Investigating the possibility of probing fundamental space-time symmetries using gravitational wave memory

As predicted by the theory of general relativity, the passage of gravitational waves can leave a measurable change in the relative positions of objects. This physical phenomenon, known as gravitational wave memory, could potentially be used to study both gravitational waves and space-time.

Researchers from the Gran Sasso Science Institute (GSSI) and the International School for Advanced Studies (SISSA) recently conducted a study investigating the possibility of using the memory of gravitational waves to measure space-time symmetries, fundamental properties of space-time that remain the same after specific transformations. Their work, published in Physical Review Letterssuggests that these symmetries could be probed using observations of displacement and spin memory.

“For a long time, I have been curious about the phenomenon of gravitational wave memory and the connection of the related low-energy physics with quantum mechanics,” Boris Goncharov, co-author of the paper, told Phys.org. “I first heard about Weinberg’s soft graviton theorem from Professor Paul Lasky of Monash University in Australia during my PhD when I was discussing gravitational wave memory. Then I learned about the so-called ‘infrared triangle’ that connects the soft theorem to gravitational wave memory and Spacetime Symmetry at Infinity from Gravitational Wave Sources.”

Weinberg’s soft graviton theorem and the “infrared triangle” are mathematical formulations outlining the same physical phenomenon: the memory of gravitational waves. As part of their recent study, Gončarov and his colleagues decided to explore the possibility of using the memory of gravitational waves to probe the symmetries of space-time.

“This phenomenon plays a role in the ongoing attempt to describe Einstein’s hundred-year-old, unsinkable, yet microscopically incompatible theory of gravity — general relativity — as a quantum field theory at the asymptotic edge of spacetime,” Goncharov said.

“This approach to unification in physics seems substantial and promising to me; I find it very exciting. Our particular project emerged while discussing new advances in this field with Prof. Laura Donnay, co-author of the paper.”

When they examined previous literature in the field, the researchers found that a growing number of distant space-time symmetries had been discussed, but it was unclear which of these symmetries and corresponding memory terms existed in nature. While several physicists have explored the possibility of detecting gravitational wave memory, Goncharov and his colleagues were unsure what physics could be constrained using their measurements.

“The idea that we could test these space-time symmetries was central to our study,” Goncharov explained. “Another aspect is that I and Professor Jan Harms are members of the Einstein Telescope collaboration, for which it was important to explore the prospects of observing the memory of gravitational waves. The Einstein Telescope is a new generation European ground-based gravitational wave detector planned for the 2030s.”

Scientists have not yet established a conventional approach to measuring space-time symmetries by observing the memory effects of gravitational waves. A recent article by Gončarov and colleagues aimed to fill this apparent gap in the literature.

“Previously, there has been much important work focused on (a) predicting when and with which instruments we will be able to detect various gravitational wave memory terms, (b) how to calculate gravitational wave memory effects analytically or using numerical relativity, and (c) how various models of space-time symmetries they provide the memory terms of gravitational waves,” Goncharov said. “However, discussion of spatiotemporal symmetries based on observed memory effects appeared to be a gap in the literature.”

Recent work by these researchers could be considered a proof of principle. In their paper, they present new observational tests that could be used to probe spatiotemporal symmetries, while outlining potential limitations of their proposed approach that could be addressed in the future.

Overall, their study suggests that the set of tests of general relativity could be expanded. In addition, it provides some useful calculations that could be performed using data collected by various gravitational wave detectors.

Goncharov and his colleagues hope that their work will open up further discussions about space-time symmetries and gravitational wave memory among others within their research community. These discussions could potentially pave the way for the unification of different physical theories.

“At the moment, Sharon Tomson (a new PhD student at my current institute, AEI in Hannover, Germany) and Dr. With Rutger van Haasteren, I begin the search for gravitational wave memory using Pulsar Timing Arrays (PTAs). ).”

PTAs are instruments for astronomical observations that collect highly stable and regular signals from pulsars (i.e. rapidly rotating neutron stars) using radio telescopes on Earth. These neutron stars act like highly accurate clocks because they are sensitive enough to pick up the delay and progression of radio pulses resulting from the propagation of gravitational waves through the Milky Way.

“PTAs are galactic-scale detectors that currently appear to be gradually picking up the collective hum of slowly inspiraling supermassive binary black holes in the nearby universe. The signal provides slow variations in pulse arrival times that are most pronounced on timescales of several years within a decade,” Gončarov added.

“One prominent merger of supermassive binary black holes in a nearby galaxy can produce a gravitational wave burst with memory, detectable by PTAs. Although such bursts are very rare, we hope to glean some useful information from the data by constraining their existence.” .”

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