Exploring the possibility of searching for fundamental space-time symmetries through gravitational wave memory

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

Researchers at the Gran Sasso Science Institute (GSSI) and the International School for Advanced Studies (SISSA) recently conducted a study exploring the possibility of using gravitational wave memory to measure spacetime symmetries, fundamental properties of spacetime that remain the same after specific transformations. Their paper, published in Physical review paperssuggests that these symmetries can be probed through the observation of displacement and rotation memory.

“For a long time, I was curious about the phenomenon of gravitational wave memory and the connection of low-energy physics related to quantum mechanics,” Boris Goncharov, co-author of the paper, told Phys.org. “I first heard about Weinberg’s soft graviton theorem from Prof. Paul Lasky at Monash University in Australia, during my PhD, when I discussed gravitational wave memory. Then I learned about the so-called ‘Infrared Triangle’ which relates the soft theorem to gravitational wave memory and spacetime symmetries at infinity from gravitational wave sources.”

Weinberg’s soft graviton theorem and the ‘infrared triangle’ are mathematical formulations that describe the same physical phenomenon: gravitational wave memory. As part of their latest study, Goncharov and his colleagues set out to explore the possibility of exploiting the memory of gravitational waves to probe space-time symmetries.

“This phenomenon plays a role in an ongoing effort to describe Einstein’s theory of gravity – General Relativity – centuries old, impregnable and yet incompatible with the microscopic world – as a quantum field theory at the asymptotic edge of space-time, ” Goncharov. said.

“I find this approach to a unification in physics essential and promising; I find it very exciting. Our specific project emerged while discussing new advances in this field with Prof. Laura Donnay, a co-author of the publication.”

When they reviewed the previous literature in the field, the researchers found that an increasing number of far-space symmetries were discussed, but it was not clear which of those symmetries and the corresponding memory terms exist in nature. While some physicists had explored the possibility of detecting gravitational wave memory, Goncharov and his colleagues weren’t sure what physics could be constrained using their measurements.

“The idea that we could test these symmetries of space was central to our study,” Goncharov explained. “Another aspect is that Prof. Jan Harms and I are members of the Einstein telescope collaboration, for which it was important to investigate the observational perspectives of gravitational wave memory. The Einstein telescope is the European next-generation gravitational wave detector planned for the 2030s”.

Until now, researchers had not yet introduced a conventional approach to measure space-time symmetries by observing the memory effects of gravitational waves. Recent work by Goncharov and colleagues aimed to fill this apparent gap in the literature.

“There was a lot of important previous work focusing on (a) predicting when and with which instruments we will be able to detect different terms of gravitational wave memory, (b) how to account for gravitational wave memory effects analytically or using numerical relativity, and (c) how different models of spacetime symmetries yield gravitational wave memory terms,” ​​Goncharov said. “However, a discussion of space-time symmetries based on observed memory effects appeared to be a gap in the literature.”

The recent work of these researchers can be seen as a proof of principle. In their paper, they present new observational tests that can be used to investigate space-time symmetries, also describing potential limitations of their suggested approach, which can be addressed in the future.

Overall, their study suggests that the test suite of the General Relativity theory can be expanded. In addition, it provides some useful calculations that can be performed using data collected by various gravitational wave detectors.

Goncharov and his colleagues hope their paper will open up further discussions about spacetime symmetries and gravitational wave memory among others within their research community. These discussions could potentially pave the way towards the unification of different theories of physics.

“At the moment, with Sharon Thomson (a young PhD student at my current institute, AEI in Hannover, Germany) and Dr. Rutger van Haasteren, I am starting a search for gravitational wave memory with Pulsar Timing Arrays (PTAs)”

PTAs are astronomical observatories that collect very stable and regular signals from pulsars (ie rapidly rotating neutron stars) using radio telescopes on Earth. These neutron stars behave like very precise clocks, as they are sensitive enough to pick up the delays and advances of radio pulses that come from the propagation of gravitational waves through the Milky Way.

“PTAs are galactic-scale detectors that currently appear to be gradually picking up a common noise of slowly exhaling binary supermassive black holes in the nearby universe. The signal yields slow variations in pulse arrival times that are more highlighted on time scales from several years to decades”, added Goncharov.

“An apparent merger of binary supermassive black holes in a nearby galaxy could cause a burst of gravitational waves with memory, distinct from PTA. Although such bursts are very rare, we hope to glean some useful information from data by imposing limitations on their existence.”

© 2024 Science X Network