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Gravitational Waves Leave Behind Memories that Could Help Detect Waves that Pass Through the Universe

Observed first in 2016, gravitational waves provide new insights about the universe, with the ability to describe everything from the time soon after the Big Bang to latest events in galaxy centers.

Moreover, while the billion-dollar Laser Interferometer Gravitational-Wave Observatory (LIGO) detector looks for gravitational waves to pass through the Earth 24/7, a new study reveals that such waves leave behind a lot of “memories” that could be used to detect them even after they leave.

That gravitational waves can leave permanent changes to a detector after the gravitational waves have passed is one of the rather unusual predictions of general relativity.

Alexander Grant, Doctoral Candidate, Cornell University

Grant is the lead author of “Persistent Gravitational Wave Observables: General Framework,” published in Physical Review D on April 26th, 2019.

For a long time, physicists have been aware of the fact that gravitational waves leave a memory on the particles that come across their path. They have even identified five such memories. Currently, scientists have discovered three more secondary effects of the passing of a gravitational wave, “persistent gravitational wave observables” that could eventually help detect waves that pass through the universe.

According to Grant, each new observable offers distinct ways of validating the theory of general relativity and enables understanding the inherent properties of gravitational waves.

The researchers stated that those properties could enable the extraction of information from the Cosmic Microwave Background—the remnant radiation from the Big Bang.

We didn’t anticipate the richness and diversity of what could be observed.

Éanna Flanagan, Edward L. Nichols Professor and Chair of Physics and Professor of Astronomy, Cornell University

What was surprising for me about this research is how different ideas were sometimes unexpectedly related,” stated Grant. “We considered a large variety of different observables, and found that often to know about one, you needed to have an understanding of the other.”

Three observables were identified by the researchers, showing the impacts of gravitational waves in a flat region in spacetime that undergoes a burst of gravitational waves, following which it returns again to being a flat region. The first observable, called “curve deviation,” is the measure of separation between two accelerating observers from one another, compared to the way observers with the same accelerations would get isolated from one another in a flat space not disturbed by a gravitational wave.

The second observable, called “holonomy,” is derived by conveying information related to a particle’s linear and angular momentum along two distinct curves through the gravitational waves, as well as comparing the two different outcomes.

The third one examines the way in which gravitational waves have an impact on the relative displacement of two particles when one of the particles includes an inherent spin.

The researchers have defined each of these observables in a way that could be evaluated by a detector. According to the researchers, the procedures for detecting the curve deviation and the spinning particles are “relatively straightforward to perform,” requiring only “a means of measuring separation and for the observers to keep track of their respective accelerations.”

It would be highly challenging to detect the holonomy observable, they wrote, “requiring two observers to measure the local curvature of spacetime (potentially by carrying around small gravitational wave detectors themselves).” With regards to the size required for LIGO to detect even a single gravitational wave, the potential to detect holonomy observables is beyond the reach of existing science, say the researchers.

But we’ve seen a lot of exciting things already with gravitational waves, and we will see a lot more. There are even plans to put a gravitational wave detector in space that would be sensitive to different sources than LIGO,” stated Flanagan.

Abraham Harte (Dublin City University, Ireland) and David Nichols (University of Amsterdam, The Netherlands) were the other researchers who contributed to this study.

The National Science Foundation and the Netherlands Organization for Scientific Research supported this study.


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