underway to deploy a gravitational wave detector in space, which would be sensitive to different sources than LIGO.”

An astrophysicist discusses the implications of the groundbreaking gravitational wave discovery—are we witnessing new physics or merging supermassive black holes?

This video details LIGO's detection of gravitational waves, shedding light on the significance of this monumental achievement in astrophysics.

Original research: “Persistent Gravitational Wave Observables: General Framework,” published April 26 in Physical Review D.